Quantum Physics (quant-ph)

Abstract: We show how to achieve strong squeezing of a microwave output field by reservoir engineering a cavity magnomechanical system, consisting of a microwave cavity, a magnon mode, and a mechanical vibration mode. The magnon mode is simultaneously driven by two microwave fields at the blue and red sidebands associated with the vibration mode. The two-tone drive induces a squeezed magnonic reservoir for the intracavity field, leading to a squeezed cavity mode due to the cavity-magnon state swapping, which further yields a squeezed cavity output field. The squeezing of the output field is stationary and substantial using currently available parameters in cavity magnomechanics. The work indicates the potential of the cavity magnomechanical system in preparing squeezed microwave fields, and may find promising applications in quantum information science and quantum metrology.

Abstract: Quantum photonic technologies, such as quantum key distribution, are already benefiting greatly from the rise of integrated photonics. However, the flexibility in design of these systems is often restricted by the properties of the integration material platforms. Here, we overcome this choice by using hybrid integration of ultra-low-loss silicon nitride waveguides with indium phosphide electro-optic modulators to produce high-performance quantum key distribution transceiver chips. Access to the best properties of both materials allows us to achieve active encoding and decoding of photonic qubits on-chip at GHz speeds and with sub-1% quantum bit error rates over long fibre distances. We demonstrate bidirectional secure bit rates of 1.82 Mbps over 10 dB channel attenuation and positive secure key rates out to 250 km of fibre. The results support the imminent utility of hybrid integration for quantum photonic circuits and the wider field of photonics.

Abstract: We show that the incompatibility of a set of measurements cannot be increased by subjecting them to a filter, namely, by combining them with a device that post-selects the incoming states on a fixed outcome of a stochastic transformation. This result holds for several measures of incompatibility, such as those based on robustness and convex weight. Expanding these ideas to Einstein-Podolsky-Rosen steering experiments, we are able to solve the problem of the maximum steerability obtained with respect to the most general local filters in a way that allows for an explicit calculation of the filter operation. Moreover, our results generalize to nonphysical maps, i.e., positive but not completely positive linear maps.

Abstract: Recent years have witnessed an explosion of interest in quantum devices for the production, storage, and transfer of energy. In this Colloquium, we concentrate on the field of quantum energy storage by reviewing recent theoretical and experimental progress in quantum batteries. We first provide a theoretical background discussing the advantages that quantum batteries offer with respect to their classical analogues. We then review the existing quantum many-body battery models and present a thorough discussion of important issues related to their open nature. We finally conclude by discussing promising experimental implementations, preliminary results available in the literature, and perspectives.

Abstract: The distribution and verification of quantum nonlocality across a network of users is essential for future quantum information science and technology applications. However, beyond simple point-to-point protocols, existing methods struggle with increasingly complex state preparation for a growing number of parties. Here, we show that, surprisingly, multiparty loophole-free quantum steering, where one party simultaneously steers arbitrarily many spatially separate parties, is achievable by constructing a quantum network from a set of qubits of which only one pair is entangled. Using these insights, we experimentally demonstrate this type of steering between three parties with the detection loophole closed. With its modest and fixed entanglement requirements, this work introduces a scalable approach to rigorously verify quantum nonlocality across multiple parties, thus providing a practical tool towards developing the future quantum internet.

Abstract: Quantum sensing protocols typically uses a quantum sensor to sense classical signals with the standard Ramsey inteferometry measurements. The classical signals are often real numbers determining the sensor Hamiltonian. However, for a senor embedded in a quantum environment, the signal to detect may be a quantum operator on a target quantum system. There is still no systematic method to detect such a quantum signal. Here we provide a general framework to sense static quantum signals with a qubit sensor by the Ramsey interferometry measurements, with the static classical signal sensing incorporated as a special case. This framework is based on a novel approach to simultaneously estimating the eigenvalues of the quantum signal operator with sequential projective measurements of the sensor, which can extract useful information about the target quantum system. The scheme can also be extended to sense ac quantum signals with dynamical decoupling control of the sensor. As an example, we show that a qubit sensor can simultaneously detect the individual coupling strengths with multiple target qubits in a spin-star model.

Abstract: We present a device that exploits spatial and spectral correlations in parametric downconversion at once. By using a ghost imaging arrangement, we have been able to reconstruct remotely the frequency profile of a composite system. The presence of distinct spectral regions is corroborated by a model-independent statistical analysis that constitutes an intriguing possibility also in the low count regime.

Abstract: The introduction of quantum methods in spectroscopy can provide enhanced performance and technical advantages in the management of noise. We investigate the application of quantum illumination in a pump and probe experiment. Thermal lensing in a suspension of gold nanorods is explored using a classical beam as the pump and the emission from parametric downconversion as the probe. We obtain an insightful description of the behaviour of the suspension under pumping with a method known to provide good noise rejection. Our findings are a further step towards investigating effects of quantum light in complex plasmonic media.

Abstract: A foundational assumption of quantum error correction theory is that quantum gates can be scaled to large processors without exceeding the error-threshold for fault tolerance. Two major challenges that could become fundamental roadblocks are manufacturing high performance quantum hardware and engineering a control system that can reach its performance limits. The control challenge of scaling quantum gates from small to large processors without degrading performance often maps to non-convex, high-constraint, and time-dependent control optimization over an exponentially expanding configuration space. Here we report on a control optimization strategy that can scalably overcome the complexity of such problems. We demonstrate it by choreographing the frequency trajectories of 68 frequency-tunable superconducting qubits to execute single- and two-qubit gates while mitigating computational errors. When combined with a comprehensive model of physical errors across our processor, the strategy suppresses physical error rates by $\sim3.7\times$ compared with the case of no optimization. Furthermore, it is projected to achieve a similar performance advantage on a distance-23 surface code logical qubit with 1057 physical qubits. Our control optimization strategy solves a generic scaling challenge in a way that can be adapted to other quantum algorithms, operations, and computing architectures.

Abstract: Quantifying entanglement in quantum systems is an important yet challenging task due to its NP-hard nature. In this work, we propose an efficient algorithm for evaluating distance-based entanglement measures. Our approach builds on Gilbert's algorithm for convex optimization, providing a reliable upper bound on the entanglement of a given arbitrary state. We demonstrate the effectiveness of our algorithm by applying it to various examples, such as calculating the squared Bures metric of entanglement as well as the relative entropy of entanglement for GHZ states, $W$ states, Horodecki states, and chessboard states. These results demonstrate that our algorithm is a versatile and accurate tool that can quickly provide reliable upper bounds for entanglement measures.

Abstract: The quantum approximate optimization algorithm (QAOA) is a leading candidate algorithm for solving optimization problems on quantum computers. However, the potential of QAOA to tackle classically intractable problems remains unclear. In this paper, we perform an extensive numerical investigation of QAOA on the Low Autocorrelation Binary Sequences (LABS) problem. The rapid growth of the problem's complexity with the number of spins $N$ makes it classically intractable even for moderately sized instances, with the best-known heuristics observed to fail to find a good solution for problems with $N \gtrapprox 200$. We perform noiseless simulations with up to 40 qubits and observe that out to this system size, the runtime of QAOA with fixed parameters and a constant number of layers scales better than branch-and-bound solvers, which are the state-of-the-art exact solvers for LABS. The combination of QAOA with quantum minimum-finding on an idealized quantum computer gives the best empirical scaling of any algorithm for the LABS problem. We demonstrate experimental progress in compiling and executing QAOA for the LABS problem using an algorithm-specific error detection scheme on Quantinuum trapped-ion processors. Our results provide evidence for the utility of QAOA as an algorithmic component when executed on an idealized quantum computer.

Abstract: In quantum illumination, various detection schemes have been proposed for harnessing remaining quantum correlations of the entanglement-based resource state. To this date, the only successful implementation in the microwave domain relies on a specific mixing operation of the respective return and idler modes, followed by single-photon counting in one of the two mixer outputs. We investigate the performance of this scheme for realistic detection parameters in terms of detection efficiency, dark count probability, and photon number resolution. Furthermore, we take into account the second mixer output and investigate the advantage of correlated photon counting (CPC) for a varying thermal background and optimum post-processing weighting in CPC. We find that the requirements for photon number resolution in the two mixer outputs are highly asymmetric due to different associated photon number expectation values.

Abstract: The passive approach to quantum key distribution (QKD) consists of eliminating all optical modulators and random number generators from QKD systems, in so reaching an enhanced simplicity, immunity to modulator side channels, and potentially higher repetition rates. In this work, we provide finite-key security bounds for a fully passive decoy-state BB84 protocol, considering a passive QKD source recently presented. With our analysis, the attainable secret key rate is comparable to that of the perfect parameter estimation limit, in fact differing from the key rate of the active approach by less than one order of magnitude. This demonstrates the practicality of fully passive QKD solutions.

Abstract: The classical causal relations between a set of variables, some observed and some latent, can induce both equality constraints (typically conditional independences) as well as inequality constraints (Instrumental and Bell inequalities being prototypical examples) on their compatible distribution over the observed variables. Enumerating a causal structure's implied inequality constraints is generally far more difficult than enumerating its equalities. Furthermore, only inequality constraints ever admit violation by quantum correlations. For both those reasons, it is important to classify causal scenarios into those which impose inequality constraints versus those which do not. Here we develop methods for detecting such scenarios by appealing to d-separation, e-separation, and incompatible supports. Many (perhaps all?) scenarios with exclusively equality constraints can be detected via a condition articulated by Henson, Lal and Pusey (HLP). Considering all scenarios with up to 4 observed variables, which number in the thousands, we are able to resolve all but three causal scenarios, providing evidence that the HLP condition is, in fact, exhaustive.

Abstract: We discuss the role of non-ideal measurements within the context of measurement engines by contrasting examples of measurement engines which have the same work output but with varying amounts of entanglement. Accounting for the cost of resetting, correlating the engine to a pointer state and also the cost of cooling the pointer state, we show that for a given work output, thermally correlated engines can outperform corresponding entanglement engines. We also show that the optimal efficiency of the thermally correlated measurement engine is achieved with a higher temperature pointer than the pointer temperature of the optimal entanglement engine.

Abstract: Experimental investigation of the interplay of dualities, generalized symmetries, and topological defects is an important challenge in condensed matter physics and quantum materials. A simple model exhibiting this physics is the transverse-field Ising model, which can host a noninvertible topological defect that performs the Kramers-Wannier duality transformation. When acting on one point in space, this duality defect imposes the duality twisted boundary condition and binds a single Majorana zero mode. This Majorana zero mode is unusual as it lacks localized partners and has an infinite lifetime, even in finite systems. Using Floquet driving of a closed Ising chain with a duality defect, we generate this Majorana zero mode in a digital quantum computer. We detect the mode by measuring its associated persistent autocorrelation function using an efficient sampling protocol and a compound strategy for error mitigation. We also show that the Majorana zero mode resides at the domain wall between two regions related by a Kramers-Wannier duality. Finally, we highlight the robustness of the isolated Majorana zero mode to integrability and symmetry-breaking perturbations. Our findings offer an experimental approach to investigating exotic topological defects in Floquet systems.

Abstract: Quantum state tomography of weak microwave signals is an important part of many protocols in the field of quantum information processing with superconducting circuits. This step typically relies on an accurate $\textit{in-situ}$ estimation of signal losses in the experimental set-up and requires a careful photon number calibration. Here, we present an improved method for the microwave loss estimation inside of a closed cryogenic system. Our approach is based on Planck's law and makes use of independent temperature sweeps of individual parts of the cryogenic set-up. Using this technique, we can experimentally resolve changes in microwave losses of less than 0.1 dB in the cryogenic environment. We discuss potential applications of this approach for precise characterization of quantum-limited superconducting amplifiers and in other prominent experimental settings.

Abstract: Single-photon sources based on semiconductor quantum dots find several applications in quantum information processing due to their high single-photon indistinguishability, on-demand generation, and low multiphoton emission. In this context, the generation of entangled photons represents a challenging task with a possible solution relying on the interference in probabilistic gates of identical photons emitted at different pulses from the same source. In this work, we implement this approach via a simple and compact design that generates entangled photon pairs in the polarization degree of freedom. We operate the proposed platform with single photons produced through two different pumping schemes, the resonant excited one and the longitudinal-acoustic phonon-assisted configuration. We then characterize the produced entangled two-photon states by developing a complete model taking into account relevant experimental parameters, such as the second-order correlation function and the Hong-Ou-Mandel visibility. Our source shows long-term stability and high quality of the generated entangled states, thus constituting a reliable building block for optical quantum technologies.

Abstract: Information causality was initially proposed as a physical principle aimed at deriving the predictions of quantum mechanics on the type of correlations observed in the Bell experiment. In the same work, information causality was famously shown to imply the Uffink inequality that approximates the set of quantum correlations and rederives Tsirelson's bound of the Clauser-Horne-Shimony-Holt inequality. This result found limited generalizations due to the difficulty of deducing implications of the information causality principle on the set of nonlocal correlations. In this paper, we present a simple technique for obtaining polynomial inequalities from information causality, bounding the set of physical correlations in any Bell scenario. To demonstrate our method, we derive a family of inequalities which non-trivially constrains the set of nonlocal correlations in Bell scenarios with binary outcomes and equal number of measurement settings. Finally, we propose an improved statement of the information causality principle, obtain tighter constraints for the simplest Bell scenario that goes beyond the Uffink inequality, and recovers a part of the boundary of the quantum set.