The accuracy of high precision and fundamental measurements of atomic transition frequencies via laser spectroscopy depends upon fitting the spectral data with a line shape. With atomic hydrogen and antihydrogen $1S\text{−}2S$ two-photon spectroscopy, computer-intensive Monte Carlo simulations have been used to compute the optical Bloch equations in order to match and interpret the experimental spectra. For the highest resolutions, one tries to minimize saturation effects going to regimes of low excitation probability, where perturbation theory can provide reliable results. Here we describe an analytical approach to the line shape based on perturbation theory accounting for the AC-Stark shift and ionization. The expressions can be used for beam experiments or integrated over the magnetic field profile for a trapped sample. Theses line shapes, providing fast results, allow for studies of many systematic effects that influence the accuracy of the determination of the central frequency. This development has relevance to hydrogen beam experiments and to trapped hydrogen and antihydrogen, as developed by the ALPHA collaboration at CERN, for tests of the charge-parity-time symmetry and the highest-accuracy measurement on antimatter.

We propose a theoretical scheme to achieve a nonreciprocal polaritonic photonic transistor by using a spinning microresonator coupled to a tapered fiber. In such a nonreciprocal device, owing to the combination of Sagnac effects and Stokes or anti-Stokes scattering, the transmission of probe light in clockwise and counterclockwise directions exhibits clockwise (counterclockwise) unidirectional amplification, bidirectional amplification, and bidirectional attenuation, simultaneously accompanied by unidirectional slow-fast light, bidirectional fast light, and bidirectional slow light, which can be effectively modulated by pump fields and couplings among excitons, photons, and phonons. In particular, we also discover perfect unidirectional transmission, i.e., the transmission in the opposite direction is completely blocked, and fast-slow light switching by appropriately regulating pump fields, which is optimal for signal isolation and controllable group advance-delay switching. Therefore, our proposed nonreciprocal photonic transistor is an excellent multifunctional device with various amplification-attenuation transmission and fast-slow light nonreciprocities. Our results provide an opportunity to engineer multifunctional nonreciprocal photonic devices, which may be useful for building integrated photonic circuits, chiral quantum technologies, and quantum communication networks.

We study the emergence of dynamical phases of a Bose-Einstein condensate that is optomechanically coupled to a dissipative cavity mode and transversally driven by a laser. We focus on the regime close to the optomechanical resonance, where the atoms' refractive index shifts the cavity into resonance, assuming fast cavity relaxation. We derive an effective model for the atomic motion, where the cavity degrees of freedom are eliminated using perturbation theory in the atom-cavity coupling and benchmark its predictions using numerical simulations based on the full model. Away from the optomechanical resonance, perturbation theory in the lowest order (adiabatic elimination) reliably describes the dynamics and predicts chaotic phases with unstable oscillations. Interestingly, the dynamics close to the optomechanical resonance are qualitatively captured only by including the corrections to next order (nonadiabatic corrections). In this regime we find limit-cycle phases that describe stable oscillations of the density with a well-defined frequency. We further show that such limit-cycle solutions are metastable configurations of the adiabatic model. Our work sheds light on the mechanisms that are required to observe dynamical phases and predict their existence in atom-cavity systems where a substantial timescale separation is present.

We study finite-dimensional open quantum systems in contact with macroscopic equilibrium systems at their boundaries such that the system density matrix $\rho$ evolves via a Lindbladian, $\dot{\rho }=-i[H,\rho ]+\mathcal{D}\rho$. Here $H$ is the Hamiltonian of the system and $\mathcal{D}$ is the dissipator. We consider the case where the system consists of two parts, the “boundary” $A$ and the “bulk” $B$, and $\mathcal{D}$ acts only on $A$, so $\mathcal{D}={\mathcal{D}}_A\otimes {\mathcal{I}}_B$, where ${\mathcal{I}}_B$ is the identity superoperator on part $B$. Let ${\mathcal{D}}_A$ be ergodic, so ${\mathcal{D}}_A{\pi }_A=0$ only for one unique density matrix ${\pi }_A$. We show that any stationary density matrix $\overline{\rho }$ on the full system which commutes with $H$ must be of the product form $\overline{\rho }={\pi }_A\otimes {\rho }_B$ for some ${\rho }_B$. This rules out finding any ${\mathcal{D}}_A$ that has the Gibbs measure ${\rho }_{\beta }=e^{-\beta H}/Z\left(\beta \right)$ as a stationary state with $\beta \ne 0$, unless there is no interaction between parts $A$ and $B$. We give criteria for the uniqueness of the stationary state $\overline{\rho }$ for systems with interactions between $A$ and $B$. Related results for nonergodic cases are also discussed.

Quantum advantage is a demonstration of a computation by a quantum computer that cannot be performed by the best classical computer in a reasonable time. A well-studied approach to demonstrating this on near-term quantum computers is to use random circuit sampling. It has been suggested that a good candidate for demonstrating quantum advantage with random circuit sampling is to use instantaneous quantum polynomial (IQP) circuits. These are quantum circuits where the unitary it implements is diagonal. In this paper, we introduce improved techniques for exactly classically simulating random IQP circuits. We find a simple algorithm to calculate an amplitude of an $n$-qubit IQP circuit with dense random two-qubit interactions in time $O\left(\frac{{log}^{2}n}{n}{2}^n\right)$, which for sparse circuits (where each qubit interacts with $O(logn)$ other qubits) runs in $o(2^n/\mathrm{poly}\left(n\right))$ for any given polynomial. Using a more complicated stabilizer decomposition approach we improve the algorithm for dense circuits to $O\left(\frac{{(logn)}^{4-\beta }}{n^{2-\beta }}{2}^n\right)$ where $\beta \approx 0.396$. We further discuss how our techniques also lead to improved simulation times for IQP circuits on restricted architectures. We benchmarked our main algorithm and found that we can simulate up to 50-qubit circuits in a couple of minutes on a laptop, with 68-qubit sparse circuits taking a couple of hours. We estimate dense 70-qubit circuits are in range for large computing clusters.

The surface code is a stabilizer code whose ground space degeneracy depends on defects in the lattice. The protocols developed to create defects in the system have previously relied on adiabatic dynamics. In this work we use techniques of quantum optimal control to overcome the requirement for adiabaticity and achieve defect creation and implemention of other important operations required for topological quantum computation at much faster timescales.

Quantum imaginary-time evolution (QITE) is a promising tool to prepare thermal or ground states of Hamiltonians, as convergence is guaranteed when the evolved state overlaps with the ground state. However, its implementation using a a hybrid quantum-classical approach, where the dynamics of the parameters of the quantum circuit are derived by McLachlan's variational principle, is impractical as the number of parameters $m$ increases, since each step in the evolution takes $\mathrm{\Theta }\left(m^2\right)$ state preparations to calculate the quantum Fisher information matrix (QFIM). In this work, we accelerate QITE by rapid estimation of the QFIM, while conserving the convergence guarantees to the extent possible. To this end, we prove that if a parameterized state is rotated by a 2-design and measured in the computational basis, then the QFIM can be inferred from partial derivative cross correlations of the probability outcomes. One sample estimate costs only $\mathrm{\Theta }\left(m\right)$ state preparations, leading to rapid QFIM estimation when a few samples suffice. The second family of estimators takes greater liberties and replace QFIMs with averaged classical Fisher information matrices (CFIMs). In an extreme special case optimized for rapid (over accurate) descent, just one CFIM sample is drawn. We justify the second estimator family by proving rapid descent. Guided by these results, we propose the random-measurement imaginary-time evolution algorithm, which we showcase and test in several molecular systems, with the goal of preparing ground states.

Two-party secure quantum remote sensing protocols enable quantum-enhanced measurements at remote locations with guaranteed security against eavesdroppers. This idea can be scaled up to networks of nodes where one party can directly measure functions of parameters at the different nodes using entangled states. However, the security on such networks decreases exponentially with the number of nodes. Here we show how this problem can be overcome in a hybrid protocol that utilizes both entangled and separable states to achieve quantum-enhanced measurement precision and security on networks of any size.

Dissipative cat qubits are known for the exponential suppression of the bit-flip rate. However, there is significant discrepancy between experimental measurements and analytical predictions of the strength of the bit-flip suppression. In this paper we resolve this discrepancy for some of the perturbations, by developing a second-order perturbation theory on top of a nonlinear dissipative Lindbladian. Following this scheme, we derive an analytical expression for the exponentially small bit-flip rate due to single-photon loss, which shows good agreement with numerical simulations. We also apply our scheme to other perturbations, such as frequency detuning and the $Z$ gate, and find the corresponding bit-flip rates, which also show good agreement with the numerical simulation.

We theoretically investigate the multiphoton ionization of the xenon atom by pairs of linearly chirped, time-delayed circularly polarized pulses. The numerical results are obtained by solving the two-dimensional time-dependent Schrödinger equation and analyzed by using perturbation theory. By comparing observables such as the ionization probabilities and the energy, momentum, and angular distributions for laser pulses with different chirp rates, we find that the ionization processes have a clear dependence on the pulse chirp. Specifically, the dependence of the energy separation between adjacent interference fringes in the photoelectron energy distributions on the chirp rate is studied in corotating pulses. In counter-rotating pulses, the analysis of the photoelectron angular distributions reveals that the photoelectron ejection angle is sensitive to the chirp rate due to chirp-induced phase difference. Moreover, the critical chirp rates for different multiphoton ionization processes are determined from the multistart spiral structures in photoelectron momentum distributions and applied for control of the photoelectron vortices. The chirp dependence of the emitted electron observables discussed here may provide an ideal diagnostic tool for characterizing the chirped laser pulses.

High-order harmonic generation (HHG) driven by infrared laser pulses in Ba and Cs atoms is studied numerically and analytically by solving the time-dependent Kohn-Sham equations. We show that the energy transfer from the returning electron, ejected from the outer $6s$ subshell, to the electrons in the fourth $\left(N\right)$ shell results in a giant broadband HHG enhancement at frequencies $\sim 100\mathrm{eV}$ in Ba and Cs similar to that in Xe, where the returning electron is ejected from the outer $5p$ subshell. The high degree of delocalization of the outer $6s$ subshell in Ba and Cs slows down the spreading of the laser-driven electron wave packet outside the atom, which ensures high amplitudes of all generated harmonics. We demonstrate that the giant enhancement region can be reached in both Ba and Cs subjected to a low-intensity mid-IR laser pulse. Also, for Ba at high intensity $(\sim {10}^{14}\mathrm{W}/{\mathrm{cm}}^2)$, singly ionized atoms contribute to HHG; thus, the giant enhancement region in Ba can be reached using intense near-IR laser pulses. For this case, the HHG yield in Ba is more than an order of magnitude higher than that in Xe. In addition to the giant enhancement, the rescattering photoelectron excites in Ba and Cs autoionizing states with vacancy on the $5p$ subshell, which results in the narrowband HHG enhancement at frequencies $\sim 20\mathrm{eV}$.

Generally, a gradient force draws a dielectric sphere towards the region of high light intensity. However, this force may repel the sphere when a Rayleigh particle's effective polarizability has a negative real part, or when a dielectric sphere's whispering gallery modes (WGMs) are excited. For a dielectric sphere larger than a Rayleigh particle but not large enough to excite WGMs, the possibility of experiencing this “anomalous” gradient force remains uncertain. Our study employs multipole expansion theory to investigate gradient forces on a dielectric sphere in an evanescent wave. We find that the sphere may be pushed away from the high-intensity region regardless of WGM excitation, due to multipole-incident field coupling. However, the underlying mechanisms differ. For an $s$-polarized incident evanescent wave, WGM excitation limits the contribution to the gradient force from a specific order of multipole. The gradient force is notably stronger for transverse magnetic mode excitation compared to transverse electric mode, due to differences in the electromagnetic energy density gradient's contribution. Without WGMs, multiple multipoles—interestingly, only magnetic ones—contribute to this gradient force. We provide an intuitive explanation for the dielectric sphere experiencing gradient forces in opposite directions by analyzing internal field distributions and the optical force density within the sphere. Additionally, we explore how evanescent wave decay rates and polarizations influence gradient forces by analyzing recoil forces and imaginary Poynting momentum density contributions. These in-depth studies deepen our understanding of near-field optical manipulation mechanisms and provide a theoretical foundation for designing innovative optical devices and systems.

The prediction of nonlocal phenomena is a key attribute of quantum mechanics that distinguishes it from classical theories. It was recently suggested that state swapping is one such effect that a fundamentally classical gravitational field cannot give rise to. The reasoning was that the phenomenology of classical fields is restricted to local operations and classical communication (LOCC), whereas swapping is a non-LOCC effect, so gravity must not be a classical channel if it can give rise to such a phenomenon. Here, we show that the Hamiltonian of semiclassical gravity results in an evolution of coherent states that is virtually indistinguishable from swapping them, both theoretically and experimentally. Our results indicate that drawing a definitive conclusion about the nature of gravity based solely on its ability to swap a pair of coherent states would be premature. We discuss the implications of these results regarding proposed experimental tests for the quantum nature of gravity and clarify some conceptual issues.

We explore a small quantum refrigerator consisting of three qubits, each of which is kept in contact with an environment. We consider two settings: one is when there is necessarily transient cooling and the other is when both steady-state and transient coolings prevail. Our primary focus, however, is on the transient cooling phenomena. We show that in the transient regime, the temperature of the cold qubit can decrease further compared to the case where all qubits are connected to Markovian environments, by replacing the environment attached to the cold qubit with a finite-size spin environment, modeled by a few quantum spins interacting with the cold qubit. We also consider refrigeration with more than one finite-size spin environment of the three-qubit refrigerating device. As expected, a steady temperature is reached only if there are at least two Markovian baths, regardless of whether the cold qubit is attached to an environment. However, distinct envelopes of temperature oscillations are observed in all cases. We investigate the effects of the finite-size environments on the cooling of the cold qubit when one, two, or all Markovian baths are replaced by finite-size environments. Additionally, we examine this effect in two- and single-qubit self-sustained devices connected to one or more finite-size environments. In deriving the dynamical equations for the qubits connected to finite-size environments, we made no assumptions about the Markovian nature of the environment. As a result, these finite-size environments inherently exhibit information backflow from the environment to the system, a hallmark of non-Markovianity. Hence, we propose a witness to detect non-Markovianity in such systems. Finally, the cooling processes are studied in presence of Markovian noise, and we analyze the response on the refrigeration of the noise strength. In particular, we find the noise strength until which refrigeration remains possible.

Exact calculation and even multiplicative error estimation of matrix permanent are challenging for both classical and quantum computers. Regarding the permanents of random Gaussian matrices, the additive error estimation is closely linked to boson sampling and achieving multiplicative error estimation requires exponentially many samplings. Our formula for matrix permanents and its corresponding quantum expression have enabled better estimation of the average additive error for random Gaussian matrices compared to Gurvits' classical sampling algorithm. The well-known Ryser formula has been converted into a quantum permanent estimator. When dealing with real random Gaussian square matrices of size $N$, the quantum estimator can approximate the matrix permanent with an additive error smaller than $\epsilon {\left(\sqrt{\mathrm{e}N}\right)}^N$, where $\epsilon$ is the estimation precision. In contrast, Gurvits' classical sampling algorithm has an estimation error of $\epsilon {\left(2\sqrt{N}\right)}^N$, which is exponentially larger ($1.2^N$) than the quantum method. As expected, the quantum additive error bound fails to reach the multiplicative error bound of ${\left(2\pi N\right)}^{1/4}\epsilon {\left(\sqrt{N/\mathrm{e}}\right)}^N$. Additionally, the quantum permanent estimator can be up to quadratically faster than the classical estimator when using quantum phase estimation-based amplitude estimation.

Reliable quantum computation requires systematic identification and correction of errors that occur and accumulate in quantum hardware. To diagnose and correct such errors, standard quantum error-correcting protocols utilize global error information across the system obtained by mid-circuit readout of ancillary qubits. We investigate circuit-level error-correcting protocols that are measurement-free and based on local error information. Such a local error correction (LEC) circuit consists of faulty multi-qubit gates to perform both syndrome extraction and ancilla-controlled error removal. We develop and implement a reinforcement learning framework that takes a fixed set of faulty gates as inputs and outputs an optimized LEC circuit. To evaluate this approach, we quantitatively characterize an extension of logical qubit lifetime by a noisy LEC circuit. For the two-dimensional (2D) classical Ising model and four-dimensional toric code, our optimized LEC circuit performs better at extending a memory lifetime compared with a conventional LEC circuit based on Toom's rule in a subthreshold gate error regime. We further show that such circuits can be used to reduce the rate of mid-circuit readouts to preserve a 2D toric code memory. Finally, we discuss the application of the LEC protocol on dissipative preparation of quantum states with topological phases.

Entanglement with single-particle states is advantageous in quantum technology because of the states' ability to encode and process information more securely than their multiparticle analogs. Three-way- and nonlocal two-way-entangled single-particle states are desirable in this context. Herein we generate genuine three-way entanglement from an initially separable state involving three degrees of freedom of a quantum particle, which evolves via a two-dimensional alternate quantum walk employing a resource-saving single-qubit coin. We achieve maximum possible values for the three-way entanglement quantified by the $\pi$-tangle between the three degrees of freedom. We also generate optimal nonlocal two-way entanglement, quantified by the negativity between the nonlocal position degrees of freedom of the particle. This prepared architecture using quantum walks can be experimentally realized with a photon.

Memristive systems exhibit dynamics that depend on their past states, making them useful as memory units. Recently, quantum memristor models have been proposed, and notably, a photonic quantum memristor (PQM) has been experimentally proven. In this work, we explore and characterize various quantum properties that emerge from this specific model of PQM. First, we find that a single PQM displays memristive dynamics on its quantum coherence. Second, we analytically show that a network made of two independent PQMs can manifest memory effects on the dynamics of both entanglement and coherence of correlated photons traveling through the network, regardless of their distance, in the hypothesis of negligible external disturbances. Additionally, we build and run a circuit-model of the PQM on a real qubit-based quantum computer (IBM-Q), showing that (1) this system can effectively be used for nonlinear quantum computing under specific conditions and (2) digital quantum simulations can reproduce the dynamics of a memristive quantum system in a non-Markovian regime.

This study investigates the implementation of measurement-device-independent multiparty quantum communication using continuous-variable Greenberger-Horne-Zeilinger (CV GHZ) states under the fast fading channel. The communication parties are connected through free space, and factors such as atmospheric turbulence cause beam drift, resulting in variations in channel transmittance according to specific probability distributions. We assume a worst-case scenario in which an eavesdropper has complete control over the channel, forcing the communicators to estimate the channel based on this probability distribution. The protocol employs CV GHZ states to achieve secure communication through quantum cryptographic conference and quantum secret sharing. For the aspect of security, independent entangling cloner attack and coherent attack are analyzed. Simulation results demonstrate that the protocol can withstand advantageous attacks from eavesdroppers.