Fault tolerant quantum computing relies on the ability to detect and correct errors, which in quantum error correction codes is typically achieved by projectively measuring multi-qubit parity operators and by conditioning operations on the observed error syndromes. Here, we experimentally demonstrate the use of an ancillary qubit to repeatedly measure the $ZZ$ and $XX$ parity operators of two data qubits and to thereby project their joint state into the respective parity subspaces. By applying feedback operations conditioned on the outcomes of individual parity measurements, we demonstrate the real-time stabilization of a Bell state with a fidelity of $F\approx 74\%$ in up to 12 cycles of the feedback loop. We also perform the protocol using Pauli frame updating and, in contrast to the case of real-time stabilization, observe a steady decrease in fidelity from cycle to cycle. The ability to stabilize parity over multiple feedback rounds with no reduction in fidelity provides strong evidence for the feasibility of executing stabilizer codes on timescales much longer than the intrinsic coherence times of the constituent qubits.

We propose a new variational scheme based on the neural-network quantum states to simulate the stationary states of open quantum many-body systems. Using the high expressive power of the variational ansatz described by the restricted Boltzmann machines, which we dub as the neural stationary state ansatz, we compute the stationary states of quantum dynamics obeying the Lindblad master equations. The mapping of the stationary-state search problem into finding a zero-energy ground state of an appropriate Hermitian operator allows us to apply the conventional variational Monte Carlo method for the optimization. Our method is shown to simulate various spin systems efficiently, i.e., the transverse-field Ising models in both one and two dimensions and the XYZ model in one dimension.

Entanglement renormalization group flow of the Haah cubic code produces another fracton model with 4 qubits per lattice site, dubbed as the Haah B-code. We provide a schema that generalizes both models to stabilizer codes on any finite group with 2q qubits per site and labeled by multi-subsets of the finite group and a symmetric binary matrix.

An order $2m$ complex tensor $\mathcal{H}$ is said to be Hermitian if \[\mathcal{H}_{i_1\cdots i_m j_1\cdots j_m}= \mathcal{H}_{j_1\cdots j_m i_1\cdots i_m} ^*\mathrm{\ for\ all\ }i_1\cdots i_m j_1\cdots j_m .\] It can be regarded as an extension of Hermitian matrix to higher order. This paper is focusing on the concepts and basis properties of Hermitian tensors, partial traces of tensors, rank-one Hermitian decomposition, nonnegative Hermitian tensors, Hermitian tensor eigenvalues and separable Hermitian tensors are introduced.

The foundations of non-linear quantum mechanics are based on six postulates and five propositions. On a first quantised level, these approaches are built on non-linear differential operators, non-linear eigenvalue equations, and the notion of non-linear observables and non-linear states. The present theory predicts that the non-linear function solution of a non-linear partial differential equation for a free particle is correct and that the commutator of the position operator and the non-linear momentum operator does not commute. Moreover, the implicit wave function of linear quantum mechanics can be determined by a non-linear wave function, which also verifies the non-linear quantum mechanics.

In this paper we introduce both a theoretical and an experimental scheme of an all-optical quantum thermal engine, where the working substance and the thermal reservoirs can be efficiently encoded in internal degrees of freedom of a single-photon. By using polarization and propagation path, we encode two quantum bits and then implement the thermodynamical steps of an Otto cycle. To illustrate the feasibility of our proposal, we experimentally realize such optical thermal engine through an intense laser beam, evaluating heat and work at each individual step of the thermodynamical cycle. In addition, from the analysis of the entropy production during the entire cycle, we can study the amount of quantum friction produced in the Otto cycle as a function of the difference of temperature between hot and cold reservoirs. Our investigation constitutes, therefore, an \textit{all-optical-based thermal machine} and opens perspectives for other optical simulations in quantum thermodynamics.

The quantum vacuum energy for a hybrid comb of Dirac $\delta$-$\delta'$ potentials is computed using the energy of the single $\delta$-$\delta'$ potential over the real line that makes up the comb. The zeta function of a comb periodic potential is the continuous sum of zeta functions over the dual primitive cell of Bloch quasi-momenta. The result obtained for the quantum vacuum energy is non-perturbative in the sense that the energy function is not analytical for small couplings

PennyLane is a Python 3 software framework for optimization and machine learning of quantum and hybrid quantum-classical computations. The library provides a unified architecture for near-term quantum computing devices, supporting both qubit and continuous-variable paradigms. PennyLane's core feature is the ability to compute gradients of variational quantum circuits in a way that is compatible with classical techniques such as backpropagation. PennyLane thus extends the automatic differentiation algorithms common in optimization and machine learning to include quantum and hybrid computations. A plugin system makes the framework compatible with any gate-based quantum simulator or hardware. We provide plugins for Strawberry Fields, Rigetti Forest, Qiskit, and ProjectQ, allowing PennyLane optimizations to be run on publicly accessible quantum devices provided by Rigetti and IBM Q. On the classical front, PennyLane interfaces with accelerated machine learning libraries such as TensorFlow, PyTorch, and autograd. PennyLane can be used for the optimization of variational quantum eigensolvers, quantum approximate optimization, quantum machine learning models, and many other applications.

A quantum vortex dipole, comprised of a closely bound pair of vortices of equal strength with opposite circulation, is a spatially localized travelling excitation of a planar superfluid that carries linear momentum, suggesting a possible analogy with ray optics. We investigate numerically and analytically the motion of a quantum vortex dipole incident upon a step-change in the background superfluid density of an otherwise uniform two-dimensional Bose-Einstein condensate. Due to the conservation of fluid momentum and energy, the incident and refracted angles of the dipole satisfy a relation analogous to Snell's law, when crossing the interface between regions of different density. The predictions of the analogue Snell's law relation are confirmed for a wide range of incident angles by systematic numerical simulations of the Gross-Piteavskii equation. Near the critical angle for total internal reflection, we identify a regime of anomalous Snell's law behaviour where the finite size of the dipole causes transient capture by the interface. Remarkably, despite the extra complexity of the surface interaction, the incoming and outgoing dipole paths obey Snell's law.

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

A ferrimagnetic yttrium-iron-garnet sphere placed inside a cavity provides a platform for investigating macroscopic quantum phenomena. By exploring the strong coupling between magnons and cavity-polaritons, as well as the magnetostrictive interaction, a scheme to create squeezed states of magnons and phonons is proposed.

[Phys. Rev. A 99, 021801(R)] Published Thu Feb 21, 2019

Author(s): Leon Droenner, Nicolas L. Naumann, Eckehard Schöll, Andreas Knorr, and Alexander Carmele

Pyragas control allows us to stabilize unstable states in applied nonlinear science. We propose to apply a quantum version of the Pyragas protocol to control individual photon probabilities in an otherwise only globally accessible photon-probability distribution of a quantum light emitter. The versa...

[Phys. Rev. A 99, 023840] Published Thu Feb 21, 2019

Author(s): A. Słapik, J. Łuczka, P. Hänggi, and J. Spiechowicz

A prerequisite for isolating diseased cells requires a mechanism for effective mass-based separation. This objective, however, is generally rather challenging because typically no valid correlation exists between the size of the particles and their mass value. We consider an inertial Brownian partic...

[Phys. Rev. Lett. 122, 070602] Published Thu Feb 21, 2019

Author(s): Lorenzo Buffoni, Andrea Solfanelli, Paola Verrucchi, Alessandro Cuccoli, and Michele Campisi

A proposed noise-tolerant approach to quantum refrigeration eliminates the need for feedback control by exploiting the invasiveness of quantum measurements.

[Phys. Rev. Lett. 122, 070603] Published Thu Feb 21, 2019

A proposed noise-tolerant approach to quantum refrigeration eliminates the need for feedback control by exploiting the invasiveness of quantum measurements.

[Physics] Published Thu Feb 21, 2019

Categories: Physics

Author(s): Marco Roth, Nikolaj Moll, Gian Salis, Marc Ganzhorn, Daniel J. Egger, Stefan Filipp, and Sebastian Schmidt

We propose a quantum simulator based on driven superconducting qubits where the interactions are generated parametrically by a bichromatic magnetic flux modulation of a tunable bus element. Using a time-dependent Schrieffer-Wolff transformation, we analytically derive a multiqubit Hamiltonian which ...

[Phys. Rev. A 99, 022323] Published Thu Feb 21, 2019

Author(s): Yan Li, Luca Pezzè, Weidong Li, and Augusto Smerzi

Sensitivity bounds for a generic interferometric phase estimation problem are the shot noise and the Heisenberg limits. The shot noise is the highest sensitivity that can be reached with separable states, while the Heisenberg limit is the ultimate bound in sensitivity which can be saturated with ent...

[Phys. Rev. A 99, 022324] Published Thu Feb 21, 2019

Author(s): Yuki Takeuchi, Yuichiro Matsuzaki, Koichiro Miyanishi, Takanori Sugiyama, and William J. Munro

Typically, the aim of quantum metrology is to sense target fields with high precision utilizing quantum properties. Unlike the typical aim, in this paper, we use quantum properties for adding a functionality to quantum sensors. More concretely, we propose a delegated quantum sensor (a client-server ...

[Phys. Rev. A 99, 022325] Published Thu Feb 21, 2019

Being motivated by the surge of fermionic quantum Monte Carlo simulations at finite temperature, we present a detailed analysis of the permutation-cycle properties of path integral Monte Carlo (PIMC) simulations of degenerate electrons. Particular emphasis is put onto the uniform electron gas in the warm dense matter regime. We carry out PIMC simulations of up to $N=100$ electrons and investigate exchange-cycle frequencies, which are found not to follow any simple exponential law even in the case of ideal fermions due to the finite size of the simulation box. Moreover, we introduce a permutation-cycle correlation function, which allows us to analyse the joint probability to simultaneously find cycles of different lengths within a single configuration. Again, we find that finite-size effects predominate the observed behaviour. Finally, we briefly consider an inhomogeneous system, namely electrons in a $2D$ harmonic trap. We expect our results to be of interest for the further development of fermionic PIMC methods, in particular to alleviate the notorious fermion sign problem.

We present a new method by which, in principle, it is possible to "see in absolute darkness", i.e., without exchanging any real quanta through quantum fields. This is possible because objects modify the mode structure of the vacuum in their vicinity. The new method probes the mode structure of the vacuum through the Unruh effect, i.e., by recording the excitation rates of quantum systems that are accelerated.

We introduce a new quantum optimization algorithm for Linear Programming (LP) problems based on Interior Point (IP) Predictor-Corrector (PC) methods whose (worst case) time complexity is $O(\sqrt{n}Ls^3 k \epsilon^{-1}\epsilon_s^{-1}) $. This represents a quantum speed-up in the number $n$ of variables in the cost function with respect to the comparable classical Interior Point (IP) algorithms that behave as $O((n+m)\sqrt{nk}L s^3\log(\epsilon^{-1})\epsilon_s^{-1})$ or $O(\sqrt{n}(n+m)L)$ depending on the technique employed, where $m$ is the number of constraints and the rest of the variables are defined in the introduction. The average time complexity of our algorithm is $O(\sqrt{n}s^3 k \epsilon^{-1}\epsilon_s^{-1})$, which equals the behaviour on $n$ of quantum Semidefinite Programming (SDP) algorithms based on multiplicative weight methods when restricted to LP problems and heavily improves on the precision $\epsilon^{-1}$ of the algorithm. Unlike the quantum SDP algorithm, the quantum PC algorithm does not depend on size parameters of the primal and dual LP problems ($R,r$), and outputs a feasible and optimal solution whenever it exists.