We explore long-lived electronic coherences in molecules using shaped ultrafast laser pulses to launch and probe entangled nuclear-electronic wave packets. We find that for particular pairs of states, the electronic phase remains well defined despite vibrational motion along many degrees of freedom. We explore the time dependence of the electronic coherence and show how it is modulated by the time-dependent vibrational overlap between the two wave functions. Our measurements are interpreted with electronic structure calculations and compared to analytic expressions for the electronic coherence.

We investigate the ground state and dynamics of a mixture of spin-1 and spin-2 Bose-Einstein condensates of ${}^{87}\mathrm{Rb}$ atoms. For the experimentally measured interaction coefficients, the ground state exhibits phase separation between the spin-1 ferromagnetic state and the spin-2 nematic state. At the interface between them, a partially polarized spin state emerges. The uniformly mixed state of the spin-1 polar state and spin-2 biaxial nematic state is metastable, and the phase separation via nucleation can be triggered by a local disturbance.

We derive a time-dependent master equation for an externally driven system, where the subsystems weakly interact with each other and are locally connected to the thermal reservoirs. The nonadiabatic equation obtained here can be viewed as a generalization of the local master equation, which has already been extensively used in describing the dynamics of a boundary-driven system. In addition, we investigate the fundamental reason underlying the thermodynamic inconsistency generated by the local and nonadiabatic master equations. We also provide a criterion to assess the consistency of these two equations with the second law of thermodynamics. Finally, we numerically confirm our results by considering a toy model consisting of two qubits and two local heat baths.

We demonstrate how to universally simulate ensemble statistics of projective local measurements on any $n$-qubit state shared among $n$ observers with classical communication and shared randomness. Our technique originates from protocols designed to simulate quantum nonlocality [in Horizons of the Mind (Springer, Cham, 2014)] and classical simulation of quantum circuits [Phys. Rev. Lett. 115, 070501 (2015)]. The protocol preserves three crucial aspects of the simulated quantum scenario in contrast to other approaches: no involvement of additional parties, none of the observers knows the global state of the system, and local measurement settings remain undisclosed. The simulation cost grows exponentially with the number of observers.

In muonic bound systems, the dominant radiative correction is due to vacuum polarization. Yet, for the interpretation of precision experiments, self-energy effects are also important. In turn, additional vacuum-polarization loops perturb the self-energy. Here we show that combined self-energy vacuum-polarization effects can perturb the bound-state self-energy at the percent level in one-muon bound systems with nuclear charges $Z=1-6$. We also update previous treatments of the corrections for muonic hydrogen, muonic deuterium, and muonic helium bound systems.

We have measured multiple differential cross sections for dissociative capture in collisions of 75-keV protons with ${\mathrm{D}}_2$. Data were analyzed for kinetic energy releases (KERs) ranging from 0 to 2 eV, where the dissociation is primarily caused by excitation of the nuclear motion to a vibrational continuum state. Pronounced signatures of two-center interference effects were observed. The data were compared to previously published cross sections for dissociative capture in $p+{\mathrm{H}}_2$ collisions, for which we reported a scattering angle dependent phase shift in the interference term. The ${\mathrm{D}}_2$ data reveal qualitative differences in the interference structure to the ${\mathrm{H}}_2$ case. Some aspects of the present results appear to support a hypothetical explanation which we offered earlier for the ${\mathrm{H}}_2$ data, but other aspects are not fully understood yet.

We discuss dressing trapped ions with the near field of a trap integrated wire. Ramping a dressing field on or off adiabatically before or after an operation changes its effective Hamiltonian. The amplitude and detuning of the dressing field act as tunable degrees of freedom we can use to customize the properties of any operation. We propose three use cases for this general tool. First, we can generate “artificial” clock states, where we eliminate the (assumed to be small) linear sensitivity of a qubit. Second, we can break the degeneracies that often complicate shelving at low quantization fields, allowing us to implement operations with linearly polarized microwaves that would otherwise require circular polarization. Finally, we can implement laser-free single-qubit gates on a set of “target” ions using fields that are separated from the rest of the computer in frequency space.

In the framework of strong-field quantum electrodynamics, the angular and spectral distribution of the polarized photons scattered from electrons interacting with few-cycle laser pulse are investigated using density-matrix theory. Effects are taken into consideration about various factors, including the ellipticity of ultrashort laser pulse, carrier envelope phase (CEP), electron wave packet, and incident electron energy. In the case of a linearly polarized laser pulse interacting with electrons of not-so-high energy, the angularly resolved spectra of the scattered photons within the polarization plane are entirely contributed by the $E$-polarized photons, which is destroyed when GeV electrons are used. Contributions from the $B$-polarized photons are weaker and located at odd harmonics in the magnetic-field plane. For a circularly polarized interacting pulse, there is a certain polar angle range within which the angularly resolved spectra near the backward-scattering direction is entirely contributed by photons having helicity opposite to that of the laser pulse. Out of this range, the angular spectrum is relatively weaker and mainly comes from polarized photons having the same helicity to the laser pulse. The broadening of the electron momentum in the transverse direction causes a extension of the radiation angle, while in the longitudinal direction, electron's momentum broadening blurs the interference structure. For an elliptically polarized pulse with CEP $\ne 0$, the maximum of frequency-integrated angular distribution in the transverse plane is no longer along the direction of the peak of the vector potential.

We introduce the generalized fractional Hermite-Gaussian (GFHG) beams, a type of structured light based on the fractionalization of Hermite functions. By incorporating a noninteger free parameter, these beams constitute a bridge between Hermite-Gaussian beams of integer order but exhibit an asymmetric light field distribution in their transverse profile. We show that the fractionalization can be seen as the result of a convolution between a Gaussian beam and a power law function. Remarkably, and in contrast to traditional Hermite-Gaussian beams of integer order, we find that to ensure light localization, there is a critical threshold value between the fractional Hermite functions and the corresponding Gaussian apodization. We demonstrate their numerical and theoretical propagation in free space and gradient-index (GRIN) media. Finally, we highlight a possible connection between these beams and nonlocal nonlinear media.

We consider a three-stroke engine in the microscopic regime where the working body of the engine is composed of a two-level system. The working body of the engine aims to withdraw heat from the hot heat bath, generate work, and discharge the surplus heat into the cold heat bath through the successive execution of three strokes. In this process, the interaction of the working body with the heat baths is assumed to be energy-conserving and thus can be described by thermal operations. While earlier studies analyzed the optimal performance of this engine when the working body could be transformed by any arbitrary thermal operation, we present closed expressions for the maximum work produced by the engine and the maximum efficiency of the engine when only a restricted class of thermal operations can be implemented on the working body. Furthermore, we explore the engine's optimal performance under two well-studied classes of restrictions: thermal operations realized via Jaynes-Cummings interaction and thermal operations realizable with finite-sized heat baths. Therefore, on one hand, our results are general, as they reproduce the optimal performance achieved when any arbitrary thermal operation can be implemented on the working body once the restriction is relaxed. On the other hand, our results allow us to determine the engine's maximum work production and efficiency in a more realistic scenario, where only a restricted class of thermal operations are possible, thereby bringing our findings closer to experimental feasibility.

Many-body quantum systems can be characterized using the notions of $k$-separability and entanglement depth. A quantum state is $k$-separable if it can be expressed as a mixture of $k$ entangled subsystems, and its entanglement depth is given by the size of the largest entangled subsystem. In this paper we propose a multipartite entanglement measure that satisfies the following criteria: (i) It can be used with both pure and mixed states; (ii) it is encoded in a single element of the density matrix, so it does not require knowledge of the full spectrum of the density matrix; (iii) it can be applied to large systems; and (iv) it can be experimentally verified. The proposed method allows the certification of non-$k$-separability of a given quantum state. We show that the proposed method successfully classifies three-qubit systems into known stochastic local operations and classical communication (SLOCC) classes, namely, bipartite, $W$-, and Greenberger-Horne-Zeilinger-type entanglement. Furthermore, we characterize the non-$k$-separability in nine known SLOCC classes of four-qubit states and absolutely maximally entangled states for five and six qubits and for arbitrary-size-qubit Dicke states.

Quantum resources have been proven to drive the measurement sensitivity to beyond standard quantum limit in quantum metrology. However, quantum advantage degrades with optical losses. Aiming at angular rotation estimation, we theoretically investigate three loss-tolerant (external loss) quantum-enhanced interferometers—the degenerate SU(1,1) interferometer (DSI), nondegenerate hybrid interferometer (NHI), and degenerate hybrid interferometer (DHI)—to achieve supersensitive angular rotation estimation, without strict requirement of external losses. Analysis results show that these interferometers are perfectly applicable to angular rotation estimation. Moreover, the sensitivity, which is the same as in the case of no external losses, can always be retrieved by taking full advantage of the second optical parametric amplifier as a phase-sensitive amplification. The sensitivity bound that originates from laser power restriction in the DSI scheme can be overcome by the employment of the NHI or DHI scheme. These results are critical in real time variation of the angle measurement and tracking an object in circular motion.

To implement a quantum error-correction protocol, we first need a scheme to prepare our state in the correct subspace of the code, and this can be done using a unitary encoding circuit. Majorana codes are special since any gates that transform such codes must preserve fermionic parity. In this paper, we present an algorithm that uses the stabilizer matrix to compute unitary encoding circuits for Majorana codes. We present two approaches, both of which use a version of Gaussian elimination with row operations replaced with elementary fermionic Clifford operations. One approach uses an additional ancilla mode and works for all Majorana stabilizer codes, while the second approach does not use an ancilla but does not work if the total parity is inside the stabilizer group.

The need for fast and robust quantum state transfer is an essential element in scalable quantum information processing, leading to widespread interest in shortcuts to adiabaticity for speeding up adiabatic quantum protocols. However, shortcuts to adiabaticity for systems with more than a few levels is occasionally challenging to compute in theory and frequently difficult to implement in experiments. In this paper, we develop a protocol for constructing shortcuts to adiabaticity through the multistate Landau-Zener approach and a stricter adiabatic condition. Importantly, our protocol only requires a few pieces of information about the energy spectrum and just adjusts the evolutionary rate of the system. It means that our protocol has broad applicability to theoretical models and does not require increasing the difficulty of the experiment. As examples, we apply our protocol to state transfer in the two-level Landau-Zener model, the non-Hermitian Su-Schrieffer-Heeger model, and the topological Thouless pump model and find that it can speed up the manipulation speed while remaining robust to Hamiltonian errors. Furthermore, based on the experimental friendliness of our findings, it can potentially be extended to many-body systems, dissipation cases, or Floquet processes. Overall, the proposed shortcut protocol offers a promising avenue for enhancing the efficiency and reliability of quantum state transfer protocols.

We use continuous, stochastic quantum trajectories within a framework of quantum state diffusion (QSD) to describe alternating measurements of two noncommuting observables. Projective measurement of an observable completely destroys memory of the outcome of a previous measurement of the conjugate observable. In contrast, measurement under QSD is not projective and it is possible to vary the rate at which information about previous measurement outcomes is lost by changing the strength of measurement. We apply our methods to a spin 1/2 system and a spin 1 system undergoing alternating measurements of the $S_z$ and $S_x$ spin observables. Performing strong $S_z$ measurements and weak $S_x$ measurements on the spin 1 system, we demonstrate return to the same eigenstate of $S_z$ to a degree beyond that expected from projective measurements and the Born rule. Such a memory effect appears to be greater for return to the $\pm 1$ eigenstates than the 0 eigenstate. Furthermore, the spin 1 system follows a measurement cascade process where an initial superposition of the three eigenstates of the observable evolves into a superposition of just two, before finally collapsing into a single eigenstate, giving rise to a distinctive pattern of evolution of the spin components.

In this paper, we introduce an information-theoretic task that we refer to as correlated sources coding where decoders possess complementary quantum side information. We present the optimal rates for lossless correlated sources coding with both complementary and less noisy quantum side information. We also generalize quantum Sgarro's coding and correlated sources coding with complementary quantum side information to scenarios with distortion.

There is an ongoing effort to find quantum speedups for learning problems. Recently [Y. Liu et al., Nat. Phys. 17, 1013 (2021)] proved an exponential speedup for quantum support vector machines by leveraging the speedup of Shor's algorithm. We expand upon this result and identify a speedup utilizing Grover's algorithm in the kernel of a support vector machine. To show the practicality of the kernel structure we apply it to a problem related to pattern matching, providing a practical yet provable advantage. Moreover, we show that combining quantum computation in a preprocessing step with classical methods for classification further improves classifier performance.