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  • 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.

  • Journal article
    Zhao H, Smith A, Mintert F, Knolle Jet al., 2021,

    Orthogonal Quantum Many-Body Scars

    , PHYSICAL REVIEW LETTERS, Vol: 127, ISSN: 0031-9007
  • Journal article
    Bray AC, Maxwell AS, Kissin Y, Ruberti M, Ciappina MF, Averbukh V, Faria CFDMet al., 2021,

    Polarization in strong-field ionization of excited helium

  • Conference paper
    Barontini G, Boyer V, Calmet X, Fitch NJ, Forgan EM, Godun RM, Goldwin J, Guarrera V, Hill IR, Jeong M, Keller M, Juipers F, Margolis HS, Newman P, Prokhorov L, Rodewald J, Sauer BE, Schioppo M, Sherrill N, Tarbutt MR, Vecchio A, Worm Set al., 2021,

    QSNET, a network of clock for measuring the stability of fundamental constants

    , Proceedings Volume 11881, Quantum Technology: Driving Commercialisation of an Enabling Science II, Publisher: SPIE, Pages: 1-4

    The QSNET consortium is building a UK network of next-generation atomic and molecular clocks that will achieve unprecedented sensitivity in testing variations of the fine structure constant, α, and the electron-to-proton mass ratio, μ. This in turn will provide more stringent constraints on a wide range of fundamental and phenomenological theories beyond the Standard Model and on dark matter models.

  • Journal article
    Alauze X, Lim J, Trigatzis MA, Swarbrick S, Collings FJ, Fitch NJ, Sauer BE, Tarbutt MRet al., 2021,

    An ultracold molecular beam for testing fundamental physics

  • Journal article
    Vovrosh J, Khosla KE, Greenaway S, Self C, Kim MS, Knolle Jet al., 2021,

    Simple mitigation of global depolarizing errors in quantum simulations.

    , Physical Review E: Statistical, Nonlinear, and Soft Matter Physics, Vol: 104, Pages: 1-8, ISSN: 1539-3755

    To get the best possible results from current quantum devices error mitigation is essential. In this work we present a simple but effective error mitigation technique based on the assumption that noise in a deep quantum circuit is well described by global depolarizing error channels. By measuring the errors directly on the device, we use an error model ansatz to infer error-free results from noisy data. We highlight the effectiveness of our mitigation via two examples of recent interest in quantum many-body physics: entanglement measurements and real-time dynamics of confinement in quantum spin chains. Our technique enables us to get quantitative results from the IBM quantum computers showing signatures of confinement, i.e., we are able to extract the meson masses of the confined excitations which were previously out of reach. Additionally, we show the applicability of this mitigation protocol in a wider setting with numerical simulations of more general tasks using a realistic error model. Our protocol is device-independent, simply implementable, and leads to large improvements in results if the global errors are well described by depolarization.

  • Journal article
    Dhar HS, Zuo Z, Rodrigues JD, Nyman RA, Mintert Fet al., 2021,

    Quest for vortices in photon condensates

    , PHYSICAL REVIEW A, Vol: 104, ISSN: 2469-9926
  • Journal article
    Barillot T, Alexander O, Cooper B, Driver T, Garratt D, Li S, Al Haddad A, Sanchez-Gonzalez A, Agåker M, Arrell C, Bearpark MJ, Berrah N, Bostedt C, Bozek J, Brahms C, Bucksbaum PH, Clark A, Doumy G, Feifel R, Frasinski LJ, Jarosch S, Johnson AS, Kjellsson L, Kolorenč P, Kumagai Y, Larsen EW, Matia-Hernando P, Robb M, Rubensson J-E, Ruberti M, Sathe C, Squibb RJ, Tan A, Tisch JWG, Vacher M, Walke DJ, Wolf TJA, Wood D, Zhaunerchyk V, Walter P, Osipov T, Marinelli A, Maxwell TJ, Coffee R, Lutman AA, Averbukh V, Ueda K, Cryan JP, Marangos JPet al., 2021,

    Correlation-driven transient hole dynamics resolved in space and time in the isopropanol molecule

    , Physical Review X, Vol: 11, Pages: 1-15, ISSN: 2160-3308

    The possibility of suddenly ionized molecules undergoing extremely fast electron hole (or, hole)dynamics prior to significant structural change was first recognized more than 20 years ago andtermed charge migration. The accurate probing of ultrafast electron hole dynamics requires measurements that have both sufficient temporal resolution and can detect the localization of a specifichole within the molecule. We report an investigation of the dynamics of inner valence hole states inisopropanol where we use an x-ray pump/x-ray probe experiment, with site and state-specific probing of a transient hole state localized near the oxygen atom in the molecule, together with an abinitio theoretical treatment. We record the signature of transient hole dynamics and make the firsttentative observation of dynamics driven by frustrated Auger-Meitner transitions. We verify thatthe effective hole lifetime is consistent with our theoretical prediction. This state-specific measurement paves the way to widespread application for observations of transient hole dynamics localizedin space and time in molecules and thus to charge transfer phenomena that are fundamental inchemical and material physics.

  • Journal article
    Bohman M, Grunhofer V, Smorra C, Wiesinger M, Will C, Borchert MJ, Devlin JA, Erlewein S, Fleck M, Gavranovic S, Harrington J, Latacz B, Mooser A, Popper D, Wursten E, Blaum K, Matsuda Y, Ospelkaus C, Quint W, Walz J, Ulmer Set al., 2021,

    Sympathetic cooling of a trapped proton mediated by an LC circuit

    , NATURE, Vol: 596, Pages: 514-+, ISSN: 0028-0836
  • Journal article
    Zhang A, Zhan H, Liao J, Zheng K, Jiang T, Mi M, Yao P, Zhang Let al., 2021,

    Quantum verification of NP problems with single photons and linear optics

    , Light: Science & Applications, Vol: 10

    <jats:title>Abstract</jats:title><jats:p>Quantum computing is seeking to realize hardware-optimized algorithms for application-related computational tasks. NP (nondeterministic-polynomial-time) is a complexity class containing many important but intractable problems like the satisfiability of potentially conflict constraints (SAT). According to the well-founded exponential time hypothesis, verifying an SAT instance of size <jats:italic>n</jats:italic> requires generally the complete solution in an <jats:italic>O</jats:italic>(<jats:italic>n</jats:italic>)-bit proof. In contrast, quantum verification algorithms, which encode the solution into quantum bits rather than classical bit strings, can perform the verification task with quadratically reduced information about the solution in <jats:inline-formula><jats:alternatives><jats:tex-math>$$\tilde O(\sqrt n )$$</jats:tex-math><mml:math xmlns:mml=""> <mml:mrow> <mml:mover> <mml:mrow> <mml:mi>O</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̃</mml:mo> </mml:mrow> </mml:mover> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:msqrt> <mml:mrow> <mml:mi>n</mml:mi> </mml:mrow> </mml:msqrt> </mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math></jats:alternatives></jats:inline-f

  • Journal article
    Driver T, Averbukh V, Frasiński LJ, Marangos JP, Edelson-Averbukh Met al., 2021,

    Two-dimensional partial covariance mass spectrometry for the top-down analysis of intact proteins.

    , Analytical Chemistry, Vol: 93, Pages: 10779-10788, ISSN: 0003-2700

    Two-dimensional partial covariance mass spectrometry (2D-PC-MS) exploits the inherent fluctuations of fragment ion abundances across a series of tandem mass spectra, to identify correlated pairs of fragment ions produced along the same fragmentation pathway of the same parent (e.g., peptide) ion. Here, we apply 2D-PC-MS to the analysis of intact protein ions in a standard linear ion trap mass analyzer, using the fact that the fragment-fragment correlation signals are much more specific to the biomolecular sequence than one-dimensional (1D) tandem mass spectrometry (MS/MS) signals at the same mass accuracy and resolution. We show that from the distribution of signals on a 2D-PC-MS map it is possible to extract the charge state of both parent and fragment ions without resolving the isotopic envelope. Furthermore, the 2D map of fragment-fragment correlations naturally separates the products of the primary decomposition pathways of the molecular ions from those of the secondary ones. We access this spectral information using an adapted version of the Hough transform. We demonstrate the successful identification of highly charged, intact protein molecules bypassing the need for high mass resolution. Using this technique, we also perform the in silico deconvolution of the overlapping fragment ion signals from two co-isolated and co-fragmented intact proteins, demonstrating a viable new method for the concurrent mass spectrometric identification of a mixture of intact protein ions from the same fragment ion spectrum.

  • Journal article
    Lee S-W, Im D-G, Kim Y-H, Nha H, Kim MSet al., 2021,

    Quantum teleportation is a reversal of quantum measurement

    , Physical Review Research, Vol: 3, Pages: 1-16, ISSN: 2643-1564

    We introduce a generalized concept of quantum teleportation in the framework of quantum measurement and reversing operation. Our framework makes it possible to find an optimal protocol for quantum teleportation enabling a faithful transfer of unknown quantum states with maximum success probability up to the fundamental limit of the no-cloning theorem. Moreover, an optimized protocol in this generalized approach allows us to overcome noise in quantum channel beyond the reach of existing teleportation protocols without requiring extra qubit resources. Our proposed framework is applicable to multipartite quantum communications and primitive functionalities in scalable quantum architectures.

  • Journal article
    Stickler BA, Hornberger K, Kim MS, 2021,

    Quantum rotations of nanoparticles

    , NATURE REVIEWS PHYSICS, Vol: 3, Pages: 589-597
  • Journal article
    Mori T, Zhao H, Mintert F, Knolle J, Moessner Ret al., 2021,

    Rigorous Bounds on the Heating Rate in Thue-Morse Quasiperiodically and Randomly Driven Quantum Many-Body Systems

    , PHYSICAL REVIEW LETTERS, Vol: 127, ISSN: 0031-9007
  • Journal article
    Self CN, Khosla KE, Smith AWR, Sauvage F, Haynes PD, Knolle J, Mintert F, Kim MSet al., 2021,

    Variational quantum algorithm with information sharing

    , npj Quantum Information, Vol: 7, ISSN: 2056-6387

    We introduce an optimisation method for variational quantum algorithms and experimentally demonstrate a 100-fold improvement in efficiency compared to naive implementations. The effectiveness of our approach is shown by obtaining multi-dimensional energy surfaces for small molecules and a spin model. Our method solves related variational problems in parallel by exploiting the global nature of Bayesian optimisation and sharing information between different optimisers. Parallelisation makes our method ideally suited to the next generation of variational problems with many physical degrees of freedom. This addresses a key challenge in scaling-up quantum algorithms towards demonstrating quantum advantage for problems of real-world interest.

  • Journal article
    Liu W, Li Z-P, Yang Y-Z, Yu S, Meng Y, Wang Z-A, Li Z-C, Guo N-J, Yan F-F, Li Q, Wang J-F, Xu J-S, Wang Y-T, Tang J-S, Li C-F, Guo G-Cet al., 2021,

    Temperature-Dependent Energy-Level Shifts of Spin Defects in Hexagonal Boron Nitride

    , ACS Photonics, Vol: 8, Pages: 1889-1895, ISSN: 2330-4022
  • Working paper
    Schofield RC, Clear C, Hoggarth RA, Major KD, McCutcheon DPS, Clark ASet al., 2021,

    Photon indistinguishability measurements under pulsed and continuous excitation

    , Publisher: arXiv

    The indistinguishability of successively generated photons from a singlequantum emitter is most commonly measured using two-photon interference at abeam splitter. Whilst for sources excited in the pulsed regime the measuredbunching of photons reflects the full wavepacket indistinguishability of theemitted photons, for continuous wave (cw) excitation the inevitable dependenceon detector timing resolution and driving strength obscures the underlyingphoton interference process. Here we derive a method to extract the photonindistinguishability from cw measurements by considering the relevantcorrelation functions. The equivalence of both methods is experimentallyverified through comparison of cw and pulsed excitation of an archetypal sourceof photons, a single molecule.

  • Journal article
    Ling Y, Mintert F, 2021,

    Deterministic preparation of nonclassical states of light in cavity optomechanics

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

    Cavity-optomechanics is an ideal platform for the generation non-Gaussian quantum states due to the anharmonic interaction between the light field and the mechanical oscillator, but it is exactly this interaction that also impedes the preparation of pure states of the light field. In this paper we derive a driving protocol that helps to exploit the anharmonic interaction for state preparation and that ensures that the state of the light field remains close to pure. This shall enable the deterministic preparation of photon Fock states or coherent superpositions thereof.

  • Journal article
    Li S, Driver T, Al Haddad A, Champenois EG, Agaker M, Alexander O, Barillot T, Bostedt C, Garratt D, Kjellsson L, Lutman AA, Rubensson J-E, Sathe C, Marinelli A, Marangos JP, Cryan JPet al., 2021,

    Two-dimensional correlation analysis for x-ray photoelectron spectroscopy

    , Journal of Physics B: Atomic, Molecular and Optical Physics, Vol: 54, Pages: 1-9, ISSN: 0953-4075

    X-ray photoelectron spectroscopy (XPS) measures the binding energy of core-level electrons, which are well-localised to specific atomic sites in a molecular system, providing valuable information on the local chemical environment. The technique relies on measuring the photoelectron spectrum upon x-ray photoionisation, and the resolution is often limited by the bandwidth of the ionising x-ray pulse. This is particularly problematic for time-resolved XPS, where the desired time resolution enforces a fundamental lower limit on the bandwidth of the x-ray source. In this work, we report a novel correlation analysis which exploits the correlation between the x-ray and photoelectron spectra to improve the resolution of XPS measurements. We show that with this correlation-based spectral-domain ghost imaging method we can achieve sub-bandwidth resolution in XPS measurements. This analysis method enables XPS for sources with large bandwidth or spectral jitter, previously considered unfeasible for XPS measurements.

  • Journal article
    Jurgilas S, Chakraborty A, Rich C, Sauer B, Frye MD, Hutson JM, Tarbutt Met al., 2021,

    Collisions in a dual-species magneto-optical trap of molecules and atoms

    , New Journal of Physics, Vol: 23, ISSN: 1367-2630

    We study inelastic collisions between CaF molecules and ⁸⁷Rb atoms in a dual-species magneto-optical trap. The presence of atoms increases the loss rate of molecules from the trap. By measuring the loss rates and density distributions, we determine a collisional loss rate coefficient k₂ = (1.43 ± 0.29) × 10‾¹⁰cm³/s at a temperature of 2.4 mK. We show that this is not substantially changed by light-induced collisions or by varying the populations of excited-state atoms and molecules. The observed loss rate is close to the universal rate expected in the presence of fast loss at short range, and can be explained by rotation-changing collisions in the ground electronic state.

  • Journal article
    Greenaway S, Sauvage F, Khosla KE, Mintert Fet al., 2021,

    Efficient assessment of process fidelity

    , Physical Review Research, Vol: 3, Pages: 1-15, ISSN: 2643-1564

    The accurate implementation of quantum gates is essential for the realisation of quantum algorithms and digital quantum simulations. This accuracy may be increased on noisy hardware through the variational optimisation of gates, however the experimental realisation of such a protocol is impeded by the large effort required to estimate the fidelity of an implemented gate. With a hierarchy of approximations we find a faithful approximation to the quantum process fidelity that can be estimated experimentally with reduced effort. Its practical use is demonstrated with the optimisation of a three-qubit quantum gate on a commercially available quantum processor.

  • Journal article
    Qvarfort S, Vanner MR, Barker PF, Bruschi DEet al., 2021,

    Master-equation treatment of nonlinear optomechanical systems with optical loss

    , PHYSICAL REVIEW A, Vol: 104, ISSN: 2469-9926
  • Journal article
    Armstrong GSJ, Khokhlova MA, Labeye M, Maxwell AS, Pisanty E, Ruberti Met al., 2021,

    Dialogue on analytical and ab initio methods in attoscience

    , The European Physical Journal D, Vol: 75, ISSN: 1434-6060

    <jats:title>Abstract</jats:title><jats:p>The perceived dichotomy between analytical and ab initio approaches to theory in attosecond science is often seen as a source of tension and misconceptions. This Topical Review compiles the discussions held during a round-table panel at the ‘Quantum Battles in Attoscience’ <jats:sc>cecam</jats:sc> virtual workshop, to explore the sources of tension and attempt to dispel them. We survey the main theoretical tools of attoscience—covering both analytical and numerical methods—and we examine common misconceptions, including the relationship between ab initio approaches and the broader numerical methods, as well as the role of numerical methods in ‘analytical’ techniques. We also evaluate the relative advantages and disadvantages of analytical as well as numerical and ab initio methods, together with their role in scientific discovery, told through the case studies of two representative attosecond processes: non-sequential double ionisation and resonant high-harmonic generation. We present the discussion in the form of a dialogue between two hypothetical theoreticians, a numericist and an analytician, who introduce and challenge the broader opinions expressed in the attoscience community.</jats:p>

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