The state comparison amplifier is a recently proposed probabilistic quantum amplifier, intended especially for amplifying coherent states. Its realization is simple and uses only linear optics and photodetectors, and the preparation of a “guess” state, typically a coherent state. Fidelity and success probability can be high compared with other probabilistic amplifiers. State comparison amplification does, however, extract information about the amplified state, which means that it is especially important to benchmark it against a simple measure-and-resend procedure. We compare state comparison quantum amplifiers to measure-and-resend strategies, and identify parameter regimes and scenarios where these can and where they cannot provide an advantage.

The analytic Sommerfeld-Maue theory, including the next-to-leading-order correction, and the relativistic Dirac partial-wave theory are probed by a systematic comparison of the bremsstrahlung spectra with available experimental data. These measurements cover a wide range of nuclear charge numbers ($13\le Z\le 79$), collision energies (1–10 MeV), and photon angles ($4^{\circ }\le {\theta }_k\le {60}^{\circ }$). It is found that the analytic model performs well for $Z\lesssim 13$ at all angles down to very low photon frequencies. For high charge numbers, at high photon energies (i.e., in the short-wavelength limit) or for the larger angles at all photon energies, the partial-wave theory has to be used. For intermediate $Z\lesssim 50$, the maximum angle up to which the analytic formulae can be used decreases with increasing $Z$.

We study effects of a potential barrier on collective modes of superfluid Bose gases in optical lattices. We assume that the barrier is created by local suppression of the hopping amplitude. When the system is in a close vicinity of the Mott transition at commensurate fillings, where an approximate particle-hole symmetry emerges, there exist bound states of Higgs amplitude mode that are localized around the barrier. By applying the Gutzwiller mean-field approximation to the Bose-Hubbard model, we analyze properties of normal modes of the system with a special focus on the Higgs bound states. We show that when the system moves away from the Mott transition point, the Higgs bound states turn into quasibound states due to inevitable breaking of the particle-hole symmetry. We use a stabilization method to compute the resonance energy and linewidth of the quasibound states. We compare the results obtained by the Gutzwiller approach with those by the Ginzburg-Landau theory. We find that the Higgs bound states survive even in a parameter region far from the Mott transition, where the Ginzburg-Landau theory fails.

We make use of numerical exact diagonalization calculations to explore the physics of $\nu =1/2$ bosonic fractional quantum Hall droplets in the presence of experimentally realistic cylindrically symmetric hard-wall potentials. This kind of confinement is found to produce very different many-body spectra compared to a harmonic trap or the so-called extremely steep limit. For a relatively weak confinement, the degeneracies are lifted and the low-lying excited states organize themselves in energy branches that can be explained in terms of their Jack polynomial representation. For a strong confinement, a strong spatial deformation of the droplet is found, with an unexpected depletion of its central density.

With rotating-wave approximation, we show in this paper that exciton transmission in a one-dimensional two-level molecule chain embedded in a cavity can be enhanced or suppressed by strong cavity-chain couplings. This exciton transmission is closely related to the number of molecules and the distribution of molecular exciton energy. In addition, we propose a proposal to enhance the exciton transmission by quantum-jump-based feedback. These results may find applications in experiments of exciton transmission in organic materials.

We show that the Aharonov-Bohm-like suppression of optical tunneling in twisted multicore fibers can persist even under highly nonlinear conditions. Our analysis indicates that the topological phase is robust and remains intact in the presence of nonlinearity. The energy exchange dynamics are analyzed theoretically via closed-form solutions in four-core ring systems. Effects arising from asymmetry are also investigated. A possible arrangement to experimentally observe this effect is suggested.

We explore theoretically and numerically extreme event excitation in stimulated Raman scattering in gases. We consider gas-filled hollow-core photonic crystal fibers as a particular system realization. We show that moderate amplitude pump fluctuations obeying Gaussian statistics lead to the emergence of heavy-tailed non-Gaussian statistics as coherent seed Stokes pulses are amplified on propagation along the fiber. We reveal the crucial role that coherent memory effects play in causing non-Gaussian statistics of the system. We discover that extreme events can occur even at the initial stage of stimulated Raman scattering when one can neglect energy depletion of an intense, strongly fluctuating Gaussian pump source. Our analytical results in the undepleted pump approximation explicitly illustrate power-law probability density generation as the input pump noise is transferred to the output Stokes pulses.

We study the rovibrational polarization response of water vapor using a fully correlated optical Bloch equation approach employing data from the HITRAN database. For a $10\text{−}\mu \mathrm{m}$ long-wave infrared pulse the resulting linear response is negative, with a negative nonlinear response at intermediate intensities and a positive value at higher intensities. For a model atmosphere comprised of the electronic response of argon combined with the rovibrational response of water vapor this leads to a weakened positive nonlinear response at intermediate intensities. Propagation simulations using a simplified noncorrelated approach show the resultant reduction in the peak filament intensity sustained during filamentation due to the presence of the water vapor.

The use of multidimensional entanglement opens new perspectives for quantum information processing. However, an important challenge in practice is to certify and characterize multidimensional entanglement from measurement data that are typically limited. Here, we report the certification and quantification of two-photon multidimensional energy-time entanglement between many temporal modes, after one photon has been stored in a crystal. We develop a method for entanglement quantification which makes use of only sparse data obtained with limited resources. This allows us to efficiently certify an entanglement of formation of 1.18 ebits after performing quantum storage. The theoretical methods we develop can be readily extended to a wide range of experimental platforms, while our experimental results demonstrate the suitability of energy-time multidimensional entanglement for a quantum repeater architecture.

We observe the breakup dynamics of an elongated cloud of condensed ${}^{85}\mathrm{Rb}$ atoms placed in an optical waveguide. The number of localized spatial components observed in the breakup is compared with the number of solitons predicted by a plane-wave stability analysis of the nonpolynomial nonlinear Schrödinger equation, an effective one-dimensional approximation of the Gross-Pitaevskii equation for cigar-shaped condensates. It is shown that the numbers predicted from the fastest growing sidebands are consistent with the experimental data, suggesting that modulational instability is the key underlying physical mechanism driving the breakup.

We design a heat engine with multiheat reservoir, an ancillary system, and quantum memory. We then derive an inequality related to the second law of thermodynamics and give a limitation on the work gain from the engine by analyzing the entropy change and quantum mutual information change during the process. In addition, remarkably, by the combination of two independent engines and with the help of the entropic uncertainty relation with quantum memory, we find that the total maximum work gained from those two heat engines should be larger than a quantity related to the quantum entanglement between the ancillary state and the quantum memory. This result provides a lower bound for the maximum work extracted, in contrast to the upper bound in the conventional second law of thermodynamics. However, the validity of this inequality depends on whether the maximum work can achieve the upper bound.

Superconducting microwave circuits form a versatile platform for storing and manipulating quantum information. A major challenge to further scalability is to find approaches for connecting these systems over long distances and at high rates. One approach is to convert the quantum state of a microwave circuit to optical photons that can be transmitted over kilometers at room temperature with little loss. Many proposals for electro-optic conversion between microwave and optics use optical driving of a weak three-wave mixing nonlinearity to convert the frequency of an excitation. Residual absorption of this optical pump leads to heating, which is problematic at cryogenic temperatures. Here we propose an alternative approach where a nonlinear superconducting circuit is driven to interconvert between microwave-frequency ($7\times {10}^9$ Hz) and millimeter-wave-frequency photons ($3\times {10}^{11}$ Hz). To understand the potential for quantum state conversion between microwave and millimeter-wave photons, we consider the driven four-wave mixing quantum dynamics of nonlinear circuits. In contrast to the linear dynamics of the driven three-wave mixing converters, the proposed four-wave mixing converter has nonlinear decoherence channels that lead to a more complex parameter space of couplings and pump powers that we map out. We consider physical realizations of such converter circuits by deriving theoretically the upper bound on the maximum obtainable nonlinear coupling between any two modes in a lossless circuit, and synthesizing an optimal circuit based on realistic materials that saturates this bound. Our proposed circuit dissipates less than ${10}^{-9}$ times the energy of current electro-optic converters per qubit. Finally, we outline the quantum link budget for optical, microwave, and millimeter-wave connections, showing that our approach is viable for realizing interconnected quantum processors for intracity or quantum data center environments.

In the present paper we analyze in detail several properties of the ${\mathrm{MgH}}^{-}$ anion and the MgH neutral molecule using accurate ab initio quantum computational methods in order to establish with a higher reliability specific molecular features like the gas-phase electron affinity, the Franck-Condon factors for excitation of the neutral and of its anion to their lower electronic states, and the general feasibility of employing the anion in photodetachment experiments after its confinement in cold ion traps. The calculations suggest that the electron affinity value is in agreement with an existing early experiment and, further, places on it an error bar smaller than that given before. Accurate zero-point-energy corrections are also included in our calculations and their effects discussed.

We consider weakly interacting bosonic gases with local and nonlocal multibody interactions. By using the Bogoliubov approximation, we first investigate contact interactions, studying the case in which the interparticle potential can be written as a sum of $N$-body $\delta$ interactions and then considering general contact potentials. Results for the quasiparticle spectrum and the stability are presented. We then examine nonlocal interactions, focusing on two different cases of three-body nonlocal interactions. Our results are used for systems with two- and three-body $\delta$ interactions and applied for realistic values of the trap parameters. Finally, the effect of conservative three-body terms in dipolar systems and soft-core potentials (that can be simulated with Rydberg dressed atoms) is also studied.

Ultrasensitive detections have been proposed as an application of optomechanical systems. Here we develop an approach to mass sensing by comparing the detected quadratures of light field coupled to a mechanical resonator, whose slight change of the mass should be precisely measured. The change in the mass of the mechanical resonator will cause the detectable difference in the evolved quadrature of the light field, to which the mechanical oscillator is coupled. It is shown that the ultrasmall change $\mathrm{\Delta }m$ from a mass $m$ can be detected up to the ratio $\mathrm{\Delta }m/m\sim {10}^{-8}-{10}^{-7}$ by choosing the feasible system parameters.

A point source model based on wave dynamics is proposed to study the fundamental light diffraction physics by a subwavelength slit and grooves in a metallic film. In this model, two opposite traveling waves are considered in each indentation; the resultant outgoing wave can propagate along the film surface to couple each other or radiate into free space as a point source. With small-system simulations, the tangential electric field at each opening determines its source temporal phase; then the energy conservation of each point source radiation and of the total radiant wave determine the source amplitudes. Besides these, this model reveals more physics regarding the wave interactions. In the strong-wave-coupling case studied, the surface waves created by the grooves flow into the slit and delay the Fabry-Pérot-like resonance. When adding the grooves concentrates the light field into a directional beam, the total transmitted energy through the slit significantly decreases. However, the energy in the original nearby grooves increases so that the groove radiation increasingly shares the transmitted energy. As the total transmitted energy decreases, the slit radiation energy decreases further due to the energy conservation. In the weak-wave-coupling cases, the groove radiation still interferes with that from the slit; as a result, the diffracted light is split into two beams. It is interesting to find that, due to the groove radiation, the slit radiation energy is enhanced to become larger than that transmitted through it. Detailed physical interpretations will be given.

We theoretically predict a giant quantized Goos-Hänchen (GH) effect on the surface of graphene in the quantum Hall regime. The giant quantized GH effect manifests itself as an angular shift whose quantized step reaches the order of mrad for light beams impinging on a graphene-on-substrate system. The quantized GH effect can be attributed to quantized Hall conductivity, which corresponds to the discrete Landau levels in the quantum Hall regime. We find that the quantized step can be greatly enhanced for incident angles near the Brewster angle. Moreover, the Brewster angle is sensitive to the Hall conductivity, and therefore the quantized GH effect can be modulated by the Fermi energy and the external magnetic field. The giant quantized GH effect offers a convenient way to determine the quantized Hall conductivity and the discrete Landau levels by a direct optical measurement.

The one-clean-qubit model (or the DQC1 model) is a restricted model of quantum computing where only a single input qubit is pure and all other input qubits are maximally mixed. In spite of the severe restriction, the model can solve several problems (such as calculating Jones polynomials) whose classical efficient solutions are not known. Furthermore, it was shown that if the output probability distribution of the one-clean-qubit model can be classically efficiently sampled with a constant multiplicative error, then the polynomial hierarchy collapses to the second level. Is it possible to improve the multiplicative error hardness result to a constant total variation distance error one like other subuniversal quantum computing models such as the IQP (Instantaneous Quantum Polynomial time) model, the boson sampling model, and the Fourier sampling model? In this paper we show that it is indeed possible if we accept a modified version of the average case hardness conjecture. Interestingly, the anticoncentration lemma can be easily shown by using the special property of the one-clean-qubit model that each output probability is so small that no concentration occurs.