We propose a simple but versatile protocol to engineer time-dependent Hamiltonians inversely for geometric quantum computation. By utilizing SU(2) transformation, a speedup goal on gate operation is achieved with more freedom to design the control parameters. As an application, this protocol enables the conventional and unconventional nonadiabatic geometric quantum gates with desired evolution paths by controlling the microwave pulses in the diamond nitrogen-vacancy center system. We show that the inversely designed Hamiltonian can fulfill the geometric gate with more economical evolution time and further reduces the influence of the environment noise on gate fidelity.

SU(1,1) interferometers, based on the usage of nonlinear elements, are superior to passive interferometers in phase sensitivity. However, the SU(1,1) interferometer cannot make full use of photons carrying phase information as the second nonlinear element annihilates some of the photons inside. Here we focus on improving phase sensitivity and propose a different protocol based on a modified SU(1,1) interferometer, where the second nonlinear element is replaced by a beam splitter. We utilize two coherent states as inputs and implement balanced homodyne measurement at the output. Our analysis suggests that the protocol we propose can achieve the sub-shot-noise-limited phase sensitivity and is robust against photon loss as well as background noise. Our work is important for practical quantum metrology using SU(1,1) interferometers.

We report on a velocity map imaging study of ${\mathrm{Cl}}^{-}$ anions resulting from the dissociative electron attachment to four unsaturated chlorohydrocarbons. In all four molecules, this process is mediated by the formation of the lowest shape resonance. The choice of the molecules was motivated by the different character of this resonance. In the planar compounds chlorobenzene and vinyl chloride, it is a ${\pi }^{*}$ resonance, which is not dissociative along the C-Cl bond without distortion of the planar geometry. In the nonplanar compounds benzyl chloride and allyl chloride, the shape resonance has a mixed ${\pi }^{*}\text{−}{\sigma }^{*}$ character and is directly dissociative without the need for any additional distortion. Our motivation was to find out whether the dissociation-allowing nuclear motion has a common imprint in the resulting fragment angular distributions. In spite of the expected similarities between the two classes of compounds, the resulting images are quite different for all four molecules. We interpret the results, especially the imprints of the bending dynamics, with the aid of a single-electronic-state model in the axial recoil approximation.

We develop the perturbation theory for bosons interacting via a weak two-body potential $V$, attractive and repulsive parts of which cancel each other. We find that the leading nonpairwise contribution to the energy emerges in the third order in $V$ and represents an effective three-body interaction, the sign of which in most cases (although not in general) is anticorrelated with the sign of the long-range tail of $V$. We apply our theory to a few particular two-body interaction potentials and calculate the leading two-body and three-body interaction corrections for tilted dipoles in quasi-low-dimensional geometries. We show that our approach is consistent with the many-body Bogoliubov treatment.

We investigate numerically the role of near-field dipole-dipole interactions on the late-emission dynamics of large disordered cold atomic samples driven by a weak field. Previous experimental and numerical studies of subradiance in macroscopic samples have focused on low-density samples of pure two-level atoms, without internal structure, which corresponds to a scalar representation of the light. The cooperative nature of the late emission of light is then governed by the resonant optical depth. Here, by considering the vectorial nature of the light, we show the detrimental role of the near-field terms on cooperativity in higher-density samples. The observed reduction in the subradiant lifetimes is interpreted as a signature of the inhomogeneous broadening due to the near-field contributions, in analogy with the van der Waals dephasing phenomenon for superradiance.

We consider a quantum sensor network of qubit sensors coupled to a field $f(\mathit{x};\mathit{\theta })$ analytically parameterized by the vector of parameters $\mathit{\theta }$. The qubit sensors are fixed at positions ${\mathit{x}}_1,\cdots ,{\mathit{x}}_d$. While the functional form of $f(\mathit{x};\mathit{\theta })$ is known, the parameters $\mathit{\theta }$ are not. We derive saturable bounds on the precision of measuring an arbitrary analytic function $q\left(\mathit{\theta }\right)$ of these parameters and construct the optimal protocols that achieve these bounds. Our results are obtained from a combination of techniques from quantum information theory and duality theorems for linear programming. They can be applied to many problems, including optimal placement of quantum sensors, field interpolation, and the measurement of functionals of parametrized fields.

We measure polarization-resolved fundamental, second, and third harmonic nonlinear Thomson scattering out the side of a laser focus with ${10}^{18}\mathrm{W}/{\mathrm{cm}}^2$. The separate measured polarization components are each associated with a distinct dimension of predicted electron figure-8 motion. Taken together, the measured angular emission patterns for the two polarizations unambiguously confirm the figure-8 motion. Electrons are donated from low-density helium (${10}^{-3}$ to 1 Torr) ionized early during the laser pulse. Time-resolved single-photon detection is used to distinguish signal from noise.

Polygonal microcavities supporting whispering-gallery modes (WGMs) are widely investigated for optoelectronic applications. Recently, polygonal microcavities with reduced symmetry of various types have been realized in a large number of materials, while the corresponding theoretical analysis of WGMs was either absent or done only for individual cavity shapes. We perform a systematic analysis of irregular polygonal microcavities in general, allowing us to obtain deterministic conclusions on WGMs for all possible parameters of all types of shape irregularities from a set of analytical equations and inequalities. We test the theory with several typical irregular hexagonal microcavities that were experimentally reported, and we discuss the loss mechanism verified by wave optics simulations. The results provide a powerful analyzing tool for experimentalists working on irregular polygonal microcavities, and they constitute a significant extension to the scope of WGM theory.

Quantum tomography is an important tool for the characterization of quantum operations. In this paper we present a framework of quantum tomography in fermionic systems. Compared with qubit systems, fermions obey the superselection rule, which sets constraints on states, processes, and measurements in a fermionic system. As a result, we can only partly reconstruct an operation that acts on a subset of fermion modes and the full reconstruction always requires at least one ancillary fermion mode in addition to the subset. We also report a protocol for the full reconstruction based on gates in a Majorana-fermion quantum computer, including a set of circuits for realizing the informationally complete state preparation and measurement.

Recent theoretical and experimental advances have demonstrated advantages of using non-Gaussian optical resources compared to the Gaussian ones in the context of quantum teleportation (QT), an important quantum information processing task. From both theoretical and experimental points of view the question of which attributes of the resources, besides entanglement, render them useful for QT is an important one. In this paper, we examine the question of whether two well-studied attributes of optical resources, viz., squeezed vacuum affinity (SVA) and Einstein-Podolsky-Rosen (EPR) correlation are necessary and/or sufficient for QT. The specific class of non-Gaussian resources that we have considered for this purpose are the two-mode entangled states generated by mixing nonclassical inputs with vacuum at the beam splitter (BS). Our analytical results show that SVA is not always nonzero and hence it cannot be considered to be a genuine attribute. Our numerical results show that there exist some BS-generated entangled states that do not give QT in spite of being EPR correlated, implying that EPR correlation is not sufficient for QT. In conjunction with the earlier observation in the literature to the effect that EPR correlation is not necessary for QT, our results lead to the conclusion that in general, EPR correlation is neither necessary nor sufficient for QT. Our results leave the question open as to what attributes, in general, may be necessary and/or sufficient for QT.

Mixing transformations in quantum field theory are nontrivial, since they are intimately related to the unitary inequivalence between Fock spaces for fields with definite mass and fields with definite flavor. Considering the superposition of two neutral scalar (spin-0) bosonic fields, we investigate some features of the emerging condensate structure of the flavor vacuum. In particular, we quantify the flavor vacuum entanglement in terms of the von Neumann entanglement entropy of the reduced state. Furthermore, in a suitable limit, we show that the flavor vacuum has a structure akin to the thermal vacuum of thermo field dynamics, with a temperature dependent on both the mixing angle and the particle mass difference.

We investigate behaviors of single-mode multiphoton polarization states under lossless random Pauli noises, where “single-mode” means that all the photons are in the same spatiotemporal mode and cannot be distinguished by any degree of freedom. We characterize the different decoherence effects of multiphoton and single-photon polarization states via the normalized linear entropy. Our results show that, contrary to single-photon states, multiphoton states cannot always evolve to the maximally mixed states. In particular, decoherence-free states and subspaces are found for the random Pauli noises. We reveal that the distinct behaviors result from the bosonic bunching effect and permutation symmetry, and hence our study may be generalized to other photon degrees of freedom and even some other bosonic systems.

In free-space quantum key distribution (QKD) in turbulent conditions, scattering and beam wandering cause intensity fluctuations which decrease the detected signal-to-noise ratio. This effect can be mitigated by rejecting received bits when the channel's transmittance is below a threshold. Thus, the overall error rate is reduced and the secure key rate increases despite the deletion of bits. In this work, we implement recently proposed selection methods focusing on the prefixed-threshold real-time selection (P-RTS) where a cutoff can be chosen prior to data collection and independently of the transmittance distribution. We perform finite-size decoy-state Bennett-Brassard 1984 QKD in a laboratory setting where we simulate the atmospheric turbulence using an acousto-optical modulator. We show that P-RTS can yield considerably higher secure key rates for a wide range of the atmospheric channel parameters. In addition, we evaluate the performance of the P-RTS method for a realistically finite sample size. We demonstrate that a near-optimal selection threshold can be predetermined even with imperfect knowledge of the channel transmittance distribution parameters.

We theoretically and numerically investigate rotational dynamics of ${\mathrm{O}}_2$ in the electronic $X{}^3\mathrm{\Sigma }_g^{-}$ ground state induced by an intense femtosecond laser field. The rotational dynamics are calculated by two different models. One of the models includes triplet splittings explicitly in rotational energy levels originated from an electronic spin (a spin-dependent model), and the other omits the triplet splittings (a spin-independent model). We find that the final rotational population after the interaction with the laser field does not depend on the models. On the other hand, we find that the rotational dynamics evaluated by alignment degrees and angular distributions of molecular axes depend on the models. Although the rotational dynamics calculated by the spin-independent model is similar to that calculated by the spin-dependent model within 5 ps, the triplet splittings in the rotational energy levels affect the rotational dynamics after 5 ps. This is explained by the inverse of the energy difference between triplet splittings in the rotational levels. We show that the multiplet energy splittings in rotational energies should be included when the rotational dynamics in the multiplet electronic state are considered. We also investigate excitation processes on the basis of time evolutions of state populations and find that there are excitation pathways with double as well as single bifurcations depending on the initial state.

We investigate the localization and topological transitions in a one-dimensional (interacting) non-Hermitian quasiperiodic lattice, which is described by a generalized Aubry-André-Harper model with irrational modulations in the off-diagonal hopping and on-site potential and with non-Hermiticities from the nonreciprocal hopping and complex potential phase. For noninteracting cases, we reveal that the nonreciprocal hopping (the complex potential phase) can enlarge the delocalization (localization) region in the phase diagrams spanned by two quasiperiodic modulation strengths. We show that the localization transition is always accompanied by a topological phase transition characterized the winding numbers of eigenenergies in three different non-Hermitian cases. Moreover, we find that a real-complex eigenenergy transition in the energy spectrum coincides with (occurs before) these two phase transitions in the nonreciprocal (complex potential) case, while the real-complex transition is absent with the coexistence of the two non-Hermiticities. For interacting spinless fermions, we demonstrate that the extended phase and the many-body localized phase can be identified by the entanglement entropy of eigenstates and the level statistics of complex eigenenergies. By making the critical scaling analysis, we further show that the many-body localization transition coincides with the real-complex transition and occurs before the topological transition in the nonreciprocal case, which are absent in the complex phase case.

We show theoretically that the dynamics of a driven quantum harmonic oscillator subject to nondissipative noise is formally equivalent to the single-particle dynamics propagating through an experimentally feasible dynamically disordered photonic network. Using this correspondence, we find that noise-assisted energy transport occurs in this network and if the noise is Markovian or delta correlated, we can obtain an analytical solution for the maximum amount of transferred energy between all network's sites at a fixed propagation distance. Beyond the Markovian limit, we further consider two different types of non-Markovian noise and show that it is possible to have efficient energy transport for larger values of the dephasing rate.

In the present report, the question of U(1) symmetry breaking in a system of atoms and electromagnetic fields, interacting inside a leaky cavity, filled with a nonlinear medium, is addressed. In particular, the $Z_2$ discrete symmetry of the system and the emergence of optical phases are fully discussed. For the nonlinearity of a general order, with an electromagnetic field of any number of modes, conditions under which the resulting field-field interactions destroy the U(1) (and $Z_2$) symmetry are determined. We then apply the theory to the case of a collection of two-level atoms and three electromagnetic modes. Taking one of the fields as a classically adjustable pumping one, it is demonstrated that the quantized field-field coupling profoundly depends upon the pump field strength. The presence of two competing phenomena, namely, the cavity photonic dissipation and nonlinearity, is shown to lead the system towards steady behavior. The steady-state solutions to the atomic population and field quadratures exhibit the normal and superradiant phases, depending on the strengths of pumping field and atom-field couplings. The conditions for the stability of such steady-state solutions are also discussed in detail. A notable result of the present article is that by adjusting the parameters involved in the system one can switch from the normal to electric and/or magnetic superradiant phases.

We calculate the field eigenmodes of the superradiant emission from an ensemble of $N$ two-level atoms and confirm the results of other authors that Dicke superradiant emission is restricted to a small number of modes, with more than 90% of the photons emitted in a single dominant mode. While numerical techniques are effective due to the symmetry of the problem, we develop also an analytical method to approximate the modes in the limit of a large number of emitters.

The Projection Postulate from Standard Quantum Mechanics relies fundamentally on measurements. But measurements implicitly suggest the existence of anthropocentric notions like measuring devices, which should rather emerge from the theory. This article proposes an alternative formulation of the Standard Quantum Mechanics, in which the Projection Postulate is replaced with a version in which measurements and observations are not assumed as fundamental. More precisely, the Wigner functions representing the quantum states on the phase space are required to be tightly constrained to regions of the classical coarse graining of the phase space. This ensures that states are quasiclassical at the macrolevel. Within a coarse-graining region, the time evolution of the Wigner functions representing the quantum system is required to obey the Liouville-von Neumann equation, the phase-space equivalent of the Schrödinger equation. The projection is postulated to happen when the system transitions from a coarse-graining region to others, by selecting one of them according to the Born rule, but without reference to a measurements. The connection with the standard formulation of Quantum Mechanics is explained, as well as the problems that the present formulation solves, in particular the Wigner's friend type of paradoxes. Experimental consequences and open problems of the proposed formulation are discussed.