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Journal articleShah R, Barrett TJ, Colcelli A, et al., 2023,
Probing the Degree of Coherence through the Full 1D to 3D Crossover.
, Phys Rev Lett, Vol: 130We experimentally study a gas of quantum degenerate ^{87}Rb atoms throughout the full dimensional crossover, from a one-dimensional (1D) system exhibiting phase fluctuations consistent with 1D theory to a three-dimensional (3D) phase-coherent system, thereby smoothly interpolating between these distinct, well-understood regimes. Using a hybrid trapping architecture combining an atom chip with a printed circuit board, we continuously adjust the system's dimensionality over a wide range while measuring the phase fluctuations through the power spectrum of density ripples in time-of-flight expansion. Our measurements confirm that the chemical potential μ controls the departure of the system from 3D and that the fluctuations are dependent on both μ and the temperature T. Through a rigorous study we quantitatively observe how inside the crossover the dependence on T gradually disappears as the system becomes 3D. Throughout the entire crossover the fluctuations are shown to be determined by the relative occupation of 1D axial collective excitations.
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Journal articleLee C, Webster SC, Toba JM, et al., 2023,
Measurement-based ground-state cooling of a trapped-ion oscillator
, PHYSICAL REVIEW A, Vol: 107, ISSN: 2469-9926 -
Journal articleCheng C, Frasinski LJ, Mogol G, et al., 2023,
Multiparticle Cumulant Mapping for Coulomb Explosion Imaging
, PHYSICAL REVIEW LETTERS, Vol: 130, ISSN: 0031-9007- Author Web Link
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- Citations: 1
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Journal articleHaug T, Self CN, Kim MS, 2023,
Quantum machine learning of large datasets using randomized measurements
, Machine Learning: Science and Technology, Vol: 4, Pages: 1-17, ISSN: 2632-2153Quantum computers promise to enhance machine learning for practical applications. Quantum machine learning for real-world data has to handle extensive amounts of high-dimensional data. However, conventional methods for measuring quantum kernels are impractical for large datasets as they scale with the square of the dataset size. Here, we measure quantum kernels using randomized measurements. The quantum computation time scales linearly with dataset size and quadratic for classical post-processing. While our method scales in general exponentially in qubit number, we gain a substantial speed-up when running on intermediate-sized quantum computers. Further, we efficiently encode high-dimensional data into quantum computers with the number of features scaling linearly with the circuit depth. The encoding is characterized by the quantum Fisher information metric and is related to the radial basis function kernel. Our approach is robust to noise via a cost-free error mitigation scheme. We demonstrate the advantages of our methods for noisy quantum computers by classifying images with the IBM quantum computer. To achieve further speedups we distribute the quantum computational tasks between different quantum computers. Our method enables benchmarking of quantum machine learning algorithms with large datasets on currently available quantum computers.
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Journal articleThomas SE, Sagona-Stophel S, Schofield Z, et al., 2023,
Single-photon-compatible telecommunications-band quantum memory in a hot atomic gas
, Physical Review Applied, Vol: 19, Pages: 1-6, ISSN: 2331-7019The efficient storage and on-demand retrieval of quantum optical states that are compatible with the telecommunications band is a requirement for future terrestrial-based quantum optical networking. Spectrum in the telecommunications band minimizes optical fiber-propagation losses, and broad optical bandwidth facilitates high-speed networking protocols. Here we report on a telecommunications-wavelength- and bandwidth-compatible quantum memory. Using the Off-Resonant Cascaded Absorption protocol in hot 87Rb vapor, we demonstrate a total internal memory efficiency of 20.90(1)% with a Doppler-limited storage time of 1.10(2) ns. We characterize the memory performance with weak coherent states and measure a signal-to-noise ratio of 1.9(1)×104 for an average input photon number of 0.084.
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Journal articleHo C, Lim J, Sauer B, et al., 2023,
Measuring the nuclear magnetic quadrupole moment in heavy polar molecules
, Frontiers in Physics, Vol: 11, Pages: 1-10, ISSN: 2296-424XTheories that extend the Standard Model of particle physics often introduce new interactions that violate charge-parity (CP) symmetry. CP-violating effects within an atomic nucleus can be probed by measuring its nuclear magnetic quadrupole moment (MQM). The sensitivity of such a measurement is enhanced when using a heavy polar molecule containing a nucleus with quadrupole deformation. We determine how the energy levels of a molecule are shifted by the MQM and how those shifts can be measured. The measurement scheme requires molecules in a superposition of magnetic sub-levels that differ by many units of angular momentum. We develop a generic scheme for preparing these states. Finally, we consider the sensitivity that can be reached, showing that this method can reduce the current uncertainties on several CP-violating parameters.
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Journal articleLee J, Park J, Kim J, et al., 2023,
Non-Gaussian entanglement criteria for atomic homodyne detection
, Physical Review A, Vol: 107, ISSN: 2469-9926Homodyne measurement is a crucial tool widely used to address continuous variables for bosonic quantum systems. While an ideal homodyne detection provides a powerful analysis, e.g., to effectively measure quadrature amplitudes of light in quantum optics, it relies on the use of a strong reference field, the so-called local oscillator, typically in a coherent state. Such a strong coherent local oscillator may not be readily available, particularly for a massive quantum system like a Bose-Einstein condensate, posing a substantial challenge in dealing with continuous variables appropriately. It is necessary to establish a practical framework that includes the effects of nonideal local oscillators for a rigorous assessment of various quantum tests and applications. We here develop entanglement criteria beyond a Gaussian regime applicable for this realistic homodyne measurement that do not require assumptions on the state of local oscillators. We discuss the working conditions of homodyne detection to effectively detect non-Gaussian quantum entanglement under various states of local oscillators.
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Journal articleHaug T, Kim M, 2023,
Scalable measures of magic for quantum computers
, PRX Quantum, Vol: 4, ISSN: 2691-3399Nonstabilizerness or magic resource characterizes the amount of non-Clifford operations needed to prepare quantum states. It is a crucial resource for quantum computing and a necessary condition for quantum advantage. However, quantifying magic resource beyond a few qubits has been a major challenge. Here, we introduce efficient measures of magic resource for pure quantum states with a sampling cost that is independent of the number of qubits. Our method uses Bell measurements over two copies of a state, which we implement in experiment together with a cost-free error-mitigation scheme. We show the transition of classically simulable stabilizer states into intractable quantum states on the IonQ quantum computer. For applications, we efficiently distinguish stabilizer and nonstabilizer states with low measurement cost even in the presence of experimental noise. Further, we propose a variational quantum algorithm to maximize our measure via the shift rule. Our algorithm can be free of barren plateaus even for highly expressible variational circuits. Finally, we experimentally demonstrate a Bell-measurement protocol for the stabilizer Rényi entropy as well as the Wallach-Meyer entanglement measure. Our results pave the way to understanding the nonclassical power of quantum computers, quantum simulators, and quantum many-body systems.
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Conference paperSun B, Sotirova A, Dela Cruz V, et al., 2023,
Ultrafast Laser Written Waveguide Chips for Quantum Applications
Ultrafast laser micro-fabrication has found a wide range of applications in the past decades. In particular, it is possible to create advanced photonics chips based on three-dimensional optical waveguides. Significant potential has been demonstrated in areas such as topological photonics[1] and quantum technologies[2]. Traditional fabrication methods create optical waveguides by focusing ultrafast laser into transparent materials, such as fused silica or borosilicate glass, producing localized refractive index modification. Here, we demonstrate that our recently reported Spherical Phase Induced Multi-Core Waveguides (SPIM-WGs)[3] overcome some constraints of traditional waveguide fabrication techniques, enabling new functionalities in quantum applications.
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Journal articleAlexander OG, Marangos JP, Ruberti M, et al., 2023,
Attosecond electron dynamics in molecular systems
, Advances in Atomic Molecular and Optical Physics, Vol: 72, Pages: 183-251, ISSN: 1049-250XIn this paper we review the topic of attosecond electron dynamics in molecular systems. We present a digest of recent research on this topic conducted by ourselves and other researchers with the intention of providing an accessible, but rigorous, account of the current state of this intriguing field of research. A short account of the background quantum theory is given before discussing recent theoretical advances on understanding correlation driven electron dynamics and electron nuclear coupling in molecules undergoing fast photoionization. We then review experimental advances, using both high harmonic generation and XFEL based ultrafast x-ray pulses, and provide three recent case studies from our own work to illustrate this. The final sections look forward to the next steps in this field: we discuss the prospect for controlling attochemistry as well as extending attosecond measurement methods to electron dynamics in larger molecules and condensed phase systems.
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Journal articleHanks M, Kim MS, 2022,
Fault tolerance in qudit circuit design
, PHYSICAL REVIEW A, Vol: 106, ISSN: 2469-9926 -
Journal articleFasoulakis A, Major KDD, Hoggarth RAA, et al., 2022,
Uniaxial strain tuning of organic molecule single photon sources
, NANOSCALE, Vol: 15, Pages: 177-184, ISSN: 2040-3364 -
Journal articleAlonso I, Alpigiani C, Altschul B, et al., 2022,
Cold atoms in space: community workshop summary and proposed road-map
, EPJ QUANTUM TECHNOLOGY, Vol: 9, ISSN: 2662-4400- Author Web Link
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- Citations: 10
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Journal articleHaug T, Kim MS, 2022,
Natural parametrized quantum circuit
, PHYSICAL REVIEW A, Vol: 106, ISSN: 2469-9926- Author Web Link
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- Citations: 4
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Journal articleRussell D, Burdiak G, Carroll-Nellenback JJ, et al., 2022,
Perpendicular subcritical shock structure in a collisional plasma experiment
, Physical Review Letters, Vol: 129, ISSN: 0031-9007We present a study of perpendicular subcritical shocks in a collisional laboratory plasma. Shocks areproduced by placing obstacles into the supermagnetosonic outflow from an inverse wire array z pinch. Wedemonstrate the existence of subcritical shocks in this regime and find that secondary shocks form in thedownstream. Detailed measurements of the subcritical shock structure confirm the absence of ahydrodynamic jump. We calculate the classical (Spitzer) resistive diffusion length and show that it isapproximately equal to the shock width. We measure little heating across the shock (< 10% of the ionkinetic energy) which is consistent with an absence of viscous dissipation.
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Journal articleBarnard J, Lee J, Alexander O, et al., 2022,
Delivery of stable ultra-thin liquid sheets in vacuum for biochemical spectroscopy
, Frontiers in Molecular Biosciences, Vol: 9, ISSN: 2296-889XThe development of ultra-thin flat liquid sheets capable of running in vacuum has provided an exciting new target for X-ray absorption spectroscopy in the liquid and solution phases. Several methods have become available for delivering in-vacuum sheet jets using different nozzle designs. We compare the sheets produced by two different types of nozzle; a commercially available borosillicate glass chip using microfluidic channels to deliver colliding jets, and an in-house fabricated fan spray nozzle which compresses the liquid on an axis out of a slit to achieve collision conditions. We find in our tests that both nozzles are suitable for use in X-ray absorption spectroscopy with the fan spray nozzle producing thicker but more stable jets than the commercial nozzle. We also provide practical details of how to run these nozzles in vacuum.
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Journal articleSchwickert D, Ruberti M, Kolorenc P, et al., 2022,
Charge-induced chemical dynamics in glycine probed with time-resolved Auger electron spectroscopy
, STRUCTURAL DYNAMICS-US, Vol: 9- Author Web Link
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- Citations: 1
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Journal articleBellini M, Kwon H, Biagi N, et al., 2022,
Demonstrating quantum microscopic reversibility using coherent states of light
, Physical Review Letters, Vol: 129, Pages: 1-6, ISSN: 0031-9007The principle of microscopic reversibility lies at the core of fluctuation theorems, which have extended our understanding of the second law of thermodynamics to the statistical level. In the quantum regime, however, this elementary principle should be amended as the system energy cannot be sharply determined at a given quantum phase space point. In this Letter, we propose and experimentally test a quantum generalization of the microscopic reversibility when a quantum system interacts with a heat bath through energy-preserving unitary dynamics. Quantum effects can be identified by noting that the backward process is less likely to happen in the existence of quantum coherence between the system’s energy eigenstates. The experimental demonstration has been realized by mixing coherent and thermal states in a beam splitter, followed by heterodyne detection in an optical setup. We verify that the quantum modification for the principle of microscopic reversibility is critical in the low-temperature limit, while the quantum-to-classical transition is observed as the temperature of the thermal field gets higher.
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Journal articleTarrant J, Khokhlova M, Averbukh V, 2022,
Interferometry of quantum revivals (vol 157, 054304, 2022)
, JOURNAL OF CHEMICAL PHYSICS, Vol: 157, ISSN: 0021-9606 -
Journal articleKoukoulekidis N, Kwon H, Jee HH, et al., 2022,
Faster Born probability estimation via gate merging and frame optimisation
, Quantum, Vol: 6, Pages: 838-838, ISSN: 2521-327XOutcome probability estimation via classical methods is an important task for validating quantum computing devices. Outcome probabilities of any quantum circuit can be estimated using Monte Carlo sampling, where the amount of negativity present in the circuit frame representation quantifies the overhead on the number of samples required to achieve a certain precision. In this paper, we propose two classical sub-routines: circuit gate merging and frame optimisation, which optimise the circuit representation to reduce the sampling overhead. We show that the runtimes of both sub-routines scale polynomially in circuit size and gate depth. Our methods are applicable to general circuits, regardless of generating gate sets, qudit dimensions and the chosen frame representations for the circuit components. We numerically demonstrate that our methods provide improved scaling in the negativity overhead for all tested cases of random circuits with Clifford+T and Haar-random gates, and that the performance of our methods compares favourably with prior quasi-probability simulators as the number of non-Clifford gates increases.
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Journal articleKoukoulekidis N, Jee H, Jennings D, et al., 2022,
Faster Born probability estimation via gate merging and frame optimisation
, Quantum, Vol: 6, Pages: 838-838, ISSN: 2521-327XOutcome probability estimation via classical methods is an important task for validating quantum computing devices. Outcome probabilities of any quantum circuit can be estimated using Monte Carlo sampling, where the amount of negativity present in the circuit frame representation quantifies the overhead on the number of samples required to achieve a certain precision. In this paper, we propose two classical sub-routines: circuit gate merging and frame optimisation, which optimise the circuit representation to reduce the sampling overhead. We show that the runtimes of both sub-routines scale polynomially in circuit size and gate depth. Our methods are applicable to general circuits, regardless of generating gate sets, qudit dimensions and the chosen frame representations for the circuit components. We numerically demonstrate that our methods provide improved scaling in the negativity overhead for all tested cases of random circuits with Clifford+T and Haar-random gates, and that the performance of our methods compares favourably with prior quasi-probability simulators as the number of non-Clifford gates increases.
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Journal articleBressanini G, Kwon H, Kim MS, 2022,
Noise thresholds for classical simulability of nonlinear boson sampling
, Physical Review A: Atomic, Molecular and Optical Physics, Vol: 106, Pages: 1-9, ISSN: 1050-2947Boson sampling, a computational problem conjectured to be hard to simulate on a classical machine, is a promising candidate for an experimental demonstration of quantum advantage using bosons. However, inevitable experimental noise and imperfections, such as loss in the interferometer and random counts at the detectors, could challenge the sampling task from entering the regime where quantum advantage is achievable. In this work we introduce higher-order nonlinearities as a means to enhance the computational complexity of the problem and the protocol's robustness against noise, i.e., to increase the noise threshold that allows us to perform an efficient classical simulation of the problem. Using a phase-space method based on the negativity volume of the relevant quasiprobability distributions, we establish a necessary nonclassicality condition that any experimental proof of quantum advantage must satisfy. Our results indicate that the addition of single-mode Kerr nonlinearity at the input-state preparation level, while retaining a linear-optical evolution, makes the boson-sampling protocol more robust against noise and consequently relaxes the constraints on the noise parameters required to show quantum advantage.
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Journal articleZhang H, Wan L, Haug T, et al., 2022,
Resource-efficient high-dimensional subspace teleportation with a quantum autoencoder.
, Science Advances, Vol: 8, Pages: 1-11, ISSN: 2375-2548Quantum autoencoders serve as efficient means for quantum data compression. Here, we propose and demonstrate their use to reduce resource costs for quantum teleportation of subspaces in high-dimensional systems. We use a quantum autoencoder in a compress-teleport-decompress manner and report the first demonstration with qutrits using an integrated photonic platform for future scalability. The key strategy is to compress the dimensionality of input states by erasing redundant information and recover the initial states after chip-to-chip teleportation. Unsupervised machine learning is applied to train the on-chip autoencoder, enabling the compression and teleportation of any state from a high-dimensional subspace. Unknown states are decompressed at a high fidelity (~0.971), obtaining a total teleportation fidelity of ~0.894. Subspace encodings hold great potential as they support enhanced noise robustness and increased coherence. Laying the groundwork for machine learning techniques in quantum systems, our scheme opens previously unidentified paths toward high-dimensional quantum computing and networking.
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Journal articleSong W, Lim Y, Jeong K, et al., 2022,
Polynomial T-depth quantum solvability of noisy binary linear problem: from quantum-sample preparation to main computation
, New Journal of Physics, Vol: 24, Pages: 1-11, ISSN: 1367-2630The noisy binary linear problem (NBLP) is known as a computationally hard problem, and therefore, it offers primitives for post-quantum cryptography. An efficient quantum NBLP algorithm that exhibits a polynomial quantum sample and time complexities has recently been proposed. However, the algorithm requires a large number of samples to be loaded in a highly entangled state and it is unclear whether such a precondition on the quantum speedup can be obtained efficiently. Here, we present a complete analysis of the quantum solvability of the NBLP by considering the entire algorithm process, namely from the preparation of the quantum sample to the main computation. By assuming that the algorithm runs on 'fault-tolerant' quantum circuitry, we introduce a reasonable measure of the computational time cost. The measure is defined in terms of the overall number of T gate layers, referred to as T-depth complexity. We show that the cost of solving the NBLP can be polynomial in the problem size, at the expense of an exponentially increasing logical qubits.
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Journal articleSempere Llagostera S, Patel RB, Walmsley IA, et al., 2022,
Experimentally finding dense subgraphs using a time-bin encoded Gaussian boson sampling device
, Physical Review X, Vol: 12, Pages: 1-12, ISSN: 2160-3308Gaussian boson sampling (GBS) is a quantum computing concept based on drawing samples from a multimode nonclassical Gaussian state using photon-number resolving detectors. It was initially posed as a near-term approach to achieve quantum advantage, and several applications have beenproposed since, including the calculation of graph features. For the first time, we use a time-bin encoded interferometer to implement GBS experimentally and extract samples to enhance the search for dense subgraphs in a graph. Our results indicate an improvement over classical methods for subgraphs of sizes three and four in a graph containing ten nodes. In addition, we numerically explore the role of imperfections in the optical circuit and on the performance of the algorithm.
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Journal articleLiu W, Ivady V, Li Z-P, et al., 2022,
Coherent dynamics of multi-spin V<sub>B</sub><SUP>-</SUP> center in hexagonal boron nitride
, NATURE COMMUNICATIONS, Vol: 13- Author Web Link
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- Citations: 20
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Journal articleZhang C, Tarbutt M, 2022,
Quantum computation in a hybrid array of molecules and Rydberg atoms
, PRX Quantum, Vol: 3, Pages: 1-17, ISSN: 2691-3399We show that an array of polar molecules interacting with Rydberg atoms is a promising hybrid system for scalable quantum computation. Quantum information is stored in long-lived hyperfine or rotational states of molecules which interact indirectly through resonant dipole-dipole interactions with Rydberg atoms. A two-qubit gate based on this interaction has a duration of 1 μs and an achievable fidelity of 99.9%. The gate has little sensitivity to the motional states of the particles – the molecules can be in thermal states, the atoms do not need to be trapped during Rydberg excitation, the gate does not heat the molecules, and heating of the atoms has a negligible effect. Within a large, static array, the gate can be applied to arbitrary pairs of molecules separated by tens of micrometres, making the scheme highly scalable. The molecule-atom interaction can also be used for rapid qubit initialization and efficient, non-destructive qubit readout, without driving any molecular transitions. Single qubit gates are driven using microwave pulses alone, exploiting the strong electric dipole transitions between rotational states. Thus, all operations required for large scale quantum computation can be done without moving the molecules or exciting them out of their ground electronic states.
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Journal articleFrasinski LJ, 2022,
Cumulant mapping as the basis of multi-dimensional spectrometry
, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 24, Pages: 20776-20787, ISSN: 1463-9076- Author Web Link
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- Citations: 2
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Journal articleZhao H, Knolle J, Moessner R, et al., 2022,
Suppression of Interband Heating for Random Driving
, PHYSICAL REVIEW LETTERS, Vol: 129, ISSN: 0031-9007- Author Web Link
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- Citations: 1
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Journal articleFerchaud C, Jarosch S, Avni T, et al., 2022,
Interaction of an intense few-cycle infrared laser pulse with an ultrathin transparent liquid sheet
, Optics Express, Vol: 30, Pages: 34684-34692, ISSN: 1094-4087We experimentally study the interaction between intense infrared few-cycle laser pulses and an ultrathin (∼2 µm) flat liquid sheet of isopropanol running in vacuum. We observe a rapid decline in transmission above a critical peak intensity of 50 TW/cm2 of the initially transparent liquid sheet, and the emission of a plume of material. We find both events are due to the creation of a surface plasma and are similar to processes observed in dielectric solids. After calculating the electron density for different laser peak intensities, we find an electron scattering rate of 0.3 fs-1 in liquid isopropanol to be consistent with our data. We study the dynamics of the plasma plume to find the expansion velocity of the plume front.
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