Author(s): Tim Byrnes, Gary Forster, and Louis Tessler

Grover’s algorithm is a quantum search algorithm that proceeds by repeated applications of the Grover operator and the Oracle until the state evolves to one of the target states. In the standard version of the algorithm, the Grover operator inverts the sign on only one state. Here we provide an exac...

[Phys. Rev. Lett. 120, 060501] Published Wed Feb 07, 2018

Author(s): Shabir Barzanjeh, Matteo Aquilina, and André Xuereb

There has been significant interest recently in using complex quantum systems to create effective nonreciprocal dynamics. Proposals have been put forward for the realization of artificial magnetic fields for photons and phonons; experimental progress is fast making these proposals a reality. Much wo...

[Phys. Rev. Lett. 120, 060601] Published Wed Feb 07, 2018

Author(s): Xin H. H. Zhang and Harold U. Baranger

We obtain photon statistics by using a quantum jump approach tailored to a system in which one or two qubits are coupled to a one-dimensional waveguide. Photons confined in the waveguide have strong interference effects, which are shown to play a vital role in quantum jumps and photon statistics. Fo...

[Phys. Rev. A 97, 023813] Published Wed Feb 07, 2018

Author(s): Andrea Macchi

Experimentalists have used ultraintense laser light to explore a fundamental problem in quantum electrodynamics: the response of an accelerated electron to the radiation it emits.

[Physics 11, 13] Published Wed Feb 07, 2018

Categories: Physics

Experiments and theory show that hairs on a bat’s tongue allow the animal to drink 10 times more nectar than it could if its tongue were smooth.

[Physics] Published Wed Feb 07, 2018

Categories: Physics

Author(s): Akshay Gaikwad, Diksha Rehal, Amandeep Singh, Arvind, and Kavita Dorai

We present the NMR implementation of a scheme for selective and efficient quantum process tomography without ancilla. We generalize this scheme such that it can be implemented efficiently using only a set of measurements involving product operators. The method allows us to estimate any element of th...

[Phys. Rev. A 97, 022311] Published Wed Feb 07, 2018

This dialogue explores the possibility of updating a probability as a consequence of unlearning, reversing the role of prior and posterior probabilities.

We present a flexible scheme to realize non-artificial non-Markovian dynamics of an electronic spin qubit, using a nitrogen-vacancy center in diamond where the inherent nitrogen spin serves as a regulator of the dynamics. By changing the population of the nitrogen spin, we show that we can smoothly tune the non-Markovianity of the electron spin's dynamic. Furthermore, we examine the decoherence dynamics induced by the spin bath to exclude other sources of non-Markovianity. The amount of collected measurement data is kept at a minimum by employing Bayesian data analysis. This allows for a precise quantification of the parameters involved in the description of the dynamics and a prediction of so far unobserved data points.

The positivity conditions of the relative entropy between two thermal equilibrium states $\hat{\rho}_1$ and $\hat{\rho}_2$ are used to obtain upper and lower bounds for the subtraction of their entropies, the Helmholtz potential and the Gibbs potential of the two systems. These limits are expressed in terms of the mean values of the Hamiltonians, number operator, and temperature of the different systems. In particular, we discuss these limits for molecules which can be represented in terms of the Franck--Condon coefficients. We emphasize the case where the Hamiltonians belong to the same system at two different times $t$ and $t'$. Finally, these bounds are obtained for a general qubit system and for the harmonic oscillator with a time dependent frequency at two different times.

Weak potential wells (or traps) in one and two dimensions, and the potential wells slightly deeper than the critical ones in three dimensions, feature shallow bound states with localization length much larger than the well radii. We address a simple fundamental question of how many repulsively interacting bosons can be localized by such traps. We find that under rather generic conditions, for both weakly and strongly repulsive particles, in two and three dimensions--but not in one-dimension!--the potential well can trap infinitely many bosons. For example, even hard-core repulsive interactions do not prevent this "trapping collapse" phenomenon from taking place. For the weakly interacting/dilute regime, the effect can be revealed by the mean-field argument, while in the case of strong correlations the evidence comes from path-integral simulations. We also discuss the possibility of having a transition between the infinite and finite number of trapped particles when strong repulsive inter-particle correlations are increased.

We present and theoretically report the influence of a class of near-parity-time-(PT-) symmetric potentials with spectral filtering parameter $\alpha_2$ and nonlinear gain-loss coefficient $\beta_2$ on solitons in the complex Ginzburg-Landau (CGL) equation. The potentials do not admit entirely-real linear spectra any more due to the existence of coefficients $\alpha_2$ or $\beta_2$. However, we find that most stable exact solitons can exist in the second quadrant of the $(\alpha_2, \beta_2)$ space, including on the corresponding axes. More intriguingly, the centrosymmetric two points in the $(\alpha_2, \beta_2)$ space possess imaginary-axis (longitudinal-axis) symmetric linear-stability spectra. Furthermore, an unstable nonlinear mode can be excited to another stable nonlinear mode by the adiabatic change of $\alpha_2$ and $\beta_2$. Other fascinating properties associated with the exact solitons are also examined in detail, such as the interactions and energy flux. These results are useful for the related experimental designs and applications.

Strong correlation effects emerge from light-matter interactions in coupled resonator arrays, such as the Mott-insulator to superfluid phase transition of atom-photon excitations. We demonstrate that the quenched dynamics of a finite-sized complex array of coupled resonators induces a first-order like phase transition. The latter is accompanied by domain nucleation that can be used to manipulate the photonic transport properties of the emerging superfluid phase; this in turn leads to an empirical scaling law. This universal behavior emerges from the light-matter interaction and the topology of the array. The validity of our results over a wide range of complex architectures might lead to to a promising device for use in scaled quantum simulations.

We present three classes of symmetric broadband composite pulse sequences. The composite phases are given by analytic formulas (rational fractions of $\pi$) valid for any number of constituent pulses. The transition probability is expressed by simple analytic formulas and the order of pulse area error compensation grows linearly with the number of pulses. Therefore, any desired compensation order can be produced by an appropriate composite sequence; in this sense, they are arbitrarily accurate. These composite pulses perform equally well or better than previously published ones. Moreover, the current sequences are more flexible as they allow total pulse areas of arbitrary integer multiples of $\pi$.

Deffner and Lutz [J. Phys. A 46, 335302 (2013) and Phys. Rev. Lett. 111, 010402 (2013).] extended the Mandelstam-Tamm bound and the Margolus-Levitin bound to time-dependent and non-Markovian systems, respectively. Although the derivation of the Mandelstam-Tamm bound is correct, we point out that thier analysis of the Margolus-Levitin bound is incorrect. The Margolus-Levitin bound has not yet been established in time-dependent quantum systems, except for the adiabatic case.

Fibonacci anyons are attractive for use in topological quantum computation because any unitary transformation of their state space can be approximated arbitrarily accurately by braiding. However there is no known braid that entangles two qubits without leaving the space spanned by the two qubits. In other words, there is no known "leakage-free" entangling gate made by braiding. In this paper, we provide a remedy to this problem by supplementing braiding with measurement operations in order to produce an exact controlled rotation gate on two qubits.

The state spaces of both classical and quantum systems have a point-asymmetry about the maximally mixed state except for bit and qubit systems. In this paper, we find an informational origin of this asymmetry: In any operationally valid probabilistic model, the state space has a point-asymmetry in order to store more than a single bit of information. In particular, we introduce a storable information as a natural measure of the storability of information and show the quantitative relation with the so-called Minkowski measure of the state space, which is an affinely invariant measure for point-asymmetry of a convex body. We also show the relation between these quantities and the dimension of the model, inducing some known results in \cite{ref:KNI} and \cite{ref:FMPT} as its corollaries. Also shown are a generalization of weaker form of the dual structure of quantum state spaces, and a generalization of the maximally mixed states as points of the critical set.

We present a mathematical framework for quantum mechanics in which the basic entities and operations have physical significance. In this framework the primitive concepts are states and effects and the resulting mathematical structure is a convex effect algebra. We characterize the convex effect algebras that are classical and those that are quantum mechanical. The quantum mechanical ones are those that can be represented on a complex Hilbert space. We next introduce the sequential product of effects to form a convex sequential effect algebra. This product makes it possible to study conditional probabilities and expectations.

We investigate the qubit in the hierarchical environment where the first level is just one lossy cavity while the second level is the N-coupled lossy cavities. In the weak coupling regime between the qubit and the first level environment, the dynamics crossovers from the original Markovian to the new non-Markovian and from no-speedup to speedup can be realized by controlling the hierarchical environment, i.e., manipulating the number of cavities or the coupling strength between two nearest-neighbor cavities in the second level environment. And we find that the coupling strength between two nearest-neighbor cavities and the number of cavities in the second level environment have the opposite effect on the non-Markovian dynamics and speedup evolution of the qubit. In addition, in the case of strong coupling between the qubit and the first level environment, we can be surprised to find that, compared with the original non-Markovian dynamics, the added second level environment cannot play a beneficial role on the speedup of the dynamics of the system.

In this paper we investigate the completeness of the Stark resonant eigenstates for a particle in a square-well potential. We find that the resonant state expansions for target functions converge inside the potential well and that the existence of this convergence does not depend on the depth of the potential well. By analyzing the asymptotic form of the terms in these expansions we prove some results on the relation between smoothness of target functions and the rate of convergence of the corresponding resonant state expansion.

We demonstrate the ability of an epitaxial semiconductor-superconductor nanowire to serve as a field-effect switch to tune a superconducting cavity. Two superconducting gatemon qubits are coupled to the cavity, which acts as a quantum bus. Using a gate voltage to control the superconducting switch yields up to a factor of 8 change in qubit-qubit coupling between the on and off states without detrimental effect on qubit coherence. High-bandwidth operation of the coupling switch on nanosecond timescales degrades qubit coherence.