## Physics

### Edge insulating topological phases in a two-dimensional long-range superconductor. (arXiv:1707.05777v1 [cond-mat.str-el])

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

We study the zero-temperature phase diagram of a two dimensional square lattice loaded by spinless fermions, with nearest neighbor hopping and algebraically decaying pairing. We find that for sufficiently long-range pairing, new phases, not continuously connected with any short-range phase, occur, signaled by the violation of the area law for the Von Neumann entropy, by semi-integer Chern numbers, and by edge modes with nonzero mass. The latter feature results in the absence of single-fermion edge conductivity, present instead in the short- range limit. The definition of a topology in the bulk and the presence of a bulk-boundary correspondence is still suggested for the long-range phases. Recent experimental proposals and advances open the stimulating possibility to probe the described long-range effects in next-future realistic set-ups.

Categories: Journals, Physics

### Generalized Uncertainty Relations in Stochastic Variational Approach. (arXiv:1208.0258v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

We discuss the generalized uncertainty relation which is applicable to stochastic systems described in the framework of the stochastic variational method (SVM). We first formulate the Hamiltonian formalism of SVM which describes quantum, classical and dissipative dynamics on an equal footing. Using this result, we define the standard deviation of the momentum for stochastic trajectories and derive the inequality satisfied for the deviations of the position and the momentum. This relation not only reproduces the Kennard inequality in quantum mechanics but also is applicable to the Gross-Pitaevskii equation and the Navier-Stokes-Fourier equation. For the case of the Navier-Stokes-Fourier equation, the obtained minimum uncertainty is two order larger than that of quantum mechanics, although it is still sufficiently small compared to the coarse-grained scale of hydrodynamics. As a non-trivial example of the application of the SVM quantization, we further investigate a time-dependent minimum uncertainty of the Kostin (Schroedinger-Langevin) equation.

Categories: Journals, Physics

### Topological magnon bound-states in periodically modulated Heisenberg XXZ chains. (arXiv:1602.03217v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

Strongly interacting topological states in multi-particle quantum systems pose great challenges to both theory and experiment. Recently, bound states of elementary spin waves (magnons) in quantum magnets have been experimentally observed in quantum Heisenberg chains comprising ultracold Bose atoms in optical lattices. Here, we explore a strongly interacting topological state called topological magnon bound-state in the quantum Heisenberg chain under cotranslational symmetry. We find that the cotranslational symmetry is the key to the definition of a topological invariant for multi-particle quantum states, which enables us to characterize the topological features of multi-magnon excitations. We calculate energy spectra, density distributions, correlations and Chern numbers of the two-magnon bound-states and show the existence of topological protected edge bound-states. Our study not only opens a new prospect to pursue topological magnon bound-states, but also gives insights into the characterization and understanding of strongly interacting topological states.

Categories: Journals, Physics

### Macroscopic Quantum Measurements of noncommuting observables. (arXiv:1605.05956v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

Assuming a well-behaving quantum-to-classical transition, measuring large quantum systems should be highly informative with low measurement-induced disturbance, while the coupling between system and measurement apparatus is "fairly simple" and weak. Here, we show that this is indeed possible within the formalism of quantum mechanics. We discuss an example of estimating the collective magnetization of a spin ensemble by simultaneous measuring three orthogonal spin directions. For the task of estimating the direction of a spin-coherent state, we find that the average guessing fidelity and the system disturbance are nonmonotonic functions of the coupling strength. Strikingly, we discover an intermediate regime for the coupling strength where the guessing fidelity is quasi-optimal, while the measured state is almost not disturbed.

Categories: Journals, Physics

### Spin and topological order in a periodically driven spin chain. (arXiv:1611.00659v3 [cond-mat.quant-gas] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

The periodically driven quantum Ising chain has recently attracted a large attention in the context of Floquet engineering. In addition to the common paramagnet and ferromagnet, this driven model can give rise to new topological phases. In this work we systematically explore its quantum phase diagram, by examining the properties of its Floquet ground state. We specifically focus on driving protocols with time-reversal invariant points, and demonstrate the existence of an infinite number of distinct phases. These phases are separated by second-order quantum phase transitions, accompanied by continuous changes of local and string order parameters, as well as sudden changes of a topological winding number and of the number of protected edge states. When one of these phase transitions is adiabatically crossed, the correlator associated to the order parameter is nonvanishing over a length scale which shows a Kibble-Zurek scaling. In some phases, the Floquet ground state spontaneously breaks the discrete time-translation symmetry of the Hamiltonian. Our findings provide a better understanding of topological phases in periodically driven clean integrable models.

Categories: Journals, Physics

### A quantum phase transition detected through one dimensional ballistic conductance. (arXiv:1611.03185v2 [cond-mat.str-el] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

A quantum phase transition is an unequivocal signature of strongly correlated many-body physics. Signatures of such phenomena are yet to be observed in ballistic transport through quantum wires. Recent developments in quantum wires have made it possible to enhance the interaction between the electrons. Here we show that hitherto unexplained anticrossing between conduction energy sub-bands, observed in such experiments, can be explained through a simple yet effective discretised model which undergoes a second order quantum phase transition within the Ising universality class. Accordingly, we observe how the charge distribution, transverse to the direction of the wire will vary across the phase transition. We show that data coming from three different samples with differing electron densities and gate voltages show a remarkable universal scaling behavior, determined by the relevant critical exponent, which is only possible near a quantum phase transition.

Categories: Journals, Physics

### Determining complementary properties with quantum clones. (arXiv:1701.04095v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

In a classical world, simultaneous measurements of complementary properties (e.g. position and momentum) give a system's state. In quantum mechanics, measurement-induced disturbance is largest for complementary properties and, hence, limits the precision with which such properties can be determined simultaneously. It is tempting to try to sidestep this disturbance by copying the system and measuring each complementary property on a separate copy. However, perfect copying is physically impossible in quantum mechanics. Here, we investigate using the closest quantum analog to this copying strategy, optimal cloning. The coherent portion of the generated clones' state corresponds to "twins" of the input system. Like perfect copies, both twins faithfully reproduce the properties of the input system. Unlike perfect copies, the twins are entangled. As such, a measurement on both twins is equivalent to a simultaneous measurement on the input system. For complementary observables, this joint measurement gives the system's state, just as in the classical case. We demonstrate this experimentally using polarized single photons.

Categories: Journals, Physics

### Topological Quantum Optics in Two-Dimensional Atomic Arrays. (arXiv:1703.04849v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

We demonstrate that two-dimensional atomic emitter arrays with subwavelength spacing constitute topologically protected quantum optical systems where the photon propagation is robust against large imperfections while losses associated with free space emission are strongly suppressed. Breaking time-reversal symmetry with a magnetic field results in gapped photonic bands with non-trivial Chern numbers and topologically protected, long-lived edge states. Due to the inherent nonlinearity of constituent emitters, such systems provide a platform for exploring quantum optical analogues of interacting topological systems.

Categories: Journals, Physics

### Multi-frequency Spin Manipulation Using Rapidly Tunable Superconducting Coplanar Waveguide Microresonators. (arXiv:1705.02413v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

In this work, we demonstrate the use of frequency-tunable superconducting NbTiN coplanar waveguide microresonators for multi-frequency pulsed electron spin resonance (ESR) experiments. By applying a bias current to the center pin, the resonance frequency ($\sim$7.6 GHz) can be continuously tuned by as much as 95 MHz in 270 ns without a change in the quality factor of 3000 at 2K. We demonstrate the ESR performance of our resonators by measuring donor spin ensembles in silicon and show that adiabatic pulses can be used to overcome magnetic field inhomogeneities and microwave power limitations due to the applied bias current. We take advantage of the rapid tunability of these resonators to manipulate both phosphorus and arsenic spins in a single pulse sequence, demonstrating pulsed double electron-electron resonance (DEER). Our NbTiN resonator design is useful for multi-frequency pulsed ESR and should also have applications in experiments where spin ensembles are used as quantum memories.

Categories: Journals, Physics

### Dynamical pruning of the multiconfiguration time-dependent Hartree method (DP-MCTDH): An efficient approach for multidimensional quantum dynamics. (arXiv:1705.02769v2 [physics.chem-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

We present two strategies for combining dynamical pruning with the multiconfiguration time-dependent Hartree method (DP-MCTDH), where dynamical pruning means on-the-fly selection of relevant basis functions. The first strategy prunes the primitive basis that represents the single-particle functions (SPFs). This is useful for smaller systems that require many primitive basis functions per degree of freedom, as we will illustrate for NO$_2$. Furthermore, this allows for higher-dimensional mode combination and partially lifts the sum-of-product-form requirement onto the structure of the Hamiltonian, as we illustrate for nonadiabatic 24-dimensional pyrazine. The second strategy prunes the set of configurations of SPF at each time step. We show that this strategy yields significant speed-ups with factors between 5 and 50 in computing time, making it competitive with the multilayer MCTDH method.

Categories: Journals, Physics

### Modulator-free coherent-one-way quantum key distribution. (arXiv:1707.04425v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

Time-bin encoding is an attractive method for transmitting photonic qubits over long distances with minimal decoherence. It allows a simple receiver for quantum key distribution (QKD) that extracts a key by measuring time of arrival of photons and detects eavesdropping by measuring interference of pulses in different time bins. In the past, coherent pulses have been generated using a CW laser and an intensity modulator. A greatly simplified transmitter is proposed and demonstrated here that works by directly modulating the laser diode. Coherence between pulses is maintained by a weak seed laser. The modulator-free source creates time-bin encoded pulses with a high extinction ratio (29.4 dB) and an interference visibility above 97 %. The resulting QKD transmitter gives estimated secure key rates up to 4.57 Mbit/s, the highest yet reported for coherent-one-way QKD, and can be programmed for all protocols using weak coherent pulses.

Categories: Journals, Physics

### Learning the Einstein-Podolsky-Rosen correlations on a Restricted Boltzmann Machine. (arXiv:1707.03114v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

We construct a hidden variable model for the EPR correlations using a Restricted Boltzmann Machine. The model reproduces the expected correlations and thus violates the Bell inequality, as required by Bell's theorem. Unlike most hidden-variable models, this model does not violate the $locality$ assumption in Bell's argument. Rather, it violates $measurement$ $independence$, albeit in a decidedly non-conspiratorial way.

Categories: Journals, Physics

### Relativistic Entanglement. (arXiv:1707.03294v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

The relativistic quantum theory of Stueckelberg, Horwitz and Piron (SHP) describes in a simple way the experiment on interference in time of an electron emitted by femtosecond laser pulses carried out by Lindner {\it et al}. In this paper, we show that, in a way similar to our study of the Lindner {\it et al} experiment (with some additional discussion of the covariant quantum mechanical description of spin and angular momentum), the experiment proposed by Palacios {\it et al} to demonstrate entanglement of a two electron state, where the electrons are separated in time of emission, has a consistent interpretation in terms of the SHP theory. We find, after a simple calculation, results in essential agreement with those of Palacios {\it et al}; but with the observed times as values of proper quantum observables.

Categories: Journals, Physics

### Optimal discrimination of optical coherent states cannot always be realized by interfering with coherent light, photon counting, and feedback. (arXiv:1706.02125v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

It is well known that a minimum error quantum measurement for arbitrary binary optical coherent states can be realized by a receiver that comprises interfering with a coherent reference light, photon counting, and feedback control. We show that, for ternary optical coherent states, a minimum error measurement cannot always be realized by such a receiver. The problem of finding an upper bound on the maximum success probability of such a receiver can be formulated as a convex programming. We derive its dual problem and numerically find the upper bound. At least for ternary phase-shift keyed coherent states, this bound does not reach that of a minimum error measurement.

Categories: Journals, Physics

### (Non) equilibrium dynamics: a (broken) symmetry of the Keldysh generating functional. (arXiv:1705.10800v2 [cond-mat.stat-mech] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

We unveil the universal (model-independent) symmetry satisfied by Schwinger-Keldysh quantum field theories whenever they describe equilibrium dynamics. This is made possible by a generalization of the Schwinger-Keldysh path-integral formalism in which the physical time can be re-parametrized to arbitrary contours in the complex plane. Strong relations between correlation functions, such as the fluctuation-dissipation theorems, are derived as immediate consequences of this symmetry of equilibrium. In this view, quantum non-equilibrium dynamics -- e.g. when driving with a time-dependent potential -- are seen as symmetry-breaking processes. The symmetry-breaking terms of the action are identified as a measure of irreversibility, defined at the level of a single quantum trajectory. Moreover, they are shown to obey quantum fluctuation theorems. These results extend stochastic thermodynamics to the quantum realm.

Categories: Journals, Physics

### Analytic Few Photon Scattering in Waveguide QED for Entanglement Generation. (arXiv:1705.07016v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

We develop an approach to light-matter coupling in waveguide QED based upon scattering amplitudes evaluated via Dyson series. For optical states containing more than single photons, terms in this series become increasingly complex and we provide a diagrammatic recipe for their evaluation, which is capable of yielding analytic results. Our method fully specifies a combined emitter-optical state that permits investigation of matter-matter entanglement generation protocols. We use our expressions to study a scheme for entangling spatially separated two-level systems and find that an entangled two-photon input provides a clear advantage over a separable single photon state.

Categories: Journals, Physics

### CV MDI QKD: Composable Security against Coherent Attacks. (arXiv:1704.07924v4 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

We present a rigorous security analysis of Continuous-Variable Measurement-Device Independent Quantum Key Distribution (CV MDI QKD) in a practical finite size scenario. We show that a unique feature of CV MDI QKD is that the whole raw key can be exploited for both parameter estimation and secret-key generation, yielding the highest rates in the finite-size regime. The security proof develops in two steps: first we assess the security against collective Gaussian attacks, then we extend to the most general class of coherent attacks via the Gaussian de Finetti reduction. Our findings confirm that CV MDI protocols allow for high QKD rates on the metropolitan scale, and achieve a nonzero secret key rate against the most general class of coherent attacks after $10^7-10^9$ quantum signal transmissions, depending on loss and noise, and on the required level of security.

Categories: Journals, Physics

### Entanglement verification of noisy N00N states. (arXiv:1704.05637v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

Entangled quantum states, such as N00N states, are of major importance for quantum technologies due to their quantum-enhanced performance. At the same time, their quantum correlations are relatively vulnerable when they are subjected to imperfections. Therefore, it is crucial to determine under which circumstances their distinct quantum features can be exploited. In this paper, we study the entanglement property of noisy N00N states. This class of states is a generalization of N00N states including various attenuation effects, such as mixing, constant or fluctuating losses, and dephasing. To verify their entanglement, we pursue two strategies: detection-based entanglement witnesses and entanglement quasiprobabilities. Both methods result from our solution of so-called separability eigenvalue equations. In particular, the entanglement quasiprobabilities allow for a full entanglement characterization. As examples of our general treatment, the cases of N00N states subjected to Gaussian dephasing and fluctuating atmospheric losses are explicitly studied. In any correlated fluctuating loss channel, entanglement is found to survive for non-zero transmissivity. In addition, an extension of our approach to multipartite systems is given, and the relation to the quantum-optical nonclassicality in phase-space is discussed.

Categories: Journals, Physics

### Secure alignment of coordinate systems by using quantum correlation. (arXiv:1704.00833v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

We show that two parties far apart can use shared entangled states and classical communication to align their coordinate systems with a very high fidelity. Moreover compared with previous methods proposed for such a task, i.e. sending parallel or anti-parallel pairs or groups of spin states, our method has the extra advantages of using single qubit measurements and also being secure, so that third parties do not extract any information about the aligned coordinate system established between the two parties. The latter property is important in many other quantum information protocols in which measurements inevitably play a significant role.

Categories: Journals, Physics

### The capacity of coherent-state adaptive decoders with interferometry and single-mode detectors. (arXiv:1703.05701v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-07-19 13:08

A class of Adaptive Decoders (AD's) for coherent-state sequences is studied, including in particular the most common technology for optical-signal processing, e.g., interferometers, coherent displacements and photon-counting detectors. More generally we consider AD's comprising adaptive procedures based on passive multi-mode Gaussian unitaries and arbitrary single-mode destructive measurements. For classical communication on quantum phase-insensitive Gaussian channels with a coherent-state encoding, we show that the AD's optimal information transmission rate is not greater than that of a single-mode decoder. Our result also implies that the ultimate classical capacity of quantum phase-insensitive Gaussian channels is unlikely to be achieved with the considered class of AD's.

Categories: Journals, Physics