A dissipative method is proposed for generating the squeezed state of a microwave field in a superconducting transmission line resonator, which is coupled to a superconducting qubit. When the resonator is subject to the parametric driving, the qubit-resonator coupling can be exponentially enhanced. We show that the dissipation of the qubit as a resource can be used to produce and stabilize the single-mode squeezed state of the superconducting resonator at steady state. Compared with previous works, our scheme has the advantage that only a single drive is used for the quantum state preparation, which can simplify the experimental operations. Combined with the quantum interference effect, we also generalize our idea to the dissipative preparation of a two-mode squeezed state in two directly coupled superconducting resonators. The present work may have many potential applications in the field of quantum information processing with superconducting circuits.

A contemporary technological milestone is to build a quantum device performing a computational task beyond the capability of any classical computer, an achievement known as quantum adversarial advantage. In what ways can the entanglement realized in such a demonstration be quantified? Inspired by the area law of tensor networks, we derive an upper bound for the minimum random circuit depth needed to generate the maximal bipartite entanglement correlations between all problem variables (qubits). This bound is lattice geometry dependent and makes explicit a nuance implicit in other proposals with physical consequence. The hardware itself should be able to support superlogarithmic ebits of entanglement across some poly($n$) number of qubit bipartitions; otherwise the quantum state itself will not possess volumetric entanglement scaling and full-lattice-range correlations. Hence, as we present a connection between quantum advantage protocols and quantum entanglement, the entanglement implicitly generated by such protocols can be tested separately to further ascertain the validity of any quantum advantage claim.

We propose a method for classical simulation of finite-dimensional quantum systems, based on sampling from a quasiprobability distribution, i.e., a generalized Wigner function. Our construction applies to all finite dimensions, with the most interesting case being that of qubits. For multiple qubits, we find that quantum computation by Clifford gates and Pauli measurements on magic states can be efficiently classically simulated if the quasiprobability distribution of the magic states is non-negative. This provides the so far missing qubit counterpart of the corresponding result [V. Veitch et al., New J. Phys. 14, 113011 (2012)] applying only to odd dimension. Our approach is more general than previous ones based on mixtures of stabilizer states. Namely, all mixtures of stabilizer states can be efficiently simulated, but for any number of qubits there also exist efficiently simulable states outside the stabilizer polytope. Further, our simulation method extends to negative quasiprobability distributions, where it provides probability estimation. The simulation cost is then proportional to a robustness measure squared. For all quantum states, this robustness is smaller than or equal to robustness of magic.

An anisotropic quantum vacuum (AQV) has been predicted to induce quantum interferences during the spontaneous emission process in an atomic $V$ transition [G. S. Agarwal, Phys. Rev. Lett. 84, 5500 (2000)]. Nevertheless, the finite lifetime of the excited states is expected to strongly limit the observability of this phenomenon. In this paper, we predict that an AQV can induce a long-lifetime coherence in an atomic $\mathrm{\Lambda }$ transition from the process of spontaneous emission, which has an additional advantage of removing the need for coherent laser excitation. We also carry out two metasurface designs and compare their respective efficiencies for creating an AQV over remote distances. The detection of this coherence induced by a metasurface, in addition to being yet another vindication of quantum electrodynamics, could pave the way towards the remote distance control of coherent coupling between quantum emitters, which is a key requirement to produce entanglement in quantum technology applications.

While Gaussian states and associated Gaussian operations are basic ingredients and convenient objects for continuous-variable quantum information, it is also realized that non-Gaussianity is an important resource for quantum information processing. The characterization and quantification of non-Gaussianity have been widely studied in the past decade, with several significant measures for non-Gaussianity introduced. In this work, by exploiting an information-theoretic refinement of the conventional Heisenberg uncertainty relation and a physical characterization of Gaussian states as minimum uncertainty states, we introduce an easily computable measure for non-Gaussianity of bosonic field states in terms of the Wigner-Yanase skew information. Fundamental properties, as well as intuitive meaning, of this measure are unveiled. The concept is illustrated by prototypical non-Gaussian states, and compared with various existent measures for non-Gaussianity. Its merit and physical significance are elucidated.

Parity meters, utilized to distinguish bipartite quantum states with even or odd parity, have many applications in quantum information processing. We propose a potentially practical scheme to construct the nondestructive Rydberg parity meter (NRPM) in ground-state subspace using Rydberg atoms. Based on the proposed NRPM, the parity of the qubit atoms could be known in a deterministic and nondestructive way, implying that the output state of the NRPM can be further used in the rest of the process for quantum information tasks. In addition, we study the applications of the proposed NRPM in entangled state fusion, entangled state analysis, and quantum teleportation, as well as quantum logic gate in a decoherence-free subspace regarding collective-dephasing errors. We analyze the performance of the NRPM under the influence of decoherence and consider more general situations for the feasibility of the scheme.

We discuss three proposed schemes of initializing circular-state Rydberg atoms via optical couplings provided by the ponderomotive effect in contrast to the current circularization methods that utilize electric-dipole interactions. In our first proposed method, a radial optical trap consisting of two Laguerre-Gaussian beams of opposite winding numbers transfers orbital angular momentum to the Rydberg atom, providing a first-order coherent coupling between an F state and a circular state. Additionally, we propose a one-dimensional ponderomotive optical lattice modulated at rf frequencies, providing quadrupolelike couplings in the hydrogenic manifold for rapid adiabatic passage through a series of intermediate Rydberg states into the circular state. For the third proposed scheme, a two-dimensional ponderomotive optical lattice with a time-orbiting trap center induces effectively the same coupling as a ${\sigma }^{+}$- or ${\sigma }^{-}$-polarized rf field of tunable purity for all-optical rapid adiabatic passage into the circular state.

We study the ionization and high-order harmonic generation (HHG) in argon by short linearly polarized laser pulses. We compare the populations of atomic orbitals and HHG spectra in a wide range of laser-pulse parameters, taking into account the dynamics of all orbitals based on the time-dependent density-functional theory (TDDFT) and only the most active orbital within the single-active electron (SAE) approximation. At high intensities of laser pulses, the atom polarization leads to suppression in the bound-state depletion due to the screening of the external field. The significant increase in the HHG yield is observed for the TDDFT calculations in comparison with the SAE calculations. The contribution of different orbitals in the TDDFT calculations of HHG spectra is discussed.

Traditionally, atomic spin orientation is achieved by the transfer of angular momentum from polarized light to an atomic system. We demonstrate the mechanism of orientation generation in room-temperature caesium vapors that combines three elements: optical pumping, nonlinear spin dynamics, and spin-exchange collisions. Through the variation of the spin-exchange relaxation rate, the transition between an aligned and an oriented atomic sample is presented. The observation is performed by monitoring the atomic radio-frequency spectra. The measurement configuration discussed paves the way to simple and robust radio-frequency atomic magnetometers that are based on a single low-power laser diode that approach the performance of multilaser pump-probe systems.

Due to the boundary coupling in a finite system, the zero modes of a standard Su-Schrieffer-Heeger (SSH) model may deviate from exact-zero energy. A recent experiment has shown that by increasing the system size or altering gain or loss strength of the SSH model with parity-time ($\mathcal{PT}$) symmetry, the real parts of the energies of the edge modes can be recovered to exact-zero value [Song et al., Phys. Rev. Lett. 123, 165701 (2019)]. To clarify the effects of $\mathcal{PT}$-symmetric potentials on the recovery of the nontrivial zero modes, we study the SSH model with $\mathcal{PT}$-symmetric potentials of different forms in both infinite and finite systems. Our results indicate that the energies of the edge modes in the infinite size case decide whether or not the success of the recovery of the zero modes by tuning the strength of $\mathcal{PT}$-symmetric potential in a finite system. If the energies of the edge modes amount to zero in the thermodynamic limit under an open boundary condition (OBC), the recovery of the zero modes will break down by increasing the gain or loss strength for a finite system. Our results can be easily examined in different experimental platforms and inspire more insightful understanding on nontrivial edge modes in topologically non-Hermitian systems.

We study the two-body problem of ultracold fermionic alkaline-earth-metal-(like) atoms in the electronic ${}^{1}S_0$ state ($g$-state) and ${}^{3}P_0$ state ($e$-state) which are confined in a quasi-one-dimensional (quasi-1D) tube simultaneously, where in the axial direction the $g$-atom experiences a 1D optical lattice and the $e$-atom is localized by a harmonic potential. Due to the nuclear-spin exchange interaction between the $g$- and $e$-atoms, one can use such a quasi-$(1+0)\mathrm{D}$ system to realize the Kondo effect. We suggest two tight-binding models for this system, for the cases that the odd-wave scattering between the $g$- and $e$-atoms is negligible or not, respectively. Moreover, we give a microscopic derivation for the interatomic interaction parameters of these models, by explicitly calculating the quasi-$(1+0)\mathrm{D}$ low-energy scattering amplitude of the $g$- and $e$-atoms in this system and matching this exact result with the ones given by tight-binding models. We illustrate our results for the experimental systems of ultracold ${}^{173}\mathrm{Yb}$ and ${}^{171}\mathrm{Yb}$ atoms and show the control effect of the confinement potentials on these model parameters. Furthermore, the validity of the simple “projection approximation” is examined. In this approximation, one derives the interaction parameters of the tight-binding models by directly projecting the three-dimensional (3D) Huang-Yang pseudopotential on the ground state of the confinement and the lowest band of the optical lattice. This approximation is supposed to be correct when the 3D interatomic scattering length $a_s$ is much smaller than the characteristic lengths (CLs) of the confinements. However, we find that for our system this approximation already does not work when $a_s$ is only of the order of 10% of the confinement CLs. Using the exact two-atom scattering amplitude, we also calculate the spin-exchanging rate (i.e., the cross section of the spin-exchanging collision between the $g$- and $e$-atoms) for the recent experiment [L. Riegger et al., Phys. Rev. Lett. 120, 143601 (2018)] of ${}^{173}\mathrm{Yb}$ atoms in this quasi-$(1+0)\mathrm{D}$ system and investigate finite-quasimomentum effect of the $g$-atoms in this experiment. Our results show that this effect is very significant.

Exploiting the simultaneous entanglement (hyperentanglement) in multiple degrees of freedom of an entangled photon pair increases the dimensionality of the Hilbert space for quantum information processing. However, the generation of hyperentangled photon pairs collinearly, while capable of high brightness, results in a smaller Hilbert space due to the two photons being in the same spatial mode. In this paper, we show that one can recover the full dimensionality of such hyperentanglement through a simple interference setup, similar to the time-reversed Hong-Ou-Mandel (TR-HOM) process. In contrast to the standard TR-HOM, we point out that a critical phase condition has to be satisfied in order to recover the hyperentanglement. We theoretically analyze the realization of this approach and discuss the feasibility of generating truly hyperentangled photon pairs. Our proposed approach does not require postselection and hence enables efficient hyperentangled photon pair generation for high-dimensional quantum applications.

Initializing quantum registers with high fidelity is a fundamental precondition for many applications like quantum information processing and sensing. The electronic and nuclear spins of a nitrogen-vacancy (NV) center in diamond form an interesting hybrid quantum register that can be initialized by a combination of laser, microwave, and radio-frequency pulses. However, the laser illumination, which is necessary for achieving electron spin polarization, also has the unwanted side effect of depolarizing the nuclear spin. Here we study how the depolarization dynamics of the ${}^{14}\mathrm{N}$ nuclear spin depends on the laser wavelength. We show experimentally that excitation with an orange laser (594 nm) causes significantly less nuclear spin depolarization compared to the green laser (532 nm) typically used for excitation and hence leads to higher nuclear spin polarization. This could be because orange light excitation inhibits ionization of ${\mathrm{NV}}^0$ into ${\mathrm{NV}}^{-}$ and therefore suppresses one source of noise acting on the nuclear spin.

A light beam carrying a spatially varying state of polarization generates a space-varying Pancharatnam-Berry geometric phase while propagating through a homogeneous anisotropic medium. We show that determination of such a space-varying geometric phase provides a unique way to quantify the space-varying polarization state of light using a single-shot interferometric measurement. We demonstrate this concept in a Mach-Zehnder interferometric arrangement using a linearly polarized reference light beam, where full information on the spatially varying polarization state is successfully recovered by quantifying the space-varying geometric phase and the contrast of interference. The proposed method enables single-shot measurement of any space-varying polarization state of light from the measured interference pattern with a polarized reference beam. This approach shows considerable potential for instantaneous mapping of complex space-varying polarization of light in diverse applications, such as astronomy, biomedical imaging, and nanophotonics, where high precision and near real-time measurement of spatial polarization patterns are desirable.

We produce a trimerized kagome lattice for ultracold atoms using an optical superlattice formed by overlaying triangular lattices generated with two colors of light at a 2:1 wavelength ratio. Adjusting the depth of each lattice tunes the strong intratrimer ($J$) and weak intertrimer ($J^{\prime }$) tunneling energies, and also the on-site interaction energy $U$. Two different trimerization patterns are distinguished using matter-wave diffraction. We characterize the coherence of a strongly interacting Bose gas in this lattice, observing persistent nearest-neighbor spatial coherence in the large $U/J^{\prime }$ limit, and that such coherence displays asymmetry between the strongly and the weakly coupled bonds.

We show how entanglement between two optically levitated nanoparticles can be generated and detected by coherent scattering of tweezer photons into a single cavity mode. Triggered by the detection of a Stokes photon, the tweezer detuning is switched from the blue to the red; entanglement is then verified by the conditioned anti-Stokes photon flux, which oscillates with the mechanical beat frequency. The proposed setup is realizable with near-future technology and opens the door to experimental observation of nonclassical center-of-mass correlations between two or more levitated nanoscale objects.

We study interferometry with Gaussian states and show that an ancilla-assisted scheme outperforms coherent state interferometry for all levels of loss. We also compare the ancilla-assisted scheme to other interferometric schemes involving squeezing, and show that it has the most advantage in the high-loss, high photon-number regime. In fact, in the presence of high loss, it outperforms many other strategies proposed to date. We find the optimal measurement observable for each scheme discussed. We also find that, with the appropriate measurement, the achievable precision of the proposal by Caves [Phys. Rev. D 23, 1693 (1981)] can be improved upon, and is less vulnerable to losses than previously thought.

In this work, we study the transmission of classical information through three completely depolarizing channels in superposition of different causal orders. We exploit the quantum 3-switch as a resource for classical communication. We perform a kind of quantum control that was not accessible to the previously treated two-channel case. The fine and full quantum control achieved using selected combinations of causal orders let us uncover several features: nonmonotonous behavior on the transmission of information with respect to the number of causal orders involved, and different values of the transmission of information depending on the specific combinations of causal orders considered. Our results are a stepping stone to assess the efficiency of coherent quantum control and optimize resources in the implementation of indefinite causal structures. Finally, we suggest an optical implementation using standard telecom technology to test our predictions.