We propose that the Fröhlich condensate of phonons can be realized in optomechanical systems. The system consists of a one-dimensional array of membranes coupled to the cavity field via a quadratic interaction, and the cavity is pumped by an external laser. Analytical and numerical results demonstrate that the high phonon occupancy of the lowest or highest mechanical mode is achievable depending on the detuning of the driving laser, the optomechanical strength, and the temperature. The feasibility of experimental implementation is discussed. Our results shed light on energy conversion and transfer, heat control, and multimode cooling using optomechanical systems.

We apply the molecular convergent close-coupling method to the calculation of linear polarization fractions for Fulcher-band fluorescence following electron-impact excitation of the ${\mathrm{H}}_2$ $d{}^3{\mathrm{\Pi }}_u$ state. The results exhibit the opposite threshold behavior compared to the only previous calculations [Meneses, Brescansin, Lee, Michelin, Machado, and Csanak, Phys. Rev. A 52, 404 (1995)], but are in agreement with the most recent measurements for the $Q\left(1\right),$ $R\left(1\right),$ and $Q\left(3\right)$ transitions [Maseberg, Bartschat, and Gay, Phys. Rev. Lett. 111, 253201 (2013)].

We introduce the notion of a reverse quantum speed limit for arbitrary quantum evolution which answers a fundamental question: How slowly can a quantum system evolve in time? Using the geometrical approach to quantum mechanics, the reverse speed limit follows from the fact that the gauge-invariant length of the reference section is always greater than the Fubini-Study distance on the projective Hilbert space of the quantum system. We illustrate the reverse speed limit for two-level quantum systems with an external driving Hamiltonian and show that our results hold well. We find several examples where our bound is tight. We also find one practical application of the reverse speed limit in the discharging process of quantum batteries, which answers the following question: How slowly can quantum batteries discharge? Our result provides a lower bound on the average discharging power of quantum batteries.

We generalize the scheme for detection of qubit-environment entanglement to qudit-environment systems. This is of relevance for many-qubit systems and the quantification of the operation of quantum algorithms under the influence of external noise, since only decoherence that is not entangling in its nature can be effectively described by quantum channels and similar methods in more complicated scenarios. The generalization involves an increase in the class of entangled states which are not detected by the scheme, but the type of entanglement which cannot be detected is also least likely to qualitatively influence decoherence. We exemplify the operation of the scheme on a realistically modeled nitrogen-vacancy-center spin qutrit interacting with an environment of nuclear spins.

We provide and experimentally demonstrate an accreditation protocol that upper bounds the variation distance between noisy and noiseless probability distributions of the outputs of arbitrary quantum computations. We accredit the outputs of 24 quantum circuits executed on programmable superconducting hardware, ranging from depth-9 circuits on 10 qubits to depth-21 circuits on 4 qubits. Our protocol requires implementing the “target” quantum circuit along with a number of random Clifford circuits and subsequently postprocessing the outputs of these Clifford circuits. Importantly, the number of Clifford circuits is chosen to obtain the bound with the desired confidence and accuracy and is independent of the size and nature of the target circuit. We thus demonstrate a practical and scalable method of ascertaining the correctness of the outputs of arbitrary-sized noisy quantum computers—the ultimate arbiter of the utility of the computer itself.

It is well known that any quantum gate can be decomposed into the universal gate set {$T,H$, cnot}, and recent results have shown that each of these gates can be implemented using a dynamic quantum walk, which is a continuous-time quantum walk on a sequence of graphs. This procedure for converting a quantum gate into a dynamic quantum walk, however, can result in long sequences of graphs. To alleviate this, in this paper, we develop a length-3 dynamic quantum walk that implements any single-qubit gate. Furthermore, we extend this result to give length-3 dynamic quantum walks that implement any single-qubit gate controlled by any number of qubits. Using these, we implement Draper's quantum addition circuit, which is based on the quantum Fourier transform, using a dynamic quantum walk.

Internal states of polar molecules can be controlled by microwave-frequency electric dipole transitions. If the applied microwave electric field has a spatial gradient, these transitions also affect the motion of these dipolar particles. This capability can be used to engineer phonon-mediated quantum gates between, e.g., trapped polar molecular ion qubits without laser illumination and without the need for cooling near the motional ground state. The result is a high-speed quantum processing toolbox for dipoles in thermal motion that combines the precision microwave control of solid-state qubits with the long coherence times of trapped ion qubits.

Electron-impact single, double, and triple ionizations are investigated for the ${\mathrm{B}}^{+}$ ion. The direct and indirect ionization processes are studied for the single and double ionization. Good agreement with measurements for the single and double ionization is obtained when the scaled distorted-wave cross sections are used to study the collisional ionization and excitation processes. It is shown that ionization-excitation-autoionization and excitation-ionization-autoionization processes have to be introduced to the total double ionization to explain the measurements. The triple ionization is investigated by considering the direct double ionization with the subsequent autoionization.

A theoretical approach is developed for the Herman-Wallis factors describing the variation of the line intensities caused by rovibrational interaction in a linear molecule. In the case of ${\nu }_2$ and ${\nu }_3$ fundamentals and the ${\nu }_1+{\nu }_3/2{\nu }_2+{\nu }_3$ resonance combination band in a ${\mathrm{CO}}_2$ molecule, our theory is capable of generating reliable numerical estimates for the linear Herman-Wallis coefficients without recourse to complicated algebra. On the assumption of nearly free rotating monomers, an analog of the Herman-Wallis factors is introduced for the dipole-forbidden ${\nu }_2+{\nu }_3$ band, which manifests itself in ${\mathrm{CO}}_2$ collision-induced absorption (CIA). The rovibrational perturbation is appropriate to explain a pronounced crossflow of intensity among rovibrational branches in a virtually rotationally unresolved CIA envelope.

The scattering of ions in solids is accompanied with momentum transfer and electronic excitations resulting in the slowing down of the ion. The amount of energy transferred in a single scattering event depends on the particular trajectory which can be traced back through the scattering angle. Performing scattering angle dependent measurements of slow highly charged Xe ions transmitted through freestanding single-, bi-, and trilayer graphene allows us to determine the charge exchange and energy loss for different minimal interatomic distances. Interestingly, the charge exchange shows an increase with scattering angle by a factor of less than 2, while the energy loss increases by more than a factor of 10 for $3^{\circ }$ compared to forward direction. Our results can be compared to a time-dependent potential model and show that determination of the stopping cross section is not straightforward even with angle-dependent data at hand.

We design a neural network to extract and process features from absorption images taken of one-dimensional Bose gases in the quasicondensate regime. Specifically, the network is trained to predict both the temperature of single realizations of the system and the uncertainty thereof. For multiple realizations, the individual predictions can be combined in an estimate of the mean temperature, improving precision. We benchmark our model on both simulated and experimentally measured data and compare it to the established method of density ripple thermometry. We find the predictions of the two methods compatible, although the neural network reaches similar precision needing many fewer realizations, thus highlighting the efficiency gain achievable when incorporating neural networks into analysis of data from cold gas experiments. Further, we study feature maps to reveal which local features of the condensate are extracted by the network and how said features correlate with properties of the system. A similar analysis could be employed to uncover physical relations in more complex systems.

As an arising hotspot in nonlinear science, soliton molecules are found to have intriguing internal dynamics originating from the nonlinear interactions between closely spaced pulses. In dissipative systems with saturable gain and loss, the soliton molecule composed of unequal pulses is inferred to be unstable due to the soliton-energy-quantization effect. We experimentally demonstrated the stable soliton molecule in the form of two long-range unequal solitons trapped by the dispersive waves, i.e., the loosely bounded asymmetric soliton molecules, and numerically explored its intracavity evolution dynamics and stability mechanism. Under the nonlinear attraction of guiding-center-soliton, both solitons have average parameters satisfying the fundamental soliton, facilitating the stable evolution of asymmetric soliton molecules. Our work sheds light on the symmetry-broken of the localized structures in nonlinear-dissipative systems, which has the potential to enhance the data-carrying capacity of optical communications.

The condensation of Cooper pairs, originating from the Fermi-surface instability due to a weakly attractive interaction between two fermions, opened a new frontier for exploring many-body physics in interdisciplinary contexts. In this work, we discuss the possible condensation of Cooper triples, which are three-body counterparts of Cooper pairs for three-component fermions with a three-body attraction. Although each composite trimer-like state obeys the Fermi-Dirac statistics, its aggregate can form a condensate at zero center-of-mass momentum in the presence of the internal degrees of freedom associated with the relative momenta of constituent particles of momenta close to the Fermi surface. Such condensation can be regarded as bosonization in infinite-component fermions. We propose a variational wave function for the condensate of Cooper triples in analogy with the Bardeen-Cooper-Schrieffer ground state, and we obtain the ground-state energy.

Wave-particle duality is a bizarre feature at the heart of quantum mechanics which refers to the mutually exclusive dual attributes of quantum objects as the wave and the particle. Quantum eraser presents a counterintuitive aspect of the wave-particle duality. In this paper, we show that quantum eraser can be quantitatively understood in terms of the recently developed duality-entanglement relation. In other words, we show that wave-particle-entanglement triality captures all the physics of the quantum erasure. We find that a controllable partial erasure of the which-path information is attainable, enabling the partial recovery of the quantum interference and extending the scope of the conventional quantum eraser protocols.

We show that quantum absorption refrigerators, which have traditionally been studied as of three qubits, each of which is connected to a thermal reservoir, can also be constructed by using three qubits and two thermal baths, where two of the qubits, including the qubit to be locally cooled, are connected to a common bath. With a careful choice of the system, bath, and qubit-bath interaction parameters within the Born-Markov and rotating-wave approximations, one of the qubits attached to the common bath achieves a cooling in the steady state. We observe that the proposed refrigerator may also operate in a parameter regime where no or negligible steady-state cooling is achieved, but there is considerable transient cooling. The steady-state temperature can be lowered significantly by an increase in the strength of the few-body interaction terms existing due to the use of the common bath in the refrigerator setup. The proposed refrigerator built with three qubits and two baths is shown to provide steady-state cooling for both Markovian qubit-bath interactions between the qubits and canonical bosonic thermal reservoirs, and a simpler reset model for the qubit-bath interactions.

It is shown that variation of the one-particle dispersion in a universal many-body system enables us to obtain Tan's adiabatic sweep theorem and its generalization. The derivation is based on the Hellmann-Feynman theorem and the variational theorem for the scattering length suggested in our previous paper [Cherny and Shanenko, Phys. Rev. E 62, 1646 (2000)]. As an example, the universal effects in the system of spinless bosons are considered. With the help of the variational theorem, we obtain the mean kinetic and interaction energies and derive the virial theorem for the homogeneous and trapped bosons. The results can easily be generalized to the two-component fermions with interactions between opposite spins.

In this paper, we investigate the light-bullet dynamics during femtosecond laser filamenation in an anomalous dispersion regime based on the experimentally observed blueshifted resonant radiation (RR) in fused silica. A numerical simulation is performed and well reproduces the pronounced asymmetric feature of the RR spectra. By applying a time-frequency analysis to the propagating laser field, this specific spectral feature is found to be closely connected to the positively chirped property of RR in the time domain. In the context of the effective three-wave mixing model, this temporal chirp character of the RR is ascribed to the variable phase-matching conditions, which can be traced back to the accelerative propagation of the light bullet during its formation. Our work provides a deep understanding of the spatiotemporal dynamics of femtosecond laser-pulse propagation in condensed media.

Dissipative cavity solitons (CSs) in a Kerr resonator have wide applications from optical communications to spectroscopy due to their broad bandwidth. Although the broadband spectrum ultrashort pulses are easy to be generated in a high-$Q$ microresonator, such kinds of solitons in the macroscopic fiber ring resonator are difficult to be obtained. Dispersion management can be a feasible method to realize soliton compression, but the Kelly-like sidebands in the frequency domain are increased inside the cavity. To achieve a perfect soliton from a macroscopic cavity, an ingenious method of nonlinear Fourier transform (NFT) is utilized here to filter out the sidebands. The CS is mapped to different components in the nonlinear spectrum and a pure soliton can be reconstructed from the resonant continuous-wave background accordingly. Numerical simulations demonstrate NFT can be an effective method for CS analysis both in the time and frequency domains. Our investigations exemplify another application of NFT in a dissipative nonlinear system.