China

Topological quantum computing promises a hardware-level solution for the problem of decoherence. In particular, quantum gates based on the braiding of Majorana zero modes in superconductors are expected to be robust against control imperfections. Here, we combine earlier ideas for initialisation, control and readout to propose a setup where the robustness of the braiding-based gate could be demonstrated. By numerical experiments, we demonstrate how quasistatic disorder degrades the quality of braiding-based gates, and how that effect is reduced by increasing the system size. Our numerical experiments shed light on various practical aspects of future experiments aiming at demonstrating topologically protected quantum dynamics.

Quantum 2020

Robust topological quantum gates in numerical experiments

topological quantum computing

majorana qubits

decoherence

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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poster

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

We theoretically investigate [1] the error mechanisms that degrade charge, spin and Majorana qubit readout based on gate reflectometry [2]. We consider the effects of charge noise and overdrive effects. We find that charge noise leads to broadening of the distribution of the parametric capacitance of the device, and hence adds uncertainty to the measured reflectance, in addition to amplifier noise. Hence charge noise leads to a reduced readout fidelity. In reflectometry, it is tempting to increase the probe strength to maximize signal-to-noise ratio and hence the readout fidelity. However, we find that in the case of Majorana qubit readout, contrast to the charge and spin qubit, there is an optimal finite probe strength leading to a maximal readout fidelity. Moreover, we show that applying a strong probe pulse causes coherent transitions between quantum states, which leads to further reduction of the readout fidelity in the form of resonant dips.
References
[1] V.Derakhshan.Maman,M.F.Gonzalez-Zalba,andA.Pályi,“Chargenoiseandoverdrive errors in reflectometry-based charge, spin and Majorana qubit readout,” arXiv: 2006.12391, 1–13, Jun. 2020.
[2] S. Plugge, et.al., , “Majorana box qubits,” New J. Phys., vol. 19, no. 1, p. 012001, Jan. 2017.

Charge noise and overdrive errors in dispersive readout of charge and Majorana qubits

Precise probing of quantum systems is one of the keys to progress in diverse quantum technologies, including quantum sensing and metrology, quantum information processing and quantum many-body manipulations. Controlled quantum spins are sensitive probes of the environment and a powerful tool for characterizing highly-complex quantum systems on a molecular-scale. We explore the ability of a qubit-probe to characterize unknown parameters of its environment. By resorting to quantum estimation theory, we analytically find the ultimate bound on the precision of estimating key parameters of a broad class of ubiquitous environmental noises which the qubit may probe. By optimizing the dynamical control on the probe under realistic constraints one may attain the maximal accuracy bound on the estimation of these parameters by the least number of measurements possible [1-5]. Applications of this protocol to quantum sensing are illustrated for quantum qubit-probes serving as thermometers [3] and sensors for imaging or monitoring biological processes [4-5]
[1] A. Zwick, G. A. Álvarez, G. Kurizki, Phys. Rev. Applied 5: 014007, 2016.
[2] A. Zwick, G. A. Álvarez, G. Kurizki, Phys. Rev. A 94: 042122, 2016.
[3] V. Mukherjee, A. Zwick, A. Ghosh, X. Chen, G. Kurizki, Commun Phys 2: 162, 2019.
[4] A. Zwick, D. Suter, G. Kurizki, G. A. Álvarez, Phys. Rev. Applied 14, 024088, 2020.
[5] M. Capiglioni, A. Zwick, P. Jiménez and G. A. Álvarez, arXiv:2006.02035, 2020.

Operating quantum sensors at their ultimate precision limit by optimization

Based on the rapid developments of quantum technology, effectively manipulating single-photon quantum state transport along the frequency domain plays an important role in the field of quantum communications. Here, we propose an atom waveguide system, which can realize controllable light-atom interaction in the single-photon level. The system is composed of a 1D-waveguide coupling to a V-type three-level atom ultrastrongly driven by an external field. Under the ultrastrong driving, the rotating-wave approximation is no longer valid between the transition of the two atomic excited states, which induces the atomic virtual transitions and quantum interferences of the output photon. We perform both theoretical calculations and simulations to study the reflection and transmission spectra of the single-photon quantum transport. By properly choosing the frequency of the incident photon and the Rabi frequency of the driven field, one can generate different frequency-correlated single-photon states and achieve unidirectional single-photon frequency conversions [ACS Photonics. 7, 2010 (2020)]. Our work demonstrates the feasibility for all-optically manipulating single-photon quantum state in an atom-waveguide experimental platform, which may promote the fundamental research on the photon-photon interaction and development of the quantum information processing.

Multifunctional frequency manipulations on single photon in an atom-waveguide system

Limiting the energy of incoming/outgoing optical signals is a central task in quantum cryptography systems. We proposed and demonstrated an optical power limiter based on the thermo-optical defocusing effect of an acrylic prism. Its potential for constructing a centralized quantum secure network is highlighted.

Securing practical quantum cryptography with optical power limiters

we experimentally demonstrate a 1.25 GHz chip-based MDI QKD where both clients possess a low-cost silicon photonic transmitter chip. The transmitter can be further integrated with the laser based on wire-bounding or hybrid integration. We perform a complete demonstration of polarization-encoding MDI-QKD and distill finite-key rates higher than the previous experiment. We also characterize the side channels observed in our high-speed chip-based MDI QKD system. This work paves the way for low-cost, wafer-scale manufactured MDI-QKD system, and represents a key step towards building QKD network with untrusted relays.

High-speed MDI-QKD with silicon photonics

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.

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

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.

Robust topological quantum gates in numerical experiments

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