We recently showed [Phys. Rev. Lett. 121, 220602 (2018)] that the average bipartite entanglement entropy of all energy eigenstates of the quantum Ising chain exhibits a universal (for translationally invariant quadratic fermionic models) leading term that scales linearly with the subsystem's volume, while in the thermodynamic limit the first subleading correction does not vanish at the critical field (it only depends on the ratio $f$ between the volume of the subsystem and volume of the system) and vanishes otherwise. Here we show, analytically for bounds and numerically for averages, that the same remains true for the spin-1/2 XY chain in a transverse magnetic field. We then tighten the bounds for the coefficient of the universal volume-law term, which is a concave function of $f$. We develop a systematic approach to compute upper and lower bounds, and provide explicit analytic expressions for up to the fourth order bounds.

We show how to create quantum squeezed states of magnons and phonons in a cavity magnomechanical system. The magnons are embodied by a collective motion of a large number of spins in a macroscopic ferrimagnet, and couple to cavity microwave photons and phonons (vibrational modes of the ferrimagnet) via the magnetic dipole interaction and magnetostrictive interaction, respectively. The cavity is driven by a weak squeezed vacuum field generated by a flux-driven Josephson parametric amplifier, which is essential to get squeezed states of the magnons and phonons. We show that the magnons can be prepared in a squeezed state via the cavity-magnon beamsplitter interaction, and by further driving the magnon mode with a strong red-detuned microwave field, the phonons are squeezed. We show optimal parameter regimes for obtaining large squeezing of the magnons and phonons, which are robust against temperature and could be realized with experimentally reachable parameters.

Here we present a flexible tensor network based simulator for quantum circuits on different topologies, including the Google Bristlecone QPU. Our simulator can compute both exact amplitudes, a task essential for the verification of the quantum hardware, as well as low-fidelity amplitudes to mimic Noisy Intermediate-Scale Quantum (NISQ) devices. While our simulator can be used to compute amplitudes of arbitrary quantum circuits, we focus on random quantum circuits (RQCs) [Boixo et al., Nature Physics 14] in the range of sizes expected for supremacy experiments. Our simulator enables the simulation of sampling on quantum circuits that were out of reach for previous approaches. For instance, our simulator is able to output single amplitudes with depth 1+32+1 for the full Bristlecone QPU in less than $(f \cdot 4200)$ hours on a single core, where $0<f\leq1$ is the target fidelity, on $2\times20$-core Intel Xeon Gold 6148 processors (Skylake). We also estimate that computing $10^6$ amplitudes (with fidelity 0.50\%) needed to sample from the full Bristlecone QPU with depth (1+32+1) would require about 3.5 days using the NASA Pleiades and Electra supercomputers combined. In addition, we discuss the hardness of the classical simulation of RQCs, as well as give evidence for the higher complexity in the simulation of Bristlecone topology as compared to other two-dimensional grids with the same number of qubits. Our analysis is supported by extensive simulations on NASA HPC clusters Pleiades and Electra. For the most computationally demanding simulation we had, namely the simulation of a $60$-qubit sub-lattice of Bristlecone, the two HPC clusters combined reached a peak of 20 PFLOPS (single precision), that is $64\%$ of their maximum achievable performance. To date, this numerical computation is the largest in terms of sustained PFLOPS and number of nodes utilized ever run on NASA HPC clusters.

There are two schools of "measurement-only quantum computation". The first using prepared entanglement (cluster states) and the using collections of anyons, which according to how they were produced, also have an entanglement pattern. We abstract the common principle behind both approaches and find the notion of a graph or even continuous family of equiangular projections. This notion is the leading character in the paper. The largest continuous family we have found, in a sense made precise in Corollary 4.2, is associated with the octonions and this example leads to a universal computational scheme. Adiabatic quantum computation also fits into this rubric as a limiting case: nearby projections are nearly equiangular, so as a gapped ground state space is slowly varied the corrections to unitarity are small.

Author(s): Randall Kamien

[Rev. Mod. Phys. 91, 010001] Published Wed Feb 13, 2019

Author(s): Jari Turunen and Frank Wyrowski

We extend the concept of the Eberly-Wódkiewicz time-dependent physical spectrum of light to electromagnetic fields by considering the observable time dependence of four appropriately defined Stokes parameters. We also define the concept of time-dependent physical degree of polarization of light by m...

[Phys. Rev. A 99, 023824] Published Wed Feb 13, 2019

Author(s): Xiaoqin Gao, Mario Krenn, Jaroslav Kysela, and Anton Zeilinger

High-dimensional degrees of freedom of photons can encode more quantum information than their two-dimensional counterparts. While the increased information capacity has advantages in quantum applications (such as quantum communication), controlling and manipulating these systems has been challenging...

[Phys. Rev. A 99, 023825] Published Wed Feb 13, 2019

Author(s): Amit Rotem, Tuvia Gefen, Santiago Oviedo-Casado, Javier Prior, Simon Schmitt, Yoram Burak, Liam McGuiness, Fedor Jelezko, and Alex Retzker

The limits of frequency resolution in nano-NMR experiments have been discussed extensively in recent years. It is believed that there is a crucial difference between the ability to resolve a few frequencies and the precision of estimating a single one. Whereas the efficiency of single frequency esti...

[Phys. Rev. Lett. 122, 060503] Published Wed Feb 13, 2019

Author(s): Varun Narasimhachar, Jayne Thompson, Jiajun Ma, Gilad Gour, and Mile Gu

The information-carrying capacity of a memory is known to be a thermodynamic resource facilitating the conversion of heat to work. Szilard’s engine explicates this connection through a toy example involving an energy-degenerate two-state memory. We devise a formalism to quantify the thermodynamic va...

[Phys. Rev. Lett. 122, 060601] Published Wed Feb 13, 2019

A proposed modification to a microfluidic cell-sorting device could separate cells by their deformability, an important marker for several diseases.

[Physics] Published Wed Feb 13, 2019

Categories: Physics

Author(s): Zixin Huang, Chiara Macchiavello, and Lorenzo Maccone

We develop a general framework for parameter estimation that allows only trusted parties to access the result and achieves optimal precision. The protocols are designed such that adversaries can access some information indeterministically, but only at the risk of getting caught (cheat sensitivity); ...

[Phys. Rev. A 99, 022314] Published Wed Feb 13, 2019

Author(s): Aditya Jain, Indranil Chakrabarty, and Sourav Chatterjee

In this work, we exhaustively investigate 1→2 local and nonlocal broadcasting of entanglement as well as correlations beyond entanglement (geometric discord) using asymmetric Pauli cloners with the most general two-qubit state as the resource. We exemplify asymmetric broadcasting of entanglement usi...

[Phys. Rev. A 99, 022315] Published Wed Feb 13, 2019

Author(s): Kenji Nakahira, Kentaro Kato, and Tsuyoshi Sasaki Usuda

We investigate unambiguous discrimination between given quantum states with a sequential measurement, which is restricted to local measurements and one-way classical communication. If the given states are binary or each of their individual state spaces is two dimensional, then it is in some cases kn...

[Phys. Rev. A 99, 022316] Published Wed Feb 13, 2019

The study of information scrambling in many-body systems has sharpened our understanding of quantum chaos, complexity and gravity. Here, we extend the framework for exploring information scrambling to infinite dimensional continuous variable (CV) systems. Unlike their discrete variable cousins, continuous variable systems exhibit two complementary domains of information scrambling: i) scrambling in the phase space of a single mode and ii) scrambling across multiple modes of a many-body system. Moreover, for each of these domains, we identify two distinct `types' of scrambling; genuine scrambling, where an initial operator localized in phase space spreads out and quasi scrambling, where a local ensemble of operators distorts but the overall phase space volume remains fixed. To characterize these behaviors, we introduce a CV out-of-time-order correlation (OTOC) function based upon displacement operators and offer a number of results regarding the CV analog for unitary designs. Finally, we investigate operator spreading and entanglement growth in random local Gaussian circuits; to explain the observed behavior, we propose a simple hydrodynamical model that relates the butterfly velocity, the growth exponent and the diffusion constant. Experimental realizations of continuous variable scrambling as well as its characterization using CV OTOCs will be discussed.

We investigate the subadditivity of the bipartite entanglement entropy (EE) of many-particle states, represented by Slater determinants, with respect to single particle excitations. By quantifying this subadditivity we identify sets of single particle states that yield zero EE if jointly excited. Such states we dub entanglement erasing partner states (EEPS). By this we have identified a mechanism that allows to disentangle two subspaces of a Hilbert space by exciting additional states. We illustrate the entanglement erasure in Anderson insulators, where we identify the EEPS, and use the underlying mechanism to explain discrete entanglement bands in the clean tight binding model. Our results indicate a direct applicability to their interacting counterparts. We further discuss how EEPS impose a universal erasure of EE for randomly excited states - independent of the Hamiltonian of interest. This feature allows to compute many-particle EE by means of the associated single particle states and the filling ratio. This novel finding can be employed to drastically reduce the computational effort in free models.

Electrons on helium form a unique two-dimensional electron system on the interface of liquid helium and vacuum. On liquid helium, trapped electrons can arrange into strongly correlated states known as Wigner molecules, which can be used to study electron interactions in the absence of disorder, or as a highly promising resource for quantum computation. Wigner molecules have orbital frequencies in the microwave regime and can therefore be integrated with circuit quantum electrodynamics (cQED), which studies light-matter interactions using microwave photons. Here, we experimentally realize a cQED platform with the orbital state of Wigner molecules on helium. We deterministically prepare one to four-electron Wigner molecules on top of a microwave resonator, which allows us to observe their unique spectra for the first time. Furthermore, we find a single-electron-photon coupling strength of g/2$\pi$ = 4.8$\pm$0.3 MHz, greatly exceeding the resonator linewidth $\kappa$/2$\pi$ = 0.5 MHz. These results pave the way towards microwave studies of strongly correlated electron states and coherent control of the orbital and spin state of Wigner molecules on helium.}

There is widespread disagreement about how the general covariance of a theory affects its quantization. Without a complete quantum theory of gravity, one can examine quantum consequences of coordinate choices only in highly idealized `toy' models. In this work, we extend our previous analysis of a self-gravitating shell model [1], and demonstrate that coordinate freedom can be retained in a reduced phase space description of the system. We first consider a family of coordinate systems discussed by Martel and Poisson [2], which have time coordinates that coincide with the proper times of ingoing and outgoing geodesics (for concreteness, we only consider the former). Included in this family are Painlev\'e-Gullstrand coordinates, related to a network of infalling observers that are asymptotically at rest, and Eddington-Finkelstein coordinates, related to a network of infalling observers that travel at the speed of light. We then introduce "inflying" coordinates - a hybrid coordinate system that allows the infalling observers to be arbitrarily boosted from one member of the aforementioned family to another. We perform a phase space reduction using inflying coordinates with an unspecified boosting function, resulting in a reduced theory with residual coordinate freedom. Finally, we discuss quantization, and comment on the utility of the reduced system for the study of coordinate effects and the role of observers in quantum gravity.

We propose a novel hybrid quantum-classical approach to calculate Graver bases, which have the potential to solve a variety of hard linear and non-linear integer programs, as they form a test set (optimality certificate) with very appealing properties. The calculation of Graver bases is exponentially hard (in general) on classical computers, so they not used for solving practical problems on commercial solvers. With a quantum annealer, however, it may be a viable approach to use them. We test two hybrid quantum-classical algorithms (on D-Wave)--one for computing Graver basis and a second for optimizing non-linear integer programs that utilize Graver bases--to understand the strengths and limitations of the practical quantum annealers available today. Our experiments suggest that with a modest increase in coupler precision--along with near-term improvements in the number of qubits and connectivity (density of hardware graph) that are expected--the ability to outperform classical best-in-class algorithms is within reach, with respect to non-linear integer optimization.

We present a method for observing the dynamical Casimir effect (DCE) induced by a mechanical motion in an optomechanical system. This employs a detuned, parametric driving to squeeze a cavity mode, so that the squeezed-cavity-mode frequency is externally tunable. The mechanical mode, with a typical resonance frequency, can parametrically and resonantly couple to the squeezed cavity mode, thus leading to a resonantly amplified DCE in the squeezed frame. This process, in the laboratory frame, can also be interpreted as anti-Stokes hyper-Raman scattering. We demonstrate observable signatures of the DCE, including an output-photon flux spectrum with a resolved resonance peak, and a monotonic increase in the maxima of this peak with the strength of a mechanical driving. Furthermore, we demonstrate that for weak squeezing, the DCE can be mapped from the squeezed to the laboratory frame, which allows for a more direct DCE detection. Our method requires neither an ultra-high mechanical frequency nor an ultrastrong single-photon optomechanical coupling and, thus, could be implemented in a wide range of physical systems.

Classical one-time-pad key can only be used once. We show in this Letter that with quantum mechanical information media classical one-time-pad key can be repeatedly used. We propose a specific realization using single photons. The reason why quantum mechanics can make the classical one-time-pad key repeatable is that quantum states can not be cloned and eavesdropping can be detected by the legitimate users. This represents a significant difference between classical cryptography and quantum cryptography and provides a new tool in designing quantum communication protocols and flexibility in practical applications.

Note added: This work was submitted to PRL as LU9745 on 29 July 2004, and the decision was returned on 11 November 2004, which advised us to resubmit to some specialized journal, probably, PRA, after revision. We publish it here in memory of Prof. Fu-Guo Deng (1975.11.12-2019.1.18), from Beijing Normal University, who died on Jan 18, 2019 after two years heroic fight with pancreatic cancer. In this work, we designed a protocol to repeatedly use a classical one-time-pad key to transmit ciphertext using single photon states. The essential idea was proposed in November 1982, by Charles H. Bennett, Gilles Brassard, Seth Breidbart, which was rejected by Fifteenth Annual ACM Symposium on Theory of Computing, and remained unpublished until 2014, when they published the article, Quantum Cryptography II: How to re-use a one-time pad safely even if P=NP, Natural Computing (2014) 13:453-458, DOI 10.1007/s11047-014-9453-6. We worked out this idea independently. This work has not been published, and was in cooperated into quant-ph 706.3791 (Kai Wen, Fu Guo Deng, Gui Lu Long, Secure Reusable Base-String in Quantum Key Distribution), and quant-ph 0711.1642 (Kai Wen, Fu-Guo Deng, Gui Lu Long, Reusable Vernam Cipher with Quantum Media).