Origin and meaning of quantum nonlocality. (arXiv:1110.4641v1 [quant-ph])

Quantum nonlocality is revisited from a novel point of view by studying the
problem of an originally classical particle immersed in the stochastic
zero-point radiation field (zpf). The entire system is left to evolve until it
reaches a state in which the radiative terms cancel each other in the mean in a
first approximation. The ensuing approximate statistical description reduced to
the particle’s configuration space contains a nonclassical term due to the
dispersion of the momentum, which depends on the density of particles {\rho}(x)
and thus is nonlocal. This description is shown to be equivalent to
Schr\”odinger’s equation and its complex conjugate. The nonlocal term is
recognized as the so-called quantum potential, thus solving the long standing
problem of the origin and meaning of this term. Further, the relationship
between the Wigner function and a true Kolmogorovian probability density in
phase space is discussed from the perspective provided by this theory.

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Spectra of Harmonium in a magnetic field using an initial value representation of the semiclassical propagator. (arXiv:1110.2900v1 [quant-ph])

For two Coulombically interacting electrons in a quantum dot with harmonic
confinement and a constant magnetic field, we show that time-dependent
semiclassical calculations using the Herman-Kluk initial value representation
of the propagator lead to eigenvalues of the same accuracy as WKB calculations
with Langer correction. The latter are restricted to integrable systems,
however, whereas the time-dependent initial value approach allows for
applications to high-dimensional, possibly chaotic dynamics and is extendable
to arbitrary shapes of the potential.

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Effective and exact holographies from symmetries and dualities. (arXiv:1110.2179v1 [cond-mat.stat-mech])

The theoretical basis of the phenomenon of effective and exact dimensional
reduction, or holographic correspondence, is investigated in a wide variety of
physical systems. We first derive general inequalities linking quantum systems
of different spatial (or spatio-temporal) dimensionality, thus establishing
bounds on arbitrary correlation functions. These bounds enforce an {\em
effective} dimensional reduction and become most potent in the presence of
certain symmetries. {\em Exact} dimensional reduction can stem from a duality
that (i) follows from properties of the local density of states, and/or (ii)
from properties of Hamiltonian-dependent algebras of interactions. Dualities of
the first type (i) are illustrated with large-$ n$ vector theories whose local
density of states may remain invariant under transformations that change the
dimension. We argue that a broad class of examples of dimensional reduction may
be understood in terms of the functional dependence of observables on the local
density of states. Dualities of the second type (ii) are obtained via {\em bond
algebras}, a recently developed algebraic tool. We apply this technique to
systems displaying topological quantum order, and also discuss the implications
of dimensional reduction for the storage of quantum information.

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Experimental Demonstration of Blind Quantum Computing. (arXiv:1110.1381v1 [quant-ph])

Quantum computers, besides offering substantial computational speedups, are
also expected to provide the possibility of preserving the privacy of a
computation. Here we show the first such experimental demonstration of blind
quantum computation where the input, computation, and output all remain unknown
to the computer. We exploit the conceptual framework of measurement-based
quantum computation that enables a client to delegate a computation to a
quantum server. We demonstrate various blind delegated computations, including
one- and two-qubit gates and the Deutsch and Grover algorithms. Remarkably, the
client only needs to be able to prepare and transmit individual photonic
qubits. Our demonstration is crucial for future unconditionally secure quantum
cloud computing and might become a key ingredient for real-life applications,
especially when considering the challenges of making powerful quantum computers
widely available.

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Steady-state entanglement of a Bose-Einstein condensate and a nanomechanical resonator. (arXiv:1109.6316v1 [quant-ph])

We analyze the steady-state entanglement between Bose-Einstein condensate
trapped inside an optical cavity with a moving end mirror (nanomechanical
resonator) driven by a single mode laser. The quantized laser field mediates
the interaction between the Bose-Einstein condensate and nanomechanical
resonator. In particular, we study the influence of temperature on the
entanglement of the coupled system, and note that the steady-state entanglement
is fragile with respect to temperature.

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Realization of Three-Qubit Quantum Error Correction with Superconducting Circuits. (arXiv:1109.4948v1 [quant-ph])

Quantum computers promise to solve certain problems exponentially faster than
possible classically but are challenging to build because of their increased
susceptibility to errors. Remarkably, however, it is possible to detect and
correct errors without destroying coherence by using quantum error correcting
codes [1]. The simplest of these are the three-qubit codes, which map a
one-qubit state to an entangled three-qubit state and can correct any single
phase-flip or bit-flip error of one of the three qubits, depending on the code
used [2]. Here we demonstrate both codes in a superconducting circuit by
encoding a quantum state as previously shown [3,4], inducing errors on all
three qubits with some probability, and decoding the error syndrome by
reversing the encoding process. This syndrome is then used as the input to a
three-qubit gate which corrects the primary qubit if it was flipped. As the
code can recover from a single error on any qubit, the fidelity of this process
should decrease only quadratically with error probability. We implement the
correcting three-qubit gate, known as a conditional-conditional NOT (CCNot) or
Toffoli gate, using an interaction with the third excited state of a single
qubit, in 63 ns. We find 85\pm1% fidelity to the expected classical action of
this gate and 78\pm1% fidelity to the ideal quantum process matrix. Using it,
we perform a single pass of both quantum bit- and phase-flip error correction
with 76\pm0.5% process fidelity and demonstrate the predicted first-order
insensitivity to errors. Concatenating these two codes and performing them on a
nine-qubit device would correct arbitrary single-qubit errors. When combined
with recent advances in superconducting qubit coherence times [5,6], this may
lead to scalable quantum technology.

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Order by Disorder in Spin-Orbit Coupled Bose-Einstein Condensates. (arXiv:1109.4945v1 [cond-mat.quant-gas])

Motivated by recent experiments, we investigate the system of
isotropically-interacting bosons with Rashba spin-orbit coupling. At the
non-interacting level, there is a macroscopic ground-state degeneracy due to
the many ways bosons can occupy the Rashba spectrum. Interactions treated at
the mean-field level restrict the possible ground-state configurations, but
there remains an accidental degeneracy not corresponding to any symmetry of the
Hamiltonian, indicating the importance of fluctuations. By finding analytical
expressions for the collective excitations in the long-wavelength limit and
through numerical solution of the full Bogoliubov- de Gennes equations, we show
that the system condenses into a single momentum state of the Rashba spectrum
via the mechanism of order by disorder. We show that in 3D the quantum
depletion for this system is small, while the thermal depletion has an infrared
logarithmic divergence, which is removed for finite-size systems. In 2D, on the
other hand, thermal fluctuations destabilize the system.

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Stability of topologically invariant order parameters at finite temperature. (arXiv:1109.3496v1 [quant-ph])

Topological entanglement entropy is a topological invariant which can detect
topological order of quantum many-body ground state. We assume an existence of
such order parameter at finite temperature which is invariant under smooth
deformation of the subsystems, and study its stability under hamiltonian
perturbation. We apply this assumption to a Gibbs state of hamiltonian which
satisfies so called `strong commuting’ condition, which we shall define in the
paper. Interesting models in this category include local hamiltonian models
based on quantum error correcting code. We prove a stability of such
topologically invariant order parameter against arbitrary perturbation which
can be expressed as a sum of geometrically local bounded-norm terms. The first
order correction against such perturbation vanishes in the thermodynamic limit.

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Ramsey Fringes and Time-domain Multiple-Slit Interference from Vacuum. (arXiv:1109.3489v1 [hep-th])

Sequences of alternating-sign time-dependent electric field pulses lead to
coherent interference effects in Schwinger vacuum pair production, producing a
Ramsey interferometer, an all-optical time-domain realization of the
multiple-slit interference effect, directly from the quantum vacuum. The
interference, obeying fermionic quantum statistics, is manifest in the momentum
dependence of the number of produced electrons and positrons along the linearly
polarized electric field. The central value grows like $ N^2$ for $ N$ pulses
[i.e., $ N$ "slits"], and the functional form is well-described by a coherent
multiple-slit expression. This behavior is generic for many driven quantum
systems.

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Stability of Frustration-Free Hamiltonians. (arXiv:1109.1588v1 [quant-ph])

We prove stability of the spectral gap for gapped, frustration-free
Hamiltonians under general, quasi-local perturbations. We present a necessary
and sufficient condition for stability, which we call “Local Topological
Quantum Order” and show that this condition implies an area law for the
entanglement entropy of the groundstate subspace. This result extends previous
work by Bravyi et al., on the stability of topological quantum order for
Hamiltonians composed of commuting projections with a common zero-energy
subspace. We conclude with a list of open problems relevant to spectral gaps
and topological quantum order.

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