A class of Hamiltonians that are experimentally feasible in several contexts within quantum optics and lead to so-called cooling by heating for fermionic as well as for bosonic systems has been analyzed numerically. We have found a large range of parameters for which cooling by heating can be observed either for the fermionic system alone or for the combined fermionic and bosonic systems. Analyzing the experimental requirements, we conclude that cooling by heating is achievable with present-day technology, especially in the context of trapped-ion and cavity QED, thus contributing to the understanding of this interesting and counterintuitive effect.

In this Rapid Communication, we experimentally demonstrate that the signal velocity, defined as the earliest time when a signal is detected above the realistic noise floor, may be altered by a region of anomalous dispersion. We encode information in the spatial degree of freedom of an optical pulse so that the imprinted information is not limited by the frequency bandwidth of the region of anomalous dispersion. We then show that the combination of superluminal pulse propagation and realistic detectors with nonideal quantum efficiency leads to a speed-up of the earliest experimentally obtainable arrival time of the transmitted signal even with the overall pulse experiencing unity gain. This speed-up is reliant upon nonideal detectors and losses, as perfect detection efficiency would result in the speed of information being equal to the speed of light in vacuum, regardless of the group velocity of the optical pulses.

We analyze the quantum Zeno dynamics that takes place when a field stored in a cavity undergoes frequent interactions with atoms. We show that repeated measurements or unitary operations performed on the atoms probing the field state confine the evolution to tailored subspaces of the total Hilbert space. This confinement leads to nontrivial field evolutions and to the generation of interesting nonclassical states, including mesoscopic field state superpositions. We elucidate the main features of the quantum Zeno mechanism in the context of a state-of-the-art cavity quantum electrodynamics experiment. A plethora of effects is investigated, from state manipulations by phase space tweezers to nearly arbitrary state synthesis. We analyze in details the practical implementation of this dynamics and assess its robustness by numerical simulations including realistic experimental imperfections. We comment on the various perspectives opened by this proposal.

Inside computer networks, different information processing tasks are necessary to deliver the user data efficiently. This processing can also be done in the quantum domain. We present simple optical quantum networks where the orbital angular momentum (OAM) of a single photon is used as an ancillary degree of freedom which controls decisions at the network level. Linear optical elements are enough to provide important network primitives such as multiplexing and routing. First we show how to build a simple multiplexer and demultiplexer which combine photonic qubits and separate them again at the receiver. We also give two different self-routing networks where the OAM of an input photon is enough to make it find its desired destination.

We propose a sequence of pulses intended to preserve the state of a qubit in the presence of strong, coherent coupling to another quantum system. The sequence can be understood as a generalized swap sequence and works in formal analogy to the well-known spin echo. Since the resulting effective decoherence rate of the qubit state is strongly influenced by the additional system, this sequence might serve to protect its quantum state as well as negating the effects of the coherent coupling. A possible area of application is in large-scale quantum computing architectures, where spectral crowding of the resources might necessitate a method to mitigate residual couplings.

We present a theoretical analysis of single ionization of He by C${}^{6+}$ at an impact energy of 100 Mev/amu and a momentum transfer $Q=0.75$ a.u. Relativistic, second-order Born, and distorted-wave effects on fully differential cross sections are examined. It is demonstrated that neither of them is able to explain the serious discrepancy observed between theory and experiment when an electron is ejected perpendicular to the direction of momentum transfer. We show, however, that experimental uncertainties, including those due to a velocity spread of the He gas atoms in a supersonic jet, can be responsible for the observed disagreement.

Long-range electrostatic, induction, and dispersion coefficients including terms of order $R^{-8}$ have been calculated by the sum over states method using time-dependent density-functional theory. We also computed electrostatic moments and static polarizabilities of the individual diatoms up to the octopole order using coupled-cluster and density-functional theory. The laboratory-frame transformed electrostatic moments and van der Waals coefficients corresponding to the alignment of the diatomic molecules were found. We use this transformation to obtain the coupling induced by an external dc electric field and present values for all $XY$ combinations of like polar alkali-metal diatomic molecules with atoms from Li to Cs. Analytic solutions to the dressed-state laboratory-frame electrostatic moments and long-range intermolecular potentials are also given for the dc low-field limit.

Spin-polarized metastable atoms of ultracold ytterbium are trapped at high density and their inelastic collisional properties are measured. We reveal that in collisions of Yb(${}^3{P}_2$) with Yb(${}^1{S}_0$) there is relatively weak inelastic loss, but with a significant spin dependence consistent with Zeeman sublevel changes as being the dominant decay process. This is in strong contrast to our observations of Yb(${}^3{P}_2$)–Yb(${}^3{P}_2$) collisional losses, which are, at low field, much more rapid and have essentially no spin dependence. Our results give a guideline to using the ${}^3{P}_2$ states in many possible applications.

In recent attempts to control strong-field phenomena such as molecular dissociation, undesired ionization sometimes seriously limited the outcome. In this work we examine the capability of quantum optimal control theory to suppress the ionization by rational pulse shaping. Using a simple model system and the ground-state occupation as the target functional, we show that optimal control generally leads to a significant suppression of the ionization, although the fluence and the pulse length are kept fixed. In the low-frequency regime the ionization is reduced mainly by avoiding high peaks in the intensity and thus preventing tunneling. In contrast, at high frequencies in the extreme ultraviolet regime the optimized pulses strongly couple with the (de)-excitations of the system, which leads to different pulse characteristics. Finally, we show that the applied target functional works, to some extent, for the enhancement of the high-order-harmonic generation, although further developments in optimal control theory to find proper target functionals are required.

The nonlinear wave propagation and scattering processes in discrete optical heterostructures is studied both analytically and numerically. The presented theory describes the reflection and tunneling of lattice solitons at interfaces. In particular, the derived expressions allow us to construct the reflected and transmitted pulses from the incident one. In the range of validity, the analytical results are in very good agreement with the numerical simulations. It is demonstrated that optical heterostructures represent an effective tool for controlling and manipulating nonlinear light pulses and beams.

The Jaynes-Cummings model without the rotating-wave approximation can be solved exactly by an extended Swain ansatz with conserved parity. Analytical approximations are then performed at different levels. The well-known rotating-wave approximation (RWA) is naturally covered in the present zeroth- and first-order approximations. A first-order correction to the RWA can be obtained in a second-order approximation, by which the effect of the counter-rotating-wave term emerges clearly. Concise analytical expressions are given explicitly and can be applicable up to the ultrastrong-coupling regime. A preliminary application to vacuum Rabi splitting is shown to be very successful.

We theoretically investigate properties of individual bright spatial solitons and their interaction in nonlocal media with competing focusing and defocusing nonlinearities. We consider the general case with both nonlinear responses characterized by different strengths and degrees of nonlocality. We employ a variational approach to analytically describe soliton properties. In particular, we prove analytically that the interplay of focusing and defocusing nonlocal nonlinearities leads to attraction or repulsion of solitons depending on their separation distance. We then study the propagation and interaction of solitons using numerical simulations of the full model of beam propagation. The numerical simulations fully confirm our analytical results.

Fourth-order coherence determines the visibility of interference fringes observed in two-photon correlation measurements. In some cases, one photon triggers the presence of its conjugate twin in the interferometer. Therefore, spatial or spectral filtering of the trigger photon may change the visibility of the interference fringes and the degree of fourth-order coherence as well. We show that, contrary to the intuition developed from series of previous experiments, it is possible to increase the fourth-order coherence by reducing the degree of filtering. We present a theoretical approach and experimental results demonstrating this effect and interpret it in terms of spatial mode parity selection.

Measurement-induced back-action, a direct consequence of the Heisenberg uncertainty principle, is the defining feature of quantum measurements. We use quantum measurement theory to analyze the recent experiment of Safavi-Naeini et al. [Phys. Rev. Lett. 108, 033602 (2012)], and show that the results of this experiment not only characterize the zero-point fluctuation of a near-ground-state nanomechanical oscillator, but also demonstrate the existence of quantum back-action noise—through correlations that exist between sensing noise and back-action noise. These correlations arise from the quantum coherence between the mechanical oscillator and the measuring device, which build up during the measurement process, and are key to improving sensitivities beyond the standard quantum limit.

We rederive the Schrödinger-Robertson uncertainty principle for the position and momentum of a quantum particle. Our derivation does not directly employ commutation relations, but works by reduction to an eigenvalue problem related to the harmonic oscillator, which can then be further exploited to find a larger class of constrained uncertainty relations. We derive an uncertainty relation under the constraint of a fixed degree of Gaussianity and prove that, remarkably, it is saturated by all eigenstates of the harmonic oscillator. This goes beyond the common knowledge that the (Gaussian) ground state of the harmonic oscillator saturates the uncertainty relation.

We study linear and nonlinear wave dynamics in the Lieb lattice, in the vicinity of an intersection point between two conical bands and a flat band. We define a pseudospin operator and derive a nonlinear equation for spin-1 waves, analogous to the spin-1/2 nonlinear Dirac equation. We then study the dynamics of wave packets that are associated with different pseudospin states, and find that they are distinguished by their linear and nonlinear conical diffraction patterns.

This paper considers quantum network coding, which is a recent technique that enables quantum information to be sent on complex networks at higher rates than straightforward routing strategies. Kobayashi et al. have recently showed the potential of this technique by demonstrating how any classical network coding protocol gives rise to a quantum network coding protocol. They nevertheless primarily focused on an abstract model, in which quantum resources such as additional quantum registers can be freely introduced at each node. In this work, we present a protocol for quantum network coding under weaker (and more practical) assumptions: Our new protocol works even for quantum networks where adjacent nodes initially share one Einstein-Podolsky-Rosen pair but cannot add any additional quantum registers or send any quantum information. A typical example of networks satisfying this assumption is quantum repeater networks, which are promising candidates for the implementation of large-scale quantum networks. Our results thus show that quantum network coding techniques can increase the transmission rate in such quantum networks as well.

We experimentally detect entanglement in modular position and momentum variables of photon pairs which have passed through $D$-slit apertures. We first employ an entanglement criteria recently proposed in [Phys. Rev. Lett. 106, 210501 (2011)], using variances of the modular variables. We then propose an entanglement witness for modular variables based on the Shannon entropy and test it experimentally. Finally, we derive criteria for Einstein-Podolsky-Rosen-steering correlations using variances and entropy functions. In both cases, the entropic criteria are more successful at identifying quantum correlations in our data.

We theoretically investigate the dephasing of a central spin-1 model. An interesting mechanism of spin decoherence is found with this model, namely, hyperfine mediated spectral diffusion. This mechanism contains the features of both the dipolar-interactions-induced spectral diffusion and the hyperfine mediated interactions. We also find an anomalous magnetic field dependence of decoherence, which is caused by the competition between crystal field splitting and Zeeman splitting of the central spin. As the external magnetic field increases, in the V type level structure regime, the decoherence rate becomes much stronger; while in the cascade type level structure regime, just like localized electron spin in quantum dots, the decoherence rate becomes much weaker.