We propose a method of generating maximally-polarization-entangled states by type-II spontaneous parametric down-conversion in Bragg reflection waveguides. Analytic expressions for the group velocities of down-converted modes are used to engineer zero group-velocity mismatch at the operating point, making the spectra of photons within a down-converted pair indistinguishable and thus leading to a maximally-polarization-entangled state. The results can be used for the creation and manipulation of polarization-entangled qubits entirely on a chip.

A deterministic quantum amplifier inevitably adds noise to an amplified signal due to the uncertainty principle in quantum physics. We here investigate how a quantum-noise-limited amplifier can be improved by additionally employing the photon subtraction, the photon addition, and a coherent superposition of the two, thereby making a probabilistic, heralded, quantum amplifier. We show that these operations can enhance the performance in amplifying a coherent state in terms of intensity gain, fidelity, and phase uncertainty. In particular, the photon subtraction turns out to be optimal for the fidelity and the phase concentration among these elementary operations, while the photon addition also provides a significant reduction in the phase uncertainty with the largest gain effect.

In this Brief Report, we propose a theoretical scheme to transfer quantum states between superconducting charge qubits and semiconductor spin qubits in a circuit QED device. Under dispersive conditions, resonator-assisted state transfer between qubits can be performed controllably only by addressing the flux bias applied to the charge qubits. The low infidelity and existing advantages show that the proposal may provide an effective route toward scalable quantum-information transfer with solid-state hybrid qubits.

We show that there are informationally complete joint measurements of two conjugated observables on a finite quantum system, meaning that they enable the identification of all quantum states from their measurement outcome statistics. We further demonstrate that it is possible to implement a joint observable as a sequential measurement. If we require minimal noise in the joint measurement, then the joint observable is unique. If $d$ is odd, then this observable is informationally complete. But if $d$ is even, then the joint observable is not informationally complete, and one has to allow more noise in order to obtain informational completeness.

We study multiple-spin coherence transfers in linear Ising spin chains with nearest-neighbor couplings. These constitute a model for efficient information transfers in future quantum computing devices and for many multidimensional experiments for the assignment of complex spectra in nuclear magnetic resonance spectroscopy. We complement prior analytic techniques for multiple-spin coherence transfers with a systematic numerical study where we obtain strong evidence that a certain analytically motivated family of restricted controls is sufficient for time optimality. In the case of a linear three-spin system, additional evidence suggests that prior analytic pulse sequences using this family of restricted controls are time optimal even for arbitrary local controls. In addition, we compare the pulse sequences for linear Ising spin chains to pulse sequences for more realistic spin systems with additional long-range couplings between nonadjacent spins. We experimentally implement the derived pulse sequences in three- and four-spin systems and demonstrate that they are applicable in realistic settings under relaxation and experimental imperfections—in particular—by deriving broadband pulse sequences which are robust with respect to frequency offsets.

Two classes of quantum channels, which we call more capable and less noisy, are introduced. The more capable class consists of channels such that the quantum capacities of the complementary channels to the environments are zero. The less noisy class consists of channels such that the private capacities of the complementary channels to the environment are zero. For the more capable class, it is clarified that the private capacity and quantum capacity coincide. For the less noisy class, it is clarified that the private capacity and quantum capacity can be single letter characterized.

Resonant solutions of the quantum Schrödinger equation occur at complex energies where the $S$ matrix becomes singular. Knowledge of such resonances is important in the study of the underlying physical system. Often the Schrödinger equation depends on some parameter and one is interested in following the path of the resonances in the complex energy plane as the parameter changes. This is particularly true in coupled-channel systems where the resonant behavior is highly influenced by the strength of the channel coupling, the energy separation of the channels, and other factors. In previous work it was shown that numerical continuation, a technique familiar in the study of dynamical systems, can be brought to bear on the problem of following the resonance path in one-dimensional problems [J. Broeckhove, P. Klosiewicz, and W. Vanroose, J. Comput. Appl. Math. 234, 1238 (2010).] and multichannel problems without energy separation between the channels [P. Kłosiewicz, J. Broeckhove, and W. Vanroose, Commun. Comput. Phys. 11, 435 (2012).]. A regularization can be defined that eliminates coalescing poles and zeros that appear in the $S$ matrix at the origin due to symmetries. Following the zeros of this regularized function then traces the resonance path. In this work we show that this approach can be extended to channels with energy separation, albeit limited to two channels. The issue here is that the energy separation introduces branch cuts in the complex energy domain that need to be eliminated with a so-called uniformization. We demonstrate that the resulting approach is suitable for investigating resonances in two-channel systems and provide an extensive example.

The resonant process of nuclear excitation by electron transition (NEET) in highly charged ions is investigated. In NEET, a bound electronic decay transition occurs with the simultaneous excitation of the nucleus, provided that the energies of the atomic and nuclear transitions match. By varying the atomic charge state, the atomic transition energy can be tuned to a better match of the nuclear transition energy. We propose a new way to create the atomic hole in highly charged ions by dielectronic capture of a free electron. In order to derive the cross section for the three-step process composed of dielectronic capture, nuclear excitation by electron transition, and the subsequent nuclear decay, a Feshbach projection operator formalism is developed. With the help of this formalism, the resonances in nuclei coupling to the atomic shell and the possible interference between several direct and resonant electronic and nuclear processes such as radiative recombination, dielectronic recombination, and nuclear excitation by electron capture are described.

We present design techniques for special optical lattices that allow quantum simulation of spin frustration in two-dimensional systems. By carefully overlaying optical lattices with different periods and orientations we are able to adjust the ratio between the nearest-neighbor and next-nearest-neighbor interaction strengths in a square spin lattice and realize frustration effects. We show that only laser beams of a single frequency are required and the parameter space reachable in our design is broad enough to study the important phases in the $J_1$-$J_2$ frustrated Heisenberg model and checkerboard antiferromagnet model. By using the polarization spectroscopy for detection, distinct quantum phases and quantum phase transition points can be characterized straightforwardly. Our design thus offers a suitable setup for simulation of frustrated spin systems.

We have created stable sodium Bose-Einstein condensates in the $|F=1,m_F=-1\rangle$ state in a one-dimensional optical lattice with a lifetime of 102 $\pm$ 42 s, which is limited by the photon scattering rate. The condensates were transferred to the upper hyperfine $F$ $=$ 2 state and the one-body and two-body loss rates were examined. Condensates in the $|2,1\rangle$ state were transferred in the optical lattice from the initially prepared condensates using a two-photon microwave–radio-frequency pulse. The two-body loss coefficients in the $|2,0\rangle$ and $|2,1\rangle$ states were found to be (2.1 $\pm$ 1.4) $\times$ 10${}^{-12}$ and (7.7 $\pm$ 1.0) $\times$ 10${}^{-13}$ cm${}^3$ s${}^{-1}$, respectively. The lifetime of the $|2,1\rangle$ state was 15 ms at the initial density of 5 $\times$ 10${}^{14}$ atoms/cm${}^3$. We constructed a Ramsey atom interferometer using BECs in the $|1,-1\rangle$, and $|2,1\rangle$ states with an interrogation time of 2 ms in the optical lattice.

The superradiant emission properties from an atomic ensemble with cascade level configuration is numerically simulated. The correlated spontaneous emissions (signal then idler fields) are purely stochastic processes which are initiated by quantum fluctuations. We utilize the positive-$P$ phase-space method to investigate the dynamics of the atoms and counterpropagating emissions. The light field intensities are calculated, and the signal-idler correlation function is studied for different optical depths of the atomic ensemble. A shorter correlation time scale for a denser atomic ensemble implies a broader spectral window needed to store or retrieve the idler pulse.

Soliton breakup occurring at the planar boundary separating two Kerr focusing and defocusing media is analyzed within the framework of the Helmholtz theory where the full angular content of the problem is preserved. We show that the number of solitons resulting from bright soliton breakup depends on the soliton angle of incidence, contrary to the predictions of previous paraxial analyses. The generation of multiple gray solitons resulting from the interaction of a dark soliton at Kerr defocusing interfaces is studied and the main properties of the output dark soliton pattern are revealed.

In this paper we present the realization of further steps toward the measurement of the magnetic birefringence of a vacuum using pulsed fields. After describing our experiment, we report the calibration of our apparatus using nitrogen gas and discuss the precision of our measurement giving a detailed error budget. Our best present vacuum upper limit is $\Delta n⩽5.0\times {10}^{-20}$ T${}^{-2}$ per 4 ms acquisition time. We finally discuss the improvements necessary to reach our final goal.

Many proposals for solid-state photonic implementations of quantum information processing utilize high-quality optical resonators to achieve strong coupling between guided fields and heterogeneously incorporated qubits. Given the practical difficulty of accurately placing quantum dots, vacancy centers, or other such atomlike emitters throughout a complex nanophotonic circuit, it would be natural to consider whether high-quality resonators could be used in conjunction with bulk optical nonlinearities to create optically coupled qubit degrees of freedom via lithographic patterning of a homogeneous medium. A recent limit theorem for quantum stochastic differential equations can be used to prove rigorously that this should be possible, in principle, using resonators incorporating a strongly Kerr-nonlinear, ${\chi }^{\left(2\right)}$-nonlinear, or two-photon-absorbing material with very low loss at the fundamental optical wavelength.

Recently, Li et al. [Phys. Rev. Lett. 107, 060501 (2011)] have demonstrated that topologically protected measurement-based quantum computation can be implemented on the thermal state of a nearest-neighbor two-body Hamiltonian with spin-2 and spin-3/2 particles provided that the temperature is smaller than a critical value, namely, threshold temperature. Here we show that the thermal state of a nearest-neighbor two-body Hamiltonian, which consists of only spin-3/2 particles, allows us to perform topologically protected measurement-based quantum computation. The threshold temperature is calculated and turns out to be comparable to that with the spin-2 and spin-3/2 system. Furthermore, we generally show that a cluster state of high connectivity can be efficiently generated from the thermal state of the spin-3/2 system without severe thermal noise accumulation.

Unlike the real part of the generalized weak value of an observable, which can in a restricted sense be operationally interpreted as an idealized conditioned average of that observable in the limit of zero measurement disturbance, the imaginary part of the generalized weak value does not provide information pertaining to the observable being measured. What it does provide is direct information about how the initial state would be unitarily disturbed by the observable operator. Specifically, we provide an operational interpretation for the imaginary part of the generalized weak value as the logarithmic directional derivative of the postselection probability along the unitary flow generated by the action of the observable operator. To obtain this interpretation, we revisit the standard von Neumann measurement protocol for obtaining the real and imaginary parts of the weak value and solve it exactly for arbitrary initial states and postselections using the quantum operations formalism, which allows us to understand in detail how each part of the generalized weak value arises in the linear response regime. We also provide exact treatments of qubit measurements and Gaussian detectors as illustrative special cases, and show that the measurement disturbance from a Gaussian detector is purely decohering in the Lindblad sense, which allows the shifts for a Gaussian detector to be completely understood for any coupling strength in terms of a single complex weak value that involves the decohered initial state.

We propose an alternative measure of quantum uncertainty for pairs of arbitrary observables in the two-dimensional case, in terms of collision entropies. We derive the optimal lower bound for this entropic uncertainty relation, which results in an analytic function of the overlap of the corresponding eigenbases. Besides, we obtain the minimum uncertainty states. We compare our relation with other formulations of the uncertainty principle.

We consider a model of a bosonic memory channel, which induces correlations among the transmitted signals. The application of suitable unitary transformations at the encoding and decoding stages allows the complete removal of correlations, thereby mapping the memory channel into a memoryless one. However, such transformations, being global over an arbitrarily large number of bosonic modes, are not realistically implementable. We then introduce a family of efficiently realizable transformations, which can be used to partially remove correlations among errors, and we quantify the reduction of the gap with memoryless channels.

Entanglement between two electrons belonging to an autoionization system and a neighbor two-level atom produced by the dipole-dipole interaction is studied. The entanglement is quantified using the quadratic negativity of a bipartite system including the continuum of states. Suitable conditions for the generation of highly entangled states of two electrons are revealed. Internal structure of the entanglement is elucidated using the spectral density of quadratic negativity.