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Journal articleGemmell NR, Ma Y, Pearce E, et al., 2024,
Coupling undetected sensing modes by quantum erasure
, APL Quantum, Vol: 1<jats:p>Imaging with undetected photons (IUP) enables the possibility of sensing changes in the phase and the transmission of a beam of light that need never be detected. This has led to the possibility of infrared sensing with visible silicon camera technology, for example. Relying on the interference of two identical pairs of photons, IUP was initially achieved using unidirectional paths through two nonlinear crystal pair sources. More recently, folded arrangements using bidirectional paths through a single-crystal have become common for their simplicity. Here, we theoretically model and experimentally implement a novel setup involving three interference paths through a single nonlinear crystal. This establishes two independent IUP sensing modes in addition to a third linear interference mode. We achieve this using a polarization state quantum eraser approach, with excellent agreement between experiment and theory. This system provides a new route to control and optimize IUP interference in a single-crystal folded arrangement by using controllable quantum erasure to balance the interferometer, opening the door to new implementations and applications for IUP.</jats:p>
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Journal articleAlexander O, Egun F, Rego L, et al., 2024,
Attosecond impulsive stimulated x-ray Raman scattering in liquid water
, Science Advances, Vol: 10, ISSN: 2375-2548We report the measurement of impulsive stimulated x-ray Raman scattering in neutral liquid water. An attosecond pulse drives the excitations of an electronic wavepacket in water molecules. The process comprises two steps: a transition to core-excited states near the oxygen atoms accompanied by transition to valence-excited states. Thus, the wavepacket is impulsively created at a specific atomic site within a few hundred attoseconds through a nonlinear interaction between the water and the x-ray pulse. We observe this nonlinear signature in an intensity-dependent Stokes Raman sideband at 526 eV. Our measurements are supported by our state-of-the-art calculations based on the polarization response of water dimers in bulk solvation and propagation of attosecond x-ray pulses at liquid density.
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Journal articleDriver T, Mountney M, Wang J, et al., 2024,
Attosecond delays in X-ray molecular ionization
, NATURE, Vol: 632, ISSN: 0028-0836 -
Journal articleBressanini G, Genoni MG, Kim MS, et al., 2024,
Multi-parameter quantum estimation of single- and two-mode pure Gaussian states
, Journal of Physics A: Mathematical and Theoretical, Vol: 57, ISSN: 1751-8113We discuss the ultimate precision bounds on the multiparameter estimation of single- and two-mode pure Gaussian states. By leveraging on previous approaches that focused on the estimation of a complex displacement only, we derive the Holevo Cramér–Rao bound (HCRB) for both displacement and squeezing parameter characterizing single and two-mode squeezed states. In the single-mode scenario, we obtain an analytical bound and find that it degrades monotonically as the squeezing increases. Furthermore, we prove that heterodyne detection is nearly optimal in the large squeezing limit, but in general the optimal measurement must include non-Gaussian resources. On the other hand, in the two-mode setting, the HCRB improves as the squeezing parameter grows and we show that it can be attained using double-homodyne detection.
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Journal articleHaug T, Kim MS, 2024,
Generalization of quantum machine learning models using quantum Fisher information metric
, Physical Review Letters, Vol: 133, ISSN: 0031-9007Generalization is the ability of machine learning models to make accurate predictions on new data by learning from training data. However, understanding generalization of quantum machine learning models has been a major challenge. Here, we introduce the data quantum Fisher information metric (DQFIM). It describes the capacity of variational quantum algorithms depending on variational ansatz, training data, and their symmetries. We apply the DQFIM to quantify circuit parameters and training data needed to successfully train and generalize. Using the dynamical Lie algebra, we explain how to generalize using a low number of training states. Counterintuitively, breaking symmetries of the training data can help to improve generalization. Finally, we find that out-of-distribution generalization, where training and testing data are drawn from different data distributions, can be better than using the same distribution. Our work provides a useful framework to explore the power of quantum machine learning models.
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Journal articleLatacz BM, Fleck M, Jaeger JI, et al., 2024,
Orders of Magnitude Improved Cyclotron-Mode Cooling for Nondestructive Spin Quantum Transition Spectroscopy with Single Trapped Antiprotons
, PHYSICAL REVIEW LETTERS, Vol: 133, ISSN: 0031-9007 -
Journal articleTofful A, Baynham CFA, Curtis EA, et al., 2024,
<SUP>171</SUP>Yb<SUP>+</SUP> optical clock with 2.2 x 10<SUP>-18</SUP> systematic uncertainty and absolute frequency measurements
, METROLOGIA, Vol: 61, ISSN: 0026-1394 -
Journal articleKnight PL, Gerry CC, Birrittella RJ, et al., 2024,
Enduring relevance of the Jaynes-Cummings model: a personal perspective [Invited]
, JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS, Vol: 41, ISSN: 0740-3224 -
Journal articleTaylor A, Bressanini G, Kwon H, et al., 2024,
Quantum error cancellation in photonic systems: undoing photon losses
, Physical Review A, Vol: 110, ISSN: 2469-9926Real photonic devices are subject to photon losses that can decohere quantum information encoded in the system. In the absence of full fault tolerance, quantum error mitigation techniques have been introduced to help manage errors in noisy quantum devices. In this paper, we introduce an error mitigation protocol inspired by probabilistic error cancellation (a popular error mitigation technique in discrete variable systems) for continuous variable systems. We show that our quantum error cancellation protocol can undo photon losses in expectation value estimation tasks. To do this, we analytically derive the (nonphysical) inverse photon loss channel and decompose it into a sum over physically realizable channels with potentially negative coefficients. The bias of our ideal expectation value estimator can be made arbitrarily small at the cost of increasing the sampling overhead. The protocol requires a noiseless amplification followed by a series of photon subtractions. While these operations can be implemented probabilistically, for certain classes of initial state one can avoid the burden of carrying out the amplification and photon subtractions by leveraging Monte Carlo methods to give an unbiased estimate of the ideal expectation value. We validate our proposed mitigation protocol by simulating the scheme on squeezed vacuum states, cat states, and entangled coherent states.
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Journal articleAthanasakis-Kaklamanakis M, Wilkins SG, Lassegues P, et al., 2024,
Radiative lifetime of the <i>A</i> <SUP>2</SUP>Π <sub>1 / 2</sub> state in RaF with relevance to laser cooling
, PHYSICAL REVIEW A, Vol: 110, ISSN: 2469-9926 -
Journal articleXiao X, Yang JJ, Millard TS, et al., 2024,
Nanofocusing in critically coupled nanogap waveguide resonators
, ACS Photonics, Vol: 11, Pages: 2836-2842, ISSN: 2330-4022Coupling between optical antenna resonances is a powerful way to control the distribution of light in nanoscale systems. When the strength of coupling is fine-tuned against resonance loss, a critical coupling condition is often met, where energy can be efficiently directed between the system’s components. In this work, we use this concept to nanofocus optical energy into the 50 nm gap of a waveguide resonator, which on its own cannot be excited by external illumination. Light couples to the waveguide antenna via Fano interference with a bar antenna dimer. As a composite antenna, the shifting of the dimer relative to the waveguide resonator enables the precise tuning of their mutual coupling. We find a critical coupling condition where light is maximally focused into the waveguide’s gap corresponding to unity coupling cooperativity. Our interpretation of critical-coupling-induced nanofocusing is supported by the simultaneous maximization of both second and third harmonic generation at the critical condition.
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Journal articleWill C, Wiesinger M, Micke P, et al., 2024,
Image-Current Mediated Sympathetic Laser Cooling of a Single Proton in a Penning Trap Down to 170 mK Axial Temperature
, PHYSICAL REVIEW LETTERS, Vol: 133, ISSN: 0031-9007 -
Journal articleJae J, Lee J, Kim MS, et al., 2024,
Contextual quantum metrology
, npj Quantum Information, Vol: 10, ISSN: 2056-6387We demonstrate that the contextuality of measurement selection can enhance the precision of quantum metrology with a simple linear optical experiment. Contextuality is a nonclassical property known as a resource for various quantum information processing tasks. Recent studies show that contextuality by anomalous weak values can be utilized to enhance metrological precision, unraveling the role of contextuality in quantum metrology. Our contextual quantum metrology (coQM) scheme can elevate the precision of the optical polarimetry as much as 6 times the precision limit given by the Quantum Fisher Information. We achieve the contextuality-enabled enhancement with two mutually complementary measurements, whereas, in the conventional method, some optimal measurements to achieve the precision limit are either theoretically challenging to find or experimentally infeasible to realize. These results highlight that the contextuality of measurement selection is applicable in practice for quantum metrology.
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Journal articleMichniewicz J, Kim MS, 2024,
Leveraging off-the-shelf silicon chips for quantum computing
, APPLIED PHYSICS LETTERS, Vol: 124, ISSN: 0003-6951 -
Journal articleHaug T, Lee S, Kim MS, 2024,
Efficient quantum algorithms for stabilizer entropies
, Physical Review Letters, Vol: 132, ISSN: 0031-9007Stabilizer entropies (SEs) are measures of nonstabilizerness or “magic” that quantify the degree to whicha state is described by stabilizers. SEs are especially interesting due to their connections to scrambling,localization and property testing. However, applications have been limited so far as previously knownmeasurement protocols for SEs scale exponentially with the number of qubits. Here, we efficiently measureSEs for integer R´enyi index n > 1 via Bell measurements. The SE of N-qubit quantum states can bemeasured with OðnÞ copies and OðnNÞ classical computational time, where for even n we additionallyrequire the complex conjugate of the state. We provide efficient bounds of various nonstabilizernessmonotones that are intractable to compute beyond a few qubits. Using the IonQ quantum computer, wemeasure SEs of random Clifford circuits doped with non-Clifford gates and give bounds for the stabilizerfidelity, stabilizer extent, and robustness of magic. We provide efficient algorithms to measure Clifford averaged 4n-point out-of-time-order correlators and multifractal flatness. With these measures we study thescrambling time of doped Clifford circuits and random Hamiltonian evolution depending on nonstabilizer ness. Counterintuitively, random Hamiltonian evolution becomes less scrambled at long times, which wereveal with the multifractal flatness. Our results open up the exploration of nonstabilizerness with quantumcomputers.
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Journal articleZhu R, Pike-Burke C, Mintert F, 2024,
Active learning for quantum mechanical measurements
, PHYSICAL REVIEW A, Vol: 109, ISSN: 2469-9926 -
Journal articleHanks M, Lee S, Kim MS, 2024,
Noise-Tailored Constructions for Spin Wigner Function Kernels
, ADVANCED PHYSICS RESEARCH, Vol: 3, ISSN: 2751-1200 -
Journal articleMok W-K, Zhang H, Haug T, et al., 2024,
Rigorous noise reduction with quantum autoencoders
, AVS QUANTUM SCIENCE, Vol: 6 -
Journal articlePopa S, Schaller S, Fielicke A, et al., 2024,
Understanding Inner-Shell Excitations in Molecules through Spectroscopy of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>4</mml:mn><mml:mi>f</mml:mi></mml:math> Hole States of YbF
, Physical Review X, Vol: 14<jats:p>Molecules containing a lanthanide atom have sets of electronic states arising from excitation of an inner-shell electron. These states have received little attention but are thought to play an important role in laser cooling of such molecules and may be a useful resource for testing fundamental physics. We study a series of inner-shell excited states in YbF using resonance-enhanced multiphoton ionization spectroscopy. We investigate the excited states of lowest energy, 8474, 9013, and <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mn>9090</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:msup><a:mrow><a:mi>cm</a:mi></a:mrow><a:mrow><a:mo>−</a:mo><a:mn>1</a:mn></a:mrow></a:msup></a:math> above the ground state, all corresponding to the configuration <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mn>4</c:mn><c:msup><c:mrow><c:mi>f</c:mi></c:mrow><c:mrow><c:mn>13</c:mn></c:mrow></c:msup><c:mn>6</c:mn><c:msup><c:mrow><c:mi>s</c:mi></c:mrow><c:mrow><c:mn>2</c:mn></c:mrow></c:msup><c:mtext> </c:mtext><c:mtext> </c:mtext><c:msub><c:mrow><c:mmultiscripts><c:mrow><c:mi>F</c:mi></c:mrow><c:mprescripts/><c:none/><c:mrow><c:mn>2</c:mn></c:mrow></c:mmultiscripts></c:mrow><c:mrow><c:mn>7</c:mn><c:mo>/</c:mo><c:mn>2</c:mn></c:mrow></c:msub></c:mrow></c:math> of the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:msup>&l
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Journal articleWalraven EF, Tarbutt MR, Karman T, 2024,
Scheme for deterministic loading of laser-cooled molecules into optical tweezers
, Physical Review Letters, Vol: 132, ISSN: 0031-9007We propose to repeatedly load laser-cooled molecules into optical tweezers, and transfer them to storage states that are rotationally excited by two additional quanta. Collisional loss of molecules in these storage states is suppressed, and a dipolar blockade prevents the accumulation of more than one molecule. Applying three cycles loads tweezers with single molecules at an 80% success rate, limited by residual collisional loss. This improved loading efficiency reduces the time needed for rearrangement of tweezer arrays, which would otherwise limit the scalability of neutral molecule quantum computers.
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Journal articleCornish SL, Tarbutt MR, Hazzard KRA, 2024,
Quantum computation and quantum simulation with ultracold molecules
, NATURE PHYSICS, Vol: 20, Pages: 730-740, ISSN: 1745-2473 -
Journal articleShi B, Mintert F, 2024,
Quantum simulations of time-dependent Hamiltonians beyond the quasistatic approximation
, Physical Review Research, Vol: 6, ISSN: 2643-1564Existing approaches to analogue quantum simulations of time-dependent quantum systems relyon perturbative corrections to quantum simulations of time-independent quantum systems. Weovercome this restriction to perturbative treatments with an approach based on flow equations anda multi-mode Fourier expansion. The potential of the quantum simulations that can be achievedwith our approach is demonstrated with the pedagogical example of a Lambda-system and thequench in finite time through a quantum phase transition of a Chern insulator in a driven noninteracting Hubbard system. The example of the Lambda-system demonstrates the ability of ourapproach to describe situations beyond the validity of adiabatic approximations.
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Journal articleTang Y, Dhar HS, Oulton RF, et al., 2024,
Breakdown of Temporal Coherence in Photon Condensates
, PHYSICAL REVIEW LETTERS, Vol: 132, ISSN: 0031-9007 -
Journal articleGuo Z, Driver T, Beauvarlet S, et al., 2024,
Experimental demonstration of attosecond pump-probe spectroscopy with an X-ray free-electron laser
, NATURE PHOTONICS, ISSN: 1749-4885 -
Journal articleLi W, Brown D, Vylegzhanin A, et al., 2024,
Atom-light interactions using optical nanofibres—a perspective
, Jphys Photonics, Vol: 6Complete control of light-matter interactions at a single quantum level is critical for quantum science applications such as precision measurement and information processing. Nanophotonic devices, developed with recent advancements in nanofabrication techniques, can be used to tailor the interactions between single photons and atoms. One example of such a nanophotonic device is the optical nanofibre, which provides an excellent platform due to the strongly confined transverse light fields, long interaction length, low loss, and diverse optical modes. This facilitates a strong interaction between atoms and guided light, revealing chiral atom-light processes and the prospect of waveguide quantum electrodynamics. This paper highlights recent advances, experimental techniques, and future perspectives of the optical nanofibre-atom hybrid quantum platform.
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Journal articleCheng C, Frasinski LJ, Allum F, et al., 2024,
Multiparticle cumulant mapping for Coulomb explosion imaging: Calculations and algorithm
, PHYSICAL REVIEW A, Vol: 109, ISSN: 2469-9926 -
Journal articlePopa S, Schaller S, Fielicke A, et al., 2024,
Understanding inner-shell excitations in molecules through spectroscopy of the 4f hole states of YbF
, Physical Review X, Vol: 14, ISSN: 2160-3308Molecules containing a lanthanide atom have sets of electronic states arising from excitation of an inner-shell electron. These states have received little attention, but are thought to play an important role in laser cooling of such molecules and may be a useful resource for testing fundamental physics. We study a series of inner-shell excited states in YbF using resonance-enhanced multi-photon ionisation spectroscopy. We investigate the excited states of lowest energy, 8474, 9013 and 9090 cm⁻¹ above the ground state, all corresponding to the configuration 4f¹³6s² ²F₇⁄₂ of the Yb⁺ ion. They are metastable, since they have no electric dipole allowed transitions to the ground state. We also characterize a state at 31050 cm¯¹ that is easily excited from both the ground and metastable states, which makes it especially useful for this spectroscopic study. Finally, we study two states at 48720 cm¯¹ and 48729 cm¯¹, which are above the ionization limit and feature strong auto-ionizing resonances that prove useful for efficient detection of the molecules and for identifying the rotational quantum number of each line in the spectrum. We resolve the rotational structures of all these states and find that they can all be described by a very simple model based on Hund’s case (c). Our study provides information necessary for laser slowing and magneto-optical trapping of YbF, which is an important species for testing fundamental physics. We also consider whether the low-lying inner-shell states may themselves be useful as probes of the electron’s electric dipole moment or of varying fundamental constants, since they are long-lived states in a laser-coolable molecule featuring closely-spaced levels of opposite parity.
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Journal articleTang Y, Dhar HS, Oulton RF, et al., 2024,
Photon-photon correlation of condensed light in a microcavity
, Physical Review A (atomic, molecular, and optical physics and quantum information), Vol: 109, ISSN: 2469-9926The study of temporal coherence in a Bose-Einstein condensate of photons can be challenging, especially in the presence of correlations between the photonic modes. In this work, we use a microscopic, multimode model of photonic condensation inside a dye-filled microcavity and the quantum regression theorem to derive an analytical expression for the equation of motion of the photon-photon correlation function. This allows us to derive the coherence time of the photonic modes and identify a nonmonotonic dependence of the temporal coherence of the condensed light with the cutoff frequency of the microcavity.
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Journal articleAlaa El-Din K, Alexander O, Frasinski L, et al., 2024,
Efficient prediction of attosecond two-colour pulses from an X-ray free-electron laser with machine learning
, Scientific Reports, Vol: 14, ISSN: 2045-2322X-ray free-electron lasers are sources of coherent, high-intensity X-rays with numerous applications in ultra-fast measurements and dynamic structural imaging. Due to the stochastic nature of the self-amplified spontaneous emission process and the difficulty in controlling injection of electrons, output pulses exhibit significant noise and limited temporal coherence. Standard measurement techniques used for characterizing two-coloured X-ray pulses are challenging, as they are either invasive or diagnostically expensive. In this work, we employ machine learning methods such as neural networks and decision trees to predict the central photon energies of pairs of attosecond fundamental and second harmonic pulses using parameters that are easily recorded at the high-repetition rate of a single shot. Using real experimental data, we apply a detailed feature analysis on the input parameters while optimizing the training time of the machine learning methods. Our predictive models are able to make predictions of central photon energy for one of the pulses without measuring the other pulse, thereby leveraging the use of the spectrometer without having to extend its detection window. We anticipate applications in X-ray spectroscopy using XFELs, such as in time-resolved X-ray absorption and photoemission spectroscopy, where improved measurement of input spectra will lead to better experimental outcomes.
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Journal articleGreenaway S, Petiziol F, Zhao H, et al., 2024,
Variational quantum gate optimization at the pulse level
, SciPost Physics, Vol: 16, ISSN: 2542-4653We experimentally investigate the viability of a variational quantum gate optimizationprotocol informed by the underlying physical Hamiltonian of fixed-frequency transmonqubits. Through the successful experimental optimization of two and three qubit quan-tum gates the utility of the scheme for obtaining gates based on static effective Hamilto-nians is demonstrated. The limits of such a strategy are investigated through the opti-mization of a time-dependent, Floquet-engineered gate, however parameter drift is iden-tified as a key limiting factor preventing the implementation of such a scheme which thevariational optimization protocol is unable to overcome.
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