We propose a nonreciprocal quantum heat engine based on a spinning optomechanical system, in which the properties of the polariton branches are modified by tuning the detuning to realize the Otto cycle, enabling the conversion of thermal energy into mechanical energy. The Sagnac effect resulting from the rotation of the resonator leads to a pronounced disparity in work output and efficiency when the resonator is driven from opposing directions, establishing a distinct nonreciprocal behavior. Additionally, the performance of the engine can be precisely modulated by controlling the frequency shift, allowing for flexible and efficient optimization of the quantum heat engine's operation while markedly reducing its reliance on cavity field frequencies. These results provide critical insights and a solid theoretical foundation for the advancement of quantum heat engines.

Simulating the time evolution of quantum field theories given some Hamiltonian $H$ requires developing algorithms for implementing the unitary operator ${\mathrm{e}}^{-iHt}$. A variety of techniques exist that accomplish this task, with the most common technique used so far being Trotterization, which is a special case of the application of a product formula. However, other techniques exist that promise better asymptotic scaling in certain parameters of the theory being simulated, the most efficient of which are based on the concept of block encoding. In this work we study the performance of such algorithms in simulating lattice field theories. We derive and compare the asymptotic gate complexities of several commonly used simulation techniques in application to Hamiltonian lattice field theories. Using the scalar ${\widehat{\phi }}^4$ theory as a test, we also perform numerical studies and compare the gate costs required by product formulas and signal-processing-based techniques to simulate time evolution. For the latter, we use the linear combination of unitaries (LCU) construction augmented with the quantum Fourier transform circuit to switch between the field and momentum eigenbases, which leads to immediate order-of-magnitude improvement in the cost of preparing the block encoding. This paper also includes a pedagogical review of the techniques used, in particular product formulas, LCU, qubitization, quantum signal processing, as well as the technique for simulating geometrically-local Hamiltonians developed by Haah, Hastings, Kothari, and Low.

We present a hybrid quantum algorithm for estimating gaps in many-body energy spectra, supported by an analytic proof of its inherent resilience to state preparation and measurement errors, as well as midcircuit multiqubit depolarizing noise. Our analysis extends to a broader class of Markovian noise, employing error mitigation strategies that optimize the utilization of quantum resources. By integrating trial-state optimization and classical signal processing into the algorithm, we amplify the signal peak corresponding to the exact target gap beyond the error threshold, thereby significantly reducing gap estimate errors. The algorithm's robustness is demonstrated through noisy simulations on the Qiskit Aer simulator and demonstrations on IBM Quantum processors. These results underscore the potential to enable accurate quantum simulations on near-term noisy quantum devices without resource-intensive error correction.

We investigate the formation of positronium (Ps) in the presence of a linearly polarized infrared laser field, via the collision of positrons with Rydberg hydrogen atoms $\mathrm{H}\left(n_{\mathrm{H}}\right)$ where $n_{\mathrm{H}}$ is the principal quantum number. We are using the classical trajectory Monte Carlo method (CTMC) which is justified for $n_{\mathrm{H}}\ge 3$. The hydrogen in excited states has an effective dipole moment which in the presence of a laser field leads to a dipolar focusing [H. B. Ambalampitiya et al., J. Phys. B: At. Mol. Opt. Phys. 57, 10LT01 (2024)]. Due to this effect, the Ps formation cross section is enhanced substantially as compared to the zero-field case. The degree of this enhancement is controlled by the positron velocity, laser frequency, and laser-field amplitude. The final-state distribution in Ps is much broader than in the zero-field case.

A weak magnetic-dipole ${\mathrm{CO}}_2$ absorption band has been recently identified in the Martian atmosphere. This unprecedented discovery raised a question of whether similar magnetic transitions could be found in other polyatomics as well. The present work proposes a simplified, but physically well-grounded approach, which is intended to help to evaluate the strength of magnetic-dipole transitions induced in the course of molecular vibrations. The suggested theory is developed assuming a pure vibrational magnetic moment arising within a combination band. Our estimate for the ${\nu }_2+{\nu }_3 {\mathrm{CO}}_2$ electric-dipole-forbidden band demonstrates good agreement of the transition matrix element with the value derived previously from both laboratory experiments and in-depth ab initio based consideration. The possibility is shown to observe magnetic transitions owing to a magnetic moment that is linear with respect to momentum. This prediction is discussed taking a molecule of type HCN as an example.

Nonequilibrium dynamics of highly controlled quantum systems is a challenging issue in statistical physics and quantum many-body physics, relevant to recent experimental developments of analog and digital quantum simulations. In this work, we develop a discrete phase-space approach for general $\mathrm{SU}(N$) spin systems that utilizes cluster mean-field equations, which capture nontrivial quantum correlations inside each cluster, beyond the capability of the standard discrete truncated Wigner approximation for individual classical spins. Our formalism, based on a cluster phase-point operator, enables efficient numerical samplings of cluster phase-space variables, where the total number of noise variables for a direct product state is independent of the specific way in which the entire system is divided into multiple equally sized finite clusters. We numerically demonstrate that the cluster discrete truncated Wigner approximation (C-dTWA) method can reproduce key results in a recent experiment on correlation propagation dynamics in a two-dimensional Bose-Hubbard system. We further compare the results of C-dTWA for clusters of $2\times 2$ sites with those from a two-dimensional tensor network method and discuss that both approaches agree very well in the short-time region, where the energy conservation is well maintained in the tensor network simulations. Since we formulate the C-dTWA method in a general form, it has the potential for application to various dynamical problems in isolated and open quantum systems, even in higher dimensions.

We theoretically investigate the effect of “glancing” collisions on the ultra-high-vacuum pressure readings of the cold-atom vacuum standard (CAVS), based on either ultracold ${}^7\mathrm{Li}$ or ${}^{87}\mathrm{Rb}$ atoms. Here, glancing collisions are those collisions between ultracold atoms and room-temperature background atoms or molecules in the vacuum that do not impart enough kinetic energy to eject an ultracold atom from its trap. Our model is wholly probabilistic and shows that the number of the ultracold atoms remaining in the trap as a function of time is nonexponential. We update the recent results of a comparison between a traditional pressure standard—a combined flow meter and dynamic expansion system—and the CAVS [D. S. Barker et al., AVS Quantum Sci. 5, 035001 (2023)] to reflect the results of our model. We find that the effect of glancing collisions shifts the theoretical predictions of the total loss rate coefficients for ${}^7\mathrm{Li}$ colliding with noble gases or ${\mathrm{N}}_2$ by up to 0.6%. Likewise, we find that in the limit of zero trap depth, the experimentally extracted loss rate coefficients for ${}^{87}\mathrm{Rb}$ colliding with noble gases or ${\mathrm{N}}_2$ shift by as much as 2.2%.

We investigate the dynamics of one-dimensional SU(2) ultracold fermions near the Tonks-Girardeau limit, confined in a box potential. The system is driven out of equilibrium by initially preparing the two spin components in a fully separated configuration, and its evolution is described by the Hamiltonian in the presence of strong repulsive interactions. Building on the results in G. Aupetit-Diallo et al., Phys. Rev. A 107, L061301 (2023), we extend the analysis to out-of-equilibrium dynamics, uncovering the emergence of time-dependent oscillating high-momentum tails in the momentum distribution. These oscillations, due to the finite size of the system, are governed by a nonlocal, high-order spin coherence term, whose amplitude and phase evolve over time. We show that this term initially grows as $t^{N/2}$ and subsequently follows the spin-mixing dynamics of the system. Notably, when the spin components are fully mixed, the amplitude of this border-to-border spin coherence reaches its maximum value.

The quantum Mpemba effect in open quantum systems has been extensively studied, but a comprehensive understanding of this phenomenon remains elusive. In this paper, we conduct an analytical investigation of the dissipative dynamics of single excitations in the mosaic model. Surprisingly, we discover that the presence of an asymptotic mobility edge, denoted as $E_c^{\infty }$, can lead to unique dissipation behavior, serving as a hallmark of the quantum Mpemba effect. Especially, it is found that the energy level $E_c^{\infty }$ exhibits a global periodicity in the real configuration, which acts to inhibit dissipation in the system. Conversely, when the system deviates from $E_c^{\infty }$, the quasidisorder sets in, leading to increased dissipative effects due to the broken of periodicity. Furthermore, we find that the rate of dissipation is closely linked to the localization of the initial state. As a result, the quantum Mpemba effect can be observed clearly by a measure of localization.

In analog and digital simulations of practically relevant quantum systems, the target dynamics can only be implemented approximately. The Trotter product formula is the most common approximation scheme as it is a generic method which allows tuning accuracy. The Trotter simulation precision will always be inexact for noncommuting operators, but it is currently unknown what the minimum possible error is. This is an important quantity because upper bounds for the Trotter error are known to often be vast overestimates. Here we present explicit lower bounds on the error, in norm and on states, allowing to derive minimum resource requirements. Numerical comparison with the true error shows that our bounds offer accurate and tight estimates.

One of the main limitations of variational quantum algorithms is the classical optimization of the highly dimensional nonconvex variational parameter landscape. To simplify this optimization, we can reduce the search space using problem symmetries and typical optimal parameters as initial points if they concentrate. In this article, we consider typical values of optimal parameters of the quantum approximate optimization algorithm for the MAXCUT problem with $d$-regular tree subgraphs and reuse them in different graph instances. We prove symmetries in the optimization landscape of several kinds of weighted and unweighted graphs, which explains the existence of multiple sets of optimal parameters. However, we observe that not all optimal sets can be successfully transferred between problem instances. We find specific transferable domains in the search space and show how to translate an arbitrary set of optimal parameters into the adequate domain using the studied symmetries. Finally, we extend these results to general classical optimization problems described by Ising Hamiltonians, the Hamiltonian variational ansatz for relevant physical models, and the recursive and multiangle quantum approximate optimization algorithms.

We propose a parity-controlled gate within a two-dimensional Rydberg atom array, enabling efficient discrimination between even and odd parities of virtually excited control atoms by monitoring the dynamic evolution of an auxiliary atom. This is achieved through the use of spin-exchange dipolar interactions between Rydberg states and coupling between ground states and Rydberg states. For practical applications, we explore its implementation in three-qubit repetition codes and rotated surface codes featuring $XZZX$ stabilizers, enabling single-shot readout of stabilizer measurements. Comprehensive numerical simulations are conducted to assess the feasibility of the proposed approach, taking into account potential experimental imperfections such as unwanted interactions between Rydberg states, atomic position fluctuations, laser phase noise, and Rabi amplitude noise. Our study highlights the inherent advantages of the physical mechanisms underlying parity measurement, demonstrating its reliability and practicality. These findings establish our protocol as a highly promising solution for quantum error detection and computation within Rydberg atom systems, with significant potential for future experimental realizations.

Quantum reservoir computing (QRC) exploits the information-processing capabilities of quantum systems to tackle time-series forecasting tasks, which is expected to be superior to their classical counterparts. By far, many QRC schemes have been theoretically proposed. However, most of these schemes involve long-time evolution of quantum systems or networks with quantum gates. This poses a challenge for practical implementation of these schemes, as precise manipulation of quantum systems is crucial, and this level of control is currently hard to achieve with the existing state of quantum technology. Here we propose a different QRC scheme, which is friendly to experimental realization. It implements the quantum version of nonlinear vector autoregression, extracting linear and nonlinear features of quantum data by measurements. Thus, the evolution of complex networks of quantum gates can be avoided. Compared to other QRC schemes, our proposal also achieves an advance by effectively reducing the necessary training data for reliable predictions in time-series forecasting tasks. Furthermore, we experimentally verify our proposal by performing the forecasting tasks, and the observation matches well with the theoretical ones. Our work opens up a different way for complex tasks to be solved by using QRC, which can herald the next generation of QRC.

Continuous-variable quantum key distribution (CVQKD) with discrete modulation combines advantages of CVQKD, such as implementability using readily available technologies, with advantages of discrete variable quantum key distribution, such as easier error correction procedures. We consider a prepare-and-measure CVQKD protocol, where Alice chooses from a set of four coherent states and Bob performs a heterodyne measurement, the result of which is discretized in both key and test rounds. We provide a security proof against coherent attacks in the finite-size regime based on the generalized entropy accumulation theorem. We compute the corresponding asymptotic key rate imposing a photon-number cutoff and recent advances in conic optimization. Our method yields significant improvements on key rates: at metropolitan distances, it provides positive key rates for block sizes of the order of ${10}^8$ rounds, orders of magnitude smaller compared to previous works.

Circuit quantum-electrodynamics-based quantum information processing relies on low noise amplification for signal readout. In the realm of microwave superconducting circuits, this amplification is often achieved via Josephson parametric amplifiers (JPAs). In the past, these amplifiers exhibited low power added efficiency (PAE), which is roughly the fraction of pump power that is converted to output signal power. This is increasingly relevant because recent attempts to build high saturation power amplifiers achieve this at the cost of very low PAE, which in turn puts a high heat load on the cryostat and limits the number of these devices that a dilution refrigerator can host. Here, we numerically investigate upper bounds on PAE. We focus on a class of parametric amplifiers that consists of a capacitor shunted by a nonlinear inductive block. We first set a benchmark for this class of amplifiers by considering nonlinear blocks described by an arbitrary polynomial current-phase relation. Next, we identify that it is important for amplifiers with inductive blocks composed of repeating elements to have monotonic current-phase relations for each element in order to avoid exciting high-frequency modes. Using this design rule, we propose two circuit implementations for repeating elements in JPA inductive blocks. Finally, we investigate polynomial amplifier chains. We find that while amplifiers with higher gain have a lower PAE, regardless of the gain there is considerable room to improve as compared with state-of-the-art devices. For example, for a degenerate amplifier with a power gain of 20 dB, the PAE is $\approx 0.1\%$ for typical JPAs, 37.9% for our simpler circuit JPAs, 42.6% for our more complex circuit JPAs, 63.3% for our arbitrary polynomial amplifiers, and at least 98% for our amplifier chains.

Suppressing the Trotter error in dynamical quantum simulation typically requires running deeper circuits, posing a great challenge for noisy near-term quantum devices. Studies have shown that the empirical error is usually much smaller than the one suggested by existing bounds, implying the actual circuit cost required is much less than the ones based on those bounds. Here, we improve the estimate of the Trotter error over existing bounds, by introducing a general framework of commutant decomposition that separates disjoint error components that have fundamentally different scaling with time. In particular we identify two error components that each scale as $O\left({\tau }^{p}t\right)$ and $O\left({\tau }^p\right)$ for a $p\mathrm{th}$-order product formula evolving to time $t$ using a fixed step size $\tau$, implying one would scale linearly with time $t$ and the other would be a constant of $t$. We show that this formalism not only straightforwardly reproduces previous results but also provides a better error estimate for higher-order product formulas. We demonstrate the improvement both analytically and numerically. We also apply the analysis to observable error relating to the heating in Floquet dynamics and thermalization, which is of independent interest.

The binding energies of the five bound rotational levels $J=0–4$ in the highest vibrational level $v=14$ in the $X^1{\mathrm{\Sigma }}_g^{+}$ ground electronic state of ${\mathrm{H}}_2$ were measured in a three-step ultraviolet-laser experiment. Two-photon UV photolysis of ${\mathrm{H}}_2\mathrm{S}$ produced population in these high-lying bound states that were subsequently interrogated at high precision via Doppler-free spectroscopy of the $F^1{\mathrm{\Sigma }}_g^{+}\text{−}{X}^1{\mathrm{\Sigma }}_g^{+}$ system. A third UV laser was used for detection through auto-ionizing resonances. The experimentally determined binding energies were found to be in excellent agreement with calculations based on nonadiabatic perturbation theory, also including relativistic and quantum electrodynamical contributions. The $s$-wave scattering length of the $\text{H}+\text{H}$ system is derived from the binding energy of the last bound $J=0$ level via a direct semi-empirical approach, yielding a value of $a_s=0.2724\left(5\right)a_0$, in good agreement with a result from a previously followed theoretical approach. The subtle effect of the $m{\alpha }^4$ relativity contribution to $a_s$ was found to be significant. In a similar manner a value for the $p$-wave scattering volume is determined via the $J=1$ binding energy yielding $a_p=-134.0000\left(6\right)a_0^3$. The binding energy of the last bound state in ${\mathrm{H}}_2$, the ($v=14, J=4$) level, is determined at 0.023 (4) ${\mathrm{cm}}^{-1}$, in good agreement with calculation. The effect of the hyperfine substructure caused by the two hydrogen atoms at large internuclear separation, giving rise to three distinct dissociation limits, is discussed.

The Hartree-Fock-Slater calculations of average transition energies, relevant to ionic states of warm dense plasmas, were compared to averaged transition energies of the NIST atomic lines database for oxygen, aluminum, iron, and molybdenum. A comprehensive error analysis shows the statistical nature of the theory-experiment differences almost independently of the atomic number, ionization degree, and the transitions’ atomic shells. Also, a method for the statistical analysis of the accuracy of the calculated transition energies for the plasmas’ opacity for a given density-temperature regime was suggested and demonstrated for the above-mentioned elements. This analysis show that a good accuracy of the relevant calculations is expected for temperatures larger than $40\text{–}50\mathrm{eV}$ for these elements within the regime of $20\text{–}200\mathrm{eV}@{10}^{19}\text{–}{10}^{22}\mathrm{atoms}/\mathrm{c}{\mathrm{m}}^3$, which covers a large number of laboratory-produced plasma experiments. Finally, a simple correction to the calculated transition energies is suggested for transitions which are included in the NIST database.