We present techniques for inertial-sensing atom interferometers which produce multiple phase measurements per experimental cycle. With these techniques, we realize two types of multiport measurements, namely, quadrature phase detection and real-time systematic phase cancellation, which address challenges in operating high-sensitivity cold-atom sensors in mobile and field applications. We confirm experimentally the increase in sensitivity due to quadrature phase detection in the presence of large phase uncertainty, and demonstrate suppression of systematic phases on a single-shot basis.

We have found the optimum range of driver wavelengths for mid-infrared ultraintense femtosecond pulses undergoing filamentation in atmospheric air. This wavelength range between 3.1 and $3.5\mu \mathrm{m}$ forms a nonlinear transparency window identified through a diligent scan of pulse central wavelengths in the range $2.2–4.7\mu \mathrm{m}$ with a best resolution of 5 nm. Each of 123 wavelengths scanned corresponds to the solution of the full $\text{three-dimensional}\hspace{0.17em}+\hspace{0.17em}\text{time}$ pulse propagation and filamentation problem on a 7–19 m path in air. Due to the discovered universal asymmetric character of the nonlinearly enhanced linear absorption in the vicinity of atmospheric molecular band, the optimum driver wavelength belongs to the long-wavelength side of the band.

Choosing the right first-quantization basis in quantum optics is critical for the interpretation of experimental results. The usual frequency basis is, for instance, inappropriate for short, subcycle waveforms. Deriving first quantization in the time domain shows that the electromagnetic field is not directly proportional, nor even causally related, to the photonic field (the amplitude probability of a photon detection). We derive the relation between the two and calculate the statistics of the electromagnetic field for specific states in the time domain, such as the single photon Fock state. We introduce the dual of the Hamiltonian in the time domain and extend the concept of quadratures to all first quantization bases.

We widen the range of applications for quantum computing by introducing digital quantum simulation methods for coherent light-matter interactions: We simulate an experiment where the emitted light from a single ion was interfering with its mirror image [Eschner et al., Nature (London) 413, 495 (2001)]. Using the quantum simulation software q1tsim, we accurately reproduce the interference pattern which had been observed experimentally and also show the effect of the mirror position on the spontaneous-emission rate of the ion. In order to minimize the number of required qubits, we implement a qubit-reinitialization technique. We show that a digital quantum simulation of complex experiments in atomic and quantum physics is feasible with no more than six qubits, a setting which is well within reach for advanced quantum computing platforms.

For indistinguishable itinerant particles subject to a superselection rule fixing their total number, a portion of the entanglement entropy under a spatial bipartition of the ground state is due to particle fluctuations between subsystems and thus is inaccessible as a resource for quantum information processing. We quantify the remaining operationally accessible entanglement in a model of interacting spinless fermions on a one-dimensional lattice via exact diagonalization and the density matrix renormalization group. We find that the accessible entanglement exactly vanishes at the first-order phase transition between a Tomonaga-Luttinger liquid and phase separated solid for attractive interactions and is maximal at the transition to the charge density wave for repulsive interactions. Throughout the phase diagram, we discuss the connection between the accessible entanglement entropy and the variance of the probability distribution describing intrasubregion particle-number fluctuations.

Superconducting nanowire single-photon detector (SNSPD) is a promising candidate for achieving high-rate quantum key distribution (QKD) over long distances, and it has been widely employed in recent QKD experiments. However, SNSPD is naturally polarization sensitive, which, if unchecked, could leave a back door for an eavesdropper to perform quantum hacking. Here, we experimentally study the polarization dependency on the detection efficiency of SNSPD and propose a quantum hacking attack to exploit this vulnerability. We experimentally characterize the polarization-dependent efficiencies of different SNSPDs and perform risk analysis on the security of a standard phase-encoding QKD implementation. Our experimental data and security analysis show that, if the effect of polarization-dependent efficiency is unnoticed, an eavesdropper can successfully hack the QKD implementation using the polarization-rotation attack. Countermeasures are also discussed. The result is important for the future design of secure implementations of QKD.

Adiabatic quantum computing (AQC) can be protected against thermal excitations via an encoding into error-detecting codes, supplemented with an energy penalty formed from a sum of commuting Hamiltonian terms. Earlier work showed that it is possible to suppress the initial thermally induced excitation out of the encoded ground state, in the case of local Markovian environments, by using an energy-penalty strength that grows only logarithmically in system size, at a fixed temperature. The question of whether this result applies beyond the initial time was left open. Here we answer this in the affirmative. We show that thermal excitations out of the encoded ground state can be suppressed at arbitrary times under the additional assumption that the total evolution time is polynomial in the system size. Thus, computational problems that can be solved efficiently by using AQC in a closed system setting can still be solved efficiently subject to coupling to a thermal environment. Our construction uses stabilizer subspace codes, which require at least 4-local interactions to achieve this result.

We propose a test on the role of relaxation properties of conduction electrons in the Casimir pressure between two parallel metal-coated plates kept at different temperatures. It is shown that for sufficiently thick metallic coatings the Casimir pressure and pressure gradient are determined by the mean of the equilibrium contributions calculated at temperatures of the two plates and by the term independent on separation. Numerical computations of the nonequilibrium pressures are performed for two parallel Au plates of finite thickness as a function of separation and temperature of one of the plates using the plasma and Drude models for extrapolation of the optical data of Au to low frequencies. The obtained results essentially depend on the extrapolation used. Modifications of the CANNEX setup, originally developed to measure the Casimir pressure and pressure gradient in thermal equilibrium, are suggested, which allow different temperatures of one of the plates. Computations of the nonequilibrium pressure and pressure gradient are performed for a realistic experimental configuration. According to our results, even with only a 10 K difference in temperature between the plates, the experiment could discriminate between different theoretical predictions for the total pressure and its gradient, as well as for the contributions to them due to nonequilibrium, at high confidence.

Laser-induced electron tunneling is one of the most fundamental strong-field phenomena. In laser-induced electron tunneling, it is usually assumed that the highest tunneling rate appears at the peak of the laser electric field corresponding to the shortest tunneling barrier. Here we show that this fundamental assumption is broken in an orthogonally polarized two-color laser field. We find that a rapid change of the instantaneous initial momentum at the tunnel exit can lead to a distinct ionization time shift between the instant of the maximum ionization rate and the peak of the electric-field strength. This ionization time shift can be either positive or negative depending on the degree of nonadiabaticity of the electron-tunneling process, and it is mapped to a measurable momentum shift in the photoelectron momentum distribution. These findings have significant implications for resolving attosecond electron dynamics and are essential to quantify possible tunneling delay time during strong-field ionization.

Pulsed driving of Kerr microresonators represents a promising avenue for the efficient generation of soliton states associated with coherent optical frequency combs. The underlying physics has not, however, yet been comprehensively investigated. Here, we report on a numerical and theoretical study of the impact of desynchronization between the periodic pump field and the train of solitons circulating in the cavity. We show that desynchronization can affect the soliton configurations that can be sustained for given parameters, and that it can be leveraged to guarantee operation in the attractive single-soliton regime. We also reveal that the interplay between pump-resonator desynchronization and stimulated Raman scattering can give rise to rich dynamics that explain salient features observed in recent experiments. Our work elucidates the dynamics of Kerr cavity solitons in the presence of pulsed driving fields, and could facilitate the development of efficient microresonator frequency comb systems.

Reactive power plays a crucial role in the design of small antenna systems, but its impact on the bandwidth of quantum emitters is typically disregarded. Here, we theoretically demonstrate that there is an intermediate domain between the usual weak- and strong-coupling regimes where the bandwidth of a quantum emitter is directly related to the dispersion properties of the reactive power. This result emphasizes that reactive power must be understood as an additional degree of freedom in engineering the bandwidth of quantum emitters. We illustrate the applicability of this concept by revisiting typical configurations of quantum emitters coupled to resonant cavities and waveguides. Analysis of the reactive power in these systems unveils functionalities including the design of efficient but narrow-band photon sources, as well as quantum emitters exhibiting bandwidths narrower than their nonradiative linewidths even under incoherent pumping.

In inertial confinement (ICF) experiments at the NIKE laser facility, the high-power krypton fluoride (KrF) laser output beams propagate through long ($\sim 75\mathrm{m}$) air paths to achieve angular multiplexing, which is required because the KrF medium does not store energy for a sufficiently long time. Recent experiments and simulations have shown that, via stimulated rotational Raman scattering, this propagation can spectrally broaden the laser beam well beyond the $\sim 1$ THz laser linewidth normally achieved by the induced spatial incoherence (ISI) technique used in NIKE. These enhanced bandwidths may be enough to suppress the laser-plasma instabilities which limit the maximum intensity that can be incident on the ICF target. In this paper we investigate an alternative technique that achieves spectral broadening by self-phase modulation in Xe gas, which has a large, negative nonlinear refractive index $\sim 248$ nm, and thus completely avoids transverse filamentation issues. The collective, nonlinear atomic response to the chaotic, nonsteady state ISI light is modeled using a two-photon vector model, and the effect of near-resonant behavior on the spectral broadening is studied.

We investigate the role of coherence and Markovianity in finding an answer to the question whether the outcomes of a projectively measured quantum stochastic process are compatible with a classical stochastic process. For this purpose we put forward an operationally motivated definition of “incoherent dynamics” applicable to any open system's dynamics. For nondegenerate observables described by rank-1 projective measurements we show that classicality always implies incoherent dynamics, whereas the converse is true only for invertible Markovian (but not necessarily time-homogeneous) dynamics. For degenerate observables the picture is somewhat reversed as classicality does no longer suffice to imply incoherent dynamics (even in the invertible Markovian case), while an incoherent, invertible Markovian dynamics still implies classicality.

We propose a scalable version of a Knill-Laflamme-Milburn (KLM) controlled-not (cnot) gate based upon integrated waveguide microring resonators (MRR), vs the original KLM approach using beam splitters. The core element of our KLM cnot gate is a nonlinear phase-shift gate (NLPSG) using three MRRs, which we examine in detail. We find an expanded parameter space for the NLPSG over that of the conventional version. Whereas in all prior proposals for bulk optical realizations of the NLPSG the optimal operating point is precisely a single zero-dimensional manifold within the parameter space of the device, we find conditions for effective transmission amplitudes which define a set of one-dimensional manifolds in the parameter spaces of the MRRs. This allows for an increased level of flexibility in operation of the NLPSG that allows for the fabrication of dynamically tunable MRR-based devices with high precision and low loss.

The ground state hyperfine splitting of ${}^{85}\mathrm{Rb}$ was measured precisely using a rubidium-85 atomic fountain clock. About $5\times {10}^5$ atoms were detected at the temperature of $4\mu \mathrm{K}$, and a Ramsey fringe was obtained with FWHM of 0.95 Hz. We present a measurement of the ${}^{85}\mathrm{Rb}$ ground state hyperfine splitting with a $5\times {10}^{-14}$ uncertainty. The measured ${}^{85}\mathrm{Rb}$ ground-state hyperfine splitting was $\upsilon =3.0357324448218\left(2\right)\text{GHz}$. This measurement is more precise than previously reported values by 4 orders of magnitude, and differs from the previously reported value by 5.8218 Hz.

We measured the strong-field ionization of diatomic iodine molecules as a function of wavelength and internuclear separation. The latter was done by launching a wave packet on the B state of iodine and probing the ionization as a function of time delay. As a function of wavelength at fixed internuclear separation and as a function time delay at fixed wavelength, critical points are found where the ionization increases significantly. All of the results are consistent with ionization from deep $\sigma$ orbitals and resonant interactions. In conclusion, we present a unified view of the strong-field ionization of diatomic molecules.

For a quantitative trajectory-based description of strong-field photoelectron holography beyond the electric dipole approximation, we develop a semiclassical model in which the initial conditions of outgoing electrons are set according to the beyond-dipole strong-field approximation for the tunnel-ionization step. Phases are evaluated following the prescription for semiclassical propagators. Comparison to the numerical solution of the time-dependent Schrödinger equation in two spatial dimensions shows that the semiclassical model reproduces correctly the nondipole shifts of the photoelectron momenta along the laser propagation axis. The position of the central holographic interference fringe can be estimated already from a simplified Coulomb-free interference model providing closed-form expressions for the beyond-dipole shifts. To address Coulomb focusing in three dimensions, we implement a beyond-dipole regularization procedure based on the concept of glory scattering. While the position of the central maximum and higher-order fringes in three dimensions can already be obtained approximately by simpler semiclassical modeling, the glory model is able to describe the shape of the distribution at the central maximum. Our results show that nondipole dynamics in holography should be observable with midinfrared fields, for which the forward and backward shifts can be comparable with the fringe spacing.

We show how strongly correlated ultracold bosonic atoms loaded in specific orbital angular momentum states of arrays of cylindrically symmetric potentials can realize a variety of spin-1/2 models of quantum magnetism. We consider explicitly the dependence of the effective couplings on the geometry of the system and demonstrate that several models of interest related to a general $XYZ$ Heisenberg model with external field can be obtained. Furthermore, we discuss how the relative strength of the effective couplings can be tuned and which phases can be explored by doing so in realistic setups. Finally, we address questions concerning the experimental readout and implementation and we argue that the stability of the system can be enhanced by using ring-shaped trapping potentials.

We investigate topological phase transitions driven by interaction and identify a topological Mott insulator state in one-dimensional fermionic optical superlattices through a numerical density-matrix renormalization-group method. Remarkably, the low-energy edge excitations change from spin-1/2 fermionic single-particle modes to spin-1 bosonic collective modes across the phase transition. Due to spin-charge separation, the low-energy theory is governed by an effective spin superexchange model, whereas the charge degree of freedom is fully gapped out. Such topological Mott state can be characterized by a spin Chern number and gapless magnon modes protected by a finite spin gap. The proposed experimental setup is simple and may pave the way for the experimental observation of exotic topological Mott states.

We report that localized surface plasmon resonance allows a single-element nanostructure to induce an extrinsic angular momentum of light in its interaction with a propagating plane wave. The recoil of the angular momentum results in an optical torque on the structure along an axis perpendicular to the optical axis, and the characteristics of this transverse torque depend on the incident polarization state, including the spin direction. Our results suggest that the designed dark plasmon mode can provide a new degree of freedom for optical manipulation of nanoparticles smaller than the diffraction limit.