We theoretically introduce the fundamentally fastest induction of a significant population and valley polarization in a monolayer of a transition metal dichalcogenide (i.e., $\mathrm{MoS_2}$ and $\mathrm{WS_2}$). This may be extended to other two-dimensional materials with the same symmetry. This valley polarization can be written and read-out by a pulse consisting of just a single optical oscillation with a duration of a few femtoseconds and an amplitude of $\sim0.2~\mathrm V/\mathrm{\AA}$. Under these conditions, we predict a new effect of {\em topological resonance}, which is due to Bloch motion of electrons in the reciprocal space where electron population textures are formed defined by non-Abelian Berry curvature. The predicted phenomena can be applied for information storage and processing in PHz-band optoelectronics.

We demonstrate, theoretically and experimentally, the generation of hexapartite modal entanglement by the optical parametric oscillator (OPO) operating above the oscillation threshold. We show that the OPO generates a rich structure of entanglement among sets of six optical sideband modes interacting through the non-linear crystal. The class of quantum states thus produced can be controlled by a single parameter, the power of the external laser that pumps the system. Our platform allows for the generation of massive entanglement among many optical modes with well defined but vastly different frequencies, potentially bridging nodes of a multicolor quantum network.

In previous works, we introduced a geometric route to define our Ehrenfest Statistical Dynamics (ESD) and we proved that, for a simple toy-model, the resulting ESD does not preserve purity. We now take a step further: we investigate decoherence and pointer basis in the ESD model by considering some uncertainty in the degrees of freedom of a simple but realistic molecular model, consisting of two classical cores and one quantum electron. The Ehrenfest model is sometimes discarded as a valid approximation to non-adiabatic coupled quantum-classical dynamics because it does not describe the decoherence in the quantum subsystem. However, any rigorous statistical analysis of the Ehrenfest dynamics, such as the described ESD formalism, proves that decoherence exists. In this article, decoherence in ESD is studied by measuring the change in the quantum subsystem purity and by analysing the appearance of the pointer basis to which the system decoheres, which for our example is composed by the eigenstates of the electronic Hamiltonian.

Laser control of solids was so far mainly discussed in the context of strong classical nonlinear light-matter coupling in a pump-probe framework. Here we propose a quantum-electrodynamical setting to address the coupling of a low-dimensional quantum material to quantized electromagnetic fields in quantum cavities. Using a protoypical model system describing FeSe/SrTiO$_3$ with electron-phonon long-range forward scattering, we study how the formation of phonon polaritons at the 2D interface of the material modifies effective couplings and superconducting properties in a Migdal-Eliashberg simulation. We find that through highly polarizable dipolar phonons, large cavity-enhanced electron-phonon couplings are possible but superconductivity is not enhanced for the forward-scattering pairing mechanism due to the interplay between coupling enhancement and mode softening. An analysis of critical temperature dependencies on couplings and mode frequencies suggests that that cavity-enhanced superconductivity is possible for more conventional short-range pairing mechanisms. Our results demonstrate that quantum cavities enable the engineering of fundamental couplings in solids paving the way to unprecedented control of material properties.

We propose a novel scheme to implement the BB84 quantum key distribution (QKD) protocol in optical fibers based on a quantum frequency-translation (QFT) process. Unlike conventional QKD systems, which rely on photon polarization/phase to encode qubits, our proposal utilizes photons of different frequencies. Qubits are thus expected to reach longer propagation distances due to the photon frequency state being more robust against mechanical and/or thermal fluctuations of the transmitting medium. Finally, we put forth an extension to a security-enhanced four-character-alphabet (qu-quarts) QKD scheme.

We study theoretically the spin fluctuations of nuclei in quantum dots. We employ the central spin model which accounts for the hyperfine interaction of the nuclei with the electron spin. We present an analytical solution in the frame of the box model approximation where all hyperfine coupling constants are assumed to be equal. These results are in good agreement with numerical simulations. We demonstrate that in rather high magnetic field the nuclear spin noise spectra has a two-peak structure centered at the nuclear Zeeman frequency with the shape of the spectrum controlled by the distribution of the hyperfine constants.

The term 'locality' is used in different contexts with different meanings. There have been claims that relational quantum mechanics is local, but it is not clear then how it accounts for the effects that go under the usual name of quantum non-locality. The present article shows that the failure of 'locality' in the sense of Bell, once interpreted in the relational framework, reduces to the existence of a common cause in an indeterministic context. In particular, there is no need to appeal to a mysterious space-like influence to understand it.

We study a class of anomalies associated with time-reversal and spatial reflection symmetry in (2+1)D topological phases of matter. In these systems, the topological quantum numbers of the quasiparticles, such as the fusion rules and braiding statistics, possess a $\mathbb{Z}_2$ symmetry which can be associated with either time-reversal (denoted $\mathbb{Z}_2^{\bf T})$ or spatial reflections. Under this symmetry, correlation functions of all Wilson loop operators in the low energy topological quantum field theory (TQFT) are invariant. However, the theories that we study possess a severe anomaly associated with the failure to consistently localize the symmetry action to the quasiparticles, precluding even defining a notion of symmetry fractionalization. We present simple sufficient conditions which determine when $\mathbb{Z}_2^{\bf T}$ symmetry localization anomalies exist. We present an infinite series of TQFTs with such anomalies, some examples of which include USp$(4)_2$ and SO$(4)_4$ Chern-Simons (CS) theory. The theories that we find with these $\mathbb{Z}_2^{\bf T}$ anomalies can be obtained by gauging the unitary $\mathbb{Z}_2$ subgroup of a different TQFT with a $\mathbb{Z}_4^{\bf T}$ symmetry. We show that the anomaly can be resolved in several ways: (1) the true symmetry of the theory is $\mathbb{Z}_4^{\bf T}$, or (2) the theory can be considered to be a theory of fermions, with ${\bf T}^2 = (-1)^{N_f}$ corresponding to fermion parity. Finally, we demonstrate that theories with the $\mathbb{Z}_2^{\bf T}$ localization anomaly can be compatible with $\mathbb{Z}_2^{\bf T}$ if they are "pseudo-realized" at the surface of a (3+1)D symmetry-enriched topological phase. The "pseudo-realization" refers to the fact that the bulk (3+1)D system is described by a dynamical $\mathbb{Z}_2$ gauge theory and thus only a subset of the quasiparticles are confined to the surface.

We investigate experimentally the dynamics of a non-spherical levitated nanoparticle in vacuum. In addition to translation and rotation motion, we observe the light torque-induced precession and nutation of the trapped particle. We provide a theoretical model, which we numerically simulate and from which we derive approximate expressions for the motional frequencies. Both, the simulation and approximate expressions, we find in good agreement with experiments. We measure a torque of $1.9 \pm 0.5 \times 10^{-23}$ Nm at $1 \times 10^{-1}$ mbar, with an estimated torque sensitivity of $3.6 \pm 1.1 \times 10^{-31}$ Nm/$\sqrt{\text{Hz}}$ at $1 \times 10^{-7}$ mbar.

Span programs characterize the quantum query complexity of \emph{binary} functions $f:\{0,1\}^n \to \{0,1\}$ up to a constant factor. In this paper we generalize the notion of span programs for functions with \emph{non-binary} input and/or output alphabets $f:[\ell]^n \to [m]$. We show that for any non-binary span program for such a function $f$ with complexity $C$, the quantum query complexity of $f$ is at most $Q(f)=O(C)$. Conversely, there exists a non-binary span program for $f$ with complexity $\sqrt{\ell-1}\,Q(f)$. Thus, we conclude that non-binary span programs characterize the quantum query complexity of $f$ up to a factor of order at most $\sqrt{\ell-1}$. By giving explicit examples, we show that this $\sqrt{\ell-1}$ factor cannot be improved. We also generalize the notion of span programs for a special class of relations and prove similar results.

Learning graphs provide another tool for designing quantum query algorithms for binary functions. In this paper, we also generalize this tool for non-binary functions.

Recently several gain-dissipative platforms based on the networks of optical parametric oscillators, lasers, and various non-equilibrium Bose-Einstein condensates have been proposed and realised as analogue Hamiltonian simulators for solving large-scale hard optimisation problems. However, in these realisations the parameters of the problem depend on the node occupancies that are not a priory known, which limits the applicability of the gain-dissipative simulators to the classes of problems easily solvable by classical computations. We show how to overcome this difficulty and formulate the principles of operation of such simulators for solving the NP-hard large-scale optimisation problems such as constant modulus continuous quadratic optimisation and quadratic binary optimisation for any general matrix. To solve such problems any gain-dissipative simulator has to implement a feedback mechanism for the dynamical adjustment of the gain and coupling strengths.

We propose a method of estimating ergodization time of a chaotic many-particle system by monitoring equilibrium noise before and after time reversal of dynamics (Loschmidt echo). The ergodization time is defined as the characteristic time required to extract the largest Lyapunov exponent from a system's dynamics. We validate the method by numerical simulation of an array of coupled Bose-Einstein condensates in the regime describable by the discrete Gross-Pitaevskii equation. The quantity of interest for the method is a counterpart of out-of-time-order correlators (OTOCs) in the quantum regime.

Strong and general entropic and geometric Heisenberg limits are obtained, for estimates of multiparameter unitary displacements in quantum metrology, such as the estimation of a magnetic field from the induced rotation of a probe state in three dimensions. A key ingredient is the Holevo bound on the Shannon mutual information of a quantum communication channel. This leads to a Bayesian bound on performance, in terms of the prior distribution of the displacement and the asymmetry of the input probe state with respect to the displacement group. A geometric measure of performance related to entropy is proposed for general parameter estimation. It is also shown how strong entropic uncertainty relations for mutually unbiased observables, such as number and phase, position and momentum, energy and time, and orthogonal spin-1/2 directions, can be obtained from elementary applications of Holevo's bound. A geometric interpretation of results is emphasised, in terms of the 'volumes' of quantum and classical statistical ensembles.

Author(s): Jie Li, Simon Gröblacher, Shi-Yao Zhu, and G. S. Agarwal

Adding excitations on a coherent state provides an effective way to observe the nonclassical properties of radiation fields. Here, we describe and analyze how to apply this concept to the motional state of a mechanical oscillator and present a full scheme to prepare non-Gaussian *phonon*-added coheren...

[Phys. Rev. A 98, 011801(R)] Published Mon Jul 02, 2018

Author(s): Robert Cook, David I. Schuster, Andrew N. Cleland, and Kurt Jacobs

Input-output theory is invaluable for treating superconducting and photonic circuits connected by transmission lines or waveguides. However, this theory cannot in general handle situations in which retroreflections from circuit components or configurations of beam splitters create loops for the trav...

[Phys. Rev. A 98, 013801] Published Mon Jul 02, 2018

Author(s): Alexander A. Zharov, Alexander A. Zharov, Jr., and Nina A. Zharova

We show that linearly polarized light in a suspension of gyrotropic nanoparticles can experience modulation instability, which leads to spatial separation of right- and left-circularly polarized waves. Such a separation preserves zero full angular momentum of the electromagnetic field; thus, the tim...

[Phys. Rev. A 98, 013802] Published Mon Jul 02, 2018

Author(s): Abraham Loeb and Julián B. Muñoz

Recent observations of hydrogen absorption that occurred when the first stars turned on may give insights into the nature of dark matter, new analyses show.

[Physics 11, 69] Published Mon Jul 02, 2018

Categories: Physics

Author(s): Junkai Zeng and Edwin Barnes

Dynamically correcting for unwanted interactions between a quantum system and its environment is vital to achieving the high-fidelity quantum control necessary for a broad range of quantum information technologies. In recent work, we uncovered the complete solution space of all possible driving fiel...

[Phys. Rev. A 98, 012301] Published Mon Jul 02, 2018

Author(s): Arun B. Aloshious and Pradeep Kiran Sarvepalli

Toric codes and color codes are two important classes of topological codes. Kubica *et al.* [A. Kubica *et al.*, New J. Phys. **17**, 083026 (2015)] showed that any D-dimensional color code can be mapped to a finite number of toric codes in D dimensions. We propose an alternate map of three-dimensional (3D...

[Phys. Rev. A 98, 012302] Published Mon Jul 02, 2018