Measurement-free quantum error correction (MFQEC) offers an alternative to standard measurement-based QEC in platforms with an unconditional qubit reset gate. We revisit the question of fault tolerance (FT) for a measurement-free variant of the Steane code that leverages multiqubit gates and redundant syndrome extraction, finding previously overlooked phase-flip errors that undermine FT. We then construct a revised MFQEC circuit that is resistant to all single-qubit errors, but which nonetheless cannot tolerate certain correlated errors. In order to investigate FT systematically, we introduce an efficient method to classically simulate MFQEC circuits with (i) Clifford gates for syndrome extraction, (ii) syndrome-controlled Pauli operations for decoding, and (iii) a Pauli noise model. We thereby find a pseudothreshold of $\sim 0.7\%$ for our revised MFQEC Steane code under a restricted noise model previously considered in the literature. We then relax noise model assumptions to identify general requirements for FT with multiqubit gates, finding that existing multiqubit neutral atom gates are incompatible with fault-tolerant syndrome extraction in a straightforward implementation of both measurement-based and measurement-free variants of the Steane code. Decomposing multiqubit gates into two-qubit gates similarly spoils FT. Finally, we discuss the theoretical ingredients that are necessary to recover FT for MFQEC codes, including single-shot FT and a recent proposal by Heußen et al. [arXiv:2307.13296] to achieve FT by “copying” errors onto an ancilla register. By combining multiqubit gates, redundant syndrome extraction, and copy-assisted FT, we construct a measurement-free and fault-tolerant variant of the Steane code with a pseudothreshold of $\sim 0.1\%$.

We report spectroscopy experiments of rubidium vapor in a high magnetic field under conditions of electromagnetically induced transparency (EIT) and optical pumping. The 1.1-T static magnetic field decouples nuclear and electronic spins and shifts each magnetic state via the Zeeman effect, allowing us to resolve individual optical transitions of the $D_2$ line in a Doppler-broadened medium. By varying the control laser power driving one leg of a spectrally isolated $\mathrm{\Lambda }$ system we tune the vapor from the EIT regime to conditions of Autler-Townes line splitting. The resulting spectra conform to simple three-level models demonstrating the effective simplification of the energetic structure. Further, we quantify the viability of state preparation via optical pumping on nuclear spin-forbidden transitions. We conclude that the “cleanliness” of this system greatly enhances the capabilities of quantum control in hot vapor, offering advantages in a broad variety of quantum applications plagued by spurious light-matter interaction processes, such as atomic quantum memories for light.

The majority of molecules proposed for laser cooling and trapping experiments have $\mathrm{\Sigma }$-type ground states. Specifically, ${}^2\mathrm{\Sigma }$ states have cycling transitions analogous to $D_1$ lines in alkali-metal atoms while ${}^1\mathrm{\Sigma }$ states offer both strong and weak cycling transitions analogous to those in alkaline-earth atoms. Despite this proposed variety, to date, only molecules with ${}^2\mathrm{\Sigma }$-type ground states have successfully been confined and cooled in magneto-optical traps. While none of the proposed ${}^1\mathrm{\Sigma }$-type molecules have been successfully laser cooled and trapped, they are expected to have various advantages in terms of exhibiting a lower chemical reactivity and an internal structure that benefits the cooling schemes. Here, we present the prospects and strategies for optical cycling in AlCl—a ${}^1\mathrm{\Sigma }$ molecule—and report on the characterization of the $A^1\mathrm{\Pi }$ state hyperfine structure. Based on these results, we carry out detailed simulations on the expected capture velocity of a magneto-optical trap for AlCl. Finally, using ab initio calculations, we identify the photodissociation via a $3^1\mathrm{\Pi }$ state and photoionization process via the $3^1{\mathrm{\Sigma }}^{+}$ state as possible loss mechanisms for a magneto-optical trap of AlCl.

The so-called proton-radius puzzle (the current discrepancy of proton radii determined from spectroscopic measurements in ordinary versus muonic hydrogen) could be addressed via an accurate measurement of the Rydberg constant because the proton radius and the Rydberg constant values are linked through high-precision optical spectroscopy. We argue that, with manageable additional experimental effort, it might be possible to improve circular Rydberg state spectroscopy, potentially leading to an important contribution to the clarification of the puzzle. Our proposal involves circular and near-circular Rydberg states of hydrogen with a principal quantum number around $n=18$, whose classical velocity on a Bohr orbit is slower than that of the fastest macroscopic man-made object, the Parker Solar Probe. We obtain improved estimates for the quality factor of pertinent transitions and illustrate a few recent improvements in instrumentation which facilitate pertinent experiments.

We study the energy loss of helium and hydrogen ions after transmission through monolayers of graphene and ${\mathrm{MoS}}_2$ and compare experimental results with different computational approaches. We find a systematically higher energy loss in experiments compared to simulations by up to a factor of 2. Additionally, our results show that neutralization processes of the particles increase the kinetic energy loss, a contribution generally not included in any computational approach.

We propose an approach to the realization of all-optical three-dimensional molecular orientation, in which a linearly polarized fundamental pulse and an elliptically polarized second-harmonic pulse with one of the polarization axes parallel to the linear polarization of the fundamental pulse are employed to orient asymmetric-top molecules, leading to higher degrees of orientation. In addition to the one-dimensional orientation realized along the linearly polarized fundamental pulse, planar molecular alignment can be naturally realized along the elliptical polarization plane of the second-harmonic pulse, resulting in the three-dimensional molecular orientation. Compared to another all-optical method to achieve three-dimensional molecular orientation, in which linearly polarized two-color laser fields with their polarizations crossed obliquely are employed, it is numerically demonstrated that our method with an elliptically polarized second-harmonic pulse is especially advantageous when the molecular orientation dynamics is nonadiabatic.

This paper presents experimental results for the photodetachment of the excited state (${}^{2}D^{\mathrm{o}}$) of the carbon anion. The animated crossed beam technique is used to measure the absolute cross section for photodetachment with photon energies ranging from 0.12 to 2.7 eV, shedding light on a long-standing discrepancy between existing theoretical approaches. A velocity map imaging spectrometer is used to determine partial cross sections and asymmetry parameters. An update of the binding energy of ${\mathrm{C}}^{-}\left({}^{2}D^{\mathrm{o}}\right)$ is provided with a much improved accuracy, ${}^e{A}_{{}^{2}D^{\mathrm{o}}}=36.87\left(3\right)$ meV.

We study nonreciprocal single-photon scattering in a giant-spin-ensemble (GSE)–waveguide magnonics system where the GSE constructed by a yttrium iron garnet sphere interacts twice with the waveguide via two separated coupling points. It is shown that the generation of nonreciprocity arises from the synergy of the breaking of time-reversal symmetry induced by the chiral coupling and the intrinsic dissipation of the magnon mode in the case of a single waveguide. The accumulated phase caused by the photon propagating between coupling points can be a powerful tool to control nonreciprocity due to quantum interference effects. Compared to the Markovian regime, single-photon transmission exhibits peculiarly nonreciprocal properties in the non-Markovian regime. Multiple narrow nonreciprocal transmission windows are observed. Non-Markovianity can break the decoupling phenomenon of the Markovian regime and non-Markovianity-induced nonreciprocity is demonstrated. We extend the study of nonreciprocity to the GSE coupled to a double-waveguide structure to explore the design of quantum devices. A high-efficiency and tunable multifrequency single-photon targeted router and circulator with narrow operational bandwidth are achieved. Our results provide an effective avenue for single-photon manipulation and have potential applications in designing magnon-based quantum devices and constructing an integrated quantum network.

We propose an electrodynamic Aharonov-Bohm (AB) scheme where a nonzero AB phase difference appears even if the interferometer paths do not enclose a magnetic flux and are subjected to negligible scalar potential differences during the propagation of the quantum charged particle. In the proposal, the current in a solenoid outside the interferometer varies in time while the quantum particle is in a superposition state inside two Faraday cages, such that it is always subjected to negligible electromagnetic fields. At first glance, this result could challenge the topological nature of the AB effect. However, by considering the topology of the electromagnetic field configuration and the possible particle trajectories in spacetime, we demonstrate the topological nature of this situation.

We study how decoherence increases the efficiency with which we can simulate the quantum dynamics of an anharmonic oscillator governed by the Kerr effect. As decoherence washes out the fine-grained sub-Planck structure associated with phase-space quantum interference in the closed quantum system, open quantum dynamics can be more efficiently simulated using a coarse-grained finite-difference numerical integration. We tie this to the way in which decoherence recovers the semiclassical truncated Wigner approximation, which strongly differs from the exact closed-system dynamics at times when quantum interference leads to cat states and more general superpositions of coherent states. The regression in quadrature measurement statistics to semiclassical dynamics becomes more pronounced as the initial amplitude of the oscillator grows, with implications for the quantum advantage that might be accessible as system size grows in noisy quantum devices. Lastly, we show that this regression does not have the form of a convex noise model, such as for a depolarizing noise channel. Instead, closed quantum system effects interact with the open-system effects, giving rise to distinct open-system behavior.

State-of-the-art quantum computers can only reliably execute circuits with limited qubit numbers and computational depth. This severely reduces the scope of algorithms that can be run. While numerous techniques have been invented to exploit few-qubit devices, corresponding schemes for depth-limited computations are less explored. This work investigates to what extent we can mimic the performance of a deeper quantum computation by repeatedly using a shallower device. We propose a method for this purpose, inspired by Feynman simulation, where a given circuit is chopped into two pieces. The first piece is executed and measured early on, and the second piece is run based on the previous outcome. This method is inefficient if applied in a straightforward manner due to the high number of possible outcomes. To mitigate this issue, we propose a shallow variational circuit, whose purpose is to maintain the complexity of the method within predefined tolerable limits, and provide an optimization method to find such a circuit. The composition of these components of the methods is called “reduce&chop.” As we discuss, this approach works for certain cases of interest. We believe this work may stimulate new research towards exploiting the potential of shallow quantum computers.

In this paper we present a quantum algorithm that uses noise as a resource. The goal of our quantum algorithm is the calculation of operator averages of an open quantum system evolving in time. Selected low-noise system qubits and noisy bath qubits represent the system and the bath of the open quantum system. All incoherent qubit noise can be mapped to bath spectral functions. The form of the spectral functions can be tuned digitally, allowing for the time evolution of a wide range of open-system models at finite temperature. We study the feasibility of this approach with a focus on the solution of the spin-boson model and assume intrinsic qubit noise that is dominated by damping and dephasing. We find that classes of open quantum systems exist where our algorithm performs very well, even with gate errors as high as 1%. In general the presented algorithm performs best if the system-bath interactions can be decomposed into native gates.

As a promising approach for optical nonreciprocity without magnetic materials, optomechanically induced nonreciprocity has great potential for all-optical controllable isolators and circulators on integrated photonic chips. However, as a very important issue in practical applications, the bandwidth for nonreciprocal transmission with high isolation has not been fully investigated yet. In this study we first review the nonreciprocity in a Brillouin optomechanical system with single cavity mode and point out the challenge in achieving broad bandwidth with high isolation. To overcome this challenge, we propose a one-dimensional optomechanical array to realize broadband optical nonreciprocity via nonreciprocal band structure. We exploit nonreciprocal band structure induced by the stimulated Brillouin scattering with directional optical pumping and show that it is possible to demonstrate optical nonreciprocity with both broad bandwidth and high isolation. Such optomechanical lattices with nonreciprocal band structure, offer an avenue to explore nonreciprocal collective effects of both photons and phonons in different frequency regimes.

Nonlinear quantum interferometry is widely used in photonic quantum sensing applications to perform measurements with undetected photons. While continuous wave (cw) lasers are typically used to generate correlated photon pairs, pulsed laser pumping can enhance the generation efficiency for photon pairs and enable time-resolved measurements. However, the finite spectral linewidth of pulsed lasers leads to imperfect frequency correlation between generated photon pairs, which can affect the sensitivity of nonlinear interferometry. In this work, we investigate the effect of the finite linewidth of a pump laser on the visibility of spectral domain interference in a nonlinear quantum interferometer where signal photons are detected using a dispersive spectrometer. The proposed theory quantitatively predicts the visibility as a function of optical path difference for a pump source with arbitrary spectrum. As a demonstration, we construct an experimental setup to obtain quantum interference using both cw and pulsed laser excitations under the same experimental conditions. At large optical path differences, a fine interference fringe pattern with high contrast is observed with cw laser excitation, whereas with pulsed laser pumping, the visibility decreases as the optical path delay increases, and the interference fringes appear almost washed out. Our quantitative analysis is useful for determining appropriate pulsed laser parameters for designing nonlinear interferometers for quantum sensing applications using a dispersive spectrometer.

One of the main advantages expected from using quantum probes as thermometers is noninvasiveness, i.e., a negligible perturbation to the thermal sample. However, invasiveness is rarely investigated explicitly. Here, focusing on a spin probe undergoing pure dephasing due to the interaction with a bosonic sample, we show that there is a nontrivial relation between the information on the temperature gained by a quantum probe and the heat absorbed by the sample due to the interaction. We show that time-optimal probing schemes obtained by considering the total experiment time as a resource also have the benefit of limiting the heat absorbed by the sample in each shot of the experiment. For such time-optimal protocols, we show that it is advantageous to have strong probe-sample coupling, since in this regime the accuracy increases linearly with the coupling strength, while the amount of heat per shot saturates to a finite value. Since in pure-dephasing models the absorbed heat corresponds to the external work needed to couple and decouple the probe and the sample, our results also represent a first step towards the analysis of the thermodynamic and energetic cost of quantum thermometry.

The Hamiltonian of an isolated quantum-mechanical system determines its dynamics and physical behavior. This study investigates the possibility of learning and utilizing a system's Hamiltonian and its variational thermal state estimation for data analysis techniques. For this purpose, we employ the method of quantum Hamiltonian-based models for the generative modeling of simulated Large Hadron Collider data and demonstrate the representability of such data as a mixed state. In a further step, we use the learned Hamiltonian for anomaly detection, showing that different sample types can form distinct dynamical behaviors once treated as a quantum many-body system. We exploit these characteristics to quantify the difference between sample types. Our findings show that the methodologies designed for field theory computations can be utilized in machine learning applications to employ theoretical approaches in data analysis techniques.

$B$-spline functions have been widely used in computational atomic physics. Recently, correlated $B$-spline basis functions (C-BSBFs), where the interelectronic coordinate $r_{12}$ is explicitly incorporated, have greatly improved the computational accuracy in determining polarizability for helium's singlet states [S. J. Yang, X. S. Mei, T. Y. Shi, and H. X. Qiao, Phys. Rev. A 95, 062505 (2017)] and Bethe logarithm [S. J. Yang, Y. B. Tang, Y. H. Zhao, T. Y. Shi, and H. X. Qiao, Phys. Rev. A 100, 042509 (2019)]. This represents a significant advancement over the traditional $B$-spline basis, which comprises a straightforward product of two $B$ splines. In this paper, we detail the extension of C-BSBFs towards calculating leading relativistic and quantum electrodynamics (QED) corrections for energy levels of the $1{}^{1}S, 2{}^{1}S, 2{}^{3}S$, and $3{}^{3}S$ states in helium. To accomplish this, the relativistic kinetic term $p_1^4$, contact potentials ${\delta }^3\left(r_1\right)$ and ${\delta }^3\left(r_{12}\right)$, and Araki-Sucher correction $\langle 1/r_{12}^3\rangle$ were calculated via the global operator method, wherein $r_{12}^n$ and $r_{12}^{n}ln{r}_{12}$ were derived from the generalized Laplace's expansions. Our computations yielded values for the ground state of $\delta E_{\mathrm{rel}}/{\alpha }^2=-1.95175476\left(6\right)$ and $\delta E_{\mathrm{QED}}/{\alpha }^3=57.2881648\left(5\right)$, aligning well with previous results. These results illustrate the potential of the C-BSBFs for computing higher-order relativistic and QED effects.

We demonstrate a single-molecule technique that is compatible with high-precision measurements and obtain the spectrum of three molecular ion species. While the current result yields a modest spectral resolution due to a broad light source, we expect the method to ultimately provide a resolution comparable to quantum logic methods with significantly less stringent requirements. Adaptations of this technique will prove useful in a wide range of precision spectroscopy arenas including the search for parity-violating effects in chiral molecules.