Kerr parametric oscillators (KPOs), which can be implemented with superconducting parametrons possessing large Kerr nonlinearity, have been attracting much attention in terms of their applications to quantum annealing, universal quantum computation, and studies of quantum many-body systems. It is of practical importance for these studies to realize fast and accurate tunable coupling between KPOs in a simple manner. We develop a simple scheme of fast tunable coupling of KPOs with high tunability in speed and amplitude using the fast transitionless rotation of a KPO in the phase space based on the shortcuts to adiabaticity. Our scheme enables rapid switching of the effective coupling between KPOs, and can be implemented with always-on linear coupling between KPOs, by controlling the phase of the pump field and the resonance frequency of the KPO without controlling the amplitude of the pump field nor using additional drive fields and couplers. We apply the coupling scheme to a two-qubit gate, and show that our scheme realizes high gate fidelity compared with a purely adiabatic one, by mitigating undesired nonadiabatic transitions.

Bell nonclassicality of a single-photon superposition in two modes, often referred to as “nonlocality of a single photon,” is one of the most striking nonclassical phenomena discussed in the context of foundations of quantum physics. Here we show how to robustly violate local realism within the weak-field homodyne measurement scheme for any superposition of one photon with vacuum. Our modification of the previously proposed setups involves tunable beam splitters at the measurement stations, and the local oscillator fields significantly varying between the settings, optimally being on or off. As photon-number-resolving measurements are now feasible, we advocate for the use of the Clauser-Horne Bell inequalities for detection events using precisely defined numbers of photons. We find a condition for optimal measurement settings for the maximal violation of the Clauser-Horne inequality with weak-field homodyne detection, which states that the reflectivity of the local beam splitter must be equal to the strength of the local oscillator field. We show that this condition holds not only for the vacuum–one-photon qubit input state, but also for the superposition of a photon pair with vacuum, which suggests its generality as a property of weak-field homodyne detection with photon-number resolution. Our findings suggest a possible path to employ such scenarios in device-independent quantum protocols.

A numerical security proof technique is used to analyze the security of continuous-variable quantum key distribution (CV-QKD) protocols with phase-shift keying (PSK) modulation against collective attacks in the asymptotic limit. We argue that it is sufficient to consider protocols with a maximum of eight signal states and analyze different postselection strategies for protocols with four (QPSK) and eight (8PSK) signal states for untrusted ideal and trusted nonideal detectors. We introduce a cross-shaped postselection strategy, and show that both cross-shaped and radial and angular postselection clearly outperform a radial postselection scheme (and no postselection) for QPSK protocols. For all strategies studied, we provide analytical results for the operators defining the respective regions in phase space. We outline several use cases of postselection, which can easily be introduced in the data processing of both new and existing CV-QKD systems. Motivated by the high computational effort for error correction, we study the case when a large fraction of the raw key is eliminated by postselection and observe that this can be achieved while increasing the secure key rate. Postselection can also be used to partially compensate the disadvantage of QPSK protocols over 8PSK protocols for high transmission distances, while being experimentally less demanding. Finally, we highlight that postselection can be used to reduce the key rate gap between scenarios with trusted and untrusted detectors while relying on less assumptions on Eve’s power.

Due to its potential for implementing a scalable quantum computer, the multiqubit Toffoli gate lies in the heart of quantum information processing. In this paper, we demonstrate a multiqubit blockade gate with atoms arranged in a three-dimensional spheroidal array. The gate performance is greatly improved by the method of optimizing control-qubit distributions on the spherical surface via evolutionary algorithm, which leads to an enhanced asymmetric Rydberg blockade. This spheroidal configuration, not only preserves the dipole blockade energy well between arbitrary control-target pairs, which keeps the asymmetric blockade error at a very low level, but also manifests an unprecedented robustness to the spatial position variations, leading to a negligible position error. Taking account of intrinsic errors and using typical experimental parameters, we numerically show that a c${}_6$not Rydberg gate can be created with a fidelity of 0.992, which is only limited by the Rydberg state decays. Our protocol opens up a platform of higher-dimensional atomic arrays for achieving multiqubit neutral-atom quantum computation.

Surface acoustic waves (SAWs) coupled to quantum dots (QDs), trapped atoms and ions, and point defects have been proposed as quantum transduction platforms, yet the requisite coupling rates and cavity lifetimes have not been experimentally established. Although the interaction mechanism varies, small acoustic cavities with large zero-point motion are required for high efficiencies. We experimentally establish the feasibility of this platform through electro- and optomechanical characterization of tightly focusing, single-mode Gaussian SAW cavities at approximately 3.6 GHz on $\mathrm{Ga}\mathrm{As}$. We explore the performance limits of the platform by fabricating SAW cavities with mode volumes approaching $6{\lambda }^3$ and linewidths less than 1 MHz. Employing strain-coupled single $\mathrm{In}\mathrm{As}$ QDs as optomechanical intermediaries, we measure single-phonon optomechanical coupling rates $g_0\approx 2\pi \times 1.2$ MHz. Sideband scattering rates thus exceed intrinsic phonon loss, indicating the potential for quantum optical readout and transduction of cavity phonon states. To demonstrate the feasibility of this platform for low-noise ground-state quantum transduction, we develop a fiber-based confocal microscope in a dilution refrigerator and perform single-QD resonance fluorescence sideband spectroscopy at millikelvin temperatures. These measurements show conversion between microwave phonons and optical photons with sub-natural linewidths.

We analyze the cross-resonance effect for fluxonium circuits and investigate a two-qubit gate scheme based on selective darkening of a transition. In this approach, two microwave pulses at the frequency of the target qubit are applied simultaneously with a proper ratio between their amplitudes to achieve a controlled-not operation. We study in detail coherent gate dynamics and calculate gate error. With nonunitary effects accounted for, we demonstrate that gate error below ${10}^{-4}$ is possible for realistic hardware parameters. This number is facilitated by long coherence times of computational transitions and strong anharmonicity of fluxoniums, which easily prevents excitation to higher excited states during the gate microwave drive.

We perform comprehensive experimental tests of various composite pulse sequences using one of the open-access IBM quantum processors, based on superconducting transmon qubits. We implement explicit pulse control of the qubit by making use of the opportunity of low-level access to the backend, provided by IBM Quantum. We obtain the excitation profiles for a huge variety of broadband, narrowband, and passband composite pulses, producing any prechosen target probabilities, ranging from zero to one. We also test universal composite pulses that compensate errors in any experimental parameter. In all experiments, we find excellent agreement between theoretical and experimental excitation profiles. This proves both the composite pulses as a very efficient and flexible quantum control tool and the high quality of the IBM quantum processor. As an extreme example, we test and observe a pronounced narrowband excitation profile for a composite sequence of as many as 1001 pulses.

Along with the scaling of dimensions in quantum systems, transitions between the system’s energy levels would become close in frequency, which are conventionally resolved by weak and lengthy pulses. Here, we extend and experimentally demonstrate analytically based swift quantum control techniques on a four-level trapped ion system, where we perform individual or simultaneous control over two pairs of spectrally nearby transitions with tailored time-varied drive, achieving operational fidelities ranging from 99.2(3)% to 99.6(3)%. We achieve approximately an order of magnitude speed up compared with the case of a weak square pulse for a general control. Therefore, our demonstration may be beneficial to a broad range of quantum systems with crowded spectrum, for spectroscopy, quantum information processing, and quantum simulation.

We present a gate-voltage-tunable transmon qubit (gatemon) based on planar $\mathrm{In}\mathrm{As}$ nanowires that are selectively grown on a high-resistivity silicon substrate using III-V buffer layers. We show that low-loss superconducting resonators with an internal quality of $2\times {10}^5$ can readily be realized using these substrates after the removal of buffer layers. We demonstrate coherent control and readout of a gatemon device with a relaxation time, $T_1\approx 700\mathrm{ns}$, and dephasing times, $T_2^{\ast }\approx 20\mathrm{ns}$ and $T_{2,\mathrm{echo}}\approx 1.3\mu \mathrm{s}$. Further, we infer a high junction transparency of $0.4$–$0.9$ from an analysis of the qubit anharmonicity.

Optical vector beams with a higher-order polarization state, represented by a point on a higher-order Poincaré sphere, have recently found advantages in various applications. However, the considered beams are usually spatially fully coherent (monochromatic), which makes them susceptible to complex environments. In this work, we introduce a coherence-engineering protocol to generate partially coherent vectorial secondary light sources in which a higher-order polarization state is encoded into the structure of electromagnetic spatial coherence. The encoded complex polarization state is well reconstructed in the far field whose global degree of polarization can be controlled with the transverse coherence length of the source. In particular, the produced partially coherent beams are highly robust against obstructions, which is demonstrated theoretically and experimentally by inserting a static obstacle against the source or introducing strong atmospheric turbulence into the transmission link. The results of this work can find useful applications in the transfer of complicated polarization-encoded information in harsh environments.

We analyze a high-fidelity two-qubit gate using fast flux pulses on superconducting fluxonium qubits. The gate is realized by temporarily detuning the magnetic flux through the fluxonium loop away from the half flux quantum sweet spot. We simulate dynamics of two capacitively coupled fluxoniums during the flux pulses and optimize the pulse parameters to obtain a highly accurate $\sqrt{i\mathrm{SWAP}}$-like entangling gate. We also evaluate the effect of the flux noise and qubit relaxation on the gate fidelity. Our results demonstrate that the gate error remains below ${10}^{-4}$ for a currently achievable magnitude of the flux noise and qubit relaxation time.

Effective low-energy theories represent powerful theoretical tools to reduce the complexity in modeling interacting quantum many-particle systems. However, common theoretical methods rely on perturbation theory, which limits their applicability to weak interactions. Here we introduce the Variational Adiabatic Gauge Transformation (VAGT), a nonperturbative hybrid quantum algorithm that can use nowadays quantum computers to learn the variational parameters of the unitary circuit that brings the Hamiltonian to either its block-diagonal or full-diagonal form. If a Hamiltonian can be diagonalized via a shallow quantum circuit, then VAGT can learn the optimal parameters using a polynomial number of runs. The accuracy of VAGT is tested through numerical simulations, as well as simulations on Rigetti and IonQ quantum computers.

Single rare-earth ions in solids show great potential for quantum applications, including single-photon emission, quantum computing, and high-precision sensing. However, the linewidths of single rare-earth ions are often broadened due to perturbations associated with the detection methods. Identifying the dominant broadening sources is key to reduce the linewidths for practical applications. We report a spectral broadening study on a single ${\mathrm{Er}}^{3+}$ ion in a $\mathrm{Si}$ nanotransistor. The single ion spectra display a Lorentzian lineshape at all light intensities considered. The linewidth remains nearly constant at 32±2 MHz in the low-intensity regime, and shows a monotonic increase with the intensity in the high-intensity regime. The power broadening does not persist over the microsecond time scales considered after resonant excitation. Nor does it depend on the resonant excitation intensity or the Zeeman shift. These observations and temperature-dependent measurements suggest that charge fluctuations are likely to be a dominant broadening source. Laser heating may also contribute to the power broadening. Charge suppression in the ${\mathrm{Er}}^{3+}$-doped region and coupling ${\mathrm{Er}}^{3+}$ ions to a confined optical mode could be implemented to reduce the spectral linewidth and to enhance the sensing precision of single ${\mathrm{Er}}^{3+}$ ions in $\mathrm{Si}$.

We present a classical algorithm to find approximate solutions to instances of quadratic unconstrained binary optimization. The algorithm can be seen as an analog of quantum annealing under the restriction of a product-state space, where the dynamical evolution in quantum annealing is replaced with a gradient-descent-based method. This formulation is able to quickly find high-quality solutions to large-scale problem instances and can naturally be accelerated by dedicated hardware such as graphics processing units. We benchmark our approach for large-scale problem instances with tunable hardness and planted solutions. We find that our algorithm offers a similar performance to current state-of-the-art approaches within a comparably simple gradient-based and nonstochastic setting.

Phase measurement constitutes a key task in many fields of science, both in the classical and quantum regime. The higher precision of such measurement offers significant advances, and can also be utilized to achieve finer estimates for quantities such as distance, the gravitational constant, electromagnetic field amplitude, etc. Here we theoretically model the use of a quantum network, composed of a randomly coupled set of two-level systems, as a processing device for phase measurement. An incoming resource state carrying the phase information interacts with the quantum network, whose emission is trained to produce a desired output signal. We demonstrate phase-precision scaling following the standard quantum limit and Heisenberg limit. This can be achieved using quantum resource states such as NOON states or other entangled states, however, we also find that classically correlated mixtures of states are alone sufficient, provided that they exhibit quantum coherence. Our proposed setup does not require conditional measurements, and is compatible with many different types of coupling between the quantum network and the phase-encoding state, hence making it attractive to a wide range of possible physical implementations.