In this paper we will discuss how to localise a quantum wave-packet due to self-gravitating meso-scopic object by taking into account gravitational self-interaction in the Schr\"odinger equation beyond General Relativity. In particular, we will study soliton-like solutions in infinite derivative ghost free theories of gravity, which resolves the gravitational $1/r$ singularity in the potential. We will show a unique feature that the quantum spread of such a gravitational system is larger than that of the Newtonian gravity, therefore enabling us a window of opportunity to test classical and quantum properties of such theories of gravity in the near future at a table-top experiment.

We demonstrate electro-mechanical control of an on-chip GaAs optical beam splitter containing a quantum dot single-photon source. The beam splitter consists of two nanobeam waveguides, which form a directional coupler (DC). The splitting ratio of the DC is controlled by varying the out-of-plane separation of the two waveguides using electro-mechanical actuation. We reversibly tune the beam splitter between an initial state, with emission into both output arms, and a final state with photons emitted into a single output arm. The device represents a compact and scalable tuning approach for use in III-V semiconductor integrated quantum optical circuits.

Entanglement-enhanced atom interferometry has the potential of surpassing the standard quantum limit and eventually reaching the ultimate Heisenberg bound. The experimental progress is, however, hindered by various technical noise sources, including the noise in the detection of the output quantum state. The influence of detection noise can be largely overcome by exploiting echo schemes, where the entanglement-generating interaction is repeated after the interferometer sequence. Here, we propose an echo protocol that uses two-axis counter-twisting as the main nonlinear interaction. We demonstrate that the scheme is robust to detection noise and its performance is superior compared to the already demonstrated one-axis twisting echo scheme. In particular, the sensitivity maintains the Heisenberg scaling in the limit of a large particle number. Finally, we show that the protocol can be implemented with spinor Bose-Einstein condensates. Our results thus outline a realistic approach to mitigate the detection noise in quantum-enhanced interferometry.

We study periodically driven scalar fields and the resulting geometries with global AdS asymptotics. These solutions describe the strongly coupled dynamics of dual finite-size quantum systems under a periodic driving which we interpret as Floquet condensates. They span a continuous two-parameter space that extends the linearized solutions on AdS. We map the regions of stability in the solution space. In a significant portion of the unstable subspace, two very different endpoints are reached depending upon the sign of the perturbation. Collapse into a black hole occurs for one sign. For the opposite sign instead one attains a regular solution with periodic modulation. We also construct quenches where the driving frequency and amplitude are continuously varied. Quasistatic quenches can interpolate between pure AdS and sourced solutions with time periodic vev. By suitably choosing the quasistatic path one can obtain boson stars dual to Floquet condensates at zero driving field. We characterize the adiabaticity of the quenching processes. Besides, we speculate on the possible connections of this framework with time crystals.

Slow variations (quenches) of the magnetic field across the paramagnetic-ferromagnetic phase transition of spin systems produce heat. In short-ranged systems the heat exhibits a universal power-law scaling as a function of the quench rate, known as Kibble-Zurek (KZ) scaling. Attempts to extend this hypothesis to long-range interacting systems have lead to seemingly contradicting results. In this work we analyse slow quenches of the magnetic field in the Lipkin-Meshkov-Glick model, which describes fully-connected quantum spins. We determine the quantum contribution to the residual heat as a function of the quench rate by means of a Bogoliubov expansion about the mean-field value and calculate the exact solution. For a quench which ends at the quantum critical point we identify two regimes: the adiabatic limit for finite-size chains, where the scaling is dominated by the Landau-Zener tunneling, and the KZ scaling. For a quench symmetric about the critical point, instead, there is no KZ scaling. Here we identify three regimes depending on the velocity of the ramp and on the size of the system: (i) the adiabatic limit for finite-size chains; (ii) the opposite regime, namely, the thermodynamic limit, where the residual heat is independent of the quench rate; and finally (iii) the intermediate regime, which is a crossover between the two solutions. We argue that this behaviour is a property of all-connected spin systems. Our findings agree with previous studies and identify the respective limits in which they hold.

We provide a complete set of identities for the symmetric monoidal category, $\sf TOF$, generated by the Toffoli gate and computational ancillary bits. We do so by demonstrating that the functor which evaluates circuits on total points, is an equivalence into the full subcategory of sets and partial isomorphisms with objects finite powers of the two element set. The structure of the proof builds -- and follows the proof of Cockett et al. -- which provided a full set of identities for the $\sf cnot$ gate with computational ancillary bits. Thus, first it is shown that $\sf TOF$ is a discrete inverse category in which all of the identities for the $\sf cnot$ gate hold; and then a normal form for the restriction idempotents is constructed which corresponds precisely to subobjects of the total points of $\sf TOF$. This is then used to show that $\sf TOF$ is equivalent to ${\sf FPinj}_2$, the full subcategory of sets and partial isomorphisms in which objects have cardinality $2^n$ for some $n \in \mathbb{N}$.

We study properties of heavy-light-heavy three-point functions in two-dimensional CFTs by using the modular invariance of two-point functions on a torus. We show that our result is non-trivially consistent with the condition of ETH (Eigenstate Thermalization Hypothesis). We also study the open-closed duality of cylinder amplitudes and derive behaviors of disk one-point functions.

Quantum key distribution (QKD) allows two remote users to establish a secret key in the presence of an eavesdropper. The users share quantum states prepared in two mutually-unbiased bases: one to generate the key while the other monitors the presence of the eavesdropper. Here, we show that a general $d$-dimension QKD system can be secured by transmitting only a subset of the monitoring states. In particular, we find that there is no loss in the secure key rate when dropping one of the monitoring states. Furthermore, it is possible to use only a single monitoring state if the quantum bit error rates are low enough. We apply our formalism to an experimental $d=4$ time-phase QKD system, where only one monitoring state is transmitted, and obtain a secret key rate of $17.4 \pm 2.8$ Mbits/s at a 4 dB channel loss and with a quantum bit error rate of $0.045\pm0.001$ and $0.044\pm0.001$ in time and phase bases, respectively, which is 58.4 % of the secret key rate that can be achieved with the full setup. This ratio can be increased, potentially up to 100 %, if the error rates in time and phase basis are reduced. Our results demonstrate that it is possible to substantially simplify the design of high-dimension QKD systems, including those that use the spatial or temporal degrees-of-freedom of the photon, and still outperform qubit-based ($d = 2$) protocols.

In this article I study different possibilities of analytically solving the Sturm-Liouville problem with variable coefficients of sufficiently arbitrary behavior with help of perturbation theory. I show how the problem can be reformulated in order to eliminate big (or divergent) corrections. I obtain correct formulas in case of smooth as well as in case of step-wise (piece-constant) coefficients. I build simple, but very accurate analytical formulae for calculating the lowest eigenvalue and the ground state eigenfunction. I advance also new boundary conditions for obtaining more precise initial approximations. I demonstrate how one can optimize the PT calculation with choosing better initial approximations and thus diminishing the perturbative corrections. Dressing, Rebuilding, and Renormalizations are discussed in Appendices 4 and 5.

We give an explicit and general description of the energy, linear momentum, angular momentum and boost momentum of a molecule to order $1/c^2$, where it necessary to take account of kinetic contributions made by the electrons and nuclei as well as electromagnetic contributions made by the intramolecular field. A wealth of interesting subtleties are encountered that are not seen at order $1/c^0$, including relativistic Hall shifts, anomalous velocities and hidden momenta. Some of these have well known analogues in solid state physics.

Author(s): Ankit K. Singh, Subir K. Ray, Shubham Chandel, Semanti Pal, Angad Gupta, P. Mitra, and N. Ghosh

Weak measurement enables faithful amplification and high-precision measurement of small physical parameters and is under intensive investigation as an effective tool in metrology and for addressing foundational questions in quantum mechanics. Here we demonstrate weak-value amplification using the as...

[Phys. Rev. A 97, 053801] Published Thu May 03, 2018

Author(s): Lei Han, Sheng Liu, Peng Li, Yi Zhang, Huachao Cheng, and Jianlin Zhao

We report on the catalystlike effect of orbital angular momentum (OAM) on local spin-state conversion within the tightly focused radially polarized beams associated with optical spin-orbit interaction. It is theoretically demonstrated that the incident OAM can lead to a conversion of purely transver...

[Phys. Rev. A 97, 053802] Published Thu May 03, 2018

Author(s): Sylvain de Léséleuc, Daniel Barredo, Vincent Lienhard, Antoine Browaeys, and Thierry Lahaye

Arrays of single atoms promoted to Rydberg states offer a versatile platform for quantum simulation and implementation of two-qubit gates. A comprehensive account of the limiting factors and sources of errors in their experimental preparation is presented.

[Phys. Rev. A 97, 053803] Published Thu May 03, 2018

Experiments demonstrate that stretching a DNA strand can untie any knots it contains.

[Physics] Published Thu May 03, 2018

Categories: Physics

Author(s): C. B. Gallagher and A. Ferraro

A possible alternative to the standard model of measurement-based quantum computation (MBQC) is offered by the sequential model of MBQC—a particular class of quantum computation via ancillae. Although these two models are equivalent under ideal conditions, their relative resilience to noise in pract...

[Phys. Rev. A 97, 052305] Published Thu May 03, 2018

In this paper, I'm going to talk about the theoretical and experimental progress in studying spin-orbit coupled spin-1 bosons. Realization of spin-orbit coupled quantum gases opens a new avenue in cold atom physics. In particular, the interplay between spin-orbit coupling and inter-atomic interaction leads to many intriguing phenomena. Moreover, the non-zero momentum of ground states can be controlled by external fields, which allows for good quantum control.

We analyze phase transitions induced by forbidding charges and fluxes in $\mathcal{D}(S_3)$, the simplest non-Abelian model among quantum doubles, a class of 2D spin lattice topological models introduced by Kitaev. Contrary to a topological quantum field theory, the lattice degrees of freedom allow to forbid charges and fluxes independently, resulting in a non-trivial effect on dyons. Forbidding charges and fluxes leads to only a subset of the original anyons remaining, forming a new theory. We interpret the processes the theory undergoes in terms of condensation, spontaneous symmetry breaking, splitting of particles. Mapping the complete phase diagram of $\mathcal{D}(S_3)$, we find two distinct groups of phases: quantum doubles of subgroups of $S_3$, and a non-trivial emergent chiral phase, $SU(2)_4$.

We address the construction of analytically integrable complex-valued potentials by linear superpositions of fundamental bright and dark optical solitons that solve cubic nonlinear Schroedinger equations. The real part of the potentials coincides with the bright soliton intensity. The imaginary part results from the convolution of the bright soliton with its concomitant, a localized dark excitation that arises from repulsive nonlinearities in the media. In general, the method leads to the Gross-Pitaevskii nonlinear differential equation, so the above results correspond to the absence of external interactions. The potentials presented here may find applications in the study of self-focussing signals that propagate in nonlinear media with balanced gain and loss since they are parity-time symmetrical.

The usefulness of a quantum system as a sensor is given by the quantum Fisher information (QFI) which quantifies the sensitivity of its quantum states to perturbations. In particular, for unitary perturbations useful quantum states are necessarily coherent. Quantum enhanced sensing with many-body states relies on multipartite entanglement (MPE), and therefore QFI is used as an entanglement witness. Here we show that for systems with a fixed local charge (for example fixed density) the connection between QFI and MPE simplifies. In this case, QFI can become a faithful witness of MPE, as a consequence of the emerging direct relation between MPE and coherence in a quantum state, and coherence (as quantified by relative entropy) becomes a faithful upper bound for the relative entropy MPE. When the local charge is not fixed but conserved, QFI becomes a faithful witness of multipartite quantum discord (i.e. quantumness) and coherence becomes its faithful upper bound. Analogously, we show how the bipartite entanglement (BPE) of a fixed-charge state can be witnessed by the QFI related to unitary perturbations of the bipartition, while the corresponding block coherence (i.e., charge asymmetry between partitions) serves as a lower bound on BPE of formation. As estimating QFI is difficult for mixed states of open quantum systems, we adapt a recently introduced protocol that measures QFI of pure states and provides a lower bound for the QFI in open systems. When conservation laws are present, this lower bound can also be a faithful witness of MPE, and furthermore a lower bound of a BPE measure. We illustrate these general results with an application to the problem of detecting the growth of entanglement in a many-body localised system with and without dissipation.

Financial derivatives are contracts that can have a complex payoff dependent upon underlying benchmark assets. In this work, we present a quantum algorithm for the Monte Carlo pricing of financial derivatives. We show how the relevant probability distributions can be prepared in quantum superposition, the payoff functions can be implemented via quantum circuits, and the price of financial derivatives can be extracted via quantum measurements. We show how the amplitude estimation algorithm can be applied to achieve a quadratic quantum speedup in the number of steps required to obtain an estimate for the price with high confidence. This work provides a starting point for further research at the interface of quantum computing and finance.