The heralded optical parametric amplifier (OPA) has emerged as a promising tool for quantum state engineering. However, its potential has so far been limited to coherent state inputs. Here we introduce a generalized heralded OPA protocol that unlocks a vastly expanded class of quantum phenomena by accepting arbitrary nonclassical inputs. With a squeezed vacuum input, the setup functions as an integrated two-photon subtractor, nondeterministically generating high-fidelity, larger-amplitude squeezed Schrödinger cat (SC) states, an operation previously requiring complex, discrete setups. Furthermore, when fed a small-amplitude SC state, the protocol acts as a non-Gaussianity amplifier, distilling it into high-purity approximations of key quantum resources like specific photon-number superpositions. This work transforms the OPA from a specialized source into a versatile platform for advanced quantum state engineering. The integrated setup enables the generation of a wide array of non-Gaussian states, providing a unified approach to quantum state synthesis.

We theoretically investigate frequency comb generation in an optomechanical system compred of a whispering-gallery-mode microresonator and a sensor. We find that tuning the coupling strength via a unidirectional waveguide and adjusting the optical phase with a phase shifter can induce frequency comb generation. The underlying mechanism for this phenomenon is steering the system parameters toward the vicinity of an exceptional point, which dynamically enhances the optomechanical nonlinearity, gives rise to frequency comb generation. Our work may have potential application value in sensing and metrology.

We propose a scheme to achieve controllable nonreciprocal Goos-Hänchen shift (NGHS) in a cavity optomagnonic system consisting of a spinning yttrium iron garnet (YIG) sphere and a degenerate optical parametric amplifier, where the physical mechanism originates from the asymmetry of the large enough anharmonicity in the energy spectrum induced by the Barnett effect. By increasing the photon-magnon coupling strength, transparency due to normal-mode splitting appears and significantly reduces the absorption of the probe field in the system. Simultaneously, when the YIG sphere rotates at a fixed angular velocity, opposite Barnett shifts can be induced by reversing the direction of the static magnetic field, which alters the dispersion of the probe field within the system. Consequently, we investigate the GHS of the reflected probe beam. The results demonstrate that when the probe field is incident at a fixed angle, the GHS in the cavity optomagnonic system exhibits different responses in different directions of the static magnetic field, indicating the occurrence of NGHS. Our work may have potential applications in designing magnetic-controlled optical switches and in measuring the Barnett field.

We benchmark the relative ordering of three important lower bounds on the Bayesian mean-square error: Bayesian quantum Cramer-Rao bound (BQCRB), non-Hermitian quantum Weiss-Weinstein bound (QWWB), and quantum Ziv-Zakai bound (QZZB) for noisy frequency estimation with a spin-1/2 under pure dephasing. We find that both the noise characteristics and the prior distribution of the unknown frequency have strong influence on the ordering of these bounds: narrow prior distribution and/or strong noise favor the ordering (i): $\text{BQCRB}>\text{QWWB}>\text{QZZB}$, otherwise the ordering (ii): $\text{QWWB}>\text{QZZB}>\text{BQCRB}$ is favored. Interestingly, QWWB is worse than BQCRB when the noise is strong, in contrast to the noiseless case where QWWB is never worse than BQCRB. Another interesting finding is that increasing the non-Markovianity of the noise usually makes ordering (i) more favorable. This work underscores the critical role of noise characteristics and prior information in the selection of these Bayesian bounds for quantum metrology in noisy environments.

Magnetic sensing is a cornerstone of precision science, yet state-of-the-art sensors often encounter sensitivity constraints imposed by intrinsic noise and limited magnetic response. Here we introduce a magnetic amplification mechanism that boosts weak signals by several orders of magnitude using hyperpolarized spins. The principle is applicable to a broad range of hyperpolarized nuclear or electron spins, including those in noble gases, alkali metals, liquids, diamonds, and semiconductors. We propose a series of magnetic amplifiers and, as an initial experimental validation, demonstrate that noble-gas spins achieve magnetic amplification exceeding three orders of magnitude. We characterize the noise introduced during the amplification, showing that the amplified signal maintains exceptionally low noise levels. Consequently, we achieve enhanced magnetic sensitivity, reaching $4–6\mathrm{fT}/{\mathrm{Hz}}^{1/2}$ using a ${}^{129}\mathrm{Xe}$ amplifier and an unprecedented $0.8\mathrm{fT}/{\mathrm{Hz}}^{1/2}$ using a ${}^3\mathrm{He}$ amplifier. This sensitivity is on par with the most advanced magnetometers available today and holds the potential to be improved to $0.01\mathrm{fT}/{\mathrm{Hz}}^{1/2}$ under current experimental conditions. This work paves the way for ultrasensitive magnetic sensing and opens intriguing possibilities for various precision measurements.

We discuss how blind quantum computing generalizes to multilevel quantum systems (qudits), which offers advantages compared with the qubit approach. Here, a quantum computing task is delegated to an untrusted server while simultaneously preventing the server from retrieving information about the computation it performs, the input, and the output, enabling secure cloud-based quantum computing. In the standard approach with qubits, measurement-based quantum computing is used: single-qubit measurements on cluster or brickwork states implement the computation, while random rotations of the resource qubits hide the computation from the server. We generalize finite-sized approximately universal gate sets to prime-power-dimensional qudits and show that qudit versions of the cluster and brickwork states enable a similar server-blind execution of quantum algorithms. Furthermore, we compare the overheads of different resource state architectures and discuss which hiding strategies apply to alternative qudit resource states beyond graph states.

We demonstrate the effect of Zeeman and hyperfine optical pumping and transverse laser cooling of a dysprosium (Dy) atomic beam on the $4f^{10}6s^2(J=8)\to 4f^{10}6s6p(J=9)$ transition at 421.291 nm. For ${}^{163}\mathrm{Dy}$, a custom-built electro-optic modulator is used to generate five frequency sidebands required to pump the atoms to the $F=10.5$ ground-state hyperfine level, and the light polarization is chosen to pump the atoms to the $m_F=10.5$ Zeeman sublevel. The atoms are simultaneously laser cooled using a standing wave orthogonal to the atomic beam. The resulting polarized and cooled atomic beam will be used in fundamental physics experiments taking advantage of the “accidental” degeneracy of excited states in Dy, including the ongoing measurement of parity violation in this system.

We investigate a two-parametric family of one-dimensional non-Hermitian complex potentials with parity-time ($\mathcal{PT}$) symmetry. We find that there exist two distinct types of phase transitions, from an unbroken phase (characterized by a real spectrum) to a broken phase (where the spectrum becomes complex). The first type involves the emergence of a pair of complex eigenvalues bifurcating from the continuous spectrum. The second type is associated with the collision of such pairs at the bottom of the continuous spectrum. The first transition type is closely related to spectral singularities (SSs), the transmission of the spectral singularities and reflection coefficients are divergent simultaneously. The second transition type is associated with the emergence of bound states. In particular, under specific parameter conditions, we construct an exact bound-state solution. Additionally, signatures of the SSs in a scattering process with wave packets are elucidated. These findings provide a fundamental theoretical framework for manipulating wave scattering in non-Hermitian systems, offering promising implications for designing advanced optical and quantum devices.

Generating macroscopic quantum systems would allow new probes of quantum gravity and improve precision sensing capabilities, but cooling heavy objects to their motional ground state has proved intractable. We propose and evaluate an experiment which could cool milligram-scale objects to their motional ground state. By developing a scheme where the common tool of cavity-frequency locking becomes equivalent to cooling, which works by diamagnetically levitating one end mirror of a cavity, we show the movable end mirror can reach phonon occupancies $n<1$. We also show through simulations that our system can use the optical spring effect to greatly ease experimental constraints on vibrational isolation and feedback strength, by boosting the extremely low trapping frequency of diamagnetic traps. Furthermore, we characterize noise sources and analytically model the feedback loop to show that this experiment is experimentally possible with current techniques. Our setup provides a possible platform for experiments probing the interface of quantum mechanics and gravity, and significantly improved accelerometry.

The precise control of nonlinear wave propagation, particularly photonic breathers, in cavity optomechanical arrays remains a challenge. Here, we address this by numerically investigating a one-dimensional optomechanical chain with a segmented distribution of position-modulated self-Kerr nonlinearity (PMKN). We demonstrate that, in contrast to the symmetric case that yields a stationary Kuznetsov-Ma soliton, an asymmetry in the PMKN coefficients between the two segments robustly steers the photonic breather into the region of stronger nonlinearity and drives it into stable back-and -forth oscillations. This oscillatory dynamics is analogous to Bloch oscillations in electronic systems, with the nonlinearity gradient acting as an effective constant force. The oscillation period and spatial confinement can be precisely tuned by the intercavity and optomechanical coupling strengths. As a result, the phonon field exhibits dual-mode dynamics, featuring a propagating component synchronized with the photonic breather and a localized mode at the chain's center. This work establishes a paradigm for controlling breathing soliton in programmable optomechanical systems, with promising applications in reconfigurable optical oscillators, signal processing, and on-chip studies of nonequilibrium dynamics.

Using the spectral method, the boundary problem of the spatiotemporal dynamics of the formation and propagation of spectrally singular modes generated by a propagating short (sub-picosecond) optical pulse within a dispersive $\mathcal{PT}$ -symmetric 1D photonic crystal is solved. The spectral amplitudes of transmitted and reflected spectrally singular waves tend to infinity. It is shown that for a certain photonic crystal thickness, when the Bragg condition is satisfied, two waves in each direction have not only the same frequencies and different projections of wave vectors, but also amplitudes equal in magnitude and opposite in sign. The sum of such waves has a standing envelope of the traveling wave amplitudes (a traveling wave with zero group velocity of the envelope). The traveling component of this wave is amplified as it propagates along the gain-loss photonic crystal; its intensity is proportional to the square of the incident pulse duration. The amplitudes of singular modes decay slowly with a lifetime inversely proportional to the mode spectrum width. Excitation of singular modes leads to a radical change in the intensity, shape, and dynamics of the excitation pulse itself. The leading edge of the pulses is formed by the spectral components of the incident pulse, and the slowly decaying trailing edge is determined by the long lifetime of the spectrally singular mode. Increasing the duration of the incident pulse leads to the emission of amplified extended pulses in the forward and diffracted directions at the frequency of spectral singularity with an almost stepped envelope profile.

Preparing magnon nonclassical states is a central topic in the study of quantum magnonics. Here we propose to generate magnon squeezed states in a hybrid cavity-magnon-qubit system by engineering an effective Rabi-type magnon-qubit interaction. This is achieved by adiabatically eliminating the cavity mode and driving the qubit with two microwave fields, of which the driving frequencies and amplitudes are properly selected. In the normal phase of the effective Rabi model associated with the ground-state superradiant phase transition, the driven magnon-qubit dynamics generates a parametric-amplification-like two-magnon interaction that produces magnon squeezing. The resulting squeezing is strongly enhanced when the system is tuned close to the critical point. We further analyze the effects of the dissipation, dephasing, and thermal noise on the magnon squeezing. Our results indicate that a moderate degree of squeezing can be produced using currently available parameters in the experiments.

Three-body resonances are ubiquitous in quantum few-body physics and are characterized by a finite lifetime before decaying into continuum states of their composing subsystems. In this work we present a theoretical study on the possibility to stabilize three-body resonances to so-called bound states in a continuum: resonances with vanishing width that do not decay. Within a two-channel approach we unveil the underlying mechanism and show how the lifetime can be made infinitely long by a continuous tuning of system parameters. The validity of our theory is illustrated in two different examples: a mass-imbalanced system in one dimension and a system of three identical bosons in three dimensions, relevant to Efimov physics. Crucially, for the latter we find that one of the parameters that can be tuned to achieve a three-body bound state in a continuum is an external magnetic field, a common tunable variable in cold-atom experiments. Due to the generality of this stabilization effect, it is expected to be applicable to a wide range of unstable few-body systems, opening perspectives for fundamental studies as well as technical applications.

Contrary to general relativity, quantum theory treats space and time in fundamentally different ways. In particular, while joint probabilities associated with space-like separated measurements are defined in terms of the Born rule, joint probabilities associated with measurements performed in sequence are defined in terms of the state-update rule. In this work, we show that one obtains a more unified perspective of space and time in quantum theory by embracing a quasiprobabilistic description of sequential measurements. More precisely, we show that there exists a unique pseudo density operator encoding canonical quasiprobabilities associated with sequential measurements in precisely the same manner that a density operator encodes joint probabilities associated with space-like separated measurements, thus providing a natural extension of the Born rule into the temporal domain. As an application, we show how such a spatiotemporal Born rule combined in conjunction with a quantum Bayes' rule yields an operational notion of time-reversal symmetry for sequential measurements on an open quantum system.

We introduce a novel tableau-based classical simulation method for quantum computation, formulated within the phase-space framework of the extended stabilizer theory of closed noncontextual operators. This method enables the efficient classical simulation of a broader class of quantum circuits beyond the stabilizer formalism. We identify additional families of input states that are efficiently simulatable within this framework, going beyond stabilizer states, and implement the simulator and benchmark its performance on basic quantum algorithms, including the hidden shift and Deutsch–Jozsa algorithms.

In this work we generalize the algebraic framework for measurement-based quantum computation (MBQC) in one-dimensional symmetry protected topological states recently developed [Quantum 7, 1215 (2023)], such that in addition to half-odd-integer spins, the integer spin chains can also be incorporated in the framework. The computational order parameter characterizing the efficiency of MBQC is identified, which, for spin-1 chains in the Haldane phase, coincides with the conventional string order parameter in condensed matter physics.

Experiments on dissociative recombination on ${\mathrm{ArH}}^{+}$ show a small but nonzero cross section at low collision energies. Therefore, dissociative recombination of ${\mathrm{ArH}}^{+}$, as well as ${\mathrm{NeH}}^{+}$, is studied theoretically. Accurate ab initio fixed-nuclei electron-scattering calculations are performed to produce $S$-matrices that are used as input into a multichannel quantum defect treatment of the dissociation process. Results are compared to available experimental data.

$K$-shell ionization and characteristic x-ray radiation (CXR) by high-energy electrons and positrons in oriented silicon crystals are studied using computer simulation. A method for this simulation has been developed and is described in detail. The evolution of the angular distribution of CXR from the upstream surface of the crystal with changes in the angle between the incident particle momentum and the crystal $\langle 100\rangle$ axis or (100) plane, as well as with changes in particle energy over a wide range (1–1000 GeV), is investigated. It is shown that in most cases this evolution is nonmonotonic. The physical mechanisms underlying this behavior are discussed. In particular, the impact of the dechanneling process on CXR produced by electrons is analyzed.