Single-photon detectors are ubiquitous and integral components of photonic quantum cryptography, communication, and computation. Many applications, however, require not only detecting the presence of any photons, but distinguishing the number present with a single shot. Here, we implement a single-shot, high-fidelity photon number-resolving detector of up to 15 microwave photons in a cavity-qubit circuit QED platform. This detector functions by measuring a series of generalized parity operators, which make up the bits in the binary decomposition of the photon number. Our protocol consists of successive, independent measurements of each bit by entangling the ancilla with the cavity, then reading out and resetting the ancilla. Photon loss and ancilla readout errors can flip one or more bits, causing nontrivial errors in the outcome, but these errors have a traceable form, which can be captured in a simple hidden Markov model. Relying on the independence of each bit measurement, we mitigate biases in ensembles of measurements, showing good agreement with the predictions of the model. The mitigation improves the average total variation distance error of Fock states from $13.5\%$–$1.1\%$. We also show that the mitigation is efficiently scalable to an $M$-mode system provided that the errors are independent and sufficiently small. Our work motivates the development of new algorithms that utilize single-shot, high-fidelity PNR detectors.

Quantum conversion between microwave and optical photons is an important area of quantum information science and technology. Here we propose a one-step method to realize a coherent conversion between a microwave cavity and an optical cavity utilizing a nitrogen-vacancy center ensemble (NV ensemble). An NV ensemble considered here acts as a quantum transducer, dispersively coupled to the microwave and optical cavities, enabling transfer of the quantum state and generating entangled states between microwave and optical cavities. With state-of-the-art technology, the numerical simulation is performed to demonstrate that cat states and Fock states can be high-fidelity transferred and a maximally entangled state can be efficiently synthesized between microwave and optical cavities.

We demonstrate that spin-orbit coupled states are generated in neutral magnetic spin-1/2 particles traveling through an electric field. The quantization axis of the orbital angular momentum is parallel to the electric field; hence both longitudinal and transverse orbital angular momentum can be created. Furthermore, we show that the total angular momentum of the particle is conserved. Finally we propose a neutron optical experiment to measure the transverse effect.

This paper explores two circuit approaches for quantum walks: The first consists of generalized controlled inversions, whereas the second one effectively replaces them with rotation operations around the basis states. We show the theoretical foundation of the rotational implementation. The rotational approach nullifies the large amount of ancilla qubits required to carry out the computation when using the inverter implementation. Our results concentrate around the comparison of the two architectures in terms of structure, benefits, and detriments, as well as the computational resources needed for each approach. We show that the inverter approach requires exponentially fewer gates than the rotations but almost half the number of qubits in the system. Finally, we execute a number of experiments using an IBM quantum computer. The experiments show the effects of noise in our circuits. Small two-qubit quantum walks evolve closer to our expectations, whereas for a larger number of steps or state space the evolution is severely affected by noise.

A compelling way to quantify the separation between classical and quantum computing is to determine how many $T$-gate magic states, $t$, a classical computer must simulate to calculate the probability of a universal quantum circuit's output. Unfortunately, efforts to determine the minimum number of stabilizer state inner products necessary to decompose $T$-gate magic states (${\chi }_t$) have proven intractable past $t=7$. By using a phase space formalism based on Wootters' discrete Weyl operator basis over a finite field, we develop an algebraic approach to determining ${\chi }_t$ for single-Pauli measurements. This allows us to extend the bounds on ${\chi }_t$ to $t=14$ for qutrits, effectively increasing the space searched by $>{10}^{{10}^4}$. Our results show that by using such phase space methods it is possible to validate noisy intermediate-scale quantum circuits of larger size than previously thought possible.

The energy loss of iodine ions at initial charge states up to 25+ and energies up to 36 MeV in self-supporting gold foils between 37 and 107 nm of thickness was measured with an electron mirror type time-of-flight detector. An excess energy loss of 130 keV was observed at 36 MeV for charge state 25+ compared to 16+, and an energy loss deficit of 100 keV was observed for charge state 8+. The charge state equilibration length for 36-MeV iodine was estimated to lie between 3 and 7 nm, corresponding to an equilibration time between 0.4 and 0.9 fs. This result is relevant both for nanostructure fabrication with MeV ion beams and for depth profiling based on ion beam analysis data in cases where the charge state of the primary ion is far from the equilibrium value in the sample under study. A comparison to published data on charge state equilibration for various projectile-target combinations and energies from 10 keV to 6 GeV indicated that the energy scaling of the equilibration length observed at high energy is invalid for projectile velocities on the order of the Bohr velocity and below. The measurement further provided equilibrium values of the electronic stopping power for iodine in gold at ten energies between 10 and 36 MeV.

Highly charged ions (HCIs) are promising candidates for the next generation of atomic clocks, owing to their tightly bound electron cloud, which significantly suppresses the common environmental disturbances to the quantum oscillator. Here we propose and pursue an experimental strategy that, while focusing on various HCIs of a single atomic element, keeps the number of candidate clock transitions as large as possible. Following this strategy, we identify four adjacent charge states of nickel HCIs that offer as many as six optical transitions. Experimentally, we demonstrated the essential capability of producing these ions in the low-energy compact Shanghai-Wuhan Electron Beam Ion Trap. We measured the wavelengths of four magnetic-dipole ($M1$) and one electric-quadrupole ($E2$) clock transitions with an accuracy of several ppm with an innovative calibration method. Compared to earlier determinations, our measurements improved wavelength accuracy by an order of magnitude. Such measurements are crucial for constraining the range of laser wavelengths for finding the “needle in a haystack” narrow lines. In addition, we calculated frequencies and quality factors, and evaluated sensitivity of these six transitions to the hypothetical variation of the electromagnetic fine structure constant $\alpha$ needed for fundamental physics applications. We argue that all the six transitions in nickel HCIs offer intrinsic immunity to all common perturbations of quantum oscillators, and one of them has the projected fractional frequency uncertainty down to the remarkable level of ${10}^{-19}$.

Because there are new projects on the determination of the hyperfine interval in the ground state of muonium, we revisit different approaches to constructing its theoretical prediction and comparison with experimental data. We discuss a controversy in the accuracy of two recent predictions [Mohr et al., Rev. Mod. Phys. 88, 035009 (2016), and Eides, Phys. Lett. B 795, 113 (2019)] and produce a constraint on possible systematic errors in the determination of the muonium hyperfine-structure interval interval theoretically or experimentally. In particular, a possible additional theoretical term, that may be either a missing “regular” theoretical contribution or a contribution due to new physics, is estimated as $-0.14\left(52\right)\mathrm{kHz}$. The constraint is based on all available data through a least-squares-adjustment procedure, including all their correlations. The result is close to the one of Eides [Phys. Lett. B 795, 113 (2019)].

We present the creation of a lithium-cesium slow beam using a two-dimensional magneto-optical trap. The two-species atomic beam is directed to load a three-dimensional magneto-optical trap in ultrahigh vacuum. We achieve a loading rate of $1.3\times {10}^7$ lithium atoms/s with the lithium oven temperature at $370{}^{\circ }\mathrm{C}$ and $2.2\times {10}^7$ cesium atoms/s with the Cs oven temperature at $20{}^{\circ }\mathrm{C}$. The maximum numbers of lithium and cesium atoms in the trap are $1.0\times {10}^9$ and $1.4\times {10}^8$, respectively. Our results show that the simple and compact two-dimensional magneto-optical trap is suitable for producing an atomic beam of two species that have a large mass ratio and very different volatilities.

Entanglement preparation and signal accumulation are essential for quantum parameter estimations, which pose significant challenges to both theories and experiments. Here, we propose how to utilize chaotic dynamics in a periodically driven Bose-Josephson system for achieving high-precision measurements beyond the standard quantum limit (SQL). Starting from an initial nonentangled state, the chaotic dynamics generates many-body quantum entanglement and simultaneously encodes the parameter to be estimated. By using suitable chaotic dynamics, the ultimate measurement precision of the estimated parameter can beat the SQL. The sub-SQL measurement precision scaling can also be reached via specific observables, such as collective spin measurement, which can be realized with state-of-art techniques. Our study not only provides insights for understanding quantum chaos and quantum-classical correspondence, but also of promising applications in entanglement-enhanced quantum metrology.

We propose experimentally observable signatures of topological Majorana quasiparticles in the few-body limit of the interacting cold-atom model of Iemini et al. [F. Iemini, L. Mazza, L. Fallani, P. Zoller, R. Fazio, and M. Dalmonte, Phys. Rev. Lett. 118, 200404 (2017)]. In this limit, the total on-site density and single-body correlations change smoothly with the model parameters, while the calculated mutual information of opposite ends of the lattice indicates a sharp transition of the system to a topological ground state. Furthermore, local-density and -parity measurements provide an experimentally viable path for observing the ground-state Majorana quasiparticles in ultracold atoms. Our results lay out a promising future for utilizing few-body systems as a testing ground for Majorana physics.

We study the massive point-vortex model introduced in Richaud et al. [A. Richaud, V. Penna, R. Mayol, and M. Guilleumas, Phys. Rev. A 101, 013630 (2020)], which describes two-dimensional point vortices of one species that have small cores of a different species. We derive the relevant Lagrangian itself, based on the time-dependent variational method with a two-component Gross-Pitaevskii (GP) trial function. The resulting Lagrangian resembles that of charged particles in a static electromagnetic field, where the canonical momentum includes an electromagnetic term. The simplest example is a single vortex with a rigid circular boundary, where a massless vortex can only precess uniformly. In contrast, the presence of a sufficiently large filled vortex core renders such precession unstable. A small core mass can also lead to small radial oscillations, which are, in turn, clear evidence of the associated inertial effect. Detailed numerical analysis of coupled two-component GP equations with a single vortex and small second-component core confirms the presence of such radial oscillations, implying that this more realistic GP vortex also acts as if it has a small massive core.

We study a single-photon transmission through a one-dimensional $N$-qubit chain. The qubits are supposed to be identical with equal distance between neighbors. We express the transfer matrix of the $N$-qubit chain in terms of Chebyshev polynomials, which allows us to obtain simple expressions for the transmission and reflection amplitudes for arbitrarily large $N$. If the distance between neighbor qubits is equal to a half wavelength, then the transmission spectrum exhibits a flat band gap structure with very steep walls. We show that, for odd $N$, the tuning of the excitation frequency of a central qubit gives rise to the appearance within a band gap of a narrow resonance with a full transmission. The position of the resonance and its width can be monitored with the frequency of a central qubit. We show that the formation of the band gap and the transmission resonance result from the overlapping the widths of individual qubits, which is due to the strong coupling between qubits and waveguide photons.

Recently it was found that scrambled information can be partially recovered by a time-reversed evolution, even after being damaged by an intruder. We reconsider the origin of the information recovery, and argue that the presence of classical chaos does not preclude it and only leads to a quantitative reduction of the recovery ratio. We also show how decoherence (i.e., entanglement with the intruder) limits the recovery, by proving an upper bound on the recovery ratio in terms of the entangling power of the intruder's action.

We consider the question of characterizing the incompatibility of sets of high-dimensional quantum measurements. We introduce the concept of measurement incompatibility in subspaces. That is, starting from a set of measurements that is incompatible, one considers the set of measurements obtained by projection onto any strict subspace of fixed dimension. We identify three possible forms of incompatibility in subspaces: (i) incompressible incompatibility—measurements that become compatible in every subspace, (ii) fully compressible incompatibility—measurements that remain incompatible in every subspace, and (iii) partly compressible incompatibility—measurements that are compatible in some subspace and incompatible in another. For each class, we discuss explicit examples. Finally, we present some applications of these ideas. First, we show that joint measurability and coexistence are two inequivalent notions of incompatibility in the simplest case of qubit systems. Second, we highlight the implications of our results for tests of quantum steering.

In quantum logical terms, Hardy-type arguments can be uniformly presented and extended as collections of intertwined contexts and their observables. If interpreted classically, those structures serve as graph-theoretic “gadgets” that enforce correlations on the respective preselected and postselected observable terminal points. The method allows the generalization and extension to other types of relational properties, in particular, to novel joint properties predicting classical equality of quantum mechanically distinct observables. It also facilitates finding faithful orthogonal representations of quantum observables.

Readout errors are a significant source of noise for near-term intermediate-scale quantum computers. Mismeasuring a qubit as $|1\rangle$ when it should be $|0\rangle$ occurs much less often than mismeasuring a qubit as $|0\rangle$ when it should have been $|1\rangle$. We make the simple observation that one can improve the readout fidelity of quantum computers by applying targeted $X$ gates prior to performing a measurement. These $X$ gates are placed so that the expected number of qubits in the $|1\rangle$ state is minimized. Classical postprocessing can undo the effect of the $X$ gates so that the expectation value of any observable is unchanged. This protocol is designed to have no effect when readout errors are symmetric. We show that the statistical uncertainty following readout error corrections is smaller when using readout rebalancing. The statistical advantage is circuit and computer dependent and is demonstrated for the $W$ state, a Grover search, and for a Gaussian state. The benefit in statistical precision is most pronounced (and nearly a factor of 2 in some cases) when states with many qubits in the excited state have high probability.

We investigate a two-dimensional Bose-Einstein condensate that is optically driven via a retro-reflecting mirror, forming a single optical feedback loop. This induces a peculiar type of long-range atomic interaction with highly oscillatory behavior, and we show here how the sign of the underlying interaction potential can be controlled by additional optical elements and external fields. This additional tunability enriches the behavior of the system substantially and gives rise to a surprising range of ground states of the condensate. In particular, we find the emergence of self-bound crystals of quantum droplets with various lattice structures, from simple and familiar triangular arrays to complex superlattice structures and crystals with entirely broken rotational symmetry. This includes mesoscopic clusters composed of small numbers of quantum droplets as well as extended crystalline structures. Importantly, such ordered states are entirely self-bound and stable without any external in-plane confinement, having no counterpart to other quantum-gas settings with long-range atomic interactions.

Disorder and structural imperfections are unavoidable and challenge wave engineering. Therefore, methods to reduce disorder and its effects are persistently sought for. Here, we theoretically explore a kind of a bulk mode that is supported by a periodic coupled resonator waveguide with odd length. Although it is a bulk mode, its eigenfrequency is unaffected by disorder, and furthermore, its corresponding eigenvector is stationary under the influence of moderate disorder. Therefore, as we prove analytically and demonstrate numerically, the resonant transmission near its eigenfrequency exhibits substantially reduced sensitivity to disorder compared to any other frequency in the passband.

We propose a scheme to achieve the periodically modulated Su-Schrieffer-Heeger model based on a one-dimensional optomechanical lattice. We show the energy-eigenvalue spectrum and the winding number to demonstrate two topologically distinct phases of the Su-Schrieffer-Heeger model. Specifically, we realize the photon-phonon conversion process via the topologically protected edge channel with a controllable conversion efficiency. By calculating the fidelities of the photon-phonon conversion, we find that our system is more robust against the on-site defect potential throughout the overall lattice sites than the edge lattice sites. Interestingly, the large defect added into the edge sites can induce additional quantum channels to achieve the photon-photon transfer and the phonon-phonon transfer. Our scheme opens an alternative avenue to investigate the topological phases and the quantum state transfer in optomechanical lattice systems.