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Modeling quantum devices is to find a model according to quantum theory that can explain the result of experiments in a quantum device. We find that usually we cannot correctly identify the model describing the actual physics of the device regardless of the experimental effort given a limited set of operations. According to sufficient conditions that we find, correctly reconstructing the model requires either a particular set of pure states and projective measurements or a set of evolution operators that can generate all unitary operators.

Quantum 2020

Quantum Devices Modeling and Verification

black-box quantum devices modeling

quantum tomography

quantum computing

The Institute of Physics and IOP Publishing, in partnership with the Chinese Physical Society (CPS) and the University of Science and Technology of China (USTC) are proud to have brought together the international quantum science community online for this inaugural conference.


Early career and leading researchers from universities, industry and government worldwide came together to share, learn and collaborate on the latest research and emerging areas of high interest across quantum science and technology. The broad themes covered by Quantum 2020 included:

Quantum communication;
Quantum computing and quantum simulation;
Quantum sensing, metrology and imaging;
Quantum information theory, software and algorithms;
Quantum foundations;
Quantum technology in industry.
The four-day event featured talks from world-renowned scientists, interactive poster sessions and two exciting panel discussions on the themes of Quantum Technology in Industry and Quantum Technology Programmes Around the World.


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The Institute of Physics and IOP Publishing, in partnership with the Chinese Physical Society (CPS) and the University of Science and Technology of China (USTC) are proud to have brought together the international quantum science community online for this inaugural conference.

Early career and leading researchers from universities, industry and government worldwide came together to share, learn and collaborate on the latest research and emerging areas of high interest across quantum science and technology. 

poster

We study the possibility of using dark-solitons in quasi one-dimensional Bose-Einstein condensates to produce two or three-level systems (qudits) by exploiting the intrinsic nonlinear and the coherent nature of the matter waves. More precisely, we intend to create the dark soliton qudits by using trapped impurities. The decoherence induced by the quantum fluctuations (phonons) produces a finite lifetime due to their intrinsic slow-time dynamics. Remarkably, the qubit lifetime is estimated to be of the order of a few seconds, being only limited by the dark-soliton death due to quantum evaporation. Further, we explore the spontaneous generation of phononic entanglement between dark soliton qubits in the dissipative process of spontaneous emission. By driving the qubits with the help of oscillating magnetic field gradient, we observe the formation of long distance steady-state concurrence. Our results suggest that dark-soliton qubits are a good candidates for quantum information protocols based purely on matter-wave phononics.

Dark Soliton Qudit and Entanglement Dynamics

Quantum entanglement is essential to the development of quantum computation, communications, and technology. The controlled SWAP test, widely used for state comparison, can be adapted to an efficient and general test for entanglement of a pure state. We show that the test can evidence the presence of entanglement (and further, genuine n-qubit entanglement), can distinguish entanglement classes, and generates the concurrence in the case of a 2-qubit state. We also propose a multipartite degree of entanglement, related to the test's probability outputs. The average number of measurements required to detect entanglement increases with decreased entanglement. Maximally entangled states require fewer measurements the larger the system, two on average for many (n≥8) qubits. Furthermore, the results are robust to second order when typical small errors are introduced to the state under investigation.

The controlled SWAP test for determining quantum entanglement


Simulation of a quantum many-body system at finite temperatures is crucially important but quite challenging. Here we present an experimentally feasible quantum algorithm assisted with continuous-variable for simulating quantum systems at finite temperatures. Our algorithm has a time complexity scaling polynomially with the inverse temperature and the desired accuracy.
We demonstrate the quantum algorithm by simulating finite temperature phase diagram of the quantum Ising and Kitaev models. It is found that the important crossover phase diagram of the Kitaev ring can be accurately simulated by a quantum computer with only a few qubits and thus the algorithm may be readily implemented on current quantum processors. We further propose a protocol implementable with superconducting or trapped ion quantum computers.

Continuous-variable assisted thermal quantum simulation

In this study, we investigated the academic literature on quantum technologies (QT) using bibliometric tools. We used a set of 49,823 articles obtained from the Web of Science (WoS) database using a search query constructed through expert opinion. Analysis of this revealed that QT is deeply rooted in physics, and the majority of the articles are published in physics journals. Keyword analysis revealed that the literature could be clustered into three distinct sets, which are (i) quantum communication/cryptography, (ii) quantum computation, and (iii) physical realizations of quantum systems. We performed a burst analysis that showed the emergence and fading away of certain key concepts in the literature. This is followed by co-citation analysis on the highly cited articles provided by the WoS, using these we devised a set of core corpus of 34 publications. Comparing the most highly cited articles in this set with respect to the initial set we found that there is a clear difference in most cited subjects. Finally, we performed co-citation analyses on country and organization levels to find the central nodes in the literature. Overall, the analyses of the datasets allowed us to cluster the literature into three distinct sets, construct the core corpus of the academic literature in QT, and to identify the key players on country and organization levels, thus offering insight into the current state of the field.

The Landscape of Academic Literature in Quantum Technologies

Solid state quantum emitters, like color centers in diamond and systems alike, offer appealing prerequisites for practicable devices that exploit quantum effects. For example, optically addressable quantum sensors, based on the ubiquitous nitrogen-vacancy defect, have been recently commercialized in magnetometers offering unprecedented spatial resolution. Another class of defects in diamond, based on group-IV elements, is currently attracting considerable interest due to potentially further improved properties. Thereby, the vast majority of experimental studies relies on identification, characterization, and manipulation of these quantum emitters by optical means - that is, in practice, by employing suitable laser light sources. Yet, the experimental demands are challenging. In frequency domain experiments, the characterization of single color defects often necessitates quasi-continuous laser frequency tuning over wide spectral ranges, while high finesse spectral resonances require sufficiently narrow linewidths. In turn, experiments that interrogate temporal dynamics, like initialization and read out of spin states, demand fast intensity modulation capability with high on/off ratios. In this context, we present principles, design challenges, and performance highlights of state-of-the-art laser light sources tailored to meet requirements typically encountered in quantum technology research. Throughout the presentation we keep an eye on emerging applications and present recent experimental data in an illustrative manner.

Novel continuous-wave lasers tailored for identification, characterization, and manipulation of single color centers in diamond

Nonlinear optical interaction at a single photon level plays a key role in quantum optical information processing. One of its important applications is optical switching with extremely low power consumption. Although optical switching controlled by a few photons has been demonstrated using strongly coupled quantum dot-photonic crystal cavity systems, the strong coupling regime brings significant fabrication challenges as high cavity quality factor and accurate QD positioning are required. This becomes more problematic for integrated photonic circuits where the cavity Q-factor is strongly degraded due to the waveguide coupling. In this work, we show that low-photon number (~2 photons) optical switching can be realized with a QD weakly coupled to a low-Q (only 600) cavity in the high Purcell factor regime. Our scheme significantly relaxes the fabrication requirement while achieving high switching speed (22.7 ps), high switching contrast (39%), and large tunability. Our work paves the way towards the practical quantum optical switch, especially for on-chip photonic circuits.

Ultrafast few-photon switching with a coherently driven quantum dot in the high Purcell factor regime

Quantum key distribution (QKD) is one of the most practical applications in quantum information processing, which can generate information-theoretical secure keys between remote parties. With the help of the wavelength-division multiplexing technique, QKD has been integrated with the classical optical communication networks. The wavelength-division multiplexing can be further improved by the mode-wavelength dual multiplexing technique with few-mode fiber (FMF), which has additional modal isolation and large effective core area of mode, and particularly is practical in fabrication and splicing technology compared with the multi-core fiber. Here, we present for the first time a QKD implementation coexisting with classical optical communication over weakly-coupled FMF using all-fiber mode-selective couplers. The co-propagation of QKD with one 100 Gbps classical data channel at -2.60 dBm launched power is achieved over 86 km FMF with 1.3 kbps real-time secure key generation. Compared with single-mode fiber using wavelength-division multiplexing, given the same fiber-input power, the Raman noise in FMF using the mode-wavelength dual multiplexing is reduced by 86% in average. Our work implements an important approach to the integration between QKD and classical optical communication and previews the compatibility of quantum communications with the next-generation mode division multiplexing networks.

Long-distance transmission of quantum key distribution coexisting with classical optical communication over a weakly-coupled few-mode fiber

The prospects of laterally-grown quasi-AlGaN/GaN/AlGaN hetero-structure High-Electron-Mobility-Avalanche-Transit-Time (HEMATT) oscillator as high-power THz-signal source is studied through an indigenously developed Quantum-Drift–Diffusion simulator and experimental verification is reported. Active region of the HEMATT oscillator consists of two-Dimensional Electron Gas (2-DEG), trapped within a GaN quantum well that lies in between AlGaN barriers on either side. Both anode and cathode consist of GaN. Under reverse-biased condition, GaN/AlGaN band offset forces minority electrons to enter into the heterostructure active region and remain confined within the GaN quantum well. Carrier current generation in this 2-DEG active region is due to the quantum confinement of minority electrons rather than conventional tunneling phenomenon. Such quantum confinement is effectively functioning as the gate-control terminal as in conventional HEMTs. Introduction of quasi-AlGaN instead of alloy AlGaN, constitutes of alternate fine layers of AlN/GaN, results in carrier mobility doubling (800cm2/V-sec to 1600cm2/V-sec). A subtle interplay between bound and quasi-bound states is observed due to the quasi-AlGaN barrier. The study reveals a spectacular optical sensitivity with designed HEMATT oscillators. This for the first time opens the prospect of newly reported HEMATT oscillators in quantum sensing to cater the needs of ever growing, sophisticated defense and industrial applications.

A 2-terminal quasi-AlGaN/GaN/AlGaN HEMATT oscillator as quanta-image sensor for single-photon detection applications in defense industry

Entanglement plays a central role in quantum information processing and quantum physics. However, few effective ways are known to detect the amount of entanglement of an unknown quantum state. Here, we propose a scheme to estimate the entanglement negativity for any bipartition of a composite system. The proposed scheme is based on the random unitary evolution and local measurements on a single-copy quantum state, which is more practical compared with former methods based on collective measurements on many copies of the identical state. Meanwhile, we generalize the scheme to quantify the total correlation. We demonstrate the efficiency of the scheme with statistical analyses and numerical simulations. Our scheme is quite suitable for state-of-the-art quantum platforms, which can serve as a useful benchmarking tool to advance quantum technologies and a probe to study fundamental quantum physics like entanglement dynamics.

Single-Copies Estimation of Entanglement Negativity

Quantum antiferromagnet is of fundamental interest as it becomes a fertile ground for studying exotic many-body states including spin liquids and high-temperature superconductors. Compared with condensed matter systems, ultracold gases in optical lattices can be microscopically engineered and provide a unique opportunity for exploring bosonic magnetism and spin dynamics. Here, we report on the creation of a one-dimensional Heisenberg antiferromagnet with ultracold bosons. In a two-component Bose-Hubbard system, we switch the sign of the spin-exchange interaction and realize the isotropic antiferromagnetic Heisenberg model in an extended 70-site chain. Starting from a low-entropy N\'eel-ordered state, the bosonic antiferromagnet is approached via an optimized adiabatic passage. We probe the evolution of staggered magnetization and spin correlations of the system, revealing the coherent establishment of the antiferromagnetism. Furthermore, the characteristic spin-rotational symmetry of the state and its robustness against decoherence are verified. Our experiment demonstrates a novel way for generating many-body spin correlations and exploring exotic forms of magnetism.

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Quantum Devices Modeling and Verification

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