We study theoretically the phonon-induced relaxation and decoherence processes in the hybrid qubit in silicon. Hybrid qubit behaves as a charge qubit when the detuning is close to zero and as spin qubit for large detuning values. It is realized starting from an electrostatically defined double quantum dot where three electrons are confined and manipulated through only electrical tuning. By employing a three-level effective model for the qubit and describing the environment bath as a series of harmonic oscillators in the thermal equilibrium states, we extract the relaxation and decoherence times as a function of the bath spectral density and of the bath temperature using the Bloch-Redfield theory. For Si quantum dots the energy dispersion is strongly affected by the physics of the valley, i.e. the conduction band minima, so we also included the contribution of the valley excitations in our analysis. Our results offer fundamental information on the system decoherence properties when the unavoidable interaction with the environment is included and temperature effects are considered.

In this paper I propose a new method of encoding discrete variables into Ising model qubits for quantum optimization. The new method is based on the physics of domain walls in one dimensional Ising spin chains. I find that these encodings and the encoding of arbitrary two variable interactions is possible with only two body Ising terms. Following on from similar results for the `one hot' method of encoding discrete variables [Hadfield et. al. Algorithms 12.2 (2019): 34] I also demonstrate that it is possible to construct two body mixer terms which do not leave the logical subspace, an important consideration for optimising using the quantum alternating operator ansatz (QAOA). I additionally discuss how, since the couplings in the domain wall encoding only need to be ferromagnetic and therefore could in principle be much stronger than anti-ferromagnetic couplers, application specific quantum annealers for discrete problems based on this construction may be beneficial. Finally, I compare embedding for synthetic scheduling and colouring problems with the domain wall and one hot encodings on two graphs which are relevant for quantum annealing, the chimera graph and the Pegasus graph. For every case I examine I find a similar or better performance from the domain wall encoding as compared to one hot, but this advantage is highly dependent on the structure of the problem. For encoding some problems, I find an advantage similar to the one found by embedding in a Pegasus graph compared to embedding in a chimera graph.

We study the quantum to classical transition in Boson Sampling by analysing how $N$-boson interference is affected by inevitable noise in an experimental setup. We adopt the Gaussian noise model of Kalai and Kindler for Boson Sampling and show that it appears from some realistic experimental imperfections. We reveal a connection between noise in Boson Sampling and partial distinguishability of bosons, which allows us to prove efficient classical simulatability of noisy no-collision Boson Sampling with finite noise amplitude $\epsilon$, i.e., $\epsilon = \Omega(1)$ as $N\to \infty$. On the other hand, using an equivalent representation of network noise as losses of bosons compensated by random (dark) counts of detectors, it is proven that for noise amplitude inversely proportional to total number of bosons, i.e., $\epsilon=O(1/N)$, noisy no-collision Boson Sampling is as hard to simulate classically as in the noiseless case. Moreover, the ratio of ``noise clicks" (lost bosons compensated by dark counts) to the total number of bosons $N$ vanishes as $N\to \infty$ for arbitrarily small noise amplitude, i.e., $\epsilon = o(1)$ as $N\to \infty$, hence, we conjecture that such a noisy Boson Sampling is also hard to simulate classically. The results significantly relax sufficient condition on noise in a network components, such as two-mode beam splitters, for classical hardness of experimental Boson Sampling.

Author(s): Junhua Dong, Qian Jiang, Qingmei Hu, Bingsuo Zou, and Yongyou Zhang

Transfer and scattering matrix theories are derived for studying single-photon (SP) transport in optical waveguide ladders (OWLs). The OWLs consist of two one-dimensional waveguides connected by Jaynes-Cummings emitters (JCEs) and have two input and two output channels. The von Neumann entropy is in...

[Phys. Rev. A 100, 013840] Published Mon Jul 22, 2019

Author(s): Mirko Amico, Oleg L. Berman, and Roman Ya. Kezerashvili

The dynamical Lamb effect is predicted to arise in superconducting circuits when the coupling of a superconducting qubit with a resonator is periodically switched “on” and “off” nonadiabatically. We show that by using a superconducting circuit which allows one to switch between longitudinal and tran...

[Phys. Rev. A 100, 013841] Published Mon Jul 22, 2019

Author(s): Sagnik Garai and J. Solomon Ivan

We study the robustness of single-mode squeezing nonclassicality against the action of single-mode Gaussian noise (Gaussian channels), first in its local single-mode form and second in its nonlocal distributed form as two-mode entanglement, with the channel acting locally on the single-mode. We find...

[Phys. Rev. A 100, 013842] Published Mon Jul 22, 2019

Author(s): Simon Milz, M. S. Kim, Felix A. Pollock, and Kavan Modi

In the classical domain, it is well known that divisibility does not imply that a stochastic process is Markovian. However, for quantum processes, divisibility is often considered to be synonymous with Markovianity. We show that completely positive divisible quantum processes can still involve non-M...

[Phys. Rev. Lett. 123, 040401] Published Mon Jul 22, 2019

Author(s): Johannes Feldmeier, Frank Pollmann, and Michael Knap

We consider the quench dynamics of a two-dimensional quantum dimer model and determine the role of its kinematic constraints. We interpret the nonequilibrium dynamics in terms of the underlying equilibrium phase transitions consisting of a Berezinskii-Kosterlitz-Thouless (BKT) transition between a c...

[Phys. Rev. Lett. 123, 040601] Published Mon Jul 22, 2019

Author(s): John Raymond

A new optical spectroscopy method can characterize the chemical composition of different layers of the material ejected from type Ia supernovae, delivering clues to the star’s history.

[Physics 12, 82] Published Mon Jul 22, 2019

Categories: Physics

50 years after the Apollo mission, a new crop of lunar exploration projects could extend the physics-related research we can do from the Moon.

[Physics 12, 84] Published Mon Jul 22, 2019

Categories: Physics

Author(s): Blayney W. Walshe, Lucas J. Mensen, Ben Q. Baragiola, and Nicolas C. Menicucci

The immense scalability of continuous-variable cluster states motivates their study as a platform for quantum computing, with fault tolerance possible given sufficient squeezing and appropriately encoded qubits [N. C. Menicucci, Phys. Rev. Lett. **112**, 120504 (2014)]. Here, we expand the scope of that...

[Phys. Rev. A 100, 010301(R)] Published Mon Jul 22, 2019

Author(s): Yi-Hao Kang, Zhi-Cheng Shi, Bi-Hua Huang, Jie Song, and Yan Xia

In this paper, we propose a protocol to realize the conversions between Greenberger-Horne-Zeilinger (GHZ) states and W states of spin qubits. By analyzing and simplifying the dynamics of the system, the control fields are designed via the inverse Hamiltonian engineering based on the Lie transforms. ...

[Phys. Rev. A 100, 012332] Published Mon Jul 22, 2019

The Lieb-Schultz-Mattis (LSM) theorem states that a spin system with translation and spin rotation symmetry and half-integer spin per unit cell does not admit a gapped symmetric ground state lacking fractionalized excitations. That is, the ground state must be gapless, spontaneously break a symmetry, or be a gapped spin liquid. Thus, such systems are natural spin-liquid candidates. In this work, we give a much more general criterion that determines when an LSM-type theorem holds in a spin system. For example, we consider quantum magnets with arbitrary space group symmetry and/or spin-orbit coupling. Our criterion is intimately connected to recent work on the general classification of topological phases with spatial symmetries and also allows for the computation of an "anomaly" associated with the existence of an LSM theorem. Moreover, our framework is also general enough to encompass recent works on "SPT-LSM" theorems where the system admits a gapped symmetric ground state without fractionalized excitations, but such a ground state must still be non-trivial in the sense of symmetry-protected topological (SPT) phases.

We study the dynamics of a periodically driven tilted Bose-Hubbard model in one dimension deep inside its Mott phase starting from a $\mathbb{Z}_2$ symmetry-broken state. We find long-time oscillations in the density-density correlation function of the bosons and relate them to the presence of quantum many-body scars in the eigenspectrum of its {\it Floquet Hamiltonian}. For large drive frequency $\omega_D$, where a standard Magnus expansion is adequate for describing the Floquet Hamiltonian, these states are identical to their equilibrium counterparts studied earlier. In contrast, for very low frequencies, where the Magnus expansion fails, we find that the dynamics of the correlators are controlled by thermal states with volume law entanglement indicating absence of scars. In between, for large drive amplitude, the system displays several reentrant transitions as a function of $\omega_D$ from regimes with scars to those with no scars. We discuss signatures of these transitions in the correlation function dynamics and evolution of fidelity of the system, provide a qualitative understanding of their origin, and chart out experiments which can test our theory.

We numerically study the real-time dynamics of a single hole created in the $t-J$ model on a square lattice. Initially, the hole spreads ballistically with a velocity proportional to the hopping matrix element. At intermediate to long times, the dimensionality as well as the spin background determine the hole dynamics. A hole created in the ground state of a two dimensional quantum antiferromagnet propagates again ballistically at long times but with a velocity proportional to the spin exchange coupling, showing the formation of a magnetic polaron. We provide an intuitive explanation of this dynamics in terms of a parton construction, which leads to a good quantitative agreement with the numerical simulations. In the limit of infinite temperature and no spin exchange couplings, the dynamics can be approximated by a quantum random walk on the Bethe lattice. Adding Ising interactions corresponds to an effective disordered potential, which can dramatically slow down the hole propagation, consistent with subdiffusive dynamics.

We present measurements of the capacitive coupling energy and the inter-dot capacitances in a linear quadruple quantum dot array in undoped Si/SiGe. With the device tuned to a regime of strong ($>$1 GHz) intra-double dot tunnel coupling, as is typical for double dot qubits, we measure a capacitive coupling energy of $20.9 \pm 0.3$ GHz. In this regime, we demonstrate a fitting procedure to extract all the parameters in the 4D Hamiltonian for two capacitively coupled charge qubits from a 2D slice through the quadruple dot charge stability diagram. We also investigate the tunability of the capacitive coupling energy, using inter-dot barrier gate voltages to tune the inter- and intra-double dot capacitances, and change the capacitive coupling energy of the double dots over a range of 15-32 GHz. We provide a model for the capacitive coupling energy based on the electrostatics of a network of charge nodes joined by capacitors, which shows how the coupling energy should depend on inter-double dot and intra-double dot capacitances in the network, and find that the expected trends agree well with the measurements of coupling energy.

We propose a modification to Nielsen's circuit complexity, where the minimum number of gates to synthesize a desired unitary operator is related to a geodesic length in circuit space. Our proposal uses the Suzuki-Trotter iteration scheme, usually used to reduce computational time cost, which provides a network like structure for the circuit. This leads to an optimized gate counting linear in the geodesic distance unlike in the original proposal. We show how a renormalization beta-function type equation can be written for the penalty factors where the role of the RG scale is played by the network depth, which itself is correlated with the tolerance. The density of gates is shown to be monotonic with the tolerance and a holographic interpretation arising from c-theorems is given. This picture appears to be closely connected with the AdS/CFT correspondence via path integral optimization.

Entanglement has been shown to be necessary for pure state quantum computation to have an advantage over classical computation. However, it remains open whether entanglement is necessary for quantum computers that use mixed states to also have an advantage. The one clean qubit model is a form of quantum computer in which the input is the maximally mixed state plus one pure qubit. Previous work has shown that there is a limited amount of entanglement present in these computations, despite the fact that they can efficiently solve some problems that are seemingly hard to solve classically. This casts doubt on the notion that entanglement is necessary for quantum speedups. In this work we show that entanglement is indeed crucial for efficient computation in this model, because without it the one clean qubit model is efficiently classically simulable.

Quantum communication is rapidly gaining popularity due to its high security and technological maturity. However, most implementations are limited to just two communicating parties (users). Quantum communication networks aim to connect a multitude of users. Here we present a fully connected quantum communication network on a city wide scale without active switching or trusted nodes. We demonstrate simultaneous and secure connections between all 28 pairings of 8 users. Our novel network topology is easily scalable to many users, allows traffic management features and minimises the infrastructure as well as the user hardware needed.

Elucidating the energy transfer between a quantum system and a reservoir is a central issue in quantum non-equilibrium thermodynamics, which could provide novel tools to engineer quantum-enhanced heat engines. The lack of information on the reservoir inherently limits the practical insight that can be gained on the exchange process. Here, we investigate the energy transfer for an open quantum system in the framework of quantum fluctuation relations. As a novel toolbox, we employ a nitrogen-vacancy center spin qubit in diamond, subject to repeated quantum projective measurements accompanied by a tunable dissipation channel. When the system is tuned to be insensitive to dissipation, we verify the closed-system quantum Jarzynski equality. In the presence of competition between dissipation and quantum projective measurements, the experimental results suggest a formulation of the energy exchange fluctuation relation that incorporates the reservoir properties in the guise of an effective temperature of the final out-of-equilibrium steady-state. Our findings pave the way to investigate energy exchange mechanisms in higher-dimension open quantum systems.