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  • 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
  • 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.
  • 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
    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
    Hofsaess S, Doppelbauer M, Wright SC, Kray S, Sartakov BG, Perez-Rios J, Meijer G, Truppe Set al., 2021,

    Optical cycling of AlF molecules

    , NEW JOURNAL OF PHYSICS, Vol: 23, ISSN: 1367-2630
  • 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>
  • Journal article
    Smith AWR, Gray J, Kim MS, 2021,

    Efficient quantum state sample tomography with basis-dependent neural networks

    , PRX Quantum, Vol: 2, Pages: 1-15, ISSN: 2691-3399

    We use a metalearning neural-network approach to analyze data from a measured quantum state. Once our neural network has been trained, it can be used to efficiently sample measurements of the state in measurement bases not contained in the training data. These samples can be used to calculate expectation values and other useful quantities. We refer to this process as “state sample tomography.” We encode the state’s measurement outcome distributions using an efficiently parameterized generative neural network. This allows each stage in the tomography process to be performed efficiently even for large systems. Our scheme is demonstrated on recent IBM Quantum devices, producing a model for a six-qubit state’s measurement outcomes with a predictive accuracy (classical fidelity) greater than 95% for all test cases using only 100 random measurement settings as opposed to the 729 settings required for standard full tomography using local measurements. This reduction in the required number of measurements scales favorably, with training data in 200 measurement settings, yielding a predictive accuracy greater than 92% for a ten-qubit state where 59 049 settings are typically required for full local measurement-based quantum state tomography. A reduction in the number of measurements by a factor, in this case, of almost 600 could allow for estimations of expectation values and state fidelities in practicable times on current quantum devices.
  • Journal article
    Petiziol F, Sameti M, Carretta S, Wimberger S, Mintert Fet al., 2021,

    Quantum Simulation of Three-Body Interactions in Weakly Driven Quantum Systems

    , PHYSICAL REVIEW LETTERS, Vol: 126, ISSN: 0031-9007
  • Journal article
    Fitch N, Tarbutt M, 2021,

    Laser-cooled molecules

    , Advances in Atomic Molecular and Optical Physics, ISSN: 1049-250X
  • Journal article
    Driver T, Bachhawat N, Frasinski L, Marangos J, Averbukh V, Edelson-Averbukh Met al., 2021,

    Chimera spectrum diagnostics for peptides using two-dimensional partial covariance mass spectrometry

    , Molecules, Vol: 26, ISSN: 1420-3049

    The rate of successful identification of peptide sequences by tandem mass spectrometry (MS/MS) is adversely affected by the common occurrence of co-isolation and co-fragmentation of two or more isobaric or isomeric parent ions. This results in so-called `chimera spectra’, which feature peaks of the fragment ions from more than a single precursor ion. The totality of the fragment ion peaks in chimera spectra cannot be assigned to a single peptide sequence, which contradicts a fundamental assumption of the standard automated MS/MS spectra analysis tools, such as protein database search engines. This calls for a diagnostic method able to identify chimera spectra to single out the cases where this assumption is not valid. Here, we demonstrate that, within the recently developed two-dimensional partial covariance mass spectrometry (2D-PC-MS), it is possible to reliably identify chimera spectra directly from the two-dimensional fragment ion spectrum, irrespective of whether the co-isolated peptide ions are isobaric up to a finite mass accuracy or isomeric. We introduce ‘3-57 chimera tag’ technique for chimera spectrum diagnostics based on 2D-PC-MS and perform numerical simulations to examine its efficiency. We experimentally demonstrate the detection of a mixture of two isomeric parent ions, even under conditions when one isomeric peptide is at one five-hundredth of the molar concentration of the second isomer.
  • Journal article
    Thomas SE, Billard M, Coste N, Wein SC, Priya, Ollivier H, Krebs O, Tazairt L, Harouri A, Lemaitre A, Sagnes I, Anton C, Lanco L, Somaschi N, Loredo JC, Senellart Pet al., 2021,

    Bright Polarized Single-Photon Source Based on a Linear Dipole

    , PHYSICAL REVIEW LETTERS, Vol: 126, ISSN: 0031-9007
  • Journal article
    Kissin Y, Ruberti M, Kolorenc P, Averbukh Vet al., 2021,

    Attosecond pump-attosecond probe spectroscopy of Auger decay

    , PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 23, Pages: 12376-12386, ISSN: 1463-9076
  • Journal article
    Toros M, Van De Kamp TW, Marshman RJ, Kim MS, Mazumdar A, Bose Set al., 2021,

    Relative acceleration noise mitigation for nanocrystal matter-wave interferometry: Applications to entangling masses via quantum gravity

    , Physical Review Special Topics: Physics Education Research, Vol: 3, Pages: 1-14, ISSN: 1554-9178

    Matter-wave interferometers with large momentum transfers, irrespective of specific implementations, will face a universal dephasing due to relative accelerations between the interferometric mass and the associated apparatus. Here we propose a solution that works even without actively tracking the relative accelerations: putting both the interfering mass and its associated apparatus in a freely falling capsule, so that the strongest inertial noise components vanish due to the equivalence principle. In this setting, we investigate two of the most important remaining noise sources: (a) the noninertial jitter of the experimental setup and (b) the gravity-gradient noise. We show that the former can be reduced below desired values by appropriate pressures and temperatures, while the latter can be fully mitigated in a controlled environment. We finally apply the analysis to a recent proposal for testing the quantum nature of gravity [S. Bose et al., Phys. Rev. Lett. 119, 240401 (2017)] through the entanglement of two masses undergoing interferometry. We show that the relevant entanglement witnessing is feasible with achievable levels of relative acceleration noise.
  • Working paper
    Sturges TJ, McDermott T, Buraczewski A, Clements WR, Renema JJ, Nam SW, Gerrits T, Lita A, Kolthammer WS, Eckstein A, Walmsley IA, Stobinska Met al., 2021,

    Quantum simulations with multiphoton Fock states

    , Publisher: NATURE RESEARCH
  • Journal article
    Im D-G, Lee C-H, Kim Y, Nha H, Kim MS, Lee S-W, Kim Y-Het al., 2021,

    Optimal teleportation via noisy quantum channels without additional qubit resources

    , npj Quantum Information, Vol: 7, Pages: 1-7, ISSN: 2056-6387

    Quantum teleportation exemplifies how the transmission of quantum information starkly differs from that of classical information and serves as a key protocol for quantum communication and quantum computing. While an ideal teleportation protocol requires noiseless quantum channels to share a pure maximally entangled state, the reality is that shared entanglement is often severely degraded due to various decoherence mechanisms. Although the quantum noise induced by the decoherence is indeed a major obstacle to realizing a near-term quantum network or processor with a limited number of qubits, the methodologies considered thus far to address this issue are resource-intensive. Here, we demonstrate a protocol that allows optimal quantum teleportation via noisy quantum channels without additional qubit resources. By analyzing teleportation in the framework of generalized quantum measurement, we optimize the teleportation protocol for noisy quantum channels. In particular, we experimentally demonstrate that our protocol enables to teleport an unknown qubit even via a single copy of an entangled state under strong decoherence that would otherwise preclude any quantum operation. Our work provides a useful methodology for practically coping with decoherence with a limited number of qubits and paves the way for realizing noisy intermediate-scale quantum computing and quantum communication.
  • Journal article
    Thekkadath GS, Sempere-Llagostera S, Bell BA, Patel RB, Kim MS, Walmsley IAet al., 2021,

    Single-shot discrimination of coherent states beyond the standard quantum limit

    , OPTICS LETTERS, Vol: 46, Pages: 2565-2568, ISSN: 0146-9592
  • Conference paper
    Schofield RC, Boissier S, Jin L, Ovvyan A, Nur S, Koppens FHL, Toninelli C, Pernice WHP, Major KD, Hinds EA, Clark ASet al., 2021,

    Coupling a Single Molecule to an Interrupted Nanophotonic Waveguide

    Single organic molecules have recently seen increased interest for use as single photon sources [1]. They emit photons with high efficiency and at favourable wavelengths for coupling to other quantum systems. While the excitation of molecules and their subsequent radiative emission is efficient [2] , the generated photons can be difficult to efficiently collect. There is therefore a large amount of ongoing work on coupling organic molecules to nanophotonic structures to modify their emission. Evanescent coupling to nanophotonic [3] , [4] and hybrid plasmonic [5] waveguides has shown promise but has limitations; the molecules must be very close to the waveguide to be in the evanescent field of the guided mode which can cause the molecules to become unstable. Here I will present our recent work on coupling organic molecules to interrupted waveguides using on chip micro-capillaries [6].
  • Journal article
    Sameti M, Lishman J, Mintert F, 2021,

    Strong-coupling quantum logic of trapped ions

    , PHYSICAL REVIEW A, Vol: 103, ISSN: 2469-9926
  • Conference paper
    Enzian G, Price JJ, Freisem L, Szczykulska M, Nunn J, Walmsley IA, Silver J, Bino LD, Zhang S, Del'Haye P, Janousek J, Buchler BC, Koy Lam P, Vanner MRet al., 2021,

    Brillouin optomechanics in whispering-gallery-mode microresonators: From strong coupling to single-phonon addition and subtraction

    We experimentally explore backward Brillouin scattering with high-frequency acoustic fields for optomechanics applications. We (i) demonstrate strong coupling between the optical and acoustic fields and (ii) perform single-phonon addition and subtraction operations.
  • Journal article
    Nyman RA, Dhar HS, Rodrigues JD, Mintert Fet al., 2021,

    Phase transitions of light in a dye-filled microcavity: observations and simulations

    , Journal of Physics: Conference Series, Vol: 1919, Pages: 012006-012006, ISSN: 1742-6588

    <jats:title>Abstract</jats:title> <jats:p>Photon thermalisation and condensation in dye-filled microcavities is a growing area of scientific interest, at the intersection of photonics, quantum optics and statistical physics. We give here a short introduction to the topic, together with an explanation of some of our more important recent results. A key result across several projects is that we have a model based on a detailed physical description which has been used to accurately describe experimental observations. We present a new open-source package in Python called PyPBEC which implements this model. The aim is to enable the reader to readily simulate and explore the physics of photon condensates themselves, so this article also includes a working example code which can be downloaded from the GitHub repository.</jats:p>
  • Journal article
    Ruberti M, 2021,

    Quantum electronic coherences by attosecond transient absorption spectroscopy: <i>ab initio</i> B-spline RCS-ADC study

    , FARADAY DISCUSSIONS, Vol: 228, Pages: 286-311, ISSN: 1359-6640
  • Journal article
    Li S, Driver T, Alexander O, Cooper B, Garratt D, Marinelli A, Cryan JP, Marangos JPet al., 2021,

    Time-resolved pump-probe spectroscopy with spectral domain ghost imaging

    , FARADAY DISCUSSIONS, Vol: 228, Pages: 488-501, ISSN: 1359-6640
  • Journal article
    Ashfold M, Chergui M, Fischer I, Ge L, Grell G, Ivanov M, Kirrander A, Kornilov O, Krishnan SR, Kupper J, Kuttner C, Makhija V, Martin F, Matsika S, Minns RS, Natan A, Neumark DM, Palacios A, Pratt S, Roder A, Rost JM, Ruberti M, Stolow A, Titov E, Young Let al., 2021,

    Time-resolved ultrafast spectroscopy: general discussion

    , FARADAY DISCUSSIONS, Vol: 228, Pages: 329-348, ISSN: 1359-6640
  • Journal article
    Danson CN, White M, Barr JRM, Bett T, Blyth P, Bowley D, Brenner C, Collins RJ, Croxford N, Dangor AEB, Devereux L, Dyer PE, Dymoke-Bradshaw A, Edwards CB, Ewart P, Ferguson AI, Girkin JM, Hall DR, Hanna DC, Harris W, Hillier DI, Hooker CJ, Hooker SM, Hopps N, Hull J, Hunt D, Jaroszynski DA, Kempenaars M, Kessler H, Knight PL, Knight S, Knowles A, Lewis CLS, Lipton KS, Littlechild A, Littlechild J, Maggs P, Malcolm GPA, Mangles SPD, Martin W, McKenna P, Moore RO, Morrison C, Najmudin Z, Neely D, New GHC, Norman MJ, Paine T, Parker AW, Penman RR, Pert GJ, Pietraszewski C, Randewich A, Rizvi NH, Seddon N, Sheng Z-M, Slater D, Smith RA, Spindloe C, Taylor R, Thomas G, Tisch JWG, Wark JS, Webb C, Wiggins SM, Willford D, Winstone Tet al., 2021,

    A history of high-power laser research and development in the United Kingdom

    , High Power Laser Science and Engineering, Vol: 9, Pages: 1-86, ISSN: 2095-4719

    The first demonstration of laser action in ruby was made in 1960 by T. H. Maiman of Hughes Research Laboratories, USA. Many laboratories worldwide began the search for lasers using different materials, operating at different wavelengths. In the UK, academia, industry and the central laboratories took up the challenge from the earliest days to develop these systems for a broad range of applications. This historical review looks at the contribution the UK has made to the advancement of the technology, the development of systems and components and their exploitation over the last 60 years.
  • Journal article
    Jurgilas S, Chakraborty A, Rich CJH, Caldwell L, Williams HJ, Fitch NJ, Sauer BE, Frye MD, Hutson JM, Tarbutt MRet al., 2021,

    Collisions between ultracold molecules and atoms in a magnetic trap

    , Physical Review Letters, Vol: 126, ISSN: 0031-9007

    We prepare mixtures of ultracold CaF molecules and Rb atoms in a magnetic trap and study theirinelastic collisions. When the atoms are prepared in the spin-stretched state and the molecules inthe spin-stretched component of the first rotationally excited state, they collide inelastically witha rate coefficient k2 = (6.6 ± 1.5) × 10−11 cm3/s at temperatures near 100 µK. We attribute thisto rotation-changing collisions. When the molecules are in the ground rotational state we see noinelastic loss and set an upper bound on the spin-relaxation rate coefficient of k2 < 5.8×10−12 cm3/swith 95% confidence. We compare these measurements to the results of a single-channel loss modelbased on quantum defect theory. The comparison suggests a short-range loss parameter close tounity for rotationally excited molecules, but below 0.04 for molecules in the rotational ground state.
  • Journal article
    Tao S-J, Wang Q-Q, Chen Z, Pan W-W, Yu S, Chen G, Xu X-Y, Han Y-J, Li C-F, Guo G-Cet al., 2021,

    Experimental optimal generation of hybrid entangled states in photonic quantum walks

    , OPTICS LETTERS, Vol: 46, Pages: 1868-1871, ISSN: 0146-9592

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