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Journal articleKoukoulekidis N, Jennings D, 2022,
Constraints on magic state protocols from the statistical mechanics of Wigner negativity
, NPJ QUANTUM INFORMATION, Vol: 8- Author Web Link
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- Citations: 6
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Journal articleGirling M, Cirstoiu C, Jennings D, 2022,
Estimation of correlations and non-separability in quantum channels via unitarity benchmarking
, Physical Review Research, Vol: 4, ISSN: 2643-1564The ability to transfer quantum information between systems is a fundamental component of quantum technologies and leads to correlations within the global quantum process. However, correlation structures in quantum channels are less studied than those in quantum states. Motivated by recent techniques in randomized benchmarking, we develop a range of results for efficient estimation of correlations within a bipartite quantum channel. We introduce subunitarity measures that are invariant under local changes of basis, generalize the unitarity of a channel, and allow for the analysis of quantum information exchange within channels. Using these, we show that unitarity is monogamous, and we provide an information-disturbance relation. We then define a notion of correlated unitarity that quantifies the correlations within a given channel. Crucially, we show that this measure is strictly bounded on the set of separable channels and therefore provides a witness of nonseparability. Finally, we describe how such measures for effective noise channels can be efficiently estimated within different randomized benchmarking protocols. We find that the correlated unitarity can be estimated in a SPAM-robust manner for any separable quantum channel, and we show that a benchmarking/tomography protocol with mid-circuit resets can reliably witness nonseparability for sufficiently small reset errors. The tools we develop provide information beyond that obtained via simultaneous randomized benchmarking and so could find application in the analysis of cross-talk errors in quantum devices.
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Journal articleBarrett TJ, Evans W, Gadge A, et al., 2022,
An environmental monitoring network for quantum gas experiments and devices
, QUANTUM SCIENCE AND TECHNOLOGY, Vol: 7, ISSN: 2058-9565 -
Journal articleSong W, Lim Y, Jeong K, et al., 2022,
Quantum solvability of noisy linear problems by divide-and-conquer strategy
, Quantum Science and Technology, Vol: 7, ISSN: 2058-9565Noisy linear problems have been studied in various science and engineering disciplines. A class of 'hard' noisy linear problems can be formulated as follows: Given a matrix $\hat{A}$ and a vector b constructed using a finite set of samples, a hidden vector or structure involved in b is obtained by solving a noise-corrupted linear equation $\hat{A}\mathbf{x}\approx \mathbf{b}+\boldsymbol{\eta }$, where η is a noise vector that cannot be identified. For solving such a noisy linear problem, we consider a quantum algorithm based on a divide-and-conquer strategy, wherein a large core process is divided into smaller subprocesses. The algorithm appropriately reduces both the computational complexities and size of a quantum sample. More specifically, if a quantum computer can access a particular reduced form of the quantum samples, polynomial quantum-sample and time complexities are achieved in the main computation. The size of a quantum sample and its executing system can be reduced, e.g., from exponential to sub-exponential with respect to the problem length, which is better than other results we are aware. We analyse the noise model conditions for such a quantum advantage, and show when the divide-and-conquer strategy can be beneficial for quantum noisy linear problems.
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Journal articleZhang C, Zhang C, Cheng L, et al., 2022,
Inner-shell excitation in the YbF molecule and its impact on laser cooling
, Journal of Molecular Spectroscopy, Vol: 386, ISSN: 0022-2852The YbF molecule is a sensitive system for measuring the electron’s electric dipole moment. The precision ofthis measurement can be improved by direct laser cooling of the molecules to ultracold temperature. However,low-lying electronic states arising from excitation of a 4f electron may hinder laser cooling. One set of these ‘‘4fhole’’ states lies below the 𝐴2𝛱1∕2 excited state used for laser cooling, and radiative decay to these intermediatelevels, even with branching ratios as small as 10−5, can be a hindrance. Other 4f hole states lie very close tothe 𝐴2𝛱1∕2 state, and a perturbation results in states of mixed character that are involved in the laser coolingcycle. This perturbation may enhance the loss of molecules to states outside of the laser cooling cycle. Wemodel the perturbation of the 𝐴2𝛱1∕2 state to determine the strength of the coupling between the states, thede-perturbed potential energy curves, and the radiative branching ratios to various vibrational levels of theground state, 𝑋2𝛴+. We use electronic structure calculations to characterize the 4f hole states and the strengthsof transitions between these states and the 𝐴2𝛱1∕2 and 𝑋2𝛴+ states. We identify a leak out of the cooling cyclewith a branching ratio of roughly 5 × 10−4, dominated by the contribution of the ground state configurationin a 4f hole state. Finally, we assess the impact of these results for laser cooling of YbF and molecules withsimilar structure.
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Working paperKoukoulekidis N, Jennings D, 2022,
Constraints on magic state protocols from the statistical mechanics of Wigner negativity
, Publisher: Nature ResearchMagic states are key ingredients in schemes to realize universalfault-tolerant quantum computation. Theories of magic states attempt toquantify this computational element via monotones and determine how thesestates may be efficiently transformed into useful forms. Here, we develop astatistical mechanical framework based on majorization to describe Wignernegative magic states for qudits of odd prime dimension processed underClifford circuits. We show that majorization allows us to both quantifydisorder in the Wigner representation and derive upper bounds for magicdistillation. These bounds are shown to be tighter than other bounds, such asfrom mana and thauma, and can be used to incorporate hardware physics, such astemperature dependence and system Hamiltonians. We also show that a subset ofsingle-shot R\'{e}nyi entropies remain well-defined on quasi-distributions, arefully meaningful in terms of data processing and can acquire negative valuesthat signal magic. We find that the mana of a magic state is the measure ofdivergence of these R\'{e}nyi entropies as one approaches the Shannon entropyfor Wigner distributions, and discuss how distillation lower bounds could beobtained in this setting. This use of majorization for quasi-distributionscould find application in other studies of non-classicality, and raises novelquestions in the context of classical statistical mechanics.
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Journal articleWill C, Bohman M, Driscoll T, et al., 2022,
Sympathetic cooling schemes for separately trapped ions coupled via image currents
, NEW JOURNAL OF PHYSICS, Vol: 24, ISSN: 1367-2630 -
Journal articleSchofield RC, Burdekin P, Fasoulakis A, et al., 2022,
Narrow and Stable Single Photon Emission from Dibenzoterrylene in <i>para</i>-Terphenyl Nanocrystals
, CHEMPHYSCHEM, Vol: 23, ISSN: 1439-4235- Author Web Link
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- Citations: 1
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Journal articleTang H, Banchi L, Wang T-Y, et al., 2022,
Generating Haar-uniform randomness using stochastic quantum walks on a photonic chip
, Physical Review Letters, Vol: 128, ISSN: 0031-9007As 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.
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Journal articleLi S, Driver T, Rosenberger P, et al., 2022,
Attosecond coherent electron motion in Auger-Meitner decay
, SCIENCE, Vol: 375, Pages: 285-+, ISSN: 0036-8075- Author Web Link
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- Citations: 17
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Journal articleMoroney N, Del Bino L, Zhang S, et al., 2022,
A Kerr polarization controller
, NATURE COMMUNICATIONS, Vol: 13- Author Web Link
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- Citations: 5
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Journal articleClark A, Clear C, Schofield R, et al., 2022,
Photon indistinguishability measurements under pulsed and continuous excitation
, Physical Review Research, Vol: 4, ISSN: 2643-1564The 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.
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Journal articleGuo N-J, Liu W, Li Z-P, et al., 2022,
Generation of Spin Defects by Ion Implantation in Hexagonal Boron Nitride.
, ACS Omega, Vol: 7, Pages: 1733-1739Optically 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.
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Journal articleAlsing PM, Birrittella RJ, Gerry CC, et al., 2022,
Extending the Hong-Ou-Mandel effect: the power of nonclassicality
, Physical Review A: Atomic, Molecular and Optical Physics, Vol: 105, ISSN: 1050-2947We 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
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Journal articleSempere-Llagostera S, Thekkadath G, Patel R, et 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: 1094-4087Multiphoton 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.
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Journal articleKwon H, Mukherjee R, Kim MS, 2022,
Reversing Lindblad dynamics via continuous Petz recovery map
, Physical Review Letters, Vol: 128, Pages: 1-7, ISSN: 0031-9007An 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.
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Journal articleThekkadath GS, Bell BA, Patel RB, et al., 2022,
Measuring the joint spectral mode of photon pairs using intensity interferometry
, Physical Review Letters, Vol: 128, Pages: 1-6, ISSN: 0031-9007The 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.
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Journal articleMa Y, Guff T, Morley GW, et al., 2022,
Limits on inference of gravitational entanglement
, Physical Review Research, Vol: 4, Pages: 1-7, ISSN: 2643-1564Combining 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.
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Journal articleBorchert MJ, Devlin JA, Erlewein SR, et al., 2022,
A 16-parts-per-trillion measurement of the antiproton-to-proton charge-mass ratio
, NATURE, Vol: 601, Pages: 53-+, ISSN: 0028-0836- Author Web Link
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- Citations: 12
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Conference paperMaimaris M, Pettipher AJ, Azzouzi M, et 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.
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Conference paperThekkadath GS, Sempere-Llagostera S, Bell BA, et 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.
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Conference paperSempere-Llagostera S, Thekkadath GS, Patel RB, et 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.
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Journal articleBulmer JFF, Bell BA, Chadwick RS, et al., 2022,
The boundary for quantum advantage in Gaussian boson sampling
, Science Advances, Vol: 8, ISSN: 2375-2548Identifying 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
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Conference paperSchofield RC, Boissier S, Clear C, et 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.
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Conference paperEnzian G, Freisem L, Price JJ, et 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 articleCorfield O, Lishman J, Lee C, et al., 2021,
Certifying multilevel coherence in the motional state of a trapped ion
, PRX Quantum, Vol: 2, ISSN: 2691-3399Quantum 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.
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Working paperBarontini G, Blackburn L, Boyer V, et al., 2021,
Measuring the stability of fundamental constants with a network of clocks
, Publisher: arXivThe 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.
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Journal articleEnzian G, Freisem L, Price JJ, et al., 2021,
Non-Gaussian Mechanical Motion via Single and Multiphonon Subtraction from a Thermal State
, PHYSICAL REVIEW LETTERS, Vol: 127, ISSN: 0031-9007- Author Web Link
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- Citations: 9
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Journal articleMukherjee R, Kuros A, Sacha K, et al., 2021,
Controlled preparation of phases in two-dimensional time crystals
, Physical Review Research, Vol: 3, ISSN: 2643-1564The 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.
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Journal articleKanari-Naish LA, Clarke J, Vanner MR, et al., 2021,
Can the displacemon device test objective collapse models?
, AVS Quantum Science, Vol: 3Testing 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.
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