XY-mixers: analytical and numerical results for QAOA, 2020
The Quantum Alternating Operator Ansatz (QAOA) is a promising gate-model meta-heuristic for combi... more The Quantum Alternating Operator Ansatz (QAOA) is a promising gate-model meta-heuristic for combinatorial optimization. Applying the algorithm to problems with constraints presents an implementation challenge for near-term quantum resources. This work explores strategies for enforcing hard constraints by using XY-Hamiltonians as mixing operators (mixers). Despite the complexity of simulating the XY model, we demonstrate that for an integer variable admitting κ discrete values represented through one-hot-encoding, certain classes of the mixer Hamiltonian can be implemented without Trotter error in depth O(κ). We also specify general strategies for implementing QAOA circuits on all-to-all connected hardware graphs and linearly connected hardware graphs inspired by fermionic simulation techniques. Performance is validated on graph coloring problems that are known to be challenging for a given classical algorithm. The general strategy of using XY-mixers is borne out numerically, demonstrating a significant improvement over the general X-mixer, and moreover the generalized W-state yields better performance than easier-to-generate classical initial states when XY mixers are used.
Moiré excitons: From programmable quantum emitter arrays to spin-orbit-coupled artificial lattices, 2017
Highly uniform and ordered nanodot arrays are crucial for high-performance quantum optoelectronic... more Highly uniform and ordered nanodot arrays are crucial for high-performance quantum optoelectronics, including new semiconductor lasers and single-photon emitters, and for synthesizing artificial lattices of interacting quasiparticles toward quantum information processing and simulation of many-body physics. Van der Waals heterostructures of two-dimensional semiconductors are naturally endowed with an ordered nanoscale landscape, that is, the moiré pattern that laterally modulates electronic and topographic structures. We find that these moiré effects realize superstructures of nanodot confinements for long-lived interlayer excitons, which can be either electrically or strain tuned from perfect arrays of quantum emitters to excitonic superlattices with giant spin-orbit coupling (SOC). Besides the wide-range tuning of emission wavelength, the electric field can also invert the spin optical selection rule of the emitter arrays. This unprecedented control arises from the gauge structure imprinted on exciton wave functions by the moiré, which underlies the SOC when hopping couples nanodots into superlattices. We show that the moiré hosts complex hopping honeycomb superlattices, where exciton bands feature a Dirac node and two Weyl nodes, connected by spin-momentum-locked topological edge modes.
Coherence depletion in the Grover quantum search algorithm, 2017
We investigate the role of quantum coherence depletion (QCD) in Grover search algorithm (GA) by u... more We investigate the role of quantum coherence depletion (QCD) in Grover search algorithm (GA) by using several typical measures of quantum coherence and quantum correlations. By using the relative entropy of coherence measure (Cr), we show that the success probability depends on the QCD. The same phenomenon is also found by using the l1 norm of coherence measure (C l 1). In the limit case, the cost performance is defined to characterize the behavior about QCD in enhancing the success probability of GA, which is only related to the number of searcher items and the scale of database, no matter using Cr or C l 1. In generalized Grover search algorithm (GGA), the QCD for a class of states increases with the required optimal measurement time. In comparison, the quantification of other quantum correlations in GA, such as pairwise entanglement, multipartite entanglement, pairwise discord and genuine multipartite discord, cannot be directly related to the success probability or the optimal measurement time. Additionally, we do not detect pairwise nonlocality or genuine tripartite nonlocality in GA since Clauser-Horne-Shimony-Holt inequality and Svetlichny's inequality are not violated.
Quantum Supremacy for Simulating A Translation-Invariant Ising Spin Model, 2017
We introduce an intermediate quantum computing model built from translation-invariant Ising-inter... more We introduce an intermediate quantum computing model built from translation-invariant Ising-interacting spins. Despite being non-universal, the model cannot be classically efficiently simulated unless the polynomial hierarchy collapses. Equipped with the intrinsic single-instance-hardness property, a single fixed unitary evolution in our model is sufficient to produce classically intractable results, compared to several other models that rely on implementation of an ensemble of different unitaries (instances). We propose a feasible experimental scheme to implement our Hamiltonian model using cold atoms trapped in a square optical lattice. We formulate a procedure to certify the correct functioning of this quantum machine. The certification requires only a polynomial number of local measurements assuming measurement imperfections are sufficiently small. A universal quantum computer is believed to be able to solve certain tasks exponentially faster than the current computers [1, 2]. Over the past several decades, there has been tremendous progress in both theoretical and experimental developments of a quantum computer. In theory, pioneering quantum algorithms, including Shor's factor-ization [3] and an algorithm for linear systems of equations [4], achieve exponential speedup compared with the best-known classical algorithms. However, formidable experimental challenges still lie ahead in building a universal quantum computer large enough to demonstrate quantum supremacy. This calls for simpler tasks to demonstrate exponential quantum speedup without the need for a universal machine. Several intermediate computing models have been developed recently for this purpose. Examples include bo-son sampling [5], quantum circuits with commuting gates (IQP) [6, 7], sparse and "fault-tolerant" IQP [8, 9], the one-clean-qubit model [10, 11], evolution of two-qubit commuting Hamiltonians [12], quantum approximate optimization algorithm [13] and random or universal quantum circuit [14, 15]. These models fall into the category of sampling problems: the task of simulating the distribution sampled from the respective quantum system is believed to be classically intractable. In particular, if a classical computer can efficiently simulate the distribution to multiplicative errors, the polynomial hierarchy, a generalization of P and NP classes, will have to collapse to the third level [16, 17], which is believed to be unlikely in complexity theory. Several experiments (e.g. [20, 21]) have been reported for realization of boson sampling in small quantum systems using photons. However, the system size is still limited, which prohibits demonstration of quantum supremacy beyond classical tractability. In this paper, we report three advancements towards demonstration of exponential quantum speedup in intermediate computing models. First, we formulate a new sampling model built from translation-invariant Ising-interacting spins, with strong connection to simulation of natural quantum many-body systems [22-25]. Our model only requires nearest-neighbor Ising-type interactions. The state preparation, the Hamiltonian and measurements are all constructed to be translation-invariant. Similar to Refs. [5, 7], we prove the distribution sampled from our model cannot be classically efficiently simulated based on complexity theory results under reasonable conjectures [6, 26-28]. An additional desirable feature of our model, which we call the 'single-instance-hardness' property, is that a single fixed circuit and measurement pattern are sufficient to produce a classically hard distribution once the system size is fixed. This differs from typical sampling problems, where an ensemble of instances (unitaries) with a large number of parameters is demanded for the hardness result to hold [5-15]. This feature offers a significant simplification for experiments since proof of quantum supremacy for this model requires implementation of only a single Hamiltonian and measurement pattern instead of a range of different real-izations (typically an exponential number or even an infinite number). Ref. [5] also discussed the single-instance-hardness possibility in an abstract quantum circuit language , but no explicit circuit has been given thus far. Second, we propose a feasible experimental scheme to realize our model with cold atoms in optical lattices. The state preparation, engineering of time evolution and measurement techniques are achievable with the state-of-the-art technology. Unlike photonic systems, cold atomic systems are much easier to scale up and reach a system size intractable to classical machines. Finally, we devise a scheme to certify our proposed quantum machine based on extension of the techniques developed in Refs. [29, 30]. Certification of functionality is critically important for a sampling quantum machine as a correct sampling is hard to be verified. Our certification scheme only requires a polynomial number of local measurements, assuming the measurement imperfections are sufficiently small.
Ising Superconductivity and Quantum Phase Transition in Macro-Size Monolayer NbSe 2, 2017
Two-dimensional (2D) transition metal dichalcogenides (TMDs) have a range of unique physics pro... more Two-dimensional (2D) transition metal dichalcogenides (TMDs) have a range of unique physics properties and could be used in the development of electronics, photonics, spintronics and quantum computing devices. The mechanical exfoliation technique of micro-size TMD flakes has attracted particular interest due to its simplicity and cost effectiveness. However, for most applications, large area and high quality films are preferred. Furthermore, when the thickness of crystalline films is down to the 2D limit (monolayer), exotic properties can be expected due to the quantum confinement and symmetry breaking. In this paper, we have successfully prepared macro-size atomically flat monolayer NbSe2 films on bilayer graphene terminated surface of 6H-SiC(0001) substrates by molecular beam epitaxy (MBE) method. The films exhibit an onset superconducting critical transition temperature (Tconset) above 6 K, 2 times higher than that of mechanical exfoliated NbSe2 flakes. Simultaneously, the transport measurements at high magnetic fields reveal that the parallel characteristic field Bc// is at least 4.5 times higher than the paramagnetic limiting field, consistent with Zeeman-protected Ising superconductivity mechanism. Besides, by ultralow temperature electrical transport measurements, the monolayer NbSe2 film shows the signature of quantum Griffiths singularity when approaching the zero-temperature quantum critical point.
Gapless Spin Excitations in the Field-Induced Quantum Spin Liquid Phase of α-RuCl 3, 2017
α-RuCl3 is a leading candidate material for the observation of physics related to the Kitaev quan... more α-RuCl3 is a leading candidate material for the observation of physics related to the Kitaev quantum spin liquid (QSL). By combined susceptibility, specific-heat, and nuclear-magnetic-resonance measurements, we demonstrate that α-RuCl3 undergoes a quantum phase transition to a QSL in a magnetic field of 7.5 T applied in the ab plane. We show further that this high-field QSL phase has gapless spin excitations over a field range up to 16 T. This highly unconventional result, unknown in either Heisenberg or Kitaev magnets, offers insight essential to establishing the physics of α-RuCl3. The quantum spin liquid (QSL) is an exotic state of matter with long-range coherence but with no spontaneous breaking of translational or spin-rotational symmetry down to zero temperature [1]. Such a state in two or higher dimensions has implications for phenomena ranging from high-temperature superconductivity [2] to quantum computation [3, 4]. QSLs have long been sought in systems with strong geometric frustration [5-8], where magnetic order is destroyed by quantum fluctuations in a highly degenerate ground manifold. A more recent avenue to QSL formation is by competing interactions with combined spin and spatial anisotropies, as in the Kitaev model, where both gapped and gapless QSLs are realized exactly in a honeycomb-lattice spin-1/2 system [9]. A pure Kitaev Hamiltonian is hard to achieve in real materials. However, the compounds A 2 IrO 3 (A = Na, Li) [10-15] and α-RuCl 3 [16-22] are candidate systems for significant Kitaev-type interactions. In each case, the 4d (Ru 3+) or 5d (Ir 4+) ions form a Mott insulator on a honeycomb lattice, whose localized electrons have an effective spin j eff = 1/2 due to strong spin-orbit coupling [10, 23-26]. In α-RuCl 3 at zero field, a finite-energy continuum of magnetic excitations [27] is suggestive of fractionalized (spinon or Majorana-fermion) excitations [9, 28-32]. However, the ground states in all cases have "zig-zag" magnetic order [33-38], indicating the presence of significant non-Kitaev terms, whose exact nature continues to occupy many authors [39-48]. While the large T N in Na 2 IrO 3 [33, 34] suggests subdominant Kitaev terms, the relatively low T N of α-RuCl 3 has sparked an intensive search for experimental [49-51] and theoretical [52, 53] evidence for "proximate Kitaev" behavior. Here we report a nuclear-magnetic-resonance (NMR) investigation of high-quality single crystals of α-RuCl 3. With additional magnetic susceptibility and specific-heat measurements, we establish the phase diagram of Fig. 1. 0 4 8 12 16 0 10 20 30 40 T (K) 0 H (T) 3 4 5 H // ab T N from C v T N from T * from NMR Gapless QSL AFM PM FIG. 1: Magnetic phase diagram of α-RuCl3 with field applied in the ab plane. TN is determined from magne-tization and specific-heat data (Fig. 2). In the QSL phase, the color map represents the exponent, α, determined from the power-law form of the NMR spin-lattice relaxation rate, 1/ 35 T1 ∝ T α (Fig. 4). T * represents the upper limit of the gapless low-T regime. Inset: schematic representation of zero-field zigzag order in the hexagonal (ab) plane. We demonstrate the presence of a field-induced QSL beyond the quantum phase transition at µ 0 H c ≃ 7.5 T. In the field range between 7.5 T and 16 T, this partially polarized QSL has a spin-lattice relaxation rate with power-law temperature dependence, indicating effectively gap-less spin excitations with line-node dispersion. Single crystals of α-RuCl 3 were grown by chemical vapor transport. The high quality of this batch of crystals is demonstrated in Ref. [54] and x-ray characterization of the NMR sample, shown in Sec. S1 of the Supplemental Material (SM) [55], demonstrated that it is a single domain and free of twinning. Magnetization measure
Discovery of Intrinsic Quantum Anomalous Hall Effect in Organic Mn-DCA Lattice, 2017
The quantum anomalous Hall (QAH) phase is a novel topological state of matter characterized by a ... more The quantum anomalous Hall (QAH) phase is a novel topological state of matter characterized by a nonzero quantized Hall conductivity without an external magnetic field. The realizations of QAH effect, however, are experimentally challengeable. Based on ab initio calculations, here we propose an intrinsic QAH phase in Mn−dicyanoanthracene (DCA) Kagome lattice. The nontrivial topology in Kagome bands are confirmed by the nonzero chern number, quantized Hall conductivity, and gapless chiral edge states of Mn-DCA lattice. A tight-binding (TB) model is further constructed to clarify the origin of QAH effect. Furthermore, its Curie temperature, estimated to be ~ 253 K using Monte-Carlo simulation, is comparable with room temperature and higher than most of two-dimensional ferromagnetic thin films. Our findings present a reliable material platform for the observation of QAH effect in covalent-organic frameworks.
Quantum key distribution enables unconditionally secure key distribution between two legitimate u... more Quantum key distribution enables unconditionally secure key distribution between two legitimate users. The information-theoretic security is guaranteed by the fundamental laws of quantum physics. Initially, the quantum key distribution protocol was proposed based on the qubits. Later on, it was found that quantum continuous variables can also be exploited for this target. The continuous variable quantum key distribution can build upon standard telecommunication technology and exhibits a higher secret key rate per pulse at a relatively short distance due to the possibility of encoding more than 1 bit per pulse. In this article, we review the current status of the continuous variable quantum key distribution research, including its basic principle, experimental implementations, security and future directions; the experimental progress in this field made by our group is also presented.
TiL 4 -Coordinated Black Phosphorus Quantum Dots as an Efficient Contrast Agent for In Vivo Photoacoustic Imaging of Cancer Photoacoustic Imaging, 2017
better than those of organic molecules in in vivo PA bioim-aging. [4] Since near-infrared (NIR) l... more better than those of organic molecules in in vivo PA bioim-aging. [4] Since near-infrared (NIR) light can penetrate tissues, various NIR absorbing nanomaterials including metals, semiconductors , and carbon nanostructures have been explored. [5] Compared to organic dyes, inorganic nanoparticles can passively accumulate in tumors at larger concentration via the well-known enhanced permeability and retention (EPR) effect. [6] However, the potential long-term toxicity of the inorganic nanomaterials hamper wider adoption [7] and consequently , a new bioimaging nanoagent with good biocompat-ibility and efficient PA performance is demanded. Atomically thin black phosphorus (BP) is a new member of 2D materials family. Different from other 2D materials such as graphene, BP has a layer-dependent bandgap between 0.3 and 2.0 eV and is more versatile in electronic and optoelectronic applications. [8] Recently, ultrasmall BP nanosheets (referred to as BP quantum dots, BPQDs) prepared by liquid exfoliation have been shown to have good biocompatiblity, photothermal performance, and nonlinear optical properties. [8f,9] Nevertheless, BP is very reactive to oxygen and water, resulting in compositional/physical changes as well as subsequent degradation in the optical properties [10] and bioapplications. In this study, by means of surface coordination, [11] a sulfonic ester of the titanium ligand (TiL 4) is coordinated with the BPQDs to improve the stability in aqueous dispersions and the use of the materials as a photoacoustic imaging (PAI) agent in in vivo bioimaging of cancer is evaluated. The TiL 4 , a titanium sulfonate ligand (Figure S1a, Supporting Information), is synthesized by reacting titanium tetraisopropoxide [Ti(OiPr) 4 ] with p-toluenesulfonic acid in ethanol. [11] The BPQDs prepared by a liquid exfoliation technique [8f] are mixed with TiL 4 in N-methyl-2-pyrrolidone (NMP) at room temperature for 15 h to generate TiL 4 @BPQDs (Figure S1b, Supporting Information). Transmission electron microscopy (TEM) and atomic force microscopy (AFM) are performed to examine the morphology of the TiL 4 @BPQDs. The TEM images in Figure 1a and Figure S2 (Supporting Information) reveal the ultrasmall TiL 4 @BPQDs and the high-resolution TEM (HR-TEM) image in the inset shows lattice fringes of 0.34 nm ascribed to the (021) plane of the BP crystal. The topographical AFM image in Figure 1c shows that the measured heights of the TiL 4 @BPQDs are 1.6, 2.1, and 2.2 nm,
Long-distance free-space quantum key distribution in daylight towards inter-satellite communication, 2017
In the past, long-distance free-space quantum communication experiments could only be implemented... more In the past, long-distance free-space quantum communication experiments could only be implemented at night. During the daytime, the bright background sunlight prohibits quantum communication in transmission under conditions of high channel loss over long distances. Here, by choosing a working wavelength of 1,550 nm and developing free-space single-mode fibre-coupling technology and ultralow-noise upconversion single-photon detectors, we have overcome the noise due to sunlight and demonstrate free-space quantum key distribution over 53 km during the day. The total channel loss is ∼48 dB, which is greater than the 40 dB channel loss between the satellite and ground and between low-Earth-orbit satellites. Our system thus demonstrates the feasibility of satellite-based quantum communication in daylight. Moreover, given that our working wavelength is located in the optical telecom band, our system is naturally compatible with ground fibre networks and thus represents an essential step towards a satellite-constellation-based global quantum network. S atellite-based quantum communication has proven to be a feasible way to achieve a global-scale quantum communication network 1-9. Very recently, a low-Earth-orbit (LEO) satellite was launched 10 for this purpose. However, with a single satellite, an inefficient 3 day period 11 is required to provide worldwide con-nectivity. On the other hand, similar to how the Iridium system 12 functions in classical communication, a satellite constellation (SC) composed of many quantum satellites could provide global real-time quantum communication. Such an SC is expected to operate with LEO satellites or high-Earth-orbit satellites such as geosynchronous orbit (GEO) satellites. The probability of a satellite being in the Earth shadow zone decreases rapidly with increasing orbit height (Fig. 1). A LEO satellite system has a probability of ∼70% of being in the sunlight zone, whereas for a GEO satellite this rises to ∼99% (ref. 13). Meanwhile, the total channel loss between a LEO satellite and the Earth and between LEO satellites is typically ∼40-45 dB (refs 14,15). Therefore, to test the feasibility of an SC-based quantum network, quantum communication through a channel with at least ≥40 dB loss in daylight is essential. There have been several pioneering experiments on daylight quantum communication before our work 16-22. Although the experiments were novel, the maximum loss calculated from them was only ∼20 dB. The main cause of the unsatisfactory performance was the strong background noise from the scattered sunlight, which was typically five orders of magnitude greater than the background noise during the night 23. We can reduce this noise in three ways: working wavelength selection, spectrum filtering and spatial filtering. Working wavelength selection We first switched the working wavelength to 1,550.14 nm from the 700-900 nm used in all previous experiments. The 1,550 nm wavelength is known to be an atmospheric window. In fact, the transmission efficiency is slightly higher at 1,550 nm than at 800 nm, as shown in Fig. 1a, and from the solar spectrum in Fig. 1b we can see that the sunlight intensity at 1,550 nm is around five times weaker than it is at 800 nm. Furthermore, the main type of scattering of solar noise for links between a satellite and Earth or between two satellites is Rayleigh scattering, the intensity of which is proportional to 1/λ 4. Therefore, Rayleigh scattering at 1,550 nm is only 7% of its value at 800 nm. In total, the background noise with 1,550 nm light can be reduced to 3% of the background noise of 800 nm light. We measured the noise count rate of 1,550 nm light in the daylight case by pointing a telescope at the sky to simulate satellite-to-Earth communication. The result was smaller by a factor of 22.5 than for 850 nm light. Note that all existing free-space experiments without a satellite, including this work, have been implemented on Earth, and the direction of the free-space communication is parallel to the Earth rather than pointing at the sky. In this situation, Mie scattering , which does not follow the 1/λ 4 relation, will be the main noise source instead of Rayleigh scattering. Moreover, 1,550 nm is the telecom-band wavelength and is widely used for fibre-optical communication. Using the same wavelength for both free-space and fibre-optical communication is an optimal choice. Upconversion detectors and spectral filtering Despite the advantages of operating at 1,550 nm, researchers have been reluctant to use this wavelength due to a lack of good
Magnetic excitations and continuum of a possibly field-induced quantum spin liquid in α-RuCl3 Wan... more Magnetic excitations and continuum of a possibly field-induced quantum spin liquid in α-RuCl3 Wang, Z.; Reschke, S.; Hüvonen, D.; Do, S.-H.; Choi, K.-Y.; Gensch, M.; Nagel, U.; Rõõm, T.; Loidl, A.;
Epitaxy of Advanced Nanowire Quantum Devices, 2017
Semiconductor nanowires provide an ideal platform for various low-dimensional quantum devices. In... more Semiconductor nanowires provide an ideal platform for various low-dimensional quantum devices. In particular, topological phases of matter hosting non-Abelian quasi-particles can emerge when a semiconductor nanowire with strong spin-orbit coupling is brought in contact with a superconductor 1,2. To fully exploit the potential of non-Abelian anyons for topological quantum computing, they need to be exchanged in a well-controlled braiding operation 3-8. Essential hardware for braiding is a network of single-crystalline nanowires coupled to superconducting islands. Here, we demonstrate a technique for generic bottom-up synthesis of complex quantum devices with a special focus on nanowire networks having a predefined number of superconducting islands. Structural analysis confirms the high crystalline quality of the nanowire junctions, as well as an epitaxial superconductor-semiconductor interface. Quantum transport measurements of nanowire "hashtags" reveal Aharonov-Bohm and weak-antilocalization effects, indicating a phase coherent system with strong spin-orbit coupling. In addition, a
Graphene quantum dots (GQDs) hold great promise for applications in electronics, optoelectronics ... more Graphene quantum dots (GQDs) hold great promise for applications in electronics, optoelectronics and bioelectronics, but the fabrication of widely tunable GQDs has remained elusive. Here, we report the fabrication of atomically precise GQDs consisting of low-bandgap N = 14 armchair graphene nanoribbon (AGNR) segments that are achieved through edge fusion of N = 7 AGNRs. The so-formed intraribbon GQDs reveal deterministically defined, atomically sharp interfaces between wide and narrow AGNR segments and host a pair of low-lying interface states. Scanning tunneling microscopy/spectroscopy measurements complemented by extensive simulations reveal that their energy splitting depends exponentially on the length of the central narrow bandgap segment. This allows tuning of the fundamental gap of the GQDs over one order of magnitude within a few nanometers length range. These results are expected to pave the way for the development of widely tunable intraribbon GQD-based devices.
Two-Dimensional Arsenene Oxide: A Realistic Large-gap Quantum Spin Hall Insulator, 2017
Searching for two-dimensional (2D) realistic materials able to realize room-temperature quantum s... more Searching for two-dimensional (2D) realistic materials able to realize room-temperature quantum spin Hall (QSH) effects is currently a growing field. Here, we through ab initio calculations to identify arsenene oxide, AsO, as an excellent candidate, which demonstrates high stability, flexibility, and tunable spin-orbit coupling (SOC) gaps. In contrast to known pristine or functionalized arsenene, the maximum nontrivial band gap of AsO reaches 89 meV, and can be further enhanced to 130 meV under biaxial strain. By sandwiching 2D AsO between BN sheets, we propose a quantum well in which the band topology of AsO is preserved with a sizeable band gap. Considering that AsO having fully oxidized surfaces are naturally stable against surface oxidization and degradation, this functionality provides a viable strategy for designing topological quantum devices operating at room temperature. One of the grand challenges in condensed matter physics and materials science is to develop dissipationless electron conduction operating at room temperature. The appearance of band topology opens the door to fields of topological quantum states (TQSs), e.g., topological Dirac semimetal 1,2 , Weyl semimetal 3,4 , and node-line semimetal 5,6 , as well as topological insulators (TIs) 7 and so on. Remarkably, two-dimensional (2D) TIs hosts quantum spin Hall (QSH) effect with one-dimensional helical edge states, which can serve as a "two-lane highway" protected by time-reversal symmetry (TRS), making it more suited for coherent spin transport than 3D TIs. 8,9 However, the * Corresponding author: C. W. Zhang: ss_zhangchw@ujn.edu.cn experimental observations of quantized Hall conductance through QSH effect are only reported in HgTe/CdTe 10,11 and InAs/GaSb 12,13 quantum-wells with a ultralow temperature (<10 K). Hence considerable effort has been devoted to design new materials with QSH effect, and a variety of large-gap 2D TIs have been proposed, including the honeycomb lattices of silicene, 14,15 germanene, 16,17 stanene, 18-21 plumbene, 22 transition-metal halide 23 , ZrTe 5 /HfTe 5 24 , III-V bilayers 25 , BiF 26 , and Bi/Sb 27 , but none of them has been directly confirmed in experiments. Searching for realistic 2D TIs with large gap that can support high-temperature applications is vitally essential.
Light-Emitting Diodes High-Performance CsPb 1Àx Sn x Br 3 Perovskite Quantum Dots for Light- Emitting Diodes, 2017
All inorganic CsPbBr 3 perovskite quantum dots (QDs) are potential emitters for electroluminescen... more All inorganic CsPbBr 3 perovskite quantum dots (QDs) are potential emitters for electroluminescent displays. We have developed af acile hot-injection method to partially replace the toxicP b 2+ with highly stable Sn 4+ .M eanwhile,t he absolute photoluminescence quantum yield of CsPb 1Àx Sn x Br 3 increased from 45 %t o8 3% with Sn IV substitution. The transient absorption (TA) exciton dynamics in undoped CsPbBr 3 and CsPb 0.67 Sn 0.33 Br 3 QDs at various excitation fluences were determined by femtosecond transient absorption, time-resolved photoluminescence,and single-dot spectroscopy, providing clear evidence for the suppression of trion generation by Sn doping. These highly luminescent CsPb 0.67 Sn 0.33 Br 3 QDs emit at 517 nm. Ad evice based on these QDs exhibited al uminance of 12 500 cd m À2 ,acurrent efficiency of 11.63 cd A À1 ,a ne xternal quantum efficiency of 4.13 %, ap ower efficiency of 6.76 lm w À1 ,a nd al ow turn-on voltage of 3.6 V, whicha re the best values among reported tin-based perovskite quantum-dot LEDs.
Experimental Bayesian Quantum Phase Estimation on a Silicon Photonic Chip, 2017
Quantum phase estimation is a fundamental subroutine in many quantum algorithms, including Shor's... more Quantum phase estimation is a fundamental subroutine in many quantum algorithms, including Shor's factorization algorithm and quantum simulation. However, so far results have cast doubt on its practicability for near-term, nonfault tolerant, quantum devices. Here we report experimental results demonstrating that this intuition need not be true. We implement a recently proposed adaptive Bayesian approach to quantum phase estimation and use it to simulate molecular energies on a silicon quantum photonic device. The approach is verified to be well suited for prethreshold quantum processors by investigating its superior robustness to noise and decoherence compared to the iterative phase estimation algorithm. This shows a promising route to unlock the power of quantum phase estimation much sooner than previously believed.
High-Dimensional Single-Photon Quantum Gates: Concepts and Experiments, 2017
Transformations on quantum states form a basic building block of every quantum information system... more Transformations on quantum states form a basic building block of every quantum information system. From photonic polarization to two-level atoms, complete sets of quantum gates for a variety of qubit systems are well known. For multi-level quantum systems beyond qubits, the situation is more challenging. The orbital angular momentum modes of photons comprise one such high-dimensional system for which generation and measurement techniques are well-studied. However, arbitrary transformations for such quantum states are not known. Here we experimentally demonstrate a four-dimensional generalization of the Pauli X-gate and all of its integer powers on single photons carrying orbital angular momentum. Together with the well-known Z-gate, this forms the first complete set of high-dimensional quantum gates implemented experimentally. The concept of the X-gate is based on independent access to quantum states with different parities and can thus be easily generalized to other photonic degrees-of-freedom, as well as to other quantum systems such as ions and superconducting circuits. Introduction-High-dimensional quantum states have recently attracted increasing attention in both fundamental and applied research in quantum mechanics [1-5]. The possibility of encoding vast amounts of information on a single photon makes them particularly interesting for large-alphabet quantum communication protocols [6-9], as well as for investigating fundamental questions concerning local realism or quantum contextuality [10, 11]. The temporal and spatial structure of a photon provides a natural multi-mode state space in which to encode quantum information. The orbital angular momentum (OAM) modes of light [12] comprise one such basis of spatial modes that has emerged as a popular choice for experiments on high-dimensional quantum information [13]. While techniques for the generation and measurement of photonic quDits carrying OAM are well known [14-16], efficient methods for their control and transformation remain a challenge. No general recipe is known so far, and experimentally feasible techniques are known only for special cases. Here we experimentally demonstrate a four-dimensional X-gate and all of its integer powers with the orbital angular momentum modes of single photons. The four-dimensional X-gate is a generalization of the two-dimensional σ x Pauli transformation and acts as a cyclic ladder operator on a four-dimensional Hilbert space. The cyclic transformation required for this gate was designed through the use of the computer algorithm MELVIN [17] and recently demonstrated with classical states of light [18]. The Z-gate for OAM quDits (the generalization of the two-dimensional σ z Pauli transformation) introduces a mode-dependent phase,
Measuring Out-of-Time-Order Correlators on a Nuclear Magnetic Resonance Quantum Simulator, 2017
The idea of the out-of-time-order correlator (OTOC) has recently emerged in the study of both con... more The idea of the out-of-time-order correlator (OTOC) has recently emerged in the study of both condensed matter systems and gravitational systems. It not only plays a key role in investigating the holographic duality between a strongly interacting quantum system and a gravitational system, it also diagnoses the chaotic behavior of many-body quantum systems and characterizes information scrambling. Based on OTOCs, three different concepts-quantum chaos, holographic duality, and information scrambling-are found to be intimately related to each other. Despite its theoretical importance, the experimental measurement of the OTOC is quite challenging, and thus far there is no experimental measurement of the OTOC for local operators. Here, we report the measurement of OTOCs of local operators for an Ising spin chain on a nuclear magnetic resonance quantum simulator. We observe that the OTOC behaves differently in the integrable and nonintegrable cases. Based on the recent discovered relationship between OTOCs and the growth of entanglement entropy in the many-body system, we extract the entanglement entropy from the measured OTOCs, which clearly shows that the information entropy oscillates in time for integrable models and scrambles for nonintgrable models. With the measured OTOCs, we also obtain the experimental result of the butterfly velocity, which measures the speed of correlation propagation. Our experiment paves a way for experimentally studying quantum chaos, holographic duality, and information scrambling in many-body quantum systems with quantum simulators.
Deconfined Quantum Critical Points: Symmetries and Dualities, 2017
The deconfined quantum critical point (QCP), separating the Néel and valence bond solid phases in... more The deconfined quantum critical point (QCP), separating the Néel and valence bond solid phases in a 2D antiferromagnet, was proposed as an example of ð2 þ 1ÞD criticality fundamentally different from standard Landau-Ginzburg-Wilson-Fisher criticality. In this work, we present multiple equivalent descriptions of deconfined QCPs, and use these to address the possibility of enlarged emergent symmetries in the low-energy limit. The easy-plane deconfined QCP, besides its previously discussed self-duality, is dual to N f ¼ 2 fermionic quantum electrodynamics, which has its own self-duality and hence may have an Oð4Þ × Z T 2 symmetry. We propose several dualities for the deconfined QCP with SU(2) spin symmetry which together make natural the emergence of a previously suggested SO(5) symmetry rotating the Néel and valence bond solid orders. These emergent symmetries are implemented anomalously. The associated infrared theories can also be viewed as surface descriptions of ð3 þ 1ÞD topological paramagnets, giving further insight into the dualities. We describe a number of numerical tests of these dualities. We also discuss the possibility of "pseudocritical" behavior for deconfined critical points, and the meaning of the dualities and emergent symmetries in such a scenario.
An arbitrary unknown quantum state cannot be precisely measured or perfectly replicated1. However... more An arbitrary unknown quantum state cannot be precisely measured or perfectly replicated1. However, quantum teleportation allows faithful transfer of unknown quantum states from one object to another over long distance2, without physical travelling of the object itself. Long-distance teleportation has been recognized as a fundamental element in protocols such as large-scale quantum networks3,4 and distributed quantum computation5,6. However, the previous teleportation experiments between distant locations7-12 were limited to a distance on the order of 100 kilometers, due to photon loss in optical fibres or terrestrial free-space channels. An outstanding open challenge for a global-scale “quantum internet”13 is to significantly extend the range for teleportation. A promising solution to this problem is exploiting satellite platform and space-based link, which can conveniently connect two remote points on the Earth with greatly reduced channel loss because most of the photons’ propagation path is in empty space. Here, we report the first quantum teleportation of independent single-photon qubits from a ground observatory to a low Earth orbit satellite—through an up-link channel—with a distance up to 1400 km. To optimize the link efficiency and overcome the atmospheric turbulence in the up-link, a series of techniques are developed, including a compact ultra-bright source of multi-photon entanglement, narrow beam divergence, high-bandwidth and high-accuracy acquiring, pointing, and tracking (APT).
XY-mixers: analytical and numerical results for QAOA, 2020
The Quantum Alternating Operator Ansatz (QAOA) is a promising gate-model meta-heuristic for combi... more The Quantum Alternating Operator Ansatz (QAOA) is a promising gate-model meta-heuristic for combinatorial optimization. Applying the algorithm to problems with constraints presents an implementation challenge for near-term quantum resources. This work explores strategies for enforcing hard constraints by using XY-Hamiltonians as mixing operators (mixers). Despite the complexity of simulating the XY model, we demonstrate that for an integer variable admitting κ discrete values represented through one-hot-encoding, certain classes of the mixer Hamiltonian can be implemented without Trotter error in depth O(κ). We also specify general strategies for implementing QAOA circuits on all-to-all connected hardware graphs and linearly connected hardware graphs inspired by fermionic simulation techniques. Performance is validated on graph coloring problems that are known to be challenging for a given classical algorithm. The general strategy of using XY-mixers is borne out numerically, demonstrating a significant improvement over the general X-mixer, and moreover the generalized W-state yields better performance than easier-to-generate classical initial states when XY mixers are used.
Moiré excitons: From programmable quantum emitter arrays to spin-orbit-coupled artificial lattices, 2017
Highly uniform and ordered nanodot arrays are crucial for high-performance quantum optoelectronic... more Highly uniform and ordered nanodot arrays are crucial for high-performance quantum optoelectronics, including new semiconductor lasers and single-photon emitters, and for synthesizing artificial lattices of interacting quasiparticles toward quantum information processing and simulation of many-body physics. Van der Waals heterostructures of two-dimensional semiconductors are naturally endowed with an ordered nanoscale landscape, that is, the moiré pattern that laterally modulates electronic and topographic structures. We find that these moiré effects realize superstructures of nanodot confinements for long-lived interlayer excitons, which can be either electrically or strain tuned from perfect arrays of quantum emitters to excitonic superlattices with giant spin-orbit coupling (SOC). Besides the wide-range tuning of emission wavelength, the electric field can also invert the spin optical selection rule of the emitter arrays. This unprecedented control arises from the gauge structure imprinted on exciton wave functions by the moiré, which underlies the SOC when hopping couples nanodots into superlattices. We show that the moiré hosts complex hopping honeycomb superlattices, where exciton bands feature a Dirac node and two Weyl nodes, connected by spin-momentum-locked topological edge modes.
Coherence depletion in the Grover quantum search algorithm, 2017
We investigate the role of quantum coherence depletion (QCD) in Grover search algorithm (GA) by u... more We investigate the role of quantum coherence depletion (QCD) in Grover search algorithm (GA) by using several typical measures of quantum coherence and quantum correlations. By using the relative entropy of coherence measure (Cr), we show that the success probability depends on the QCD. The same phenomenon is also found by using the l1 norm of coherence measure (C l 1). In the limit case, the cost performance is defined to characterize the behavior about QCD in enhancing the success probability of GA, which is only related to the number of searcher items and the scale of database, no matter using Cr or C l 1. In generalized Grover search algorithm (GGA), the QCD for a class of states increases with the required optimal measurement time. In comparison, the quantification of other quantum correlations in GA, such as pairwise entanglement, multipartite entanglement, pairwise discord and genuine multipartite discord, cannot be directly related to the success probability or the optimal measurement time. Additionally, we do not detect pairwise nonlocality or genuine tripartite nonlocality in GA since Clauser-Horne-Shimony-Holt inequality and Svetlichny's inequality are not violated.
Quantum Supremacy for Simulating A Translation-Invariant Ising Spin Model, 2017
We introduce an intermediate quantum computing model built from translation-invariant Ising-inter... more We introduce an intermediate quantum computing model built from translation-invariant Ising-interacting spins. Despite being non-universal, the model cannot be classically efficiently simulated unless the polynomial hierarchy collapses. Equipped with the intrinsic single-instance-hardness property, a single fixed unitary evolution in our model is sufficient to produce classically intractable results, compared to several other models that rely on implementation of an ensemble of different unitaries (instances). We propose a feasible experimental scheme to implement our Hamiltonian model using cold atoms trapped in a square optical lattice. We formulate a procedure to certify the correct functioning of this quantum machine. The certification requires only a polynomial number of local measurements assuming measurement imperfections are sufficiently small. A universal quantum computer is believed to be able to solve certain tasks exponentially faster than the current computers [1, 2]. Over the past several decades, there has been tremendous progress in both theoretical and experimental developments of a quantum computer. In theory, pioneering quantum algorithms, including Shor's factor-ization [3] and an algorithm for linear systems of equations [4], achieve exponential speedup compared with the best-known classical algorithms. However, formidable experimental challenges still lie ahead in building a universal quantum computer large enough to demonstrate quantum supremacy. This calls for simpler tasks to demonstrate exponential quantum speedup without the need for a universal machine. Several intermediate computing models have been developed recently for this purpose. Examples include bo-son sampling [5], quantum circuits with commuting gates (IQP) [6, 7], sparse and "fault-tolerant" IQP [8, 9], the one-clean-qubit model [10, 11], evolution of two-qubit commuting Hamiltonians [12], quantum approximate optimization algorithm [13] and random or universal quantum circuit [14, 15]. These models fall into the category of sampling problems: the task of simulating the distribution sampled from the respective quantum system is believed to be classically intractable. In particular, if a classical computer can efficiently simulate the distribution to multiplicative errors, the polynomial hierarchy, a generalization of P and NP classes, will have to collapse to the third level [16, 17], which is believed to be unlikely in complexity theory. Several experiments (e.g. [20, 21]) have been reported for realization of boson sampling in small quantum systems using photons. However, the system size is still limited, which prohibits demonstration of quantum supremacy beyond classical tractability. In this paper, we report three advancements towards demonstration of exponential quantum speedup in intermediate computing models. First, we formulate a new sampling model built from translation-invariant Ising-interacting spins, with strong connection to simulation of natural quantum many-body systems [22-25]. Our model only requires nearest-neighbor Ising-type interactions. The state preparation, the Hamiltonian and measurements are all constructed to be translation-invariant. Similar to Refs. [5, 7], we prove the distribution sampled from our model cannot be classically efficiently simulated based on complexity theory results under reasonable conjectures [6, 26-28]. An additional desirable feature of our model, which we call the 'single-instance-hardness' property, is that a single fixed circuit and measurement pattern are sufficient to produce a classically hard distribution once the system size is fixed. This differs from typical sampling problems, where an ensemble of instances (unitaries) with a large number of parameters is demanded for the hardness result to hold [5-15]. This feature offers a significant simplification for experiments since proof of quantum supremacy for this model requires implementation of only a single Hamiltonian and measurement pattern instead of a range of different real-izations (typically an exponential number or even an infinite number). Ref. [5] also discussed the single-instance-hardness possibility in an abstract quantum circuit language , but no explicit circuit has been given thus far. Second, we propose a feasible experimental scheme to realize our model with cold atoms in optical lattices. The state preparation, engineering of time evolution and measurement techniques are achievable with the state-of-the-art technology. Unlike photonic systems, cold atomic systems are much easier to scale up and reach a system size intractable to classical machines. Finally, we devise a scheme to certify our proposed quantum machine based on extension of the techniques developed in Refs. [29, 30]. Certification of functionality is critically important for a sampling quantum machine as a correct sampling is hard to be verified. Our certification scheme only requires a polynomial number of local measurements, assuming the measurement imperfections are sufficiently small.
Ising Superconductivity and Quantum Phase Transition in Macro-Size Monolayer NbSe 2, 2017
Two-dimensional (2D) transition metal dichalcogenides (TMDs) have a range of unique physics pro... more Two-dimensional (2D) transition metal dichalcogenides (TMDs) have a range of unique physics properties and could be used in the development of electronics, photonics, spintronics and quantum computing devices. The mechanical exfoliation technique of micro-size TMD flakes has attracted particular interest due to its simplicity and cost effectiveness. However, for most applications, large area and high quality films are preferred. Furthermore, when the thickness of crystalline films is down to the 2D limit (monolayer), exotic properties can be expected due to the quantum confinement and symmetry breaking. In this paper, we have successfully prepared macro-size atomically flat monolayer NbSe2 films on bilayer graphene terminated surface of 6H-SiC(0001) substrates by molecular beam epitaxy (MBE) method. The films exhibit an onset superconducting critical transition temperature (Tconset) above 6 K, 2 times higher than that of mechanical exfoliated NbSe2 flakes. Simultaneously, the transport measurements at high magnetic fields reveal that the parallel characteristic field Bc// is at least 4.5 times higher than the paramagnetic limiting field, consistent with Zeeman-protected Ising superconductivity mechanism. Besides, by ultralow temperature electrical transport measurements, the monolayer NbSe2 film shows the signature of quantum Griffiths singularity when approaching the zero-temperature quantum critical point.
Gapless Spin Excitations in the Field-Induced Quantum Spin Liquid Phase of α-RuCl 3, 2017
α-RuCl3 is a leading candidate material for the observation of physics related to the Kitaev quan... more α-RuCl3 is a leading candidate material for the observation of physics related to the Kitaev quantum spin liquid (QSL). By combined susceptibility, specific-heat, and nuclear-magnetic-resonance measurements, we demonstrate that α-RuCl3 undergoes a quantum phase transition to a QSL in a magnetic field of 7.5 T applied in the ab plane. We show further that this high-field QSL phase has gapless spin excitations over a field range up to 16 T. This highly unconventional result, unknown in either Heisenberg or Kitaev magnets, offers insight essential to establishing the physics of α-RuCl3. The quantum spin liquid (QSL) is an exotic state of matter with long-range coherence but with no spontaneous breaking of translational or spin-rotational symmetry down to zero temperature [1]. Such a state in two or higher dimensions has implications for phenomena ranging from high-temperature superconductivity [2] to quantum computation [3, 4]. QSLs have long been sought in systems with strong geometric frustration [5-8], where magnetic order is destroyed by quantum fluctuations in a highly degenerate ground manifold. A more recent avenue to QSL formation is by competing interactions with combined spin and spatial anisotropies, as in the Kitaev model, where both gapped and gapless QSLs are realized exactly in a honeycomb-lattice spin-1/2 system [9]. A pure Kitaev Hamiltonian is hard to achieve in real materials. However, the compounds A 2 IrO 3 (A = Na, Li) [10-15] and α-RuCl 3 [16-22] are candidate systems for significant Kitaev-type interactions. In each case, the 4d (Ru 3+) or 5d (Ir 4+) ions form a Mott insulator on a honeycomb lattice, whose localized electrons have an effective spin j eff = 1/2 due to strong spin-orbit coupling [10, 23-26]. In α-RuCl 3 at zero field, a finite-energy continuum of magnetic excitations [27] is suggestive of fractionalized (spinon or Majorana-fermion) excitations [9, 28-32]. However, the ground states in all cases have "zig-zag" magnetic order [33-38], indicating the presence of significant non-Kitaev terms, whose exact nature continues to occupy many authors [39-48]. While the large T N in Na 2 IrO 3 [33, 34] suggests subdominant Kitaev terms, the relatively low T N of α-RuCl 3 has sparked an intensive search for experimental [49-51] and theoretical [52, 53] evidence for "proximate Kitaev" behavior. Here we report a nuclear-magnetic-resonance (NMR) investigation of high-quality single crystals of α-RuCl 3. With additional magnetic susceptibility and specific-heat measurements, we establish the phase diagram of Fig. 1. 0 4 8 12 16 0 10 20 30 40 T (K) 0 H (T) 3 4 5 H // ab T N from C v T N from T * from NMR Gapless QSL AFM PM FIG. 1: Magnetic phase diagram of α-RuCl3 with field applied in the ab plane. TN is determined from magne-tization and specific-heat data (Fig. 2). In the QSL phase, the color map represents the exponent, α, determined from the power-law form of the NMR spin-lattice relaxation rate, 1/ 35 T1 ∝ T α (Fig. 4). T * represents the upper limit of the gapless low-T regime. Inset: schematic representation of zero-field zigzag order in the hexagonal (ab) plane. We demonstrate the presence of a field-induced QSL beyond the quantum phase transition at µ 0 H c ≃ 7.5 T. In the field range between 7.5 T and 16 T, this partially polarized QSL has a spin-lattice relaxation rate with power-law temperature dependence, indicating effectively gap-less spin excitations with line-node dispersion. Single crystals of α-RuCl 3 were grown by chemical vapor transport. The high quality of this batch of crystals is demonstrated in Ref. [54] and x-ray characterization of the NMR sample, shown in Sec. S1 of the Supplemental Material (SM) [55], demonstrated that it is a single domain and free of twinning. Magnetization measure
Discovery of Intrinsic Quantum Anomalous Hall Effect in Organic Mn-DCA Lattice, 2017
The quantum anomalous Hall (QAH) phase is a novel topological state of matter characterized by a ... more The quantum anomalous Hall (QAH) phase is a novel topological state of matter characterized by a nonzero quantized Hall conductivity without an external magnetic field. The realizations of QAH effect, however, are experimentally challengeable. Based on ab initio calculations, here we propose an intrinsic QAH phase in Mn−dicyanoanthracene (DCA) Kagome lattice. The nontrivial topology in Kagome bands are confirmed by the nonzero chern number, quantized Hall conductivity, and gapless chiral edge states of Mn-DCA lattice. A tight-binding (TB) model is further constructed to clarify the origin of QAH effect. Furthermore, its Curie temperature, estimated to be ~ 253 K using Monte-Carlo simulation, is comparable with room temperature and higher than most of two-dimensional ferromagnetic thin films. Our findings present a reliable material platform for the observation of QAH effect in covalent-organic frameworks.
Quantum key distribution enables unconditionally secure key distribution between two legitimate u... more Quantum key distribution enables unconditionally secure key distribution between two legitimate users. The information-theoretic security is guaranteed by the fundamental laws of quantum physics. Initially, the quantum key distribution protocol was proposed based on the qubits. Later on, it was found that quantum continuous variables can also be exploited for this target. The continuous variable quantum key distribution can build upon standard telecommunication technology and exhibits a higher secret key rate per pulse at a relatively short distance due to the possibility of encoding more than 1 bit per pulse. In this article, we review the current status of the continuous variable quantum key distribution research, including its basic principle, experimental implementations, security and future directions; the experimental progress in this field made by our group is also presented.
TiL 4 -Coordinated Black Phosphorus Quantum Dots as an Efficient Contrast Agent for In Vivo Photoacoustic Imaging of Cancer Photoacoustic Imaging, 2017
better than those of organic molecules in in vivo PA bioim-aging. [4] Since near-infrared (NIR) l... more better than those of organic molecules in in vivo PA bioim-aging. [4] Since near-infrared (NIR) light can penetrate tissues, various NIR absorbing nanomaterials including metals, semiconductors , and carbon nanostructures have been explored. [5] Compared to organic dyes, inorganic nanoparticles can passively accumulate in tumors at larger concentration via the well-known enhanced permeability and retention (EPR) effect. [6] However, the potential long-term toxicity of the inorganic nanomaterials hamper wider adoption [7] and consequently , a new bioimaging nanoagent with good biocompat-ibility and efficient PA performance is demanded. Atomically thin black phosphorus (BP) is a new member of 2D materials family. Different from other 2D materials such as graphene, BP has a layer-dependent bandgap between 0.3 and 2.0 eV and is more versatile in electronic and optoelectronic applications. [8] Recently, ultrasmall BP nanosheets (referred to as BP quantum dots, BPQDs) prepared by liquid exfoliation have been shown to have good biocompatiblity, photothermal performance, and nonlinear optical properties. [8f,9] Nevertheless, BP is very reactive to oxygen and water, resulting in compositional/physical changes as well as subsequent degradation in the optical properties [10] and bioapplications. In this study, by means of surface coordination, [11] a sulfonic ester of the titanium ligand (TiL 4) is coordinated with the BPQDs to improve the stability in aqueous dispersions and the use of the materials as a photoacoustic imaging (PAI) agent in in vivo bioimaging of cancer is evaluated. The TiL 4 , a titanium sulfonate ligand (Figure S1a, Supporting Information), is synthesized by reacting titanium tetraisopropoxide [Ti(OiPr) 4 ] with p-toluenesulfonic acid in ethanol. [11] The BPQDs prepared by a liquid exfoliation technique [8f] are mixed with TiL 4 in N-methyl-2-pyrrolidone (NMP) at room temperature for 15 h to generate TiL 4 @BPQDs (Figure S1b, Supporting Information). Transmission electron microscopy (TEM) and atomic force microscopy (AFM) are performed to examine the morphology of the TiL 4 @BPQDs. The TEM images in Figure 1a and Figure S2 (Supporting Information) reveal the ultrasmall TiL 4 @BPQDs and the high-resolution TEM (HR-TEM) image in the inset shows lattice fringes of 0.34 nm ascribed to the (021) plane of the BP crystal. The topographical AFM image in Figure 1c shows that the measured heights of the TiL 4 @BPQDs are 1.6, 2.1, and 2.2 nm,
Long-distance free-space quantum key distribution in daylight towards inter-satellite communication, 2017
In the past, long-distance free-space quantum communication experiments could only be implemented... more In the past, long-distance free-space quantum communication experiments could only be implemented at night. During the daytime, the bright background sunlight prohibits quantum communication in transmission under conditions of high channel loss over long distances. Here, by choosing a working wavelength of 1,550 nm and developing free-space single-mode fibre-coupling technology and ultralow-noise upconversion single-photon detectors, we have overcome the noise due to sunlight and demonstrate free-space quantum key distribution over 53 km during the day. The total channel loss is ∼48 dB, which is greater than the 40 dB channel loss between the satellite and ground and between low-Earth-orbit satellites. Our system thus demonstrates the feasibility of satellite-based quantum communication in daylight. Moreover, given that our working wavelength is located in the optical telecom band, our system is naturally compatible with ground fibre networks and thus represents an essential step towards a satellite-constellation-based global quantum network. S atellite-based quantum communication has proven to be a feasible way to achieve a global-scale quantum communication network 1-9. Very recently, a low-Earth-orbit (LEO) satellite was launched 10 for this purpose. However, with a single satellite, an inefficient 3 day period 11 is required to provide worldwide con-nectivity. On the other hand, similar to how the Iridium system 12 functions in classical communication, a satellite constellation (SC) composed of many quantum satellites could provide global real-time quantum communication. Such an SC is expected to operate with LEO satellites or high-Earth-orbit satellites such as geosynchronous orbit (GEO) satellites. The probability of a satellite being in the Earth shadow zone decreases rapidly with increasing orbit height (Fig. 1). A LEO satellite system has a probability of ∼70% of being in the sunlight zone, whereas for a GEO satellite this rises to ∼99% (ref. 13). Meanwhile, the total channel loss between a LEO satellite and the Earth and between LEO satellites is typically ∼40-45 dB (refs 14,15). Therefore, to test the feasibility of an SC-based quantum network, quantum communication through a channel with at least ≥40 dB loss in daylight is essential. There have been several pioneering experiments on daylight quantum communication before our work 16-22. Although the experiments were novel, the maximum loss calculated from them was only ∼20 dB. The main cause of the unsatisfactory performance was the strong background noise from the scattered sunlight, which was typically five orders of magnitude greater than the background noise during the night 23. We can reduce this noise in three ways: working wavelength selection, spectrum filtering and spatial filtering. Working wavelength selection We first switched the working wavelength to 1,550.14 nm from the 700-900 nm used in all previous experiments. The 1,550 nm wavelength is known to be an atmospheric window. In fact, the transmission efficiency is slightly higher at 1,550 nm than at 800 nm, as shown in Fig. 1a, and from the solar spectrum in Fig. 1b we can see that the sunlight intensity at 1,550 nm is around five times weaker than it is at 800 nm. Furthermore, the main type of scattering of solar noise for links between a satellite and Earth or between two satellites is Rayleigh scattering, the intensity of which is proportional to 1/λ 4. Therefore, Rayleigh scattering at 1,550 nm is only 7% of its value at 800 nm. In total, the background noise with 1,550 nm light can be reduced to 3% of the background noise of 800 nm light. We measured the noise count rate of 1,550 nm light in the daylight case by pointing a telescope at the sky to simulate satellite-to-Earth communication. The result was smaller by a factor of 22.5 than for 850 nm light. Note that all existing free-space experiments without a satellite, including this work, have been implemented on Earth, and the direction of the free-space communication is parallel to the Earth rather than pointing at the sky. In this situation, Mie scattering , which does not follow the 1/λ 4 relation, will be the main noise source instead of Rayleigh scattering. Moreover, 1,550 nm is the telecom-band wavelength and is widely used for fibre-optical communication. Using the same wavelength for both free-space and fibre-optical communication is an optimal choice. Upconversion detectors and spectral filtering Despite the advantages of operating at 1,550 nm, researchers have been reluctant to use this wavelength due to a lack of good
Magnetic excitations and continuum of a possibly field-induced quantum spin liquid in α-RuCl3 Wan... more Magnetic excitations and continuum of a possibly field-induced quantum spin liquid in α-RuCl3 Wang, Z.; Reschke, S.; Hüvonen, D.; Do, S.-H.; Choi, K.-Y.; Gensch, M.; Nagel, U.; Rõõm, T.; Loidl, A.;
Epitaxy of Advanced Nanowire Quantum Devices, 2017
Semiconductor nanowires provide an ideal platform for various low-dimensional quantum devices. In... more Semiconductor nanowires provide an ideal platform for various low-dimensional quantum devices. In particular, topological phases of matter hosting non-Abelian quasi-particles can emerge when a semiconductor nanowire with strong spin-orbit coupling is brought in contact with a superconductor 1,2. To fully exploit the potential of non-Abelian anyons for topological quantum computing, they need to be exchanged in a well-controlled braiding operation 3-8. Essential hardware for braiding is a network of single-crystalline nanowires coupled to superconducting islands. Here, we demonstrate a technique for generic bottom-up synthesis of complex quantum devices with a special focus on nanowire networks having a predefined number of superconducting islands. Structural analysis confirms the high crystalline quality of the nanowire junctions, as well as an epitaxial superconductor-semiconductor interface. Quantum transport measurements of nanowire "hashtags" reveal Aharonov-Bohm and weak-antilocalization effects, indicating a phase coherent system with strong spin-orbit coupling. In addition, a
Graphene quantum dots (GQDs) hold great promise for applications in electronics, optoelectronics ... more Graphene quantum dots (GQDs) hold great promise for applications in electronics, optoelectronics and bioelectronics, but the fabrication of widely tunable GQDs has remained elusive. Here, we report the fabrication of atomically precise GQDs consisting of low-bandgap N = 14 armchair graphene nanoribbon (AGNR) segments that are achieved through edge fusion of N = 7 AGNRs. The so-formed intraribbon GQDs reveal deterministically defined, atomically sharp interfaces between wide and narrow AGNR segments and host a pair of low-lying interface states. Scanning tunneling microscopy/spectroscopy measurements complemented by extensive simulations reveal that their energy splitting depends exponentially on the length of the central narrow bandgap segment. This allows tuning of the fundamental gap of the GQDs over one order of magnitude within a few nanometers length range. These results are expected to pave the way for the development of widely tunable intraribbon GQD-based devices.
Two-Dimensional Arsenene Oxide: A Realistic Large-gap Quantum Spin Hall Insulator, 2017
Searching for two-dimensional (2D) realistic materials able to realize room-temperature quantum s... more Searching for two-dimensional (2D) realistic materials able to realize room-temperature quantum spin Hall (QSH) effects is currently a growing field. Here, we through ab initio calculations to identify arsenene oxide, AsO, as an excellent candidate, which demonstrates high stability, flexibility, and tunable spin-orbit coupling (SOC) gaps. In contrast to known pristine or functionalized arsenene, the maximum nontrivial band gap of AsO reaches 89 meV, and can be further enhanced to 130 meV under biaxial strain. By sandwiching 2D AsO between BN sheets, we propose a quantum well in which the band topology of AsO is preserved with a sizeable band gap. Considering that AsO having fully oxidized surfaces are naturally stable against surface oxidization and degradation, this functionality provides a viable strategy for designing topological quantum devices operating at room temperature. One of the grand challenges in condensed matter physics and materials science is to develop dissipationless electron conduction operating at room temperature. The appearance of band topology opens the door to fields of topological quantum states (TQSs), e.g., topological Dirac semimetal 1,2 , Weyl semimetal 3,4 , and node-line semimetal 5,6 , as well as topological insulators (TIs) 7 and so on. Remarkably, two-dimensional (2D) TIs hosts quantum spin Hall (QSH) effect with one-dimensional helical edge states, which can serve as a "two-lane highway" protected by time-reversal symmetry (TRS), making it more suited for coherent spin transport than 3D TIs. 8,9 However, the * Corresponding author: C. W. Zhang: ss_zhangchw@ujn.edu.cn experimental observations of quantized Hall conductance through QSH effect are only reported in HgTe/CdTe 10,11 and InAs/GaSb 12,13 quantum-wells with a ultralow temperature (<10 K). Hence considerable effort has been devoted to design new materials with QSH effect, and a variety of large-gap 2D TIs have been proposed, including the honeycomb lattices of silicene, 14,15 germanene, 16,17 stanene, 18-21 plumbene, 22 transition-metal halide 23 , ZrTe 5 /HfTe 5 24 , III-V bilayers 25 , BiF 26 , and Bi/Sb 27 , but none of them has been directly confirmed in experiments. Searching for realistic 2D TIs with large gap that can support high-temperature applications is vitally essential.
Light-Emitting Diodes High-Performance CsPb 1Àx Sn x Br 3 Perovskite Quantum Dots for Light- Emitting Diodes, 2017
All inorganic CsPbBr 3 perovskite quantum dots (QDs) are potential emitters for electroluminescen... more All inorganic CsPbBr 3 perovskite quantum dots (QDs) are potential emitters for electroluminescent displays. We have developed af acile hot-injection method to partially replace the toxicP b 2+ with highly stable Sn 4+ .M eanwhile,t he absolute photoluminescence quantum yield of CsPb 1Àx Sn x Br 3 increased from 45 %t o8 3% with Sn IV substitution. The transient absorption (TA) exciton dynamics in undoped CsPbBr 3 and CsPb 0.67 Sn 0.33 Br 3 QDs at various excitation fluences were determined by femtosecond transient absorption, time-resolved photoluminescence,and single-dot spectroscopy, providing clear evidence for the suppression of trion generation by Sn doping. These highly luminescent CsPb 0.67 Sn 0.33 Br 3 QDs emit at 517 nm. Ad evice based on these QDs exhibited al uminance of 12 500 cd m À2 ,acurrent efficiency of 11.63 cd A À1 ,a ne xternal quantum efficiency of 4.13 %, ap ower efficiency of 6.76 lm w À1 ,a nd al ow turn-on voltage of 3.6 V, whicha re the best values among reported tin-based perovskite quantum-dot LEDs.
Experimental Bayesian Quantum Phase Estimation on a Silicon Photonic Chip, 2017
Quantum phase estimation is a fundamental subroutine in many quantum algorithms, including Shor's... more Quantum phase estimation is a fundamental subroutine in many quantum algorithms, including Shor's factorization algorithm and quantum simulation. However, so far results have cast doubt on its practicability for near-term, nonfault tolerant, quantum devices. Here we report experimental results demonstrating that this intuition need not be true. We implement a recently proposed adaptive Bayesian approach to quantum phase estimation and use it to simulate molecular energies on a silicon quantum photonic device. The approach is verified to be well suited for prethreshold quantum processors by investigating its superior robustness to noise and decoherence compared to the iterative phase estimation algorithm. This shows a promising route to unlock the power of quantum phase estimation much sooner than previously believed.
High-Dimensional Single-Photon Quantum Gates: Concepts and Experiments, 2017
Transformations on quantum states form a basic building block of every quantum information system... more Transformations on quantum states form a basic building block of every quantum information system. From photonic polarization to two-level atoms, complete sets of quantum gates for a variety of qubit systems are well known. For multi-level quantum systems beyond qubits, the situation is more challenging. The orbital angular momentum modes of photons comprise one such high-dimensional system for which generation and measurement techniques are well-studied. However, arbitrary transformations for such quantum states are not known. Here we experimentally demonstrate a four-dimensional generalization of the Pauli X-gate and all of its integer powers on single photons carrying orbital angular momentum. Together with the well-known Z-gate, this forms the first complete set of high-dimensional quantum gates implemented experimentally. The concept of the X-gate is based on independent access to quantum states with different parities and can thus be easily generalized to other photonic degrees-of-freedom, as well as to other quantum systems such as ions and superconducting circuits. Introduction-High-dimensional quantum states have recently attracted increasing attention in both fundamental and applied research in quantum mechanics [1-5]. The possibility of encoding vast amounts of information on a single photon makes them particularly interesting for large-alphabet quantum communication protocols [6-9], as well as for investigating fundamental questions concerning local realism or quantum contextuality [10, 11]. The temporal and spatial structure of a photon provides a natural multi-mode state space in which to encode quantum information. The orbital angular momentum (OAM) modes of light [12] comprise one such basis of spatial modes that has emerged as a popular choice for experiments on high-dimensional quantum information [13]. While techniques for the generation and measurement of photonic quDits carrying OAM are well known [14-16], efficient methods for their control and transformation remain a challenge. No general recipe is known so far, and experimentally feasible techniques are known only for special cases. Here we experimentally demonstrate a four-dimensional X-gate and all of its integer powers with the orbital angular momentum modes of single photons. The four-dimensional X-gate is a generalization of the two-dimensional σ x Pauli transformation and acts as a cyclic ladder operator on a four-dimensional Hilbert space. The cyclic transformation required for this gate was designed through the use of the computer algorithm MELVIN [17] and recently demonstrated with classical states of light [18]. The Z-gate for OAM quDits (the generalization of the two-dimensional σ z Pauli transformation) introduces a mode-dependent phase,
Measuring Out-of-Time-Order Correlators on a Nuclear Magnetic Resonance Quantum Simulator, 2017
The idea of the out-of-time-order correlator (OTOC) has recently emerged in the study of both con... more The idea of the out-of-time-order correlator (OTOC) has recently emerged in the study of both condensed matter systems and gravitational systems. It not only plays a key role in investigating the holographic duality between a strongly interacting quantum system and a gravitational system, it also diagnoses the chaotic behavior of many-body quantum systems and characterizes information scrambling. Based on OTOCs, three different concepts-quantum chaos, holographic duality, and information scrambling-are found to be intimately related to each other. Despite its theoretical importance, the experimental measurement of the OTOC is quite challenging, and thus far there is no experimental measurement of the OTOC for local operators. Here, we report the measurement of OTOCs of local operators for an Ising spin chain on a nuclear magnetic resonance quantum simulator. We observe that the OTOC behaves differently in the integrable and nonintegrable cases. Based on the recent discovered relationship between OTOCs and the growth of entanglement entropy in the many-body system, we extract the entanglement entropy from the measured OTOCs, which clearly shows that the information entropy oscillates in time for integrable models and scrambles for nonintgrable models. With the measured OTOCs, we also obtain the experimental result of the butterfly velocity, which measures the speed of correlation propagation. Our experiment paves a way for experimentally studying quantum chaos, holographic duality, and information scrambling in many-body quantum systems with quantum simulators.
Deconfined Quantum Critical Points: Symmetries and Dualities, 2017
The deconfined quantum critical point (QCP), separating the Néel and valence bond solid phases in... more The deconfined quantum critical point (QCP), separating the Néel and valence bond solid phases in a 2D antiferromagnet, was proposed as an example of ð2 þ 1ÞD criticality fundamentally different from standard Landau-Ginzburg-Wilson-Fisher criticality. In this work, we present multiple equivalent descriptions of deconfined QCPs, and use these to address the possibility of enlarged emergent symmetries in the low-energy limit. The easy-plane deconfined QCP, besides its previously discussed self-duality, is dual to N f ¼ 2 fermionic quantum electrodynamics, which has its own self-duality and hence may have an Oð4Þ × Z T 2 symmetry. We propose several dualities for the deconfined QCP with SU(2) spin symmetry which together make natural the emergence of a previously suggested SO(5) symmetry rotating the Néel and valence bond solid orders. These emergent symmetries are implemented anomalously. The associated infrared theories can also be viewed as surface descriptions of ð3 þ 1ÞD topological paramagnets, giving further insight into the dualities. We describe a number of numerical tests of these dualities. We also discuss the possibility of "pseudocritical" behavior for deconfined critical points, and the meaning of the dualities and emergent symmetries in such a scenario.
An arbitrary unknown quantum state cannot be precisely measured or perfectly replicated1. However... more An arbitrary unknown quantum state cannot be precisely measured or perfectly replicated1. However, quantum teleportation allows faithful transfer of unknown quantum states from one object to another over long distance2, without physical travelling of the object itself. Long-distance teleportation has been recognized as a fundamental element in protocols such as large-scale quantum networks3,4 and distributed quantum computation5,6. However, the previous teleportation experiments between distant locations7-12 were limited to a distance on the order of 100 kilometers, due to photon loss in optical fibres or terrestrial free-space channels. An outstanding open challenge for a global-scale “quantum internet”13 is to significantly extend the range for teleportation. A promising solution to this problem is exploiting satellite platform and space-based link, which can conveniently connect two remote points on the Earth with greatly reduced channel loss because most of the photons’ propagation path is in empty space. Here, we report the first quantum teleportation of independent single-photon qubits from a ground observatory to a low Earth orbit satellite—through an up-link channel—with a distance up to 1400 km. To optimize the link efficiency and overcome the atmospheric turbulence in the up-link, a series of techniques are developed, including a compact ultra-bright source of multi-photon entanglement, narrow beam divergence, high-bandwidth and high-accuracy acquiring, pointing, and tracking (APT).
Uploads
Papers by Hong Wang