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Eavesdropper's ability to attack a free-space quantum-key-distribution receiver in atmospheric turbulence

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Physical Review A 2019年第6期99卷 062315-062315页

作者： Poompong Chaiwongkhot Katanya B. Kuntz Yanbao Zhang Anqi Huang Jean-Philippe Bourgoin Shihan Sajeed Norbert Lütkenhaus Thomas Jennewein Vadim Makarov Institute for Quantum Computing University of Waterloo Waterloo ON Canada N2L 3G1 Department of Physics and Astronomy University of Waterloo Waterloo ON Canada N2L 3G1 NTT Basic Research Laboratories NTT Corporation Atsugi Kanagawa 243-0198 Japan NTT Research Center for Theoretical Quantum Physics NTT Corporation Atsugi Kanagawa 243-0198 Japan Institute for Quantum Information and State Key Laboratory of High Performance Computing College of Computer National University of Defense Technology Changsha 410073 People's Republic of China Department of Electrical and Computer Engineering University of Waterloo Waterloo ON Canada N2L 3G1 Aegis Quantum Waterloo ON Canada Department of Electrical and Computer Engineering University of Toronto Toronto Canada M5S 3G4 Quantum Information Science Program Canadian Institute for Advanced Research Toronto ON Canada M5G 1Z8 Russian Quantum Center Skolkovo Moscow 143025 Russia Shanghai Branch National Laboratory for Physical Sciences at Microscale and CAS Center for Excellence in Quantum Information University of Science and Technology of China Shanghai 201315 People's Republic of China NTI Center for Quantum Communications National University of Science and Technology MISiS Moscow 119049 Russia

The ability of an eavesdropper (Eve) to perform an intercept-resend attack on a free-space quantum-key-distribution (QKD) receiver by precisely controlling the incidence angle of an attack laser has been previously demonstrated. However, such an attack could be ineffective in the presence of atmospheric turbulence due to beam wander and spatial mode aberrations induced by the air's varying index of refraction. We experimentally investigate the impact turbulence has on Eve's attack on a free-space polarization-encoding QKD receiver by emulating atmospheric turbulence with a spatial light modulator. Our results identify how well Eve would need to compensate for turbulence to perform a successful attack by either reducing her distance to the receiver or using beam wavefront correction via adaptive optics. Furthermore, we use an entanglement-breaking scheme to find a theoretical limit on the turbulence strength that hinders Eve's attack.

关键词： quantum cryptography

Thresholded quantum LIDAR: Exploiting Photon-Number-Resolving Detection

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Physical Review Letters 2019年第20期123卷 203601-203601页

作者： Lior Cohen Elisha S. Matekole Yoni Sher Daniel Istrati Hagai S. Eisenberg Jonathan P. Dowling Hearne Institute for Theoretical Physics and Department of Physics and Astronomy Louisiana State University Baton Rouge Louisiana 70803 USA School of Computer Science and Engineering Hebrew University of Jerusalem Jerusalem 91904 Israel Racah Institue for Physics Hebrew University of Jerusalem Jerusalem 91904 Israel NYU-ECNU Institute of Physics at NYU Shanghai 3663 Zhongshan Road North Shanghai 200062 China CAS-Alibaba Quantum Computing Laboratory CAS Center for Excellence in Quantum Information and Quantum Physics University of Science and Technology of China Shanghai 201315 China National Institute of Information and Communications Technology 4-2-1 Nukui-Kitamachi Koganei Tokyo 184-8795 Japan

We present a technique that improves the signal-to-noise-ratio (SNR) of range-finding, sensing, and other light-detection applications. The technique filters out low photon numbers using photon-number-resolving detectors. This technique has no classical analog and cannot be done with classical detectors. We investigate the properties of our technique and show under what conditions the scheme surpasses the classical SNR. Finally, we simulate the operation of a rangefinder, showing improvement with a low number of signal samplings and confirming the theory with a high number of signal samplings.

关键词： Photon statistics quantum measurements quantum sensing

Structural Monoclinicity and its coupling to layered magnetism in few-layer CrI3

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arXiv 2021年

作者： Guo, Xiaoyu Jin, Wencan Ye, Zhipeng Ye, Gaihua Xie, Hongchao Yang, Bowen Kim, Hyun Ho Yan, Shaohua Fu, Yang Tian, Shangjie Lei, Hechang Tsen, Adam W. Sun, Kai Yan, Jia-An He, Rui Zhao, Liuyan Department of Physics University of Michigan 450 Church Street Ann ArborMI48109 United States Department of Physics Auburn University 380 Duncan Drive AuburnAL36849 United States Department of Electrical and Computer Engineering Texas Tech University 910 Boston Ave LubbockTX79409 United States Institute for Quantum Computing Department of Physics and Astronomy Department of Chemistry University of Waterloo ONN2L 3G1 Canada School of Materials Science and Engineering Kumoh National Institute of Technology Gumi39177 Korea Republic of Department of Physics Beijing Key Laboratory of Opto-electronic Functional Materials & Micro-nano Devices Renmin University of China Beijing100872 China Department of Physics Astronomy & Geosciences Towson University TowsonMD21252 United States

Using polarization-resolved Raman spectroscopy, we investigate layer number, temperature, and magnetic field dependence of Raman spectra in one- to four-layer CrI3. Layer-number-dependent Raman spectra show that in the paramagnetic phase a doubly degenerated Eg mode of monolayer CrI3 splits into one Ag and one Bg mode in N-layer (N > 1) CrI3 due to the monoclinic stacking. Their energy separation increases in thicker samples until an eventual saturation. Temperature-dependent measurements further show that the split modes tend to merge upon cooling but remain separated until 10 K, indicating a failed attempt of the monoclinic-to-rhombohedral structural phase transition that is present in the bulk crystal. Magnetic-field-dependent measurements reveal an additional monoclinic distortion across the magnetic-field-induced layered antiferromagnetism-to-ferromagnetism phase transition. We propose a structural change that consists of both a lateral sliding toward the rhombohedral stacking and a decrease in the interlayer distance to explain our experimental observations. Copyright © 2021, The Authors. All rights reserved.

关键词： Chromium compounds

Proving expected sensitivity of probabilistic programs with randomized variable-dependent termination time

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arXiv 2019年

作者： Wang, Peixin Fu, Hongfei Chatterjee, Krishnendu Deng, Yuxin Xu, Ming Basics Lab Shanghai Jiao Tong University Shanghai China John Hopcroft Center for Computer Science Shanghai Jiao Tong University Shanghai China IST Austria Institute of Science and Technology Austria Klosterneuburg Austria Shanghai Key Laboratory of Trustworthy Computing East China Normal University Shanghai China Center for Quantum Computing Pengcheng Laboratory Shenzhen China

The notion of program sensitivity (aka Lipschitz continuity) specifies that changes in the program input result in proportional changes to the program output. For probabilistic programs the notion is naturally extended to expected sensitivity. A previous approach develops a relational program logic framework for proving expected sensitivity of probabilistic while loops, where the number of iterations is fixed and bounded. In this work, we consider probabilistic while loops where the number of iterations is not fixed, but randomized and depends on the initial input values. We present a sound approach for proving expected sensitivity of such programs. Our sound approach is martingale-based and can be automated through existing martingale-synthesis algorithms. Furthermore, our approach is compositional for sequential composition of while loops under a mild side condition. We demonstrate the effectiveness of our approach on several classical examples from Gambler's Ruin, stochastic hybrid systems and stochastic gradient descent. We also present experimental results showing that our automated approach can handle various probabilistic programs in the literature. Copyright © 2019, The Authors. All rights reserved.

关键词： Stochastic systems

Electron-phonon coupling in d-electron solids: A temperature-dependent study of rutile TiO2 by first-principles theory and two-photon photoemission

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arXiv 2020年

作者： Shang, Honghui Argondizzo, Adam Tan, Shijing Zhao, Jin Rinke, Patrick Carbogno, Christian Scheffler, Matthias Petek, Hrvoje Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 46 Berlin-DahlemD-14195 Germany Department of Physics and Astronomy Pittsburgh Quantum Institute University of Pittsburgh PittsburghPA15260 United States Department of Physics University of Science and Technology of China Hefei230026 China Department of Applied Physics Aalto University P.O. Box 11100 AaltoFI-00076 Finland State Key Laboratory of Computer Architecture Institute of Computing Technology Chinese Academy of Sciences Beijing100190 China

Rutile TiO2 is a paradigmatic transition-metal oxide with applications in optics, electronics, photocatalysis, etc., that are subject to pervasive electron-phonon interaction. To understand how energies of its electronic bands, and in general semiconductors or metals where the frontier orbitals have a strong d-band character, depend on temperature, we perform a comprehensive theoretical and experimental study of the effects of electron-phonon (e-p) interactions. In a two-photon photoemission (2PP) spectroscopy study we observe an unusual temperature dependence of electronic band energies within the conduction band of reduced rutile TiO2, which is contrary to the well-understood sp-band semiconductors and points to a so far unexplained dichotomy in how the e-p interactions affect differently the materials where the frontier orbitals are derived from the sp- and d orbitals. To develop a broadly applicable model, we employ state-of-the-art first-principles calculations that explain how phonons promote interactions between the Ti-3d orbitals of the conduction band within the octahedral crystal field. The characteristic difference in e-p interactions experienced by the Ti-3d orbitals of rutile TiO2 crystal lattice are contrasted with the more familiar behavior of the Si-2s orbitals of stishovite SiO2 polymorph, in which the frontier 2s orbital experiences a similar crystal field with the opposite effect. The findings of this analysis of how e-p interactions affect the d- and sp-orbital derived bands can be generally applied to related materials in a crystal field. The calculated temperature dependence of d-orbital derived band energies agrees well with and explains the temperature-dependent inter-d-band transitions recorded in 2PP spectroscopy of TiO2. The general understanding of how e-pinteractions affect d-orbital derived bands is likely to impact the understanding of temperature-dependent properties of highly correlated materials. Copyright © 2020, The Authors. All r

关键词： Temperature distribution

Universal constraints on energy flow and SYK thermalization

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arXiv 2019年

作者： Almheiri, Ahmed Milekhin, Alexey Swingle, Brian Institute for Advanced Study PrincetonNJ United States Physics Department Princeton University PrincetonNJ United States Condensed Matter Theory Center Maryland Center for Fundamental Physics Joint Institute for Quantum Information and Computer Science Department of Physics University of Maryland College Park United States

We study the dynamics of a quantum system in thermal equilibrium that is suddenly coupled to a bath at a different temperature, a situation inspired by a particular black hole evaporation protocol. We prove a universal positivity bound on the integrated rate of change of the system energy which holds perturbatively in the system-bath coupling. Applied to holographic systems, this bound implies a particular instance of the averaged null energy condition. We also study in detail the particular case of two coupled SYK models in the limit of many fermions using the Schwinger-Keldysh non-equilibrium formalism. We solve the resulting Kadanoff-Baym equations both numerically and analytically in various limits. In particular, by going to low temperature, this setup enables a detailed study of the evaporation of black holes in JT gravity. Copyright © 2019, The Authors. All rights reserved.

关键词： quantum optics

Wigner negativity in spin-j systems

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arXiv 2020年

作者： Davis, Jack Kumari, Meenu Mann, Robert B. Ghose, Shohini Department of Physics and Astronomy University of Waterloo WaterlooONN2L 3G1 Canada Institute for Quantum Computing University of Waterloo WaterlooONN2L 3G1 Canada Perimeter Institute for Theoretical Physics 31 Caroline St N WaterlooONN2L 2Y5 Canada Department of Physics and Computer Science Wilfrid Laurier University WaterlooONN2L 3C5 Canada

The nonclassicality of simple spin systems as measured by Wigner negativity is studied on a spherical phase space. Several SU(2)-covariant states with common qubit representations are addressed: spin coherent, spin cat (GHZ/N00N), and Dicke (W). We derive a bound on the Wigner negativity of spin cat states that rapidly approaches the true value as spin increases beyond j & 5. We find that spin cat states are not significantly Wigner-negative relative to their Dicke state counterparts of equal dimension. We also find, in contrast to several entanglement measures, that the most Wigner-negative Dicke basis element is spin-dependent, and not the equatorial state |j, 0i (or |j, ±1/2i for half-integer spins). These results underscore the influence that dynamical symmetry has on nonclassicality, and suggest a guiding perspective for finding novel quantum computational applications. Copyright © 2020, The Authors. All rights reserved.

关键词： Phase space methods

Efficient verification of continuous-variable quantum states and devices without assuming identical and independent operations

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arXiv 2020年

作者： Wu, Ya-Dong Bai, Ge Chiribella, Giulio Liu, Nana QICI Quantum Information and Computation Initiative Department of Computer Science The University of Hong Kong Pokfulam Road Hong Kong The University of Hong Kong Shenzhen Institute of Research and Innovation 5/F Key Laboratory Platform Building No.6 Yuexing 2nd Rd. Nanshan Shenzhen518057 China Department of Computer Science Parks Road OxfordOX1 3QD United Kingdom Perimeter Institute for Theoretical Physics WaterlooONN2L 2Y5 Canada Institute of Natural Sciences Shanghai Jiao Tong University Shanghai200240 China Ministry of Education Key Laboratory in Scientific and Engineering Computing Shanghai Jiao Tong University Shanghai200240 China University of Michigan Shanghai Jiao Tong University Joint Institute Shanghai200240 China

Continuous-variable quantum information, encoded into infinite-dimensional quantum systems, is a promising platform for the realization of many quantum information protocols, including quantum computation, quantum metrology, quantum cryptography, and quantum communication. To successfully demonstrate these protocols, an essential step is the certification of multimode continuous-variable quantum states and quantum devices. This problem is well studied under the assumption that multiple uses of the same device result in identical and independently distributed (i.i.d.) operations. However, in realistic scenarios, identical and independent state preparation and calls to the quantum devices cannot be generally guaranteed. Important instances include adversarial scenarios and instances of time-dependent and correlated noise. In this paper, we propose the first set of reliable protocols for verifying multimode continuous-variable entangled states and devices in these non-i.i.d scenarios. Although not fully universal, these protocols are applicable to Gaussian quantum states, non-Gaussian hypergraph states, as well as amplification, attenuation, and purification of noisy coherent states. Copyright © 2020, The Authors. All rights reserved.

关键词： quantum optics

Exploring entanglement and optimization within the Hamiltonian variational Ansatz

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arXiv 2020年

作者： Wiersema, Roeland Zhou, Cunlu de Sereville, Yvette Carrasquilla, Juan Felipe Kim, Yong Baek Yuen, Henry Vector Institute MaRS Centre TorontoONM5G 1M1 Canada Department of Physics and Astronomy University of Waterloo OntarioN2L 3G1 Canada Department of Computer Science University of Toronto ONM5T 3A1 Canada Institute for Quantum Computing University of Waterloo OntarioN2L 3G1 Canada Department of Physics University of Toronto ONM5S 1A7 Canada Department of Computer Science and Department of Mathematics University of Toronto ONM5T 3A1 Canada

quantum variational algorithms are one of the most promising applications of near-term quantum computers;however, recent studies have demonstrated that unless the variational quantum circuits are configured in a problem-specific manner, optimization of such circuits will most likely fail. In this paper, we focus on a special family of quantum circuits called the Hamiltonian Variational Ansatz (HVA), which takes inspiration from the quantum approximation optimization algorithm and adiabatic quantum computation. Through the study of its entanglement spectrum and energy gradient statistics, we find that HVA exhibits favorable structural properties such as mild or entirely absent barren plateaus and a restricted state space that eases their optimization in comparison to the well-studied "hardware-efficient ansatz." We also numerically observe that the optimization landscape of HVA becomes almost trap free, i.e., no sub-optimal minima, when the ansatz is overparameterized. We observe a size-dependent "computational phase transition as the number of layers in the HVA circuit is increased where the optimization crosses over from a hard to an easy region in terms of the quality of the approximations and speed of convergence to a good solution. In contrast with the analogous transitions observed in the learning of random unitaries which occur at a number of layers that grows exponentially with the number of qubits, our Variational quantum Eigensolver experiments suggest that the threshold to achieve the over-parameterization phenomenon scales at most polynomially in the number of qubits for the transverse field Ising and XXZ models. Lastly, as a demonstration of its entangling power and effectiveness, we show that HVA can find accurate approximations to the ground states of a modified Haldane-Shastry Hamiltonian on a ring, which has long-range interactions and has a power-law entanglement scaling. Copyright © 2020, The Authors. All rights reserved.

关键词： Hamiltonians