## Journals

### Suppression of heating in quantum spin clusters under periodic driving as a dynamic localization effect. (arXiv:1712.10028v3 [cond-mat.stat-mech] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

We investigate numerically and analytically the heating process in ergodic clusters of interacting spins 1/2 subjected to periodic pulses of external magnetic field. Our findings indicate that there is a threshold for the pulse strength below which the heating is suppressed. This threshold decreases with the increase of the cluster size, approaching zero in the thermodynamic limit; yet it should be observable in clusters with fairly large Hilbert spaces. We obtain the above threshold quantitatively as a condition for the breakdown of the golden rule in the second-order perturbation theory. It is caused by the phenomenon of dynamic localization.

Categories: Journals, Physics

### Solving quantum impurity problems in and out of equilibrium with variational approach. (arXiv:1801.05825v4 [cond-mat.str-el] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

A versatile and efficient variational approach is developed to solve in- and out-of-equilibrium problems of generic quantum spin-impurity systems. Employing the discrete symmetry hidden in spin-impurity models, we present a new canonical transformation that completely decouples the impurity and bath degrees of freedom. Combining it with Gaussian states, we present a family of many-body states to efficiently encode nontrivial impurity-bath correlations. We demonstrate its successful application to the anisotropic and two-lead Kondo models by studying their spatiotemporal dynamics and universal behavior in the correlations, relaxation times and the differential conductance. We compare them to previous analytical and numerical results. In particular, we apply our method to study new types of nonequilibrium phenomena that have not been studied by other methods, such as long-time crossover in the ferromagnetic easy-plane Kondo model. The present approach will be applicable to a variety of unsolved problems in solid-state and ultracold-atomic systems.

Categories: Journals, Physics

### 64-Qubit Quantum Circuit Simulation. (arXiv:1802.06952v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

Classical simulations of quantum circuits are limited in both space and time when the qubit count is above 50, the realm where quantum supremacy reigns. However, recently, for the low depth circuit with more than 50 qubits, there are several methods of simulation proposed by teams at Google and IBM. Here, we present a scheme of simulation which can extract a large amount of measurement outcomes within a short time, achieving a 64-qubit simulation of a universal random circuit of depth 22 using a 128-node cluster, and 56- and 42-qubit circuits on a single PC. We also estimate that a 72-qubit circuit of depth 23 can be simulated in about 16 h on a supercomputer identical to that used by the IBM team. Moreover, the simulation processes are exceedingly separable, hence parallelizable, involving just a few inter-process communications. Our work enables simulating more qubits with less hardware burden and provides a new perspective for classical simulations.

Categories: Journals, Physics

### Optimal work extraction from quantum states by photo-assisted Cooper pair tunneling. (arXiv:1802.10572v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

The theory of quantum thermodynamics predicts fundamental bounds on work extraction from quantum states. As these bounds are derived in a very general and abstract setting, it is unclear how relevant they are in an experimental context, where control is typically limited. Here we address this question by showing that optimal work extraction is possible for a realistic engine. The latter consists of a superconducting circuit, where a LC-resonator is coupled to a Josephson junction. The oscillator state fuels the engine, providing energy absorbed by Cooper pairs, thus producing work in the form of an electrical current against an external voltage bias. We show that this machine can extract the maximal amount of work from all Gaussian and Fock states. Furthermore, we consider work extraction from a continuously stabilized oscillator state. In both scenarios, coherence between energy eigenstates is beneficial, increasing the power output of the machine. This is possible because the phase difference across the Josephson junction provides a phase reference.

Categories: Journals, Physics

### Symplectic realisation of electric charge in fields of monopole distributions. (arXiv:1803.00405v3 [hep-th] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

We construct a symplectic realisation of the twisted Poisson structure on the phase space of an electric charge in the background of an arbitrary smooth magnetic monopole density in three dimensions. We use the extended phase space variables to study the classical and quantum dynamics of charged particles in arbitrary magnetic fields by constructing a suitable Hamiltonian that reproduces the Lorentz force law for the physical degrees of freedom. In the source-free case the auxiliary variables can be eliminated via Hamiltonian reduction, while for non-zero monopole densities they are necessary for a consistent formulation and are related to the extra degrees of freedom usually required in the Hamiltonian description of dissipative systems. We obtain new perspectives on the dynamics of dyons and motion in the field of a Dirac monopole, which can be formulated without Dirac strings. We compare our associative phase space formalism with the approach based on nonassociative quantum mechanics, reproducing extended versions of the characteristic translation group three-cocycles and minimal momentum space volumes, and prove that the two approaches are formally equivalent. We also comment on the implications of our symplectic realisation in the dual framework of non-geometric string theory and double field theory.

Categories: Journals, Physics

### Refrigeration beyond weak internal coupling. (arXiv:1803.02002v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

We investigate the performance of a three-spin quantum absorption refrigerator using a refined open quantum system model valid across all inter-spin coupling strengths. It describes the transition between previous approximate models for the weak and the ultrastrong coupling limit, and it predicts optimal refrigeration for moderately strong coupling, where both approximations are inaccurate. Two effects impede a more effective cooling: the coupling between the spins no longer reduces to a simple resonant energy exchange (the rotating wave approximation fails), and the interactions with the thermal baths become sensitive to the level splitting, thus opening additional heat channels between the reservoirs. We identify the modified conditions of refrigeration as a function of the inter-spin coupling strength, and we show that, contrary to intuition, a high-temperature work reservoir thwarts refrigeration in the strong coupling regime.

Categories: Journals, Physics

### Localization due to topological stochastic disorder in active networks. (arXiv:1803.05176v2 [cond-mat.stat-mech] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

An active network is a prototype model in non-equilibrium statistical mechanics. It can represent, for example, a system with particles that have a self-propulsion mechanism. Each node of the network specifies a possible location of a particle, and its orientation. The orientation (which is formally like a spin degree of freedom) determines the self-propulsion direction. The bonds represent the possibility to make transitions: to hop between locations; or to switch the orientation. In systems of experimental interest (Janus particles), the self-propulsion is induced by illumination. An emergent aspect is the topological stochastic disorder (TSD). It is implied by the non-uniformity of the illumination. In technical terms the TSD reflects the local non-zero circulations (affinities) of the stochastic transitions. This type of disorder, unlike non-homogeneous magnetic field, is non-hermitian, and can lead to the emergence of a complex relaxation spectrum. It is therefore dramatically distinct from the conservative Anderson-type or Sinai-type disorder. We discuss the consequences of having TSD. In particular we illuminate 3~different routes to under-damped relaxation, and show that localization plays a major role in the analysis. Implications of the bulk-edge correspondence principle are addressed too.

Categories: Journals, Physics

### Hypercontractivity and logarithmic Sobolev Inequality for non-primitive quantum Markov semigroups and estimation of decoherence rates. (arXiv:1803.05379v3 [math-ph] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

We generalize the concepts of weak quantum logarithmic Sobolev inequality (LSI) and weak hypercontractivity (HC), introduced in the quantum setting by Olkiewicz and Zegarlinski, to the case of non-primitive quantum Markov semigroups (QMS). The originality of this work resides in that this new notion of hypercontractivity is given in terms of the so-called amalgamated $\mathbb{L}_p$ norms introduced recently by Junge and Parcet in the context of operator spaces theory. We make three main contributions. The first one is a version of Gross' integration lemma: we prove that (weak) HC implies (weak) LSI. Surprisingly, the converse implication differs from the primitive case as we show that LSI implies HC but with a weak constant equal to the cardinal of the center of the decoherence-free algebra. Building on the first implication, our second contribution is the fact that strong LSI and therefore strong HC do not hold for non-trivially primitive QMS. This implies that the amalgamated $\mathbb{L}_p$ norms are not uniformly convex for $1\leq p \leq 2$. As a third contribution, we derive universal bounds on the (weak) logarithmic Sobolev constants for a QMS on a finite dimensional Hilbert space, using a similar method as Diaconis and Saloff-Coste in the case of classical primitive Markov chains, and Temme, Pastawski and Kastoryano in the case of primitive QMS. This leads to new bounds on the decoherence rates of decohering QMS. Additionally, we apply our results to the study of the tensorization of HC in non-commutative spaces in terms of the completely bounded norms (CB norms) recently introduced by Beigi and King for unital and trace preserving QMS. We generalize their results to the case of a general primitive QMS and provide estimates on the (weak) constants.

Categories: Journals, Physics

### Spatial-Translation-Induced Discrete Time Crystals. (arXiv:1804.01291v2 [cond-mat.mes-hall] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

A discrete time crystal is a phase unique to nonequilibrium systems, where discrete time translation symmetry is spontaneously broken. Most of conventional time crystals proposed so far rely on spontaneous breaking of on-site symmetries and their corresponding on-site symmetry operations. In this Letter, we propose a new time crystal dubbed "spatial-translation-induced discrete time crystal (STI-DTC)", which is realized by spatial translation and its symmetry breaking. Owing to the properties of spatial translation, in this new time crystal, various time crystal orders can emerge only by changing the filling but not changing the driving protocol. We demonstrate that local transport of charges or spins shows a nontrivial oscillation, enabling detection and applications of time crystal orders. Our proposal opens up a new avenue of realizing time crystal orders by spatial translation.

Categories: Journals, Physics

### Fast and noise-resistant construction of controlled-phase gates in Rydberg atoms. (arXiv:1807.02808v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

Based on Rydberg single-photon excitation process, one proposal is proposed to fast construct the noise-resistant two-qubit controlled-phase gate (CPG) in Rydberg atoms with the Lewis-Riesenfeld (LR) invariant method in two-level system. The LR phases during the evolution process of the gate are exactly offset or limited to zero. Consequently and correspondingly, the two-qubit $\pi$ or arbitrary-phase CPGs are constructed. Then the proposal is generalized to multi-qubit $\pi$ CPG with one control and multiple target qubits. Numerical analysis shows that the $\pi$ CPGs can still have high fidelities after considering atomic spontaneous emission and dephasings noises. In contrast to three-level case, the two-level proposal to construct two-qubit $\pi$ CPG is simple and has a higher fidelity while a little less robust against decoherence. And the proposal is also feasible in the intermediate Rydberg interaction regime. For two groups of reasonable parameters, the two-qubit $\pi$ CPG can be constructed within 4$\mu$s with error probability on the order of $10^{-6}$ or 0.4$\mu$s with error probability on the order of $10^{-3}$.

Categories: Journals, Physics

### Benchmarking neural networks for quantum computation. (arXiv:1807.03253v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

The power of quantum computers is still somewhat speculative. While they are certainly faster than classical ones, the class of problems they can efficiently solve has not been mapped definitively onto known classical complexity theory, and there is a paucity of truly quantum algorithms. This is partly because finding algorithms that take advantage of the quantum nature of reality is very difficult. In previous work over the past three decades we have proposed, and developed, the idea of using techniques of machine learning to address this problem. Here we compare the performance of standard real- and complex-valued classical neural networks with that of one of our models for a quantum neural network, on both classical problems and on an archetypal quantum problem: the computation of an entanglement witness. The quantum network is shown to be considerably more powerful.

Categories: Journals, Physics

### Exact holographic tensor networks for the Motzkin spin chain. (arXiv:1806.09626v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

The study of low-dimensional quantum systems has proven to be a particularly fertile field for discovering novel types of quantum matter. When studied numerically, low-energy states of low-dimensional quantum systems are often approximated via a tensor-network description. The tensor network's utility in studying short range correlated states in 1D have been thoroughly investigated, with numerous examples where the treatment is essentially exact. Yet, despite the large number of works investigating these networks and their relations to physical models, examples of exact correspondence between the ground state of a quantum critical system and an appropriate scale-invariant tensor network have eluded us so far. Here we show that the features of the quantum-critical Motzkin model can be faithfully captured by an analytic tensor network that exactly represents the ground state of the physical Hamiltonian. In particular, our network offers a two-dimensional representation of this state by a correspondence between walks and a type of tiling of a square lattice. We discuss connections to renormalization and holography.

Categories: Journals, Physics

### Non-Markovianity and negative entropy production rates. (arXiv:1806.09101v2 [cond-mat.stat-mech] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

Entropy production plays a fundamental role in nonequilibrium thermodynamics to quantify the irreversibility of open systems. Its positivity can be ensured for a wide class of setups, but the entropy production rate can become negative sometimes. This is often taken as an indicator of non-Markovian dynamics. We make this link precise by showing under which conditions a negative entropy production rate implies non-Markovian dynamics and when it does not. This is established within a unified language for two setups: (i) the dynamics resulting from a coarse-grained description of a system in contact with a single heat bath described by a Markovian master equation and (ii) the classical Hamiltonian dynamics of a driven system, which is coupled arbitrary strongly to a single heat bath. The quantum version of the latter result is shown not to hold despite the fact that the integrated thermodynamic description is formally equivalent to the classical case. The instantaneous steady state of a non-Markovian dynamics plays an important element in our study. Our key contribution is to provide a consistent theoretical framework to study the finite-time thermodynamics of a large class of dynamics with a precise link to its non-Markovianity.

Categories: Journals, Physics

### Quantum computing cryptography: Unveiling cryptographic Boolean functions with quantum annealing. (arXiv:1806.08706v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

As the building block in symmetric cryptography, designing Boolean functions satisfying multiple properties is an important problem in sequence ciphers, block ciphers, and hash functions. However, the search of $n$-variable Boolean functions fulfilling global cryptographic constraints is computationally hard due to the super-exponential size $\mathcal{O}(2^{2^n})$ of the space. Here, we introduce a codification of the cryptographically relevant constraints in the ground state of an Ising Hamiltonian, allowing us to naturally encode it in a quantum annealer, which seems to provide a quantum speedup. Additionally, we benchmark small $n$ cases in a D-Wave machine, showing its capacity of devising bent functions, the most relevant set of cryptographic Boolean functions. We have complemented it with local search and chain repair to improve the D-Wave quantum annealer performance related to the low connectivity. This work shows how to codify super-exponential cryptographic problems into quantum annealers and paves the way for reaching quantum supremacy with an adequately designed chip.

Categories: Journals, Physics

### Tightening the Quantum Speed Limit for Almost All Processes. (arXiv:1806.08742v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

Starting from a geometric perspective, we derive a quantum speed limit for arbitrary open quantum evolution, which could be Markovian or non-Markovian. Our methods rely on measuring angles and distances between(mixed) states in the generalized Bloch sphere. We show that our bound is easier to compute and measure than other quantum speed limits for open evolution. Moreover, our bound is tighter than the previous bounds for almost all open processes. Finally, we discuss the physical interpretation of quantum speed limits.

Categories: Journals, Physics

### Limitations on the indistinguishability of photons from remote solid state sources. (arXiv:1806.08213v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

In the present work, we derive a formalism that can be used to predict and interpret the time structure and achievable visibilities for two-photon interference (TPI) experiments using photons from two separate sources. The treatment particularly addresses photons stemming from solid state quantum emitters, which are often subject to pure dephasing (PD) and spectral diffusion (SD). Therefore, it includes the impact of phase- and emission frequency-jitter besides the influence of differing radiative lifetimes and a relative spectral detuning. While the treatment is mainly aimed at interference experiments after Hong-Ou-Mandel (HOM), we additionally offer generalized equations that are applicable to arbitrary linear optical gates, which rely on TPI.

Categories: Journals, Physics

### Bounding the outcome of a two-photon interference measurement using weak coherent states. (arXiv:1806.05012v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

Interference of two photons at a beamsplitter is at the core of many quantum photonic technologies, such as quantum key distribution or linear-optics quantum computing. Observing high-visibility interference is challenging because of the difficulty of realizing indistinguishable single-photon sources. Here, we perform a two-photon interference experiment using phase-randomized weak coherent states with different mean photon numbers. We place a tight upper bound on the expected coincidences for the case when the incident wavepackets contain single photons, allowing us to observe the Hong-Ou-Mandel effect. We find that the interference visibility is at least as large as 0.995$^{+0.005}_{-0.013}$.

Categories: Journals, Physics

### Achieving High-Fidelity Single-Qubit Gates in a Strongly Driven Charge Qubit with $1\!/\!f$ Charge Noise. (arXiv:1806.02413v2 [cond-mat.mes-hall] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

Charge qubits formed in double quantum dots represent quintessential two-level systems that enjoy both ease of control and efficient readout. Unfortunately, charge noise can cause rapid decoherence, with typical single-qubit gate fidelities falling below $90\%$. Here, we develop analytical methods to study the evolution of strongly driven charge qubits, for general and $1\!/\!f$ charge-noise spectra. We show that special pulsing techniques can simultaneously suppress errors due to strong driving and charge noise, yielding single-qubit gates with fidelities above $99.9\%$. These results demonstrate that quantum dot charge qubits provide a potential route to high-fidelity quantum computation.

Categories: Journals, Physics

### A tunable quantum dissipator for active resonator reset in circuit QED. (arXiv:1806.01880v2 [cond-mat.mes-hall] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

We propose a method for fast, deterministic resonator reset based on tunable dissipative modes. The dissipator is based on a Josephson junction with relatively low quality factor. When the dissipator is tuned into resonance with a high quality microwave resonator, resonator photons are absorbed by the dissipator at a rate orders of magnitude faster than the resonator relaxation rate. We determine the optimal parameters for realization of the tunable dissipator, and examine application of the dissipator to removing spurious photon population in the qubit readout resonator in circuit quantum electrodynamics. We show that even in the nonlinear large photon occupation regime, this enhanced resonator decay rate can be attained by appropriate modulation of the dissipator frequency.

Categories: Journals, Physics

### Exact search algorithm to factorize large biprimes and a triprime on IBM quantum computer. (arXiv:1805.10478v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Sat, 2018-07-14 00:33

Factoring large integers using a quantum computer is an outstanding research problem that can illustrate true quantum advantage over classical computers. Exponential time order is required in order to find the prime factors of an integer by means of classical computation. However, the order can be drastically reduced by converting the factorization problem to an optimization one and solving it using a quantum computer. Recent works involving both theoretical and experimental approaches use Shor's algorithm, adiabatic quantum computation and quantum annealing principles to factorize integers. However, our work makes use of the generalized Grover's algorithm as proposed by Liu, with an optimal version of classical algorithm/analytic algebra. We utilize the phase-matching property of the above algorithm for only amplitude amplification purposes to avoid an inherent phase factor that prevents perfect implementation of the algorithm. Here we experimentally demonstrate the factorization of two bi-primes, 4088459 and 966887 using IBM's 5- and 16-qubit quantum processors, hence making those the largest numbers that have been factorized on a quantum device. Using the above 5-qubit processor, we also realize the factorization of a tri-prime integer 175, which had not been achieved to date. We observe good agreement between experimental and theoretical results with high fidelities. The difficulty of the factorization experiments has been analyzed and it has been concluded that the solution to this problem depends on the level of simplification chosen, not the size of the number factored. In principle, our results can be extended to factorize any multi-prime integer with minimum quantum resources.

Categories: Journals, Physics