Physics

Time evolution of quantum systems with time-dependent non-Hermitian Hamiltonian and the pseudo Hermitian invariant operator. (arXiv:1705.08298v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

We study the time evolution of quantum systems with a time-dependent non-Hermitian Hamiltonian given by a linear combination of SU(1,1) and SU(2) generators.With a time-dependent metric, the pseudo-Hermitian invariant operator is constructed in the same manner as for both the SU(1,1) and SU(2) systems. The exact common solutions of the Schr\"odinger equations for both the SU(1,1) and SU(2) systems are obtained in terms of eigenstates of the pseudo-Hermitian invariant operator.

Categories: Journals, Physics

An impossibility theorem for Parameter Independent hidden variable theories. (arXiv:1705.08341v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

Recently, Roger Colbeck and Renato Renner (C&R) have claimed that '[n]o extension of quantum theory can have improved predictive power'. If correct, this is a spectacular impossibility theorem for hidden variable theories, which is more general than the theorems of Bell and Leggett. C&R's claim essentially means that in any hidden variable theory that is compatible with quantum-mechanical predictions, probabilities of measurement outcomes are independent of these hidden variables. On closer inspection, however, the generality and validity of the claim can be contested. First, it is based on an assumption called 'Freedom of Choice'. As the name suggests, this assumption involves the independence of an experimenter's choice of measurement settings. But in the way C&R define this assumption, a no-signalling condition is surreptitiously presupposed, making the assumption less innocent than it sounds. When using this definition, any hidden variable theory violating Parameter Independence, such as Bohmian Mechanics, is immediately shown to be incompatible with quantum-mechanical predictions. Also, the argument of C&R is hard to follow and their mathematical derivation contains several gaps, some of which cannot be closed in the way they suggest. We shall show that these gaps can be filled. The issue with the 'Freedom of Choice' assumption can be circumvented by explicitly assuming Parameter Independence. This makes the result less general, but better founded. We then obtain an impossibility theorem for hidden variable theories satisfying Parameter Independence only. So, while quantum mechanics itself satisfies Parameter Independence, if a variable is added that changes the outcome probabilities, however slightly, Parameter Independence must be violated.

Categories: Journals, Physics

Photonic topological pumping through the edges of a dynamical four-dimensional quantum Hall system. (arXiv:1705.08361v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

When a two-dimensional electron gas is exposed to a perpendicular magnetic field and an in-plane electric field, its conductance becomes quantized in the transverse in-plane direction: this is known as the quantum Hall (QH) effect. This effect is a result of the nontrivial topology of the system's electronic band structure, where an integer topological invariant known as the first Chern number leads to the quantization of the Hall conductance. Interestingly, it was shown that the QH effect can be generalized mathematically to four spatial dimensions (4D), but this effect has never been realized for the obvious reason that experimental systems are bound to three spatial dimensions. In this work, we harness the high tunability and control offered by photonic waveguide arrays to experimentally realize a dynamically-generated 4D QH system using a 2D array of coupled optical waveguides. The inter-waveguide separation is constructed such that the propagation of light along the device samples over higher-dimensional momenta in the directions orthogonal to the two physical dimensions, thus realizing a 2D topological pump. As a result, the device's band structure is associated with 4D topological invariants known as second Chern numbers which support a quantized bulk Hall response with a 4D symmetry. In a finite-sized system, the 4D topological bulk response is carried by localized edges modes that cross the sample as a function of of the modulated auxiliary momenta. We directly observe this crossing through photon pumping from edge-to-edge and corner-to-corner of our system. These are equivalent to the pumping of charge across a 4D system from one 3D hypersurface to the opposite one and from one 2D hyperedge to another, and serve as first experimental realization of higher-dimensional topological physics.

Categories: Journals, Physics

Exploring 4D Quantum Hall Physics with a 2D Topological Charge Pump. (arXiv:1705.08371v1 [cond-mat.quant-gas])

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

The discovery of topological states of matter has profoundly augmented our understanding of phase transitions in physical systems. Instead of local order parameters, topological phases are described by global topological invariants and are therefore robust against perturbations. A prominent example thereof is the two-dimensional integer quantum Hall effect. It is characterized by the first Chern number which manifests in the quantized Hall response induced by an external electric field. Generalizing the quantum Hall effect to four-dimensional systems leads to the appearance of a novel non-linear Hall response that is quantized as well, but described by a 4D topological invariant - the second Chern number. Here, we report on the first observation of a bulk response with intrinsic 4D topology and the measurement of the associated second Chern number. By implementing a 2D topological charge pump with ultracold bosonic atoms in an angled optical superlattice, we realize a dynamical version of the 4D integer quantum Hall effect. Using a small atom cloud as a local probe, we fully characterize the non-linear response of the system by in-situ imaging and site-resolved band mapping. Our findings pave the way to experimentally probe higher-dimensional quantum Hall systems, where new topological phases with exotic excitations are predicted.

Categories: Journals, Physics

Coherent many-body spin dynamics in a long-range interacting Ising chain. (arXiv:1705.08372v1 [physics.atom-ph])

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

Coherent many-body quantum dynamics lies at the heart of quantum simulation and quantum computation. Both require coherent evolution in the exponentially large Hilbert space of an interacting many-body system. To date, trapped ions have defined the state of the art in terms of achievable coherence times in interacting spin chains. Here, we establish an alternative platform by reporting on the observation of coherent, fully interaction-driven quantum revivals of the magnetization in Rydberg-dressed Ising spin chains of atoms trapped in an optical lattice. We identify partial many-body revivals at up to about ten times the characteristic time scale set by the interactions. At the same time, single-site-resolved correlation measurements link the magnetization dynamics with inter-spin correlations appearing at different distances during the evolution. These results mark an enabling step towards the implementation of Rydberg atom based quantum annealers, quantum simulations of higher dimensional complex magnetic Hamiltonians, and itinerant long-range interacting quantum matter.

Categories: Journals, Physics

Teleportation identities for discrete and continuous systems. (arXiv:1705.08390v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

We give a streamlined derivation of "teleportation identities" for discrete and continuous systems. The identities do not depend on the choice of Bell basis and so are "coordinate free". The unitaries Bob needs to apply to recover Alice's unknown state is the product of the unitaries Alice and Bob use to generate a common Bell basis. The case of qubit, qudits and continuous systems are all treated on the same footing.

Categories: Journals, Physics

Killing (absorption) versus survival in random motion. (arXiv:1705.08402v1 [cond-mat.stat-mech])

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

We address anew the random dynamics in a bounded domain, while focusing on somewhat unexplored affinities between the presence of absorbing and/or inaccessible boundaries. For the Brownian motion (L\'{e}vy-stable cases are briefly mentioned) model-independent features of the long-term survival, including these of the permanent trapping, are established. Generalization of these arguments to stochastic processes with killing in an unbounded domain is provided as well. We demonstrate that the adopted conditioning method, based on the notion of the Bernstein transition density, works properly also in cases, when the spectrum of the motion generator goes down to zero and no isolated minimal eigenvalue is in existence.

Categories: Journals, Physics

Hamilton-Jacobi approach to Potential Functions in Information Geometry. (arXiv:1608.06584v2 [math-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

The search for a potential function $S$ allowing to reconstruct a given metric tensor $g$ and a given symmetric covariant tensor $T$ on a manifold $\mathcal{M}$ is formulated as the Hamilton-Jacobi problem associated with a canonically defined Lagrangian on $T\mathcal{M}$. The connection between this problem, the geometric structure of the space of pure states of quantum mechanics, and the theory of contrast functions of classical information geometry is outlined.

Categories: Journals, Physics

Duality and helicity: the photon wave function approach. (arXiv:1608.08573v4 [hep-th] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

The photon wave equation proposed in terms of the Riemann-Silberstein vector is derived from a first-order Dirac/Weyl-type action principle. It is symmetric w.r.t. duality transformations, but the associated Noether quantity vanishes. Replacing the fields by potentials and using instead a quadratic Klein-Gordon-type Lagrangian allows us to recover the double-Chern-Simons expression of conserved helicity and is shown to be equivalent to recently proposed alternative frameworks. Applied to the potential-modified theory the Dirac/Weyl-type approach yields again zero conserved charge, whereas the Klein-Gordon-type approach applied to the original setting yields Lipkin's "zilch".

Categories: Journals, Physics

Local Pauli stabilizers of symmetric hypergraph states. (arXiv:1609.01306v4 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

Hypergraph states of many quantum bits share the rich interplay between simple combinatorial description and nontrivial entanglement properties enjoyed by the graph states that they generalize. In this paper, we consider hypergraph states that are also permutationally invariant. We characterize the states in this class that have nontrivial local Pauli stabilizers and give applications to nonlocality and error correction.

Categories: Journals, Physics

Area law and its violation: A microscopic inspection into the structure of entanglement and fluctuations. (arXiv:1506.00545v4 [cond-mat.quant-gas] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

Quantum fluctuations of local quantities can be a direct signature of entanglement in an extended quantum many-body system. Hence they may serve as a theoretical (as well as an experimental) tool to detect the spatial properties of the entanglement entropy of a subsystem. In the ground state of quantum many-body systems, its scaling is typically linear in the boundary of the subsystem (area law). Here we propose a microscopic insight into the spatial structure of entanglement and fluctuations using the concept of \emph{contour}, recently introduced to decompose the bipartite entanglement entropy of lattice free fermions between two extended regions $A$ and $B$ into contributions from single sites in $A$. We generalize the notion of contour to the entanglement of any quadratic (bosonic or fermionic) lattice Hamiltonian, as well as to particle-number fluctuations. The entanglement and fluctuations contours are found to generally decay when moving away from the boundary between $A$ and $B$. We show that in the case of free fermions the decay of the entanglement contour follows closely that of the fluctuation contour. In the case of Bose-condensed interacting bosons, treated via the Bogoliubov and spin-wave approximations, such a link cannot be established -- fluctuation and entanglement contours are found to be radically different, as they lead to a logarithmically violated area law for fluctuations, and to a strict area law of entanglement. Analyzing in depth the role of the zero-energy Goldstone mode of spin-wave theory, and of the corresponding lowest-energy mode in the entanglement spectrum, we unveil a subtle interplay between the special contour and energy scaling of the latter, and universal additive logarithmic corrections to entanglement area law discussed extensively in the recent literature.

Categories: Journals, Physics

Vibrational assisted conduction in a molecular wire. (arXiv:1612.01809v2 [cond-mat.mes-hall] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

We present a detailed study of the conduction properties of a molecular wire where hopping processes between electronic sites are coupled to a vibrational mode of the molecule. The latter is sandwiched between two electronic leads at finite temperatures. We show that the electro-mechanical coupling can lead to a strong enhancement of the lead-to-lead conduction. Moreover, under suitable driving of the molecular vibrational mode, the device can act as a transistor passing sharply from enhanced conduction to short-circuit configuration.

Categories: Journals, Physics

Single-photon interference due to motion in an atomic collective excitation. (arXiv:1612.05467v2 [physics.atom-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

We experimentally demonstrate the generation of heralded bi-chromatic single photons from an atomic collective spin excitation (CSE). The photon arrival times display collective quantum beats, a novel interference effect resulting from the relative motion of atoms in the CSE. A combination of velocity-selective excitation with strong laser dressing and the addition of a magnetic field allows for exquisite control of this collective beat phenomenon. The present experiment uses a diamond scheme with near-IR photons that can be extended to include telecommunications-wavelengths or modified to allow storage and retrieval in an inverted-Y scheme.

Categories: Journals, Physics

Phase rotation symmetry and the topology of oriented scattering networks. (arXiv:1612.05769v2 [cond-mat.mes-hall] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

We investigate the topological properties of dynamical states evolving on periodic oriented graphs. This evolution, that encodes the scattering processes occurring at the nodes of the graph, is described by a single-step global operator, in the spirit of the Ho-Chalker model. When the successive scattering events follow a cyclic sequence, the corresponding scattering network can be equivalently described by a discrete time-periodic unitary evolution, in line with Floquet systems. Such systems may present anomalous topological phases where all the first Chern numbers are vanishing, but where protected edge states appear in a finite geometry. To investigate the origin of such anomalous phases, we introduce the phase rotation symmetry, a generalization of usual symmetries which only occurs in unitary systems (as opposed to Hamiltonian systems). Equipped with this new tool, we explore a possible explanation of the pervasiveness of anomalous phases in scattering network models, and we define bulk topological invariants suited to both equivalent descriptions of the network model, which fully capture the topology of the system. We finally show that the two invariants coincide, again through a phase rotation symmetry arising from the particular structure of the network model.

Categories: Journals, Physics

Critical behavior at dynamical phase transition in the generalized Bose-Anderson model. (arXiv:1701.03332v2 [cond-mat.quant-gas] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

Critical properties of the dynamical phase transition in the quenched generalized Bose-Anderson impurity model are studied in the mean-field limit of an infinite number of channels. The transition separates the evolution toward ground state and toward the branch of stable excited states. We perform numerically exact simulations of a close vicinity of the critical quench amplitude. The relaxation constant describing the asymptotic evolution toward ground state, as well as asymptotic frequency of persistent phase rotation and number of cloud particles at stable excited state are power functions of the detuning from the critical quench amplitude. The critical evolution (separatrix between the two regimes) shows a non-Lyapunov power-law instability arising after a certain critical time. The observed critical behavior is attributed to the irreversibility of the dynamics of particles leaving the cloud and to memory effects related to the low-energy behavior of the lattice density of states.

Categories: Journals, Physics

Efficient classical verification of quantum computations. (arXiv:1705.02817v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

We propose an efficient scheme for verifying quantum computations in the `high complexity' regime i.e. beyond the remit of classical computers. Previously proposed schemes remarkably provide confidence against arbitrarily malicious adversarial behaviour in the misfunctioning of the quantum computing device. Our scheme is not secure against arbitrarily adversarial behaviour, but may nevertheless be sufficiently acceptable in many practical situations. With this concession we gain in manifest simplicity and transparency, and in contrast to previous schemes, our verifier is entirely classical. It is based on the fact that adaptive Clifford circuits on general product state inputs provide universal quantum computation, while the same processes without adaptation are always classically efficiently simulatable.

Categories: Journals, Physics

Magnetometry via spin-mechanical coupling in levitated optomechanics. (arXiv:1705.07453v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

We analyze magnetometry using an optically levitated nanodiamond. We consider a configuration where a magnetic field gradient couples the mechanical oscillation of the diamond with its spin degree of freedom provided by a Nitrogen vacancy center. First, we investigate measurement of the position spectrum of the mechanical oscillator. We find that conditions of ultrahigh vacuum and feedback cooling allow a magnetic field gradient sensitivity of 50 $\mu$Tm$^{-1}$/$\sqrt{\mbox{Hz}}$. At high pressure and room temperature, this sensitivity degrades by approximately a factor of two. Subsequently, we characterize the magnetic field gradient sensitivity obtainable by maneuvering the spin degrees of freedom using Ramsey interferometry. We find that this technique can offer photon-shot noise and spin-projection noise limited magnetic field gradient sensitivity of 100 $\mu$Tm$^{-1}$/$\sqrt{\mbox{Hz}}$. For experimentally accessible parameters, the effects of decoherence in the spin-based magnetometry turn out to be negligible. We conclude that this hybrid levitated nanomechanical magnetometer provides a favorable and versatile platform for sensing applications.

Categories: Journals, Physics

Deriving robust noncontextuality inequalities from algebraic proofs of the Kochen-Specker theorem: the Peres-Mermin square. (arXiv:1704.01153v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

When a measurement is compatible with each of two other measurements that are incompatible with one another, these define distinct contexts for the given measurement. The Kochen-Specker theorem rules out models of quantum theory that satisfy a particular assumption of context-independence: that sharp measurements are assigned outcomes both deterministically and independently of their context. This notion of noncontextuality is not suited to a direct experimental test because realistic measurements always have some degree of unsharpness due to noise. However, a generalized notion of noncontextuality has been proposed that is applicable to any experimental procedure, including unsharp measurements, but also preparations as well, and for which a quantum no-go result still holds. According to this notion, the model need only specify a probability distribution over the outcomes of a measurement in a context-independent way, rather than specifying a particular outcome. It also implies novel constraints of context-independence for the representation of preparations. In this article, we describe a general technique for translating proofs of the Kochen-Specker theorem into inequality constraints on realistic experimental statistics, the violation of which witnesses the impossibility of a noncontextual model. We focus on algebraic state-independent proofs, using the Peres-Mermin square as our illustrative example. Our technique yields the necessary and sufficient conditions for a particular set of correlations (between the preparations and the measurements) to admit a noncontextual model. The inequalities thus derived are demonstrably robust to noise. We specify how experimental data must be processed in order to achieve a test of these inequalities. We also provide a criticism of prior proposals for experimental tests of noncontextuality based on the Peres-Mermin square.

Categories: Journals, Physics

Rotating Gaussian wave packets in weak external potentials. (arXiv:1703.06413v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

We address the time evolution of two- and three-dimensional nonrelativistic Gaussian wave packets in the presence of a weak external potential of arbitrary functional form. The focus of our study is the phenomenon of rotation of a Gaussian wave packet around its center of mass, as quantified by mean angular momentum computed relative to the wave packet center. Using a semiclassical approximation of the eikonal type, we derive an explicit formula for a time-dependent change of mean angular momentum of a wave packet induced by its interaction with a weak external potential. As an example, we apply our analytical approach to the scenario of a two-dimensional quantum particle crossing a tilted ridge potential barrier. In particular, we demonstrate that the initial orientation of the particle wave packet determines the sense of its rotation, and report a good agreement between analytical and numerical results.

Categories: Journals, Physics

On the Stochastic Origin of Quantum Mechanics. (arXiv:1703.00140v4 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-05-24 09:45

The quantum Liouville equation, which describes the phase space dynamics of a quantum system of fermions, is analyzed from statistical point of view as a particular example of the Kramers-Moyal expansion. Quantum mechanics is extended to the relativistic domain by generalizing the Wigner-Moyal equation. Thus, an expression is derived for the relativistic mass in the Wigner quantum phase space presentation. The diffusion with an imaginary diffusion coefficient is also discussed. An imaginary stochastic process is proposed as the origin of quantum mechanics.

Categories: Journals, Physics
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