We demonstrate and evaluate an on-demand source of single itinerant microwave photons. Photons are generated using a highly coherent, fixed-frequency qubit-cavity system, and a protocol where the microwave control field is far detuned from the photon emission frequency. By using a Josephson parametric amplifier (JPA), we perform efficient single-quadrature detection of the state emerging from the cavity. We characterize the imperfections of the photon generation and detection, including detection inefficiency and state infidelity caused by measurement back-action over a range of JPA gains from 17 to 33 dB. We observe that both detection efficiency and undesirable back-action increase with JPA gain. We find that the density matrix has its maximum single-photon component ${\rho }_{11}=0.36\pm 0.01$ at 29 dB JPA gain. At this gain, back-action of the JPA creates cavity photon number fluctuations that we model as a thermal distribution with an average photon number $\overline{n}=0.041\pm 0.003$.

An arbitrary optical waveform can be measured using a perturbative approach known as the petahertz optical oscilloscope, in which an electron trajectory in the process of high-order-harmonic generation is used as a temporal gate. Here, we report that the petahertz optical oscilloscope exhibits a significant frequency dependence for the short wavelength signal field due to the finite time span of the electron trajectory. We show that the intrinsic response of the atom to the signal field can be used to correct the frequency dependence of the measurement. It is shown that the optical waveform having the spectral components from the second harmonic to the fourth harmonic of the fundamental field can accurately be reconstructed after the correction of the frequency dependence.

Nonclassicality is a key ingredient for quantum enhanced technologies and experiments involving macroscopic quantum coherence. Considering various exactly solvable quantum-oscillator systems, we address the role played by the anharmonicity of their potential in the establishment of nonclassical features. Specifically, we show that a monotonic relation exists between the entropic nonlinearity of the considered potentials and their ground-state nonclassicality, as quantified by the negativity of the Wigner function. In addition, in order to clarify the role of squeezing, which is not captured by the negativity of the Wigner function, we focus on the Glauber-Sudarshan $P$ function and address the nonclassicality-nonlinearity relation using the entanglement potential. Finally, we consider the case of a generic sixth-order potential confirming the idea that nonlinearity is a resource for the generation of nonclassicality and may serve as a guideline for the engineering of quantum oscillators.

The role of the side channels in the continuous-variable quantum key distribution is studied. It is shown how the information leakage through a side channel from the trusted sender station increases the vulnerability of the protocols to the eavesdropping in the main quantum communication channel. Moreover, the untrusted noise infusion by an eavesdropper on the trusted receiving side breaks the security even for a purely attenuating main quantum channel. As a method to compensate for the effect of the side-channel leakage on the sender side, we suggest several types of manipulations on the side-channel input. It is shown that by applying the modulated coherent light on the input of the side channel that is optimally correlated to the modulation on the main signal and optionally introducing additional squeezing in the case of the squeezed-state protocol, the negative influence of the lossy side channel on the sender side can be completely removed. For the trusted receiving side, the method of optimal monitoring of the residual noise from the side-channel noise infusion is suggested and shown to be able to completely eliminate the presence of the noisy side channel. We therefore prove that the side-channel effects can be completely removed using feasible operations if the trusted parties access the respective parts of the side channels.

We consider a coupled-cavity array, where each cavity interacts with an atom under the rotating-wave approximation. For a staggered pattern of intercavity couplings, a pair of field normal modes, each bilocalized at the two array ends, arises. A rich structure of dynamical regimes can hence be addressed, depending on which resonance condition is set between the atom and the field modes. We show that this can be harnessed to carry out high-fidelity quantum-state transfer (QST) of photonic, atomic, or polaritonic states. Moreover, by partitioning the array into coupled modules of shorter length, the QST time can be substantially shortened without significantly affecting the fidelity.

Single and multiple electron removal processes (capture and ionization) in proton-${\mathrm{H}}_2\mathrm{O}$ collisions have been investigated applying the continuum distorted wave with eikonal initial-state model within the framework of independent electron approach. Probabilities and cross sections for electron capture are derived from the same quantities evaluated for ionization using the continuity of transition quantities across the ionization threshold. Dissociation and fragmentation cross sections for the ${\mathrm{H}}_2{\mathrm{O}}^{q+}$ ($q=1$–3) ions have been evaluated by considering branching ratios that include the effect of multiple electron removal transitions. The results are compared with experimental and other theoretical data in the range of impact energies from 30 kev to 5 MeV. Generally, the evaluated cross sections and fragmentation yields show good agreement with experiments at impact energies above 100–150 keV.

We have studied the decay of Rydberg excitations in a cold Rb gas. A 10 ns, pulsed-dye-amplified diode laser excites Rb atoms at $70\mu \mathrm{K}$ in a magneto-optical trap to $ns$ or $np$ Rydberg states with principal quantum numbers $26\le n\le 40$. Time-delayed state-selective field ionization is used to directly monitor the population in the initial and neighboring Rydberg levels. The measured time dependence of the Rydberg population is well described by numerical simulations which consider only spontaneous emission and population transfer by blackbody radiation. No evidence for collective decay is found at atom densities up to $3\times {10}^9{\mathrm{cm}}^{-3}$. This result is in contrast to a previous study [Wang et al., Phys. Rev. A 75, 033802 (2007)], in which superradiant decay was theoretically predicted and experimentally inferred for atom density and laser focal volume conditions very similar to those considered here. Suppression of collective emission is likely due to variations in transition energies within the atom sample, dominated by inhomogeneities in dipole-dipole exchange interactions for initial $s$ states, or by a combination of dipole-dipole and electric field inhomogeneities for the initial $p$ states.

We investigate the dynamic properties of bouncing and penetration in colliding binary and ternary Bose-Einstein condensates comprised of different Zeeman or hyperfine states of ${}^{87}\mathrm{Rb}$. Through the application of magnetic field gradient pulses, two- or three-component condensates in an optical trap are spatially separated and then made to collide. The subsequent evolutions are classified into two categories: repeated bouncing motion and mutual penetration after damped bounces. We experimentally observed mutual penetration for immiscible condensates, bouncing between miscible condensates, and domain formation for miscible condensates. From numerical simulations of the Gross-Pitaevskii equation, we find that the penetration time can be tuned by slightly changing the atomic interaction strengths.

A non-Abelian anyon can only occur in the presence of ground-state degeneracy in the plane. It is conceivable that for some strange anyon with quantum dimension $>1$ that the resulting representations of all $n$-strand braid groups $B_n$ are overall phases, even though the ground-state manifolds for $n$ such anyons in the plane are in general Hilbert spaces of dimensions $>1$. We observe that degeneracy is all that is needed: For an anyon with quantum dimension $>1$ the non-Abelian statistics cannot all be overall phases on the degeneracy ground-state manifold. Therefore, degeneracy implies non-Abelian statistics, which justifies defining a non-Abelian anyon as one with quantum dimension $>1$. Since non-Abelian statistics presumes degeneracy, degeneracy is more fundamental than non-Abelian statistics.

In the topic of perfect local distinguishability of orthogonal multipartite quantum states, most results obtained so far pertain to bipartite systems whose subsystems are of specific dimensions. In contrast very few results for bipartite systems whose subsystems are of arbitrary dimensions, are known. This is because a rich variety of (algebraic or geometric) structure is exhibited by different sets of orthogonal states owing to which it is difficult to associate some common property underlying them all, i.e., a common property that would play a crucial role in the local distinguishability of these states. In this Rapid Communication, I propose a framework for the distinguishability by one-way local operations and classical communication (1-LOCC) of sets of orthogonal bipartite states in a $d_A\otimes d_B$ bipartite system, where $d_A,d_B$ are the dimensions of both subsytems, labeled as $A$ and $B$. I show that if the $i\mathrm{th}$ party (where $i=A,B$) can initiate a 1-LOCC protocol to perfectly distinguish among a set of orthogonal bipartite states, then the information of the existence of such a 1-LOCC protocol lies in a subspace of $d_i\times d_i$ Hermitian matrices, denoted by ${\mathcal{T}}_{\perp }^{\left(i\right)}$, and that the method to extract this information (of the existence of this 1-LOCC protocol) from ${\mathcal{T}}_{\perp }^{\left(i\right)}$ depends on the value of $\dim {\mathcal{T}}_{\perp }^{\left(i\right)}$. In this way one can give sweeping results for the 1-LOCC (in)distinguishability of all sets of orthogonal bipartite states corresponding to certain values of $\dim {\mathcal{T}}_{\perp }^{\left(i\right)}$. Thus I propose that the value of $\dim {\mathcal{T}}_{\perp }^{\left(i\right)}$ gives the common underlying property based on which sweeping results for the 1-LOCC (in)distinguishability of orthogonal bipartite quantum states can be made.

We study measures of decoherence and thermalization of a quantum system $S$ in the presence of a quantum environment (bath) $E$. The entirety $S+E$ is prepared in a canonical-thermal state at a finite temperature; that is, the entirety is in a steady state. Both our numerical results and theoretical predictions show that measures of the decoherence and the thermalization of $S$ are generally finite, even in the thermodynamic limit, when the entirety $S+E$ is at finite temperature. Notably, applying perturbation theory with respect to the system-environment coupling strength, we find that under common Hamiltonian symmetries, up to first order in the coupling strength it is sufficient to consider $S$ uncoupled from $E$, but entangled with $E$, to predict decoherence and thermalization measures of $S$. This decoupling allows closed-form expressions for perturbative expansions for the measures of decoherence and thermalization in terms of the free energies of $S$ and of $E$. Large-scale numerical results for both coupled and uncoupled entireties with up to 40 quantum spins support these findings.

Coherence arises from the superposition principle and plays a key role in quantum mechanics. Recently, Baumgratz et al. [T. Baumgratz, M. Cramer, and M. B. Plenio, Phys. Rev. Lett. 113, 140401 (2014)] established a rigorous framework for quantifying the coherence of finite-dimensional quantum states. In this work we provide a framework for quantifying the coherence of Gaussian states and explicitly give a coherence measure for Gaussian states based on the relative entropy.

We study nonequilibrium steady states (NESS) of spin chains with boundary Markovian dissipation from the computational complexity point of view. We focus on $XX$ chains whose NESS are matrix product operators, i.e., with coefficients of a tensor operator basis described by transition amplitudes in an auxiliary space. Encoding quantum algorithms in the auxiliary space, we show that estimating expectations of operators, being local in the sense that each acts on disjoint sets of few spins covering all the system, provides the answers of problems at least as hard as, and believed by many computer scientists to be much harder than, those solved by quantum computers. We draw conclusions on the hardness of the above estimations.

In order to improve the key rate of the decoy-state method, we need to jointly study yields of different bases. Given the delicate fact that pulses of the same preparation state can have different counting rates if they are measured in different bases, for example, those vacuum pulses and those single-photon pulses, existing results of decoy-state quantum key distribution using biased bases are actually flawed by assuming that they are equal. We fix this flaw through using the idea that yields of pulses prepared in different bases are the same provided that they are prepared in the same state and also measured in the same basis, for example, those single-photon pulses prepared in different bases but measured in the same basis. Based on this, we present correct formulas for the decoy-state method using biased bases. Taking the effects of statistical fluctuations into account, we then numerically study the key rates of different protocols with all parameters being fully optimized. Our result confirms the prior conclusion that the decoy-state method using biased bases can have an advantage over the symmetric protocol with unbiased bases. We obtain high key rates of our four-intensity protocol without using any vacuum source.

To implement fault-tolerant quantum computation, entanglement generation with low error probability and high success probability is required. In a previous paper, we proposed the use of squeezed coherent light as a probe to generate entanglement between two atoms by communication and showed that the error probability is reduced well below the threshold of fault-tolerant quantum computation [F. Matsuoka et al., Phys. Rev. A 88, 022313 (2013)]. In this paper, we investigate the effect of photon loss mainly due to finite coupling efficiency to the cavity. The error probability with photon loss is calculated using a beam-splitter model for homodyne measurement of probe light. We examine the optimum conditions of the amplitude of the probe light and the degree of squeezing to minimize the error probability. We show that the phase-squeezed probe light yields lower error probability than a coherent-light probe, even with photon losses. A fault-tolerant quantum computation algorithm can be implemented under 87% transmittance by concatenating a seven-qubit error correction code for the phase-flip error.

The two-photon $1s^{2}2s2p{}^3{P}_0\to 1s^2{s}^2{}^1{S}_0$ transition in berylliumlike ions is investigated theoretically within a fully relativistic framework and a second-order perturbation theory. We focus our analysis on how electron correlation, as well as the negative-energy spectrum, can affect the forbidden $E1M1$ decay rate. For this purpose, we include the electronic correlation via an effective local potential and within a single-configuration-state model. Due to its experimental interest, evaluations of decay rates are performed for berylliumlike xenon and uranium. We find that the negative-energy contribution can be neglected at the present level of accuracy in the evaluation of the decay rate. On the other hand, if contributions of electronic correlation are not carefully taken into account, it may change the lifetime of the metastable state by up to 20%. By performing a fully relativistic $jj$-coupling calculation, we find a decrease of the decay rate by two orders of magnitude compared to nonrelativistic $LS$-coupling calculations, for the selected heavy ions.

We theoretically demonstrate temporal and spatial manipulation of electron wave packets involved in strong-field photoelectron holography (SFPH) with the orthogonally polarized two-color laser fields. By varying the relative phase of the two-color fields, the recollision time of the returning wave packet can be accurately controlled, which allows us to switch off and on the holographic interference. Moreover, the recollision angles of the returning electron wave packet can be arbitrarily controlled via changing the relative intensity of the two-color fields, and thus the structure information of the target is encoded in the hologram by the recollision electron wave packet from different angles. This makes the SFPH a powerful technique of imaging the molecular structure as well as ultrafast dynamics on an attosecond time scale.

We investigate the dynamics of angular momentum states for a single ultracold atom trapped in two-dimensional systems of sided coupled ring potentials. The symmetries of the system show that tunneling amplitudes between different ring states with variation of the winding number are complex. In particular, we demonstrate that in a triangular ring configuration the complex nature of the cross-couplings can be used to geometrically engineer spatial dark states to manipulate the transport of orbital angular momentum states via quantum interference.