Author(s): Francesco Mori, Satya N. Majumdar, and Grégory Schehr

We present an exact solution for the probability density function P(τ=tmin−tmax|T) of the time difference between the minimum and the maximum of a one-dimensional Brownian motion of duration T. We then generalize our results to a Brownian bridge, i.e., a periodic Brownian motion of period T. We demo...

[Phys. Rev. Lett. 123, 200201] Published Fri Nov 15, 2019

Author(s): Francesco Albarelli, Jamie F. Friel, and Animesh Datta

Only with the simultaneous estimation of multiple parameters are the quantum aspects of metrology fully revealed. This is due to the incompatibility of observables. The fundamental bound for multiparameter quantum estimation is the Holevo Cramér-Rao bound (HCRB) whose evaluation has so far remained ...

[Phys. Rev. Lett. 123, 200503] Published Fri Nov 15, 2019

Author(s): David Ehrenstein

The unusual wing flapping of submerged “sea butterflies” is similar to that of birds and insects and may provide signs of climate stress.

[Physics 12, 127] Published Fri Nov 15, 2019

Categories: Physics

Author(s): Takaaki Matsuo, Clément Durand, and Rodney Van Meter

Establishing end-to-end quantum connections requires quantified link characteristics, and operations need to coordinate decision making between nodes across a network. We introduce the RuleSet-based communication protocol for supporting quantum operations over distant nodes to minimize classical pac...

[Phys. Rev. A 100, 052320] Published Fri Nov 15, 2019

We discuss the simulation of non-perturbative cavity-QED effects using systems of trapped ions. Specifically, we address the implementation of extended Dicke models with both collective dipole-field and direct dipole-dipole interactions, which represent a minimal set of models for describing light-matter interactions in the ultrastrong and deep-strong coupling regime. We show that this approach can be used in state-of-the-art trapped ion setups to investigate excitation spectra or the transition between sub- and superradiant ground states, which are currently not accessible in any other physical system. Our analysis also reveals the intrinsic difficulty of accessing this non-perturbative regime with larger numbers of dipoles, which makes the simulation of many-dipole cavity QED a particularly challenging test case for future quantum simulation platforms.

We formulate a state-dependent definition of operator size that captures the effective size of an operator acting on a reference state. We apply our definition to the SYK model and holographic 2-dimensional CFTs, generalizing the Qi-Streicher formula to a large class of geometries which includes pure AdS$_3$ and BTZ black holes. In pure AdS$_3$, the operator size is proportional to the global Hamiltonian at leading order in $1/N$, mirroring the results of Lin-Maldacena-Zhao in AdS$_2$. For BTZ geometries, it is given by the sum of the Kruskal momenta. Higher $1/N$ corrections become relevant when backreaction gets large, and we expect a transition in the growth pattern that depends on the transverse profile of the excitation. We propose a bulk dual that captures this profile dependence and exhibits saturation at a size of order the black hole entropy. This bulk dual is an averaged eikonal phase over a class of scattering events, and it can be interpreted as the "number of virtual gravitons" in the gravitational field created by an infaller.

We describe a method to map the standing-wave pattern inside a Fabry-Perot optical cavity with sub-wavelength resolution by perturbing it with a commercially available scanning near-field optical microscope (SNOM) tip. The method is applied to a fiber Fabry-Perot microcavity. We demonstrate its use to determine the relative position of the antinodes at two different wavelengths. In addition, we use the SNOM tip as a point-like source allowing precise positioning of a microscope objective with respect to the cavity mode.

We introduce a multireference selected quantum Krylov (MRSQK) algorithm suitable for quantum simulation of many-body problems. MRSQK is a low-cost alternative to the quantum phase estimation algorithm that generates a target state as a linear combination of non-orthogonal Krylov basis states. This basis is constructed from a set of reference states via real-time evolution avoiding the numerical optimization of parameters. An efficient algorithm for the evaluation of the off-diagonal matrix elements of the overlap and Hamiltonian matrices is discussed and a selection procedure is introduced to identify a basis of orthogonal references that ameliorates the linear dependency problem. Preliminary benchmarks on linear H$_6$, H$_8$, and BeH$_2$ indicate that MRSQK can predict the energy of these systems accurately using very compact Krylov bases.

A central challenge in developing quantum computers and long-range quantum networks lies in the distribution of entanglement across many individually controllable qubits. Colour centres in diamond have emerged as leading solid-state 'artificial atom' qubits, enabling on-demand remote entanglement, coherent control of over 10 ancillae qubits with minute-long coherence times, and memory-enhanced quantum communication. A critical next step is to integrate large numbers of artificial atoms with photonic architectures to enable large-scale quantum information processing systems. To date, these efforts have been stymied by qubit inhomogeneities, low device yield, and complex device requirements. Here, we introduce a process for the high-yield heterogeneous integration of 'quantum micro-chiplets' (QMCs) -- diamond waveguide arrays containing highly coherent colour centres -- with an aluminium nitride (AlN) photonic integrated circuit (PIC). Our process enables the development of a 72-channel defect-free array of germanium-vacancy (GeV) and silicon-vacancy (SiV) colour centres in a PIC. Photoluminescence spectroscopy reveals long-term stable and narrow average optical linewidths of 54 MHz (146 MHz) for GeV (SiV) emitters, close to the lifetime-limited linewidth of 32 MHz (93 MHz). Additionally, inhomogeneities in the individual qubits can be compensated in situ with integrated tuning of the optical frequencies over 100 GHz. The ability to assemble large numbers of nearly indistinguishable artificial atoms into phase-stable PICs provides an architecture toward multiplexed quantum repeaters and general-purpose quantum computers.

We study the distribution of quantum entanglement for continuous variables among causally disconnected open charts in de Sitter space. It is found that genuine tripartite entanglement is generated among the open chart modes under the influence of curvature of de Sitter space for any nonzero squeezing. Bipartite entanglement is also generated when the curvature is strong enough, even though the observers are separated by the event horizon. This provides a clearcut interpretation of the two-mode squeezing mechanism in the de Sitter space. In addition, the curvature generated genuine tripartite entanglement is found to be less sensitive to the mass parameter than the generated bipartite entanglement. The effects of the curvature of de Sitter space on the generated entanglement become more apparent in the limit of conformal and massless scalar fields.

It has recently been reported [\textit{PNAS} \textbf{114}, 2303 (2017)] that, under an operational definition of time, quantum clocks would get entangled through gravitational effects. Here we study an alternative scenario: the clocks have different masses and energy gaps, which would produce time difference via gravitational interaction. The proposal of quantum clock synchronization for the gravity-induced time difference is discussed. We illustrate how the stability of measurement probability in the quantum clock synchronization proposal is influenced by the gravitational interaction induced by the clock themselves. It is found that the precision of clock synchronization depends on the energy gaps of the clocks and the improvement of precision in quantum metrology is in fact an indicator of entanglement generation. We also present the quantum enhanced estimation of time difference and find that the quantum Fisher information is very sensitive to the distance between the clocks.

This paper investigates the experimental performance of a discrete portfolio optimization problem relevant to the financial services industry on the gate-model of quantum computing. We implement and evaluate a portfolio rebalancing use case on an idealized simulator of a gate-model quantum computer. The characteristics of this exemplar application include trading in discrete lots, non-linear trading costs, and the investment constraint. We design a novel problem encoding and hard constraint mixers for the Quantum Alternating Operator Ansatz, and compare to its predecessor the Quantum Approximate Optimization Algorithm. Experimental analysis demonstrates the potential tractability of this application on Noisy Intermediate-Scale Quantum (NISQ) hardware, identifying portfolios within 5% of the optimal adjusted returns and with the optimal risk for a small eight-stock portfolio.

The distinguishing of the multiphoton quantum interference effect from the classical one forms one of the most important issues in modern quantum mechanics and experimental quantum optics. For a long time, the two-photon interference (TPI) of correlated photons has been recognized as a pure quantum effect that cannot be simulated with classical lights. In the meantime, experiments have been carried out to investigate the classical analogues of the TPI. In this study, we conduct TPI experiments with uncorrelated photons with different center frequencies from a luminescent light source, and we compare our results with the previous ones of correlated photons. The observed TPI fringe can be expressed in the form of three phase terms related to the individual single-photon and two-photon states, and the fringe pattern is strongly affected by the two single-photon-interference fringes and also by their visibilities. With the exception of essential differences such as valid and accidental coincidence events within a given resolving time and the two-photon spectral bandwidth, the interference phenomenon itself exhibits the same features for both correlated and uncorrelated photons in the single-photon counting regime.

Non-Markovian dynamics detection is one of the most popular subjects in the quantum information science. In this paper, a linear-entropy-based non-Markovianity witness scheme is constructed. The positive definiteness of the Choi state will be broken in the non-Markovian evolution, which can be witnessed by its linear entropy. Thus, the linear entropy of the Choi state can be used to witness the non-Markovian dynamics. The effectiveness of the proposed method is verified by an example of the pure dephasing channel. Also, it is shown that this method can be extended to the one based on R\'enyi entropy.

The anomalous heating of ions has been a major obstacle for quantum information processing (QIP) based on surface ion trap. Recent results indicated that the adatoms contamination on trap surface can generate contact potential, which gives rise to the fluctuating patch potential. By investigating the contamination from trap surface adatoms induced in loading process, we present the direct physical image of the contamination process and conclude the relationship between the capacitance change and contamination from surface adatoms through the theory and experiment. According to this relationship, the contamination from surface adatoms and the effect of in-situ treatment process can be monitored by the capacitance between electrodes in real time. This paper provides a research method of anomalous heating of ions, which has practicable value to QIP based on surface ion trap.

During the last 20 years, the advance of communication technologies has generated multiple exciting applications. However, classical cryptography, commonly adopted to secure current communication systems, can be jeopardized by the advent of quantum computers. Quantum key distribution (QKD) is a promising technology aiming to solve such a security problem. Unfortunately, current implementations of QKD systems show relatively low key rates, demand low channel noise and use ad hoc devices. In this work, we picture how to overcome the rate limitation by using a 37-core fibre to generate 2.86 Mbit/s per core that can be space multiplexed into the highest secret key rate of 105.7 Mbit/s to date. We also demonstrate, with off-the-shelf equipment, the robustness of the system by co-propagating a classical signal at 370 Gbit/s, paving the way for a shared quantum and classical communication network.

We study the entanglement distillation in continuous variable systems when a photon replacement protocol is employed. A cascaded protocol is studied and we find that the resultant entanglement increases by increasing the number of repetitions. Interestingly, the entanglement enhancement is not sensitive to the asymmetry of the protocol and gives the same result for any arrangement in the absence of loss. The non-Gaussianity of the outcome state is also studied and it is found that the non-Gaussianity of the state dramatically depends on the experimental arrangements. By providing practical information on photon replacement operation, this work is one step towards realization of universal quantum computation. In particular, in setups where deGaussifying protocols are only applicable to one of the parties.

We report a theoretical and experimental study on the role of indistinguishability in the estimation of an interferometric phase. In particular, we show that the quantum Fisher information, which limits the maximum precision achievable in the parameter estimation, increases linearly with respect to the degree of indistinguishability between the input photons in a two-port interferometer, in the ideal case of a pure probe state. We experimentally address the role played by the indistinguishability for the case of two photons entering a polarization-based interferometer, where the degree of indistinguishability is characterized by the overlap between two spatial modes. The experimental results support the fact that, even in the presence of white noise, a quantum enhancement in the interferometric phase estimation can be obtained from a minimum degree of indistinguishability.

It's known that if $d^2$ vectors from $d$-dimensional Hilbert space $H$ form a SIC-POVM (SIC for short) then tensor square of those vectors form an equiangular tight frame on the symmetric subspace of $H\otimes H$. We prove that for any SIC of WH-type (Weyl-Heisenberg group covariant) this squared frame can be obtained as a projection of WH-type basis of $H\otimes H$ onto the symmetric subspace. We give a full description of the set of all WH-type bases, so this set could be used as a search space for SIC solutions. Also we show that a particular element of this set is close to a SIC solution in some structural sense. Finally we give a geometric construction of a SIC-related symmetric tight fusion frames that were discovered in odd dimensions.