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Hong-Ou-Mandel (HOM) interference plays the essential role in many quantum information processing protocols, such as Bell state measurement, quantum teleportation, measurement-device-independent quantum key distribution (MDI-QKD) and so on. Fiber-based HOM interference has been successfully implemented over a distance of 404 km. However, long distance free-space-based HOM interference remains challenging owing to the atmospheric turbulence. Here, by developing a robust adaptive optics system, a high precision time synchronization system and a frequency locking system, a HOM interference demonstration is implemented over a free-space channel of 19.2 km. A visibility of 0.474 ± 0.010 (0.412 ± 0.001) is observed with (without) intensity post-selection, which is used to match the intensity of the two interfering weak coherent pulses. Based on the excellent interference visibility, a MDI-QKD is demonstrated, and a secure key rate of 6.11 bit per second is observed.

Quantum 2020

Long distance free-space Hong-Ou-Mandel interference and measurement-device-independent quantum key distribution

mdi-qkd

hom interference

free-space

long distance

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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technical paper

Building on our previous work [Phys. Rev. Lett. 123, 140602] that explains the mechanism behind the dynamical detection of quantum phases via out-of-time-order correlators (OTOCs) at zero temperature, here we ask whether OTOCs could capture quantum order of the ground state at infinite temperature. Although the common intuition dictates that low energy quantum effects should be washed away by strong thermal fluctuations at high temperatures, in particular in nonintegrable systems that can host quantum chaos, we show that protection of quantum information against information scrambling at infinite temperature is possible under Ising (Z2) symmetry in the transverse-field Ising (Kitaev) 1D Hamiltonians. By utilizing Majorana basis, we demonstrate that the infinite-temperature OTOCs of edge observables probe zero-temperature quantum phases via detecting the presence of Majorana zero modes at the ends of the chain that is associated with 1D Z2 topological order. Therefore, we establish a connection between information scrambling and topological order which leads to a new phenomenon in the scrambling of nonintegrable models: topological order induced prescrambling that introduces a new timescale for the OTOC restricted from decaying for long times. In the prescrambling regime, OTOC demonstrates a well-defined plateau that encodes quantum order, paralleling the prethermalization of two-time correlators.

Topologically induced prescrambling of out-of-time-order correlators


Title: Experimental falsification of the pilot-wave interpretation of Quantum Mechanics and other consequences of a new model of ultra-thin gas optical transmittance
Authors: Jakub Ratajczak - University of Warsaw
Session: QF-1: Quantum Foundations - Default Location, 19/10/2020, 10:00 PM - 11:00 PM
Quantum Mechanics still lacks a widely recognized interpretation. Many are considered because they are hard to falsify experimentally. In this talk, we show that the results of a recent experiment (arXiv:2008.02799) go against one of them: the pilot-wave interpretation. This is because the key assumption of this interpretation on particles locality is in contradiction with the experiment's underlying model (arXiv:2008.04413v2) assumption of wave function non-locality. The recently developed model of ultra-thin gas optical transmittance takes into account the quantum mechanical effects of spontaneous spreading of the wave functions of individual gas particles. In the experiment we show that measured optical transmittance of such a gas depends on the detector size. The result of the experiment shows, with >5σ statistical significance, higher transmittances when the measurement is performed using smaller detectors. It is qualitatively in agreement with the developed model.

This is a rare example of a quantum model and an experiment not indifferent to the interpretation of Quantum Mechanics. Moreover, in this experiment a smooth transition from macroscopic to microscopic observers affecting the transmittance measurement gives a possibility of examining the quantum measurement problem.

We will also briefly address other conclusions regarding other phenomena predicted by the model.

Experimental falsification of the pilot-wave interpretation of Quantum Mechanics and other consequences of a new model of ultra-thin gas optical transmittance

With an ever-expanding ecosystem of noisy and intermediate-scale quantum devices, exploring their possible applications is a rapidly growing field of quantum information science. In this work, we demonstrate that variational quantum algorithms feasible on such devices address a challenge central to the field of quantum metrology: The identification of near-optimal probes and measurement operators for noisy multi-parameter estimation problems. We first introduce a general framework which allows for sequential updates of variational parameters to improve probe states and measurements and is widely applicable to both discrete and continuous-variable settings. We then demonstrate the practical functioning of the approach through numerical simulations, showcasing how tailored probes and measurements improve over standard methods in the noisy regime. Along the way, we prove the validity of a general parameter-shift rule for noisy evolutions, expected to be of general interest in variational quantum algorithms. In our approach, we advocate the mindset of quantum-aided design, exploiting quantum technology to learn close to optimal, experimentally feasible quantum metrology protocols.



Improving Quantum Sensing with Quantum Computers

Adiabatic evolutions of many-body quantum systems are very important in many fields of physics, from quantum computing to quantum thermodynamics to atomic and molecular physics, to name a few. 
However, current methods for characterizing adiabaticity are designed for pure states at zero temperature, while most applications are based on many-body systems at finite temperature, and so new insights and techniques are needed.
We demonstrate [1], [2] that appropriate metrics for the system state and for its corresponding local particle-density can be used to quantitatively determine the degree of adiabaticity of the dynamics of quantum many-body systems, at zero and finite temperature. This is important for quantum technologies and quantum thermodynamics related protocols. We show that the Bures and the trace distance, both well defined at finite temperatures, are reliable measures of adiabaticity, both qualitatively and quantitatively. In addition, adiabaticity can be characterized using only the local particle density distance. The local particle density is a much more accessible quantity, both experimentally and computationally, especially when handling many-body systems.
Metrics retain memory effects, and the importance of considering them is discussed by comparing the metrics' results to the ones obtained by extending the quantum adiabatic criterion to finite temperatures: the latter may produce false readings being quasi-Markovian by construction.
The possibility of characterizing the degree of adiabatic evolution via the system local particle densities, makes this method potentially applicable to very large many-body systems and to experiments.

[1] A. H. Skelt, R. W. Godby, and I. D'Amico, Phys. Rev. A 98, 012104 (2018)
[2] A. H. Skelt, and I. D'Amico, Adv. Quantum Technol. 3, 1900139 (2020)

Density and state metrics to measure adiabaticity in quantum many-body systems at zero and finite temperature

Oblivious Transfer (OT) is a cryptographic primitive where a sender Alice sends some information to a receiver Bob, who can only learn some but not all of this information, without Alice knowing what he has learned. One variant of OT is XOR OT (XOT), where Alice sends two bits, and Bob can either learn the first bit, the second bit, or their XOR. Perfect quantum XOT is impossible, but imperfect variants remain possible. We have investigated two quantum XOT protocols based on quantum state elimination. Alice sends one of four states to Bob and, depending on which two states he excludes, Bob can deduce one of the three aforementioned possibilities. 
Examining cheating probabilities in the protocols, we found that in one of them Bob has a relatively high cheating probability and Alice can cheat perfectly, unless Bob randomly tests the state Alice sends to him which decreases her cheating probability. In the other protocol, however, both Bob’s and Alice’s cheating probabilities are considerably lower and Alice’s cheating probability does not depend on any test done by Bob.
It remains an open problem what the lowest possible cheating probabilities are for quantum XOT, and for quantum OT in general.

XOR oblivious transfer based on quantum state elimination

In cellular communication, the main goal of resource allocation is to optimally assign the limitedly available radio resource blocks to the mobile user equipment (UE) present within the cell coverage region, based on the metrics dependent on the channel quality experienced by the UE such that it satisfies all the given constraints by maximizing the overall sum of user-perceived valuations. As this combinatorial optimization problem had been proved as an NP-hard problem, only approximate classical algorithms exist till date and they are also computationally expensive. Furthermore, the resource units must be reassigned at the beginning of each Transmission Time Interval (TTI) which is usually in milliseconds and so there is a certain need to perform this optimal allocation repeatedly in a faster manner. We solve this problem in LTE-B networks based on Orthogonal Frequency-Division Multiple Access (OFDMA), efficiently using a quantum annealer, by converting it to a Quadratic Unconstrained Binary Optimization (QUBO) problem. Moreover, we propose a quantum circuit which can solve this problem using the universal gate-based quantum computer. The experimental results prove that our novel quantum approach producing optimal/near-optimal results outperforms the existing classical algorithms in terms of computational speed by harnessing the power of quantum computers.

Quantum Technology for Allocating Radio Resources in Cellular Network

Recently, there has been much interest in the efficient preparation of complex quantum states using low-depth quantum circuits, such as Quantum Approximate Optimization Algorithm (QAOA). While it has been numerically shown that such algorithms prepare certain correlated states of quantum spins with surprising accuracy, a systematic way of quantifying the efficiency of QAOA in general classes of models has been lacking. Here, we propose that the success of QAOA in preparing ordered states is related to the interaction distance of the target state, which measures how close that state is to the manifold of all Gaussian states in an arbitrary basis of single-particle modes. We numerically verify this for the ground state of the quantum Ising model with arbitrary transverse and longitudinal field strengths, a canonical example of a non-integrable model. Our results suggest that the structure of the entanglement spectrum, as witnessed by the interaction distance, correlates with the success of QAOA state preparation. We conclude that QAOA typically finds a solution that perturbs around the closest free-fermion state.

Quantifying the efficiency of state preparation via quantum variational eigensolvers

Memory effects in the dynamics of open systems have been the subject of significant interest in the last decades. The methods involved in quantifying this effect, however, are often difficult to compute and may lack analytical insight. With this in mind, we study collisional models} where non-Markovianity is introduced by means of additional interactions between neighboring environmental units. 
We show that the dynamics can be cast in terms of a Markovian Embedding of the covariance matrix, which yields closed-form expressions for the memory kernel that governs the dynamics, a quantity that can seldom be computed analytically. 
The same is also possible for a divisibility monotone, based on the complete positivity of intermediate maps. By focusing on continuous-variables Gaussian dynamics, we are able to analytically study models of arbitrary size. 
We analyze in detail two types of interactions, a beam-splitter implementing a partial SWAP and a two-mode squeezing, which entangles the ancillas and, at the same time, feeds excitations into the system. By analyzing the memory kernel and divisibility for these two representative scenarios, our results help to shed light on the intricate mechanisms behind memory effects in the quantum domain.

Memory Kernel and CP-Divisibility in Gaussian Collisional Models

In this work some results of the investigation of the new crystalline Hall effect, proposed by Libor et. al., in the compound CoNb3S6 for the antiferromagnetic collinear phase are presented. The electronic properties of the compound are determined by first principle methods, including orbital spin coupling. The topological properties present in the band structure of the crystal were analyzed and the presence of an "Hourglass" dispersion relation was found, due to the presence of non-symmetrical rotation symmetry axes. The relationship between the symmetries of the compound and the conductivity tensor was also analyzed, finding that the point magnetic group of the antiferromagnetic compound with Néel vector [100] is 2'2'2, so the conductivity tensor indicates the presence of a crystalline Hall effect. Using Wannier functions and Kubo equation, the Berry curvature and anomalous Hall conductivity were derived. Being a collinear antiferromagnetic material, a zero Hall conductivity would be expected for all components, but due to the spontaneous break of symmetry generated by the non-magnetic sulfur atoms surrounding the cobalt atoms, an anomalous Hall conductivity appears, which could be explained by the proposed mechanism of the crystalline Hall effect.

Crystal Hall Effect in Collinear Antiferromagnets: symmetry and first principles analysis of CoNb3S6


Compared to the circuit model of quantum computation, there has been only limited work on error correction for continuous-time quantum computing. Building on an idea proposed in [Young et al., Phys. Rev. A88, 062314, 2013], multiple copies k of a small Ising spin model can be linked together via ferromagnetic or anti-ferromagnetic links. This increases the logical system's robustness to error. Numerical methods show that copies connected via anti-ferromagnetic links perform better than those connected via ferromagnetic links in some cases. A protocol enables us to determine for which models anti-ferromagnetic links are beneficial, allowing us to harness this performance improvement. This technique shows promise for increasing the robustness and precision in continuous-time quantum computing.

Long distance free-space Hong-Ou-Mandel interference and measurement-device-independent quantum key distribution

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