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  • Journal article
    Tang H, Banchi L, Wang T-Y, Shang X-W, Tan X, Zhou W-H, Feng Z, Pal A, Li H, Hu C-Q, Kim MS, Jin X-Met al., 2022,

    Generating Haar-uniform randomness using stochastic quantum walks on a photonic chip

    , Physical Review Letters, Vol: 128, ISSN: 0031-9007

    As random operations for quantum systems are intensively used in various quantum information tasks, a trustworthy measure of the randomness in quantum operations is highly demanded. The Haar measure of randomness is a useful tool with wide applications, such as boson sampling. Recently, a theoretical protocol was proposed to combine quantum control theory and driven stochastic quantum walks to generate Haar-uniform random operations. This opens up a promising route to converting classical randomness to quantum randomness. Here, we implement a two-dimensional stochastic quantum walk on the integrated photonic chip and demonstrate that the average of all distribution profiles converges to the even distribution when the evolution length increases, suggesting the 1-pad Haar-uniform randomness. We further show that our two-dimensional array outperforms the one-dimensional array of the same number of waveguide for the speed of convergence. Our Letter demonstrates a scalable and robust way to generate Haar-uniform randomness that can provide useful building blocks to boost future quantum information techniques.

  • Journal article
    Li S, Driver T, Rosenberger P, Champenois EG, Duris J, Al-Haddad A, Averbukh V, Barnard JCT, Berrah N, Bostedt C, Bucksbaum PH, Coffee RN, DiMauro LF, Fang L, Garratt D, Gatton A, Guo Z, Hartmann G, Haxton D, Helml W, Huang Z, LaForge AC, Kamalov A, Knurr J, Lin M-F, Lutman AA, MacArthur JP, Marangos JP, Nantel M, Natan A, Obaid R, O'Neal JT, Shivaram NH, Schori A, Walter P, Li Wang A, Wolf TJA, Zhang Z, Kling MF, Marinelli A, Cryan JPet al., 2022,

    Attosecond coherent electron motion in Auger-Meitner decay

    , SCIENCE, Vol: 375, Pages: 285-+, ISSN: 0036-8075
  • Journal article
    Moroney N, Del Bino L, Zhang S, Woodley MTM, Hill L, Wildi T, Wittwer VJ, Sudmeyer T, Oppo G-L, Vanner MR, Brasch V, Herr T, Del'Haye Pet al., 2022,

    A Kerr polarization controller

  • Journal article
    Clark A, Clear C, Schofield R, McCutcheon D, Hoggarth R, Major Ket al., 2022,

    Photon indistinguishability measurements under pulsed and continuous excitation

    , Physical Review Research, Vol: 4, ISSN: 2643-1564

    The indistinguishability of successively generated photons from a single quantum emitter is most commonly measured using two-photon interference at a beam splitter. Whilst for sources excited in the pulsed regime the measured bunching of photons reflects the full wave-packet indistinguishability of the emitted photons, for continuous wave (cw) excitation, the inevitable dependence on detector timing resolution and driving strength obscures the underlying photon interference process. Here we derive a method to extract full photon wave-packet indistinguishability from cw measurements by considering the relevant correlation functions. The equivalence of both methods is experimentally verified through a comparison of cw and pulsed excitation measurements on an archetypal source of photons, a single molecule.

  • Journal article
    Guo N-J, Liu W, Li Z-P, Yang Y-Z, Yu S, Meng Y, Wang Z-A, Zeng X-D, Yan F-F, Li Q, Wang J-F, Xu J-S, Wang Y-T, Tang J-S, Li C-F, Guo G-Cet al., 2022,

    Generation of Spin Defects by Ion Implantation in Hexagonal Boron Nitride.

    , ACS Omega, Vol: 7, Pages: 1733-1739

    Optically addressable spin defects in wide-band-gap semiconductors as promising systems for quantum information and sensing applications have recently attracted increased attention. Spin defects in two-dimensional materials are expected to show superiority in quantum sensing due to their atomic thickness. Here, we demonstrate that an ensemble of negatively charged boron vacancies (VB -) with good spin properties in hexagonal boron nitride (hBN) can be generated by ion implantation. We carry out optically detected magnetic resonance measurements at room temperature to characterize the spin properties of ensembles of VB - defects, showing a zero-field splitting frequency of ∼3.47 GHz. We compare the photoluminescence intensity and spin properties of VB - defects generated using different implantation parameters, such as fluence, energy, and ion species. With the use of the proper parameters, we can successfully create VB - defects with a high probability. Our results provide a simple and practicable method to create spin defects in hBN, which is of great significance for realizing integrated hBN-based devices.

  • Journal article
    Sempere-Llagostera S, Thekkadath G, Patel R, Kolthammer WS, Walmsley Iet al., 2022,

    Reducing $g^{(2)}(0)$ of a parametric down-conversion source via photon-number resolution with superconducting nanowire detectors

    , Optics Express, Vol: 30, Pages: 3138-3147, ISSN: 1094-4087

    Multiphoton contributions pose a significant challenge for the realisation of heralded single-photon sources (HSPS) based on nonlinear processes. In this work, we improve the quality of single photons generated in this way by harnessing the photon-number resolving (PNR) capabilities of commercial superconducting nanowire single-photon detectors (SNSPDs). We report a 13 ± 0.4% reduction of g(2)(τ = 0), even with a collection efficiency in the photon source of only 29.6%. Our work demonstrates the first application of the PNR capabilities of SNSPDs and shows improvement in the quality of an HSPS with widely available technology.

  • Journal article
    Alsing PM, Birrittella RJ, Gerry CC, Mimih J, Knight PLet al., 2022,

    Extending the Hong-Ou-Mandel effect: the power of nonclassicality

    , Physical Review A: Atomic, Molecular and Optical Physics, Vol: 105, ISSN: 1050-2947

    We show that the parity (evenness or oddness) of a nonclassical state of light has a dominant influence on the interference effects at a balanced beam splitter, irrespective of the state initially occupying the other input mode. Specifically, the parity of the nonclassical state gives rise to destructive interference effects that result in deep valleys in the output joint number distribution of which the Hong-Ou-Mandel (HOM) effect is a limiting case. The counterintuitive influence of even a single photon to control the output of a beam splitter illuminated by any field, be it a coherent or even a noisy thermal field, demonstrates the extraordinary power of nonclassicality. The canonical example of total destructive interference of quantum amplitudes leading to the absence of coincidence counts from a 50:50 beam splitter (BS) is the celebrated HOM effect, characterized by the vanishing of the joint probability of detecting singe photons in each of the output beams. We show that this is a limiting case of more general input states upon which a 50:50 BS can create total, or near total, destructive interference of quantum amplitudes. For the case of an odd photon-number input Fock state of arbitrary value n>0 we show that the joint photon-number probabilities vanish when detecting identical photon numbers in each output beams. We specifically examine the mixing of photon-number states of n=1, 2, and 3 with a continuous-variable state, such as a coherent state of arbitrary amplitude, and a thermal state. These vanishing joint probabilities form what we call a central nodal line: A contiguous set of zeros representing complete destructive interference of quantum amplitudes. We further show that with odd or even photon-number Fock states n, with n>1, there will be additional off-diagonal curves along which the joint photon-number probabilities are either zero, or near zero, which we call pseudonodal curves, which constitute a near, but not complete, destructive inte

  • Journal article
    Kwon H, Mukherjee R, Kim MS, 2022,

    Reversing Lindblad dynamics via continuous Petz recovery map

    , Physical Review Letters, Vol: 128, Pages: 1-7, ISSN: 0031-9007

    An important issue in developing quantum technology is that quantum states are so sensitive to noise. We propose a protocol that introduces reverse dynamics, in order to precisely control quantum systems against noise described by the Lindblad master equation. The reverse dynamics can be obtained by constructing the Petz recovery map in continuous time. By providing the exact form of the Hamiltonian and jump operators for the reverse dynamics, we explore the potential of utilizing the near-optimal recovery of the Petz map in controlling noisy quantum dynamics. While time-dependent dissipation engineering enables us to fully recover a single quantum trajectory, we also design a time-independent recovery protocol to protect encoded quantum information against decoherence. Our protocol can efficiently suppress only the noise part of dynamics thereby providing an effective unitary evolution of the quantum system.

  • Journal article
    Ma Y, Guff T, Morley GW, Pikovski I, Kim MSet al., 2022,

    Limits on inference of gravitational entanglement

    , Physical Review Research, Vol: 4, Pages: 1-7, ISSN: 2643-1564

    Combining gravity with quantum mechanics remains one of the biggest challenges of physics. In the past years, experiments with opto-mechanical systems have been proposed that may give indirect clues about the quantum nature of gravity. In a recent variation of such tests [D. Carney et al., Phys.Rev.X Quantum 2, 030330 (2021)], the authors ropose to gravitationally entangle an atom interferometer with a mesoscopic oscillator. The interaction results in periodic drops and revivals of the interferometeric visibility, which under specific assumptions indicate the gravitational generation of entanglement. Here we study semi-classical models of the atom interferometer that can reproduce the same effect. We show that the core signature – periodic collapses and revivals of the visibility – can appear if the atom is subject to a random unitary channel, including the casewhere the oscillator is fully classical and situations even without explicit modelling of the oscillator. We also show that the non-classicality of the oscillator vanishes unless the system is very close to its ground state, and even when the system is in the ground state, the non-classicality is limitedby the coupling strength. Our results thus indicate that deducing ntanglement from the proposed experiment is very challenging, since fulfilling and verifying the non-classicality assumptions is a significant challenge on its own right.

  • Journal article
    Thekkadath GS, Bell BA, Patel RB, Kim MS, Walmsley IAet al., 2022,

    Measuring the joint spectral mode of photon pairs using intensity interferometry

    , Physical Review Letters, Vol: 128, Pages: 1-6, ISSN: 0031-9007

    The ability to manipulate and measure the time-frequency structure of quantum light is useful for information processing and metrology. Measuring this structure is also important when developing quantum light sources with high modal purity that can interfere with other independent sources. Here, we present and experimentally demonstrate a scheme based on intensity interferometry to measure the joint spectral mode of photon pairs produced by spontaneous parametric down-conversion. We observe correlations in the spectral phase of the photons due to chirp in the pump. We show that our scheme can be combined with stimulated emission tomography to quickly measure their mode using bright classical light. Our scheme does not require phase stability, nonlinearities, or spectral shaping and thus is an experimentally simple way of measuring the modal structure of quantum light.

  • Journal article
    Borchert MJ, Devlin JA, Erlewein SR, Fleck M, Harrington JA, Higuchi T, Latacz BM, Voelksen F, Wursten EJ, Abbass F, Bohman MA, Mooser AH, Popper D, Wiesinger M, Will C, Blaum K, Matsuda Y, Ospelkaus C, Quint W, Walz J, Yamazaki Y, Smorra C, Ulmer Set al., 2022,

    A 16-parts-per-trillion measurement of the antiproton-to-proton charge-mass ratio

    , NATURE, Vol: 601, Pages: 53-+, ISSN: 0028-0836
  • Conference paper
    Schofield RC, Boissier S, Clear C, Bogusz DP, Burdekin P, Nur S, Jin L, Ovvyan A, Koppens FHL, Toninelli C, Pernice WHP, Hinds EA, Major KD, McCutcheon DPS, Hoggarth RA, Clark ASet al., 2022,

    Continuous-Wave Characterisation of Photon Indistinguishability and Nanophotonic Coupling

    We use continuous-wave excitation of a single molecule to measure the full temporal wavepacket indistinguishability of emitted photons, and show that continuous-wave light can determine the coupling of quantum emitters to arbitrary nanophotonic structures.

  • Conference paper
    Sempere-Llagostera S, Thekkadath GS, Patel RB, Kolthammer WS, Walmsley IAet al., 2022,

    Reducing g<sup>(2)</sup>(0) of a Parametric Down-Conversion Source via Photon-Number Resolution with Superconducting Nanowire Detectors

    We demonstrate a 13±0.4% reduction of the second-order correlation function g(2)(0) by harnessing the photon-number resolving capabilities of commercial superconducting nanowire single-photon detectors, improving the quality of a heralded single-photon source.

  • Conference paper
    Thekkadath GS, Sempere-Llagostera S, Bell BA, Patel RB, Kim MS, Walmsley IAet al., 2022,

    Experimental demonstration of Gaussian boson sampling with displacement

    We inject squeezed vacuum and weak coherent light into a multiport interferometer and measure the output photon statistics. Our work explores the capabilities of a displacement field in Gaussian boson sampling.

  • Conference paper
    Maimaris M, Pettipher AJ, Azzouzi M, Walke DJ, Zheng X, Gorodetsky A, Dong Y, Tuladhar PS, Crespo H, Nelson J, Tisch JWG, Bakulin AAet al., 2022,

    Sub-10fs Photocurrent and Photoluminescence Action Spectroscopies of Organic Optoelectronic Devices Reveals Ultrafast Formation of Bound Excitonic States

    We apply ultrafast pump-push-photocurrent and pump-push-photoluminescence spectroscopies to polyfluorene organic diode to track in time the bound exciton formation. ‘Cold’-excitons become bound within 10-fs while ‘hot’-excitons can dissociate spontaneously within 50-fs before acquiring bound character.

  • Journal article
    Bulmer JFF, Bell BA, Chadwick RS, Jones AE, Moise D, Rigazzi A, Thorbecke J, Haus U-U, Van Vaerenbergh T, Patel RB, Walmsley IA, Laing Aet al., 2022,

    The boundary for quantum advantage in Gaussian boson sampling

    , Science Advances, Vol: 8, ISSN: 2375-2548

    Identifying the boundary beyond which quantum machines provide a computational advantage over their classical counterparts is a crucial step in charting their usefulness. Gaussian boson sampling (GBS), in which photons are measured from a highly entangled Gaussian state, is a leading approach in pursuing quantum advantage. State-of-the-art GBS experiments that run in minutes would require 600 million years to simulate using the best preexisting classical algorithms. Here, we present faster classical GBS simulation methods, including speed and accuracy improvements to the calculation of loop hafnians. We test these on a ∼100,000-core supercomputer to emulate GBS experiments with up to 100 modes and up to 92 photons. This reduces the simulation time for state-of-the-art GBS experiments to several months, a nine–orders of magnitude improvement over previous estimates. Last, we introduce a distribution that is efficient to sample from classically and that passes a variety of GBS validation methods

  • Conference paper
    Enzian G, Freisem L, Price JJ, Svela AO, Clarke J, Szczykulska M, Nunn J, Walmsley I, Silver J, Del Bino L, Zhang S, Del'Haye P, Shajilal B, Janousek J, Buchler BC, Lam PK, Vanner MRet al., 2022,

    Brillouin optomechanics: From strong coupling to single-phonon-level operations

    , Conference on Optical and Quantum Sensing and Precision Metrology II, Publisher: SPIE-INT SOC OPTICAL ENGINEERING, ISSN: 0277-786X
  • Journal article
    Corfield O, Lishman J, Lee C, Toba JM, Porter G, Heinrich JM, Webster SC, Mintert F, Thompson RCet al., 2021,

    Certifying multilevel coherence in the motional state of a trapped ion

    , PRX Quantum, Vol: 2, ISSN: 2691-3399

    Quantum coherence is the foundation of almost all departures from classical physics and is exhibited when a quantum system is in a superposition of different basis states. Here, the coherent superposition of three motional Fock states of a single trapped ion is experimentally certified, with a procedure that does not produce false positives. As the motional state cannot be directly interrogated, our scheme uses an interference pattern generated by projective measurement of the coupled qubit state. The minimum number of coherently superposed states is inferred from a series of threshold values based on analysis of the interference pattern. This demonstrates that high-level coherence can be verified and investigated with simple nonideal control methods that are well suited to noisy intermediate-scale quantum devices.

  • Working paper
    Barontini G, Blackburn L, Boyer V, Butuc-Mayer F, Calmet X, Lopez-Urrutia JRC, Curtis EA, Darquie B, Dunningham J, Fitch NJ, Forgan EM, Georgiou K, Gill P, Godun RM, Goldwin J, Guarrera V, Harwood A, Hill IR, Hendricks RJ, Jeong M, Johnson MYH, Keller M, Sajith LPK, Kuipers F, Margolis HS, Mayo C, Newman P, Parsons AO, Prokhorov L, Robertson BI, Rodewald J, Safronova MS, Sauer BE, Schioppo M, Sherrill N, Stadnik YV, Szymaniec K, Tarbutt MR, Thompson RC, Tofful A, Tunesi J, Vecchio A, Wang Y, Worm Set al., 2021,

    Measuring the stability of fundamental constants with a network of clocks

    , Publisher: arXiv

    The detection of variations of fundamental constants of the Standard Modelwould provide us with compelling evidence of new physics, and could lift theveil on the nature of dark matter and dark energy. In this work, we discuss howa network of atomic and molecular clocks can be used to look for suchvariations with unprecedented sensitivity over a wide range of time scales.This is precisely the goal of the recently launched QSNET project: A network ofclocks for measuring the stability of fundamental constants. QSNET will includestate-of-the-art atomic clocks, but will also develop next-generation molecularand highly charged ion clocks with enhanced sensitivity to variations offundamental constants. We describe the technological and scientific aims ofQSNET and evaluate its expected performance. We show that in the range ofparameters probed by QSNET, either we will discover new physics, or we willimpose new constraints on violations of fundamental symmetries and a range oftheories beyond the Standard Model, including dark matter and dark energymodels.

  • Journal article
    Enzian G, Freisem L, Price JJ, Svela AO, Clarke J, Shajilal B, Janousek J, Buchler BC, Lam PK, Vanner MRet al., 2021,

    Non-Gaussian Mechanical Motion via Single and Multiphonon Subtraction from a Thermal State

    , PHYSICAL REVIEW LETTERS, Vol: 127, ISSN: 0031-9007
  • Journal article
    Mukherjee R, Kuros A, Sacha K, Mintert Fet al., 2021,

    Controlled preparation of phases in two-dimensional time crystals

    , Physical Review Research, Vol: 3, ISSN: 2643-1564

    The study of phases is useful for understanding novel states of matter. One such state of matter aretime crystals which constitute periodically driven interacting many-body systems that spontaneouslybreak time translation symmetry. Time crystals with arbitrary periods (and dimensions) can berealized using the model of Bose-Einstein condensates bouncing on periodically-driven mirror(s). Inthis work, we identify the different phases that characterize the two-dimensional time crystal. Bydetermining the optimal initial conditions and value of system parameters, we provide a practicalroute to realize a specific phase of the time crystal. These different phases can be mapped tothe many-body states existing on a two-dimensional Hubbard lattice model, thereby opening upinteresting opportunities for quantum simulation of many-body physics in time lattices.

  • Journal article
    Kanari-Naish LA, Clarke J, Vanner MR, Laird EAet al., 2021,

    Can the displacemon device test objective collapse models?

    , AVS Quantum Science, Vol: 3

    Testing the limits of the applicability of quantum mechanics will deepen our understanding of the universe and may shed light on the interplay between quantum mechanics and gravity. At present there is a wide range of approaches for such macroscopic tests spanning from matter-wave interferometry of large molecules to precision measurements of heating rates in the motion of micro-scale cantilevers. The "displacemon"is a proposed electromechanical device consisting of a mechanical resonator flux-coupled to a superconducting qubit enabling generation and readout of mechanical quantum states. In the original proposal, the mechanical resonator was a carbon nanotube, containing 106 nucleons. Here, in order to probe quantum mechanics at a more macroscopic scale, we propose using an aluminum mechanical resonator on two larger mass scales, one inspired by the Marshall-Simon-Penrose-Bouwmeester moving-mirror proposal, and one set by the Planck mass. For such a device, we examine the experimental requirements needed to perform a more macroscopic quantum test and thus feasibly detect the decoherence effects predicted by two objective collapse models: Diósi-Penrose and continuous spontaneous localization. Our protocol for testing these two theories takes advantage of the displacemon architecture to create non-Gaussian mechanical states out of equilibrium with their environment and then analyzes the measurement statistics of a superconducting qubit. We find that with improvements to the fabrication and vibration sensitivities of these electromechanical devices, the displacemon device provides a new route to feasibly test decoherence mechanisms beyond standard quantum theory.

  • Journal article
    Toninelli C, Gerhardt I, Clark AS, Reserbat-Plantey A, Goetzinger S, Ristanovic Z, Colautti M, Lombardi P, Major KD, Deperasinska I, Pernice WH, Koppens FHL, Kozankiewicz B, Gourdon A, Sandoghdar V, Orrit Met al., 2021,

    Single organic molecules for photonic quantum technologies

    , NATURE MATERIALS, Vol: 20, Pages: 1615-1628, ISSN: 1476-1122
  • Journal article
    Smith AWR, Khosla KE, Self CN, Kim MSet al., 2021,

    Qubit readout error mitigation with bit-flip averaging

    , Science Advances, Vol: 7, Pages: 1-10, ISSN: 2375-2548

    Quantum computers are becoming increasingly accessible, and may soonoutperform classical computers for useful tasks. However, qubit readout errorsremain a significant hurdle to running quantum algorithms on current devices.We present a scheme to more efficiently mitigate these errors on quantumhardware and numerically show that our method consistently gives advantage overprevious mitigation schemes. Our scheme removes biases in the readout errorsallowing a general error model to be built with far fewer calibrationmeasurements. Specifically, for reading out $n$-qubits we show a factor of$2^n$ reduction in the number of calibration measurements without sacrificingthe ability to compensate for correlated errors. Our approach can be combinedwith, and simplify, other mitigation methods allowing tractable mitigation evenfor large numbers of qubits.

  • Journal article
    Driver T, Bachhawat N, Pipkorn R, Frasinski LJ, Marangos JP, Edelson-Averbukh M, Averbukh Vet al., 2021,

    Proteomic Database Search Engine for Two-Dimensional Partial Covariance Mass Spectrometry

    , ANALYTICAL CHEMISTRY, Vol: 93, Pages: 14946-14954, ISSN: 0003-2700
  • Journal article
    Enzian G, Freisem L, Price J, Svela A, Clarke J, Shajilal B, Janousek J, Buchler B, Lam PK, Vanner Met al., 2021,

    Non-Gaussian mechanical motion via single and multi-phonon subtraction from a thermal state

    , Physical Review Letters, ISSN: 0031-9007

    Quantum optical measurement techniques offer a rich avenue for quantum control of mechanical oscillators via cavity optomechanics. In particular, a powerful yet little explored combination utilizes optical measurements to perform heralded non-Gaussian mechanical state preparation followed by tomography to determine the mechanical phase-space distribution. Here, we experimentally perform heralded single- and multi-phonon subtraction via photon counting to a laser-cooled mechanical thermal state with a Brillouin optomechanical system at room temperature, and use optical heterodyne detection to measure the s-parameterized Wigner distribution of the non-Gaussian mechanical states generated. The techniques developed here advance the state-of-the-art for optics-based tomography of mechanical states and will be useful for a broad range of applied and fundamental studies that utilize mechanical quantum-state engineering and tomography.

  • Journal article
    Xu L, Xu H, Jiang T, Xu F, Zheng K, Wang B, Zhang A, Zhang Let al., 2021,

    Direct Characterization of Quantum Measurements Using Weak Values

    , Physical Review Letters, Vol: 127, ISSN: 0031-9007
  • Journal article
    Ma Y, Kim MS, Stickler BA, 2021,

    Torque-free manipulation of nanoparticle rotations via embedded spins

    , Physical Review B: Condensed Matter and Materials Physics, Vol: 104, ISSN: 1098-0121

    Spin angular momentum and mechanical rotation both contribute to the total angular momentum of rigid bodies, leading to spin-rotational coupling via the Einstein–de Haas and Barnett effects. Here, we show that the revolutions of symmetric nanorotors can be strongly affected by a small number of intrinsic spins. The resulting dynamics are observable with freely rotating nanodiamonds with embedded nitrogen-vacancy centers and persist for realistically shaped near-symmetric particles, opening the door to torque-free schemes to control their rotations at the quantum level.

  • Journal article
    Haug T, Bharti K, Kim MS, 2021,

    Capacity and quantum geometry of parametrized quantum circuits

    , PRX Quantum, Vol: 2, Pages: 1-14, ISSN: 2691-3399

    To harness the potential of noisy intermediate-scale quantum devices, it is paramount to find the best type of circuits to run hybrid quantum-classical algorithms. Key candidates are parametrized quantum circuits that can be effectively implemented on current devices. Here, we evaluate the capacity and trainability of these circuits using the geometric structure of the parameter space via the effective quantum dimension, which reveals the expressive power of circuits in general as well as of particular initialization strategies. We assess the expressive power of various popular circuit types and find striking differences depending on the type of entangling gates used. Particular circuits are characterized by scaling laws in their expressiveness. We identify a transition in the quantum geometry of the parameter space, which leads to a decay of the quantum natural gradient for deep circuits. For shallow circuits, the quantum natural gradient can be orders of magnitude larger in value compared to the regular gradient; however, both of them can suffer from vanishing gradients. By tuning a fixed set of circuit parameters to randomized ones, we find a region where the circuit is expressive but does not suffer from barren plateaus, hinting at a good way to initialize circuits. We show an algorithm that prunes redundant parameters of a circuit without affecting its effective dimension. Our results enhance the understanding of parametrized quantum circuits and can be immediately applied to improve variational quantum algorithms.

  • Journal article
    Walton A, Ghesquiere A, Brumpton G, Jennings D, Varcoe Bet al., 2021,

    Thermal state quantum key distribution

    , Journal of Physics B: Atomic, Molecular and Optical Physics, Vol: 54, ISSN: 0953-4075

    We analyse a central broadcast continuous variable quantum key distribution protocol in which a beam produced by a thermal source is used to create a secret key between two parties, Alice and Bob. A beam splitter divides the initial beam into a pair of output beams, which are sent to Alice and Bob, with Eve intercepting Bob's beam. We investigate the protocol in detail, calculating mutual informations through a pair of analytic methods and comparing the results to the outputs of a Monte Carlo simulation of the protocol. In a lossless system, we find that a lower bound on the key rate remains positive in the protocol under a beam splitter attack, provided Bob receives a nonzero proportion of the beam initially sent to him. This suggests that the thermal state protocol could be used experimentally to produce secure keys.

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