Polarization-dependent loss (PDL) is a serious problem that hinders the transfer of polarization qubits through quantum networks. Recently it has been shown that the detrimental effects of PDL on qubit fidelity can be compensated for with the introduction of an additional passive PDL element that rebalances the polarization modes of the transmitted qubit. This procedure works extremely well when the output of the system is postselected on photon detection. However, in cases where the qubit might be needed for further analysis this procedure introduces unwanted vacuum terms into the state. Here we present procedures for the compensation of the effects of PDL using noiseless amplification and attenuation. Each of these techniques introduces a heralding signal into the correction procedure that significantly reduces the vacuum terms in the final state. When detector inefficiency and dark counts are included in the analysis noiseless amplification remains superior, in terms of the fidelity of the final state, to both noiseless attenuation and passive PDL compensation for detector efficiencies greater than 40%.

It is usually difficult to generate mechanical squeezing in standard optomechanical systems when the decay rate of the cavity or the effective coupling strength of the system is too large. Here we investigate the mechanical squeezing beyond resolved sideband and weak-coupling limits in a standard optomechanical system by using frequency modulation acting on both the cavity field and the mechanical oscillator. It is shown that in the presence of frequency modulation, the beyond 3 dB strong mechanical squeezing can be achieved better than the standard optomechanical system without frequency modulation. Furthermore, the proposed scheme is feasible even in the unstable region of the standard optomechanical system, and a detailed numerical analysis is carried out from the physical essence to illuminate the reason for achieving better mechanical squeezing with frequency modulation. The scheme may open promising perspectives for studying the quantum effect in strong coupling regions without the limitations of system stability and the resolved sideband regime.

Coherence manipulation is fundamental in the study of the resource theory of quantum coherence. It is aimed at identifying when it is possible to convert a given coherent state to another using only incoherent operations. In this paper, it is shown that a finite number of measure conditions are not sufficient to characterize coherence manipulation on general mixed states. Further, a finite number of measure conditions to classify coherence manipulation on subspace-independent states are found. For the input pure state, we also furnish a structural characterization of coherence manipulation in terms of a finite number of measure conditions.

We introduce a protocol for authenticated teleportation, which can be proven secure even when the receiver does not trust his or her measurement devices, and which is experimentally accessible. We use the technique of self-testing from the device-independent approach to quantum information, where we can characterize quantum states and measurements from the exhibited classical correlations alone. First, we derive self-testing bounds for the Bell state and Pauli ${\sigma }_X,{\sigma }_Z$ measurements, that are robust enough to be implemented in the laboratory. Then, we use these to determine a lower bound on the fidelity of an untested entangled state to be used for teleportation. Finally, we apply our results to propose a protocol for one-sided device-independent authenticated teleportation that is experimentally feasible in both the number of copies and fidelities required. This can be interpreted as a practical authentication of a quantum channel, with additional one-sided device independence.

In a recent work [Pallister, Linden, and Montanaro, Phys. Rev. Lett. 120, 170502 (2018)], Pallister et al. proposed an optimal strategy to verify nonmaximally entangled two-qubit pure states under the constraint that the accessible measurements are locally projective and nonadaptive. Their good result leads naturally to the following question: What is the optimal strategy among general local operations and classical communication (LOCC) measurements? In this paper, we answer this problem completely for two-qubit pure states. To be specific, we give the optimal strategy for each of the following available classes of measurements: (i) local operations and one-way classical communication (one-way LOCC) measurements; (ii) local operations and two-way classical communication (two-way LOCC) measurements; and (iii) separable measurements. Surprisingly, our results reveal that for the two-qubit pure state verification problem two-way LOCC measurements remarkably outperform one-way LOCC measurements and have the same power as the separable measurements.

We propose practical and efficient protocols for verifying bipartite pure states for any finite dimension, which can also be applied to fidelity estimation. Our protocols are based on adaptive local projective measurements with either one-way or two-way communications, which are very easy to implement in practice. They can extract the key information much more efficiently than known protocols based on tomography or direct fidelity estimation, and their efficiencies are comparable to the best protocols based on entangling measurements. These protocols highlight the significance of mutually unbiased bases and complex projective 2-designs in quantum state verification and fidelity estimation. Moreover, our protocols can be applied to the adversarial scenario, in which states are controlled by a malicious adversary. In this case, surprisingly, one of our protocols based on local measurements is even optimal among protocols without locality restriction.

The direct variational determination of the two-electron reduced-density matrix (2-RDM) corresponding to an atomic or molecular system is usually carried out in a basis of real-valued atom-centered Gaussian basis functions, under the assumption that the 2-RDM is a real-valued quantity. However, for systems that possess orbital angular momentum symmetry, the description of states with a well-defined, nonzero $z$ projection of the orbital angular momentum requires a computational framework generalized to include either complex basis functions or a complex-valued 2-RDM. We consider a semidefinite program suitable for the direct optimization of a complex-valued 2-RDM and explore the role of orbital angular momentum constraints in systems that possess the relevant symmetries. For atomic systems, constraints on the expectation values of the square and $z$ projection of the orbital angular momentum operator allow one to optimize 2-RDMs for multiple orbital angular momentum states. Similarly, in linear molecules, orbital angular momentum projection constraints enable the description of multiple electronic states and, moreover, the application of such constraints is essential for a qualitatively correct description of the electronic structure. For example, in the case of molecular oxygen, we demonstrate that orbital angular momentum constraints are necessary to recover the correct energy ordering of the lowest-energy singlet and triplet states near the equilibrium geometry. However, care must still be taken in the description of the dissociation limit, because the 2-RDM-based approach is not size consistent, and the size-consistency error varies dramatically, depending on the $z$ projections of the spin and orbital angular momenta.

The scenario of a single-mode cavity with harmonically modulated frequency is revisited in the presence of strongly detuned qubit or cyclic qutrit. It is found that when the qubit frequency is close to $3\nu$ there is a peak in the photon generation rate via four-photon transitions for the modulation frequency $4\nu$, where $\nu$ is the average cavity frequency. Effective five-photon processes can occur for the modulation frequency $5\nu$ in the presence of a cyclic qutrit, and the corresponding transition rates exhibit series of peaks. Closed analytical description is derived for the unitary evolution, and numeric simulations indicate the feasibility of multiphoton dynamical Casimir effect under weak dissipation.

We present interferometeric measurements of the $f=1$ to $f=2$ interhyperfine scattering lengths in a single-domain spinor Bose-Einstein condensate of ${}^{87}\mathrm{Rb}$. The interhyperfine interaction leads to a strong and state-dependent modification of the spin-mixing dynamics with respect to a noninteracting description. We employ hyperfine-specific Faraday-rotation probing to reveal the evolution of the transverse magnetization in each hyperfine manifold for different state preparations, and a comagnetometer strategy to cancel laboratory magnetic noise. The method allows precise determination of interhyperfine scattering length differences, calibrated to intrahyperfine scattering length differences. We report $(a_3^{\left(12\right)}-a_2^{\left(12\right)})/(a_2^{\left(1\right)}-a_0^{\left(1\right)})=-1.27\left(15\right)$ and $(a_1^{\left(12\right)}-a_2^{\left(12\right)})/(a_2^{\left(1\right)}-a_0^{\left(1\right)})=-1.31\left(13\right)$, limited by atom number uncertainty. With achievable control of atom number, we estimate precisions of $\approx 0.3\%$ should be possible with this technique.

We approach the strong-coupling thermodynamic limit in the response of the open driven Jaynes-Cummings (JC) oscillator. We do so by highlighting the role of quantum fluctuations against the semiclassical response in three distinct regimes of operation. We begin by demonstrating the persistence of photon blockade, predicted in Carmichael [Phys. Rev. X 5, 031028 (2015)], as a manifestation of the inherently quantum and nonlinear JC spectrum revealed for vanishing dissipation. We then proceed to discuss the importance of bistability, which, despite being present in photon blockades, is able to provide an alignment between the semiclassical nonlinearity and quantum dynamics only for a driving amplitude having the same order of magnitude as the light-matter coupling strength. This resolution brings us to the critical point of the well-known quantum-phase transition of second order on resonance, where the quantum and semiclassical pictures are once more contrasted for a varying participation of the two coherent interactions when going through the collapse of the quasienergy spectrum.

Inspired by the power of abstraction in information theory, we consider quantum rebound protocols as a way of providing a unifying perspective to deal with several information-processing tasks related to and extending quantum channel discrimination to the Shannon-theoretic regime. Such protocols, defined in the most general quantum-physical way possible, have been considered in the physical context of the Das-Wilde (DW) model of quantum reading [arXiv:1703.03706]. In [arXiv:1901.05895], it was discussed how such protocols apply in the different physical context of round-trip communication from one party to another and back. The common point for all quantum rebound tasks is that the decoder himself has access to both the input and output of a randomly selected sequence of channels, and the goal is to determine a message encoded into the channel sequence. As employed in the DW model of quantum reading, the most general quantum-physical strategy that a decoder can employ is an adaptive strategy, in which general quantum operations are executed before and after each call to a channel in the sequence. We determine lower and upper bounds on the quantum rebound capacities in various scenarios of interest, and we also discuss cases in which adaptive schemes provide an advantage over nonadaptive schemes in zero-error quantum rebound protocols.

Given the initial state of the whole system and the spectral density function of the environment, the dynamics of a non-Markovian open system can be exactly determined. However, it has barely been studied how to obtain, in reverse, the spectral density function from the system dynamics. In this work, focusing on the dissipation dynamics of a single-mode non-Markovian open system, we present a simple formalism to determine both the spectral density function and the system mode frequency from only the system dynamics. As examples, we demonstrate the calculation of spectral densities with arbitrary dynamics. Also, in a one-dimensional open chain of bosonic modes, we show how to determine all of its parameters iteratively from only the dynamics on the first mode. This formalism not only reveals features of non-Markovian open systems, but it is also applicable to single-qubit spectroscopy and the estimation of parameters in a structured system.

In analog gravity the recent experiment of Drori et al. [Phys. Rev. Lett. 122, 010404 (2019)] is impressive, as it shows how the emission of two Hawking quanta emitted in opposite directions leads to measurable consequences in the medium's rest system in a straightforward way. This result raises, however, the following problem: how can this experiment be explained in terms of classical electrodynamics? There must necessarily exist such an explanation (the experiment is after all classical); otherwise, classical electrodynamics would be an incomplete theory. This is the main topic of the present paper. We propose that the measured effect is a demonstration of the spacelike character of the Minkowski four-momentum. Moreover, we extend the discussion by analyzing a Gedanken experiment (making use of the Kerr effect as a formal agency) to illustrate the transition from subluminal to superluminal phenomena in a straightforward way. Finally, we emphasize the close relationship that exists between the spacelike Minkowski momentum and the anomalous Doppler effect.

Using the Lieb-Robinson inequality and the continuity property of the quantum capacities in terms of the diamond norm, we derive an upper bound on the values that these capacities can attain in spin-network communication i.i.d. models of arbitrary topology. Different from previous results we make no assumptions about the encoding mechanisms that the sender of the messages adopts in loading information on the network.

We return to the problem of evaluation of the light-by-light contribution to the energy levels of the hydrogen atom. We find an additional contribution directly related to the Delbrück scattering amplitude. The new correction is larger than the previously included light-by-light terms at order ${\alpha }^2{\left(Z\alpha \right)}^{6}ln\left(Z\alpha \right)m_e$. We consider the effective potential in position space using an effective field theory approach and evaluate light-by-light corrections to the energy levels of states with nonzero orbital momentum as well as to the weighted difference of $s$ states. We also determine the large distance asymptotic behavior of the effective potential induced by the light-by-light scattering in muonic atoms.

A natural orbital functional for electronic systems with any value of the spin is proposed. This energy functional is based on an alternative reconstruction for the two-particle reduced density matrix (2RDM) of the multiplet, that is, of the mixed quantum state that allows all possible spin projections. The mixed states of maximum spin multiplicity are considered. This approach differs from the methods routinely used in electronic structure calculations that focus on the high-spin component or break the spin symmetry. In the ensemble, there are no interactions between electrons with opposite spins in singly occupied orbitals as well as new interpair $\alpha \beta$ contributions are proposed in the 2RDM. The proposed 2RDM fulfills (2,2) positivity necessary $N$-representability conditions and guarantees the conservation of the total spin. The natural orbital functional (NOF) for multiplets is able to recover the nondynamic and the intrapair dynamic electron correlation. The missing dynamic correlation is recovered by the NOF-second-order Møller-Plesset method. Calculation of ionization potentials of the first-row transition-metal atoms is presented as test case. The values obtained agree with those reported at the coupled cluster singles and doubles level of theory with perturbative triples and experimental data.

Angular and energy-dependent photon distributions emitted from $50\text{−}\mathrm{keV}$ to $2.5\text{−}\mathrm{MeV}$ electrons colliding with targets ranging from Cu to Pb are calculated using a consistent higher order analytical approach as well as the exact Dirac partial-wave theory. By comparison with exact numerical results for neutral atoms, the validity of the Olsen-Maximon-Wergeland additivity rule for handling the screening correction is probed, and its applicability for collision energies above $100\mathrm{keV}$, if the photon frequency and emission angle are both small, is established. The comparison for a point-Coulomb field, aimed at testing the accuracy of the underlying analytical theory, reveals in most cases a deviation below $60\%$ for Au, decreasing with target nuclear charge down to $10–20\%$ for Cu.