## Physics

### Universal Dynamical Scaling of Two-dimensional Vortices in a Strongly Interacting Fermionic Superfluid. (arXiv:1902.07558v1 [cond-mat.quant-gas])

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

Dynamical formulation and annihilation of vortices play a key role in the celebrated Berezinskii-Kosterlitz-Thouless (BKT) theory, a universal topological mechanism describing exotic states of matter in low dimensions. Here we study the annihilation dynamics of a large number of vortices and anti-vortices generated by thermally quenching a Fermionic superfluid of $^6$Li atoms in an oblate optical geometry. Universal algebraic scaling laws in both time and space are experimentally revealed over a wide interaction range, from the attractive to the repulsive side across the Feshbach resonance, and further found to agree with a Glauber dynamics in Monte Carlo simulation of the classical XY model and with field-theoretical calculations. Our work provides a direct demonstration of the universal vortex dynamics underlying the BKT theory.

Categories: Journals, Physics

### Transferring the quantum state of electrons across a Fermi sea with Coulomb interaction. (arXiv:1902.07569v1 [cond-mat.mes-hall])

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

The Coulomb interaction generally limits the quantum propagation of electrons. However, it can also provide a mechanism to transfer their quantum state over larger distances. Here, we demonstrate such a form of teleportation, across a metallic island within which the electrons are trapped much longer than their quantum lifetime. This effect originates from the low temperature freezing of the island's charge $Q$ which, in the presence of a single connected electronic channel, enforces a one-to-one correspondence between incoming and outgoing electrons. Such high-fidelity quantum state imprinting is established between well-separated injection and emission locations, through two-path interferences in the integer quantum Hall regime. The added electron quantum phase of $2\pi Q/e$ can allow for strong and decoherence-free entanglement of propagating electrons, and notably of flying qubits.

Categories: Journals, Physics

### A dynamical proof of the second law of thermodynamics. (arXiv:1902.07591v1 [math-ph])

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

We provide a dynamical proof of the second law of thermodynamics, along the lines of an argument of Penrose and Gibbs, making crucial use of the upper semicontinuity of the mean entropy proved by Robinson and Ruelle and Lanford and Robinson. An example is provided by a class of models of quantum spin systems introduced by Emch and Radin. Possible extensions to quantum continuous systems are discussed.

Categories: Journals, Physics

### Simulating frustrated antiferromagnets with quadratically driven QED cavities. (arXiv:1902.07631v1 [quant-ph])

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

We propose a class of quantum simulators for antiferromagnetic spin systems, based on coupled photonic cavities in presence of two-photon driving and dissipation. By modeling the coupling between the different cavities through a hopping term with negative amplitude, we solve numerically the quantum master equation governing the dynamics of the open system and determine its non-equilibrium steady state. Under suitable conditions, the steady state can be described in terms of the degenerate ground states of an antiferromagnetic Ising model. When the geometry of the cavity array is incommensurate with the antiferromagnetic coupling, the steady state presents properties which bear full analogy with those typical of the spin liquid phases arising in frustrated magnets.

Categories: Journals, Physics

### Entropies for Coupled Harmonic Oscillators and Temperature. (arXiv:1902.07645v1 [quant-ph])

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

We study two entropies of a system composed of two coupled harmonic oscillators which is brought to a canonical thermal equilibrium with a heat-bath at temperature $T$. Using the purity function, we explicitly determine the R\'enyi and van Newmon entropies in terms of different physical parameters. We will numerically analyze these two entropies under suitable conditions and show their relevance.

Categories: Journals, Physics

### Optimized cavity-mediated dispersive two-qubit gates between spin qubits. (arXiv:1902.07649v1 [cond-mat.mes-hall])

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

The recent realization of a coherent interface between a single electron in a silicon quantum dot and a single photon trapped in a superconducting cavity opens the way for implementing photon-mediated two-qubit entangling gates. In order to couple a spin to the cavity electric field some type of spin-charge hybridization is needed, which impacts spin control and coherence. In this work we propose a cavity-mediated two-qubit gate and calculate cavity-mediated entangling gate fidelities in the dispersive regime, accounting for errors due to the spin-charge hybridization, as well as photon- and phonon-induced decays. By optimizing the degree of spin-charge hybridization, we show that two-qubit gates mediated by cavity photons are capable of reaching fidelities exceeding 90% in present-day device architectures. High iSWAP gate fidelities are achievable even in the presence of charge noise at the level of $2\,\mu\text{eV}$.

Categories: Journals, Physics

### An understanding of some properties of wave functions in PT-symmetry using (2x2) matrix model. (arXiv:1902.07673v1 [quant-ph])

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

We propose a new CCS(complex-conjugate-space) to understand the behaviour of wave functions in non-hermitian PT-symmetry model in quantum mechanics.As an example of this ,we consider previous Bender,Brody and Jones model PT-symmetry operaor . In non-conventional way one can notice that wave functions in a PTsymmetry model satifies similar relations as in hermitian operator .

Categories: Journals, Physics

### Universality of EPR pairs in Entanglement-Assisted Communication Complexity, and the Communication Cost of State Conversion. (arXiv:1902.07699v1 [quant-ph])

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

Entanglement assistance is known to reduce the quantum communication complexity of evaluating functions with distributed inputs. But does the type of entanglement matter, or are EPR pairs always sufficient? This is a natural question because in several other settings maximally entangled states are known to be less useful as a resource than some partially entangled state. These include non-local games, tasks with quantum communication between players and referee, and simulating bipartite unitaries or communication channels. By contrast, we prove that the bounded-error entanglement-assisted quantum communication complexity of a function cannot be improved by more than a constant factor by replacing maximally entangled states with arbitrary entangled states. In particular, we show that every quantum communication protocol using $Q$ qubits of communication and arbitrary shared entanglement can be $\epsilon$-approximated by a protocol using $O(Q/\epsilon+\log(1/\epsilon)/\epsilon)$ qubits of communication and only EPR pairs as shared entanglement.

Our second result concerns an old question in quantum information theory: How much quantum communication is required to approximately convert one pure bipartite entangled state into another? We show that the communication cost of converting between two bipartite quantum states is upper bounded, up to a constant multiplicative factor, by a natural and efficiently computable quantity which we call the $\ell_{\infty}$-Earth Mover's Distance (EMD) between those two states. Furthermore, we prove a complementary lower bound on the cost of state conversion by the $\epsilon$-smoothed $\ell_{\infty}$-EMD, which is a natural smoothing of the $\ell_{\infty}$-EMD that we will define via a connection with optimal transport theory.

Categories: Journals, Physics

### Velocimetry of cold atoms by matterwave interferometry. (arXiv:1802.02190v4 [physics.atom-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

We present an elegant application of matterwave interferometry to the velocimetry of cold atoms whereby, in analogy to Fourier transform spectroscopy, the 1-D velocity distribution is manifest in the frequency domain of the interferometer output. By using stimulated Raman transitions between hyperfine ground states to perform a three-pulse interferometer sequence, we have measured the velocity distributions of clouds of freely-expanding $^{85}$Rb atoms with temperatures of 33 $\mu$K and 17 $\mu$K. Quadrature measurement of the interferometer output as a function of the temporal asymmetry yields velocity distributions with excellent fidelity. Our technique, which is particularly suited to ultracold samples, compares favourably with conventional Doppler and time-of-flight techniques, and reveals artefacts in standard Raman Doppler methods. The technique is related to, and provides a conceptual foundation of, interferometric matterwave accelerometry, gravimetry and rotation sensing.

Categories: Journals, Physics

### Classical and quantum dissipative dynamics in Josephson junctions: an Arnold problem, bifurcation and capture into resonance. (arXiv:1803.03743v2 [cond-mat.stat-mech] UPDATED)

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

We theoretically study the phase dynamics in Josephson junctions, which maps onto the oscillatory motion of a point-like particle in the washboard potential. Under appropriate driving and damping conditions, the Josephson phase undergoes intriguing bistable dynamics near a saddle point in the quasienergy landscape. The bifurcation mechanism plays a critical role in superconducting quantum circuits with relevance to non-demolition measurements such as high-fidelity readout of qubit states. We address the question `what is the probability of capture into either basin of attraction' and answer it concerning both classical and quantum dynamics. Consequently, we derive the Arnold probability and numerically analyze its implementation of the controlled dynamical switching between two steady states under the various nonequilibrium conditions.

Categories: Journals, Physics

### Quantum metrology with one-dimensional superradiant photonic states. (arXiv:1805.00712v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

Photonic states with large and fixed photon numbers, such as Fock states, enable quantum-enhanced metrology but remain an experimentally elusive resource. A potentially simple, deterministic and scalable way to generate these states consists of fully exciting $N$ quantum emitters equally coupled to a common photonic reservoir, which leads to a collective decay known as Dicke superradiance. The emitted $N$-photon state turns out to be a highly entangled multimode state, and to characterise its metrological properties in this work we: (i) develop theoretical tools to compute the Quantum Fisher Information of general multimode photonic states; (ii) use it to show that Dicke superradiant photons in 1D waveguides achieve Heisenberg scaling, which can be saturated by a parity measurement; (iii) and study the robustness of these states to experimental limitations in state-of-art atom-waveguide QED setups.

Categories: Journals, Physics

### Thermalization in the Quantum Ising Model - Approximations, Limits, and Beyond. (arXiv:1805.04934v2 [cond-mat.quant-gas] UPDATED)

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

We present quantitative predictions for quantum simulator experiments on Ising models from trapped ions to Rydberg chains and show how the thermalization, and thus decoherence times, can be controlled by considering common, independent, and end-cap couplings to the bath. We find (i) independent baths enable more rapid thermalization in comparison to a common one; (ii) the thermalization timescale depends strongly on the position in the Ising phase diagram; (iii) for a common bath larger system sizes show a significant slow down in the thermalization process; and (iv) finite-size scaling indicates a subradiance effect slowing thermalization rates toward the infinite spin chain limit. We find it is necessary to treat the full multi-channel Lindblad master equation rather than the commonly used single-channel local Lindblad approximation to make accurate predictions on a classical computer. This method reduces the number of qubits one can practically classical simulate by at least a factor of 4, in turn showing a quantum advantage for such thermalization problems at a factor of 4 smaller qubit number for open quantum systems as opposed to closed ones. Thus, our results encourage open quantum system exploration in noisy intermediate-scale quantum technologies.

Categories: Journals, Physics

### A single photon detection system for visible and infrared spectrum range. (arXiv:1807.04273v2 [physics.ins-det] UPDATED)

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

We demonstrate niobium nitride based superconducting single-photon detectors sensitive in the spectral range 452 nm - 2300 nm. The system performance was tested in a real-life experiment with correlated photons generated by means of spontaneous parametric down conversion, where one of photon was in the visible range and the other was in the infrared range. We measured a signal to noise ratio as high as $4\times 10^4$ in our detection setting. A photon detection efficiency as high as 64% at 1550 nm and 15% at 2300 nm was observed.

Categories: Journals, Physics

### Epsilon-Near-Zero Grids for On-chip Quantum Networks. (arXiv:1808.04272v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

Realization of an on-chip quantum network is a major goal in the field of integrated quantum photonics. A typical network scalable on-chip demands optical integration of single photon sources, optical circuitry and detectors for routing and processing of quantum information. Current solutions either notoriously experience considerable decoherence or suffer from extended footprint dimensions limiting their on-chip scaling. Here we propose and numerically demonstrate a robust on-chip quantum network based on an epsilon-near-zero (ENZ) material, whose dielectric function has the real part close to zero. We show that ENZ materials strongly protect quantum information against decoherence and losses during its propagation in the dense network. As an example, we model a feasible implementation of an ENZ network and demonstrate that quantum information can be reliably sent across a titanium nitride grid with a coherence length of 434 nm, operating at room temperature, which is more than 40 times larger than state-of-the-art plasmonic analogs. Our results facilitate practical realization of large multi-node quantum photonic networks and circuits on-a-chip.

Categories: Journals, Physics

### More on complexity of operators in quantum field theory. (arXiv:1809.06678v2 [hep-th] UPDATED)

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

Recently it has been shown that the complexity of SU($n$) operator is determined by the geodesic length in a bi-invariant Finsler geometry, which is constrained by some symmetries of quantum field theory. It is based on three axioms and one assumption regarding the complexity in continuous systems. By relaxing one axiom and an assumption, we find that the complexity formula is naturally generalized to the Schatten $p$-norm type. We also clarify the relation between our complexity and other works. First, we show that our results in a bi-invariant geometry are consistent with the ones in a right-invariant geometry such as $k$-local geometry. Here, a careful analysis of the sectional curvature is crucial. Second, we show that our complexity can concretely realize the conjectured pattern of the time-evolution of the complexity: the linear growth up to saturation time. The saturation time can be estimated by the relation between the topology and curvature of SU($n$) groups.

Categories: Journals, Physics

### Charge-Density-Wave Transitions of Dirac Fermions Coupled to Phonons. (arXiv:1809.07903v2 [cond-mat.str-el] UPDATED)

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

The spontaneous generation of charge-density-wave order in a Dirac fermion system via the natural mechanism of electron-phonon coupling is studied in the framework of the Holstein model on the honeycomb lattice. Using two independent and unbiased quantum Monte Carlo methods, the phase diagram as a function of temperature and coupling strength is determined. It features a quantum critical point as well as a line of thermal critical points. Finite-size scaling appears consistent with fermionic Gross-Neveu-Ising universality for the quantum phase transition, and bosonic Ising universality for the thermal phase transition. The critical temperature has a maximum at intermediate couplings. Our findings motivate experimental efforts to identify or engineer Dirac systems with sufficiently strong and tunable electron-phonon coupling.

Categories: Journals, Physics

### Quantum critical regime in a quadratically-driven nonlinear photonic lattice. (arXiv:1809.10138v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

We study an array of coupled optical cavities in presence of two-photon driving and dissipation. The system displays a critical behavior similar to that of a quantum Ising model at finite temperature. Using the corner-space renormalization method, we compute the steady-state properties of finite lattices of varying size, both in one- and two-dimensions. From a finite-size scaling of the average of the photon number parity, we highlight the emergence of a critical point in regimes of small dissipations, belonging to the quantum Ising universality class. For increasing photon loss rates, a departure from this universal behavior signals the onset of a quantum critical regime, where classical fluctuations induced by losses compete with long-range quantum correlations.

Categories: Journals, Physics

### Engineering $\mathrm{SU}(1,1) \otimes \mathrm{SU}(1,1)$ vibrational states. (arXiv:1810.08531v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

We propose an ideal scheme for preparing vibrational $\mathrm{SU(1,1)} \otimes \mathrm{SU(1,1)}$ states in a two-dimensional ion trap using red and blue second sideband resolved driving of two orthogonal vibrational modes. Symmetric and asymmetric driving provide two regimes to realize quantum state engineering of the vibrational modes. In one regime, we show that time evolution synthesizes so-called $\mathrm{SU}(1,1)$ Perelomov coherent states, that is separable squeezed states and their superposition too. The other regime allows engineering of lossless 50/50 $\mathrm{SU}(2)$ beam splitter states that are entangled states. These ideal dynamics are reversible, thus, the non-classical and entangled states produced by our schemes might be used as resources for interferometry.

Categories: Journals, Physics

### Manipulation of Spin Dynamics by Deep Reinforcement Learning Agent. (arXiv:1901.08748v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

We implement the reinforcement learning agent for a spin-1 atomic system to prepare spin squeezed state from given initial state. Proximal policy gradient (PPO) algorithm is used to deal with continuous external control field and final optimized protocol is given by a stochastic policy. In both mean-field system and two-body quantum system, RL agent finds the optimal policies. In many-body quantum system, it also gives polices that outperform purely greedy policy and optimized adiabatic passage. These polices given by RL agent have good physical interpretability in phase space and may help us to understand quantum dynamics. In fact, RL could be highly versatile in quantum optimal control problems.

Categories: Journals, Physics

### Microscopic Identification of Fractional Quantum Hall States on Higher-Genus Surfaces Induced by Gapped Boundaries on Lattices. (arXiv:1901.05119v2 [cond-mat.str-el] CROSS LISTED)

arXiv.org: Quantum Physics - Thu, 2019-02-21 14:02

We present a detailed microscopic investigation of fractional quantum Hall states on effective higher-genus surfaces emerging in a coupled bilayer lattice model featuring holes whose counter propagating chiral edge states are hybridized and gapped out. Although the holes distort the original band structure and lead to ingap remnants of the continuum edge modes, we find that a lowest nearly flat band representing a higher-genus system may naturally form by controlling the local hopping terms that gap out the boundaries. Remarkably, even in the extreme lattice limit where each hole consists of just a single removed site, local interactions in this new flat band lead to various Abelian and non-Abelian fractional quantum Hall states on the emergent higher-genus surfaces which we identify by extracting the non-trivial topological ground-state degeneracies and the fractional statistics of quasiparticles. These results demonstrate that our microscopic lattice model is a promising platform to realize novel fractional quantum Hall states with gapped boundaries, thus enabling a possible new route towards universal topological quantum computation.

Categories: Journals, Physics