526 research outputs found
Monitoring currents in cold-atom circuits
Complex circuits of cold atoms can be exploited to devise new protocols for
the diagnostics of cold-atoms systems. Specifically, we study the quench
dynamics of a condensate confined in a ring-shaped potential coupled with a
rectilinear guide of finite size. We find that the dynamics of the atoms inside
the guide is distinctive of the states with different winding numbers in the
ring condensate. We also observe that the depletion of the density, localized
around the tunneling region of the ring condensate, can decay in a pair of
excitations experiencing a Sagnac effect. In our approach, the current states
of the condensate in the ring can be read out by inspection of the rectilinear
guide only, leaving the ring condensate minimally affected by the measurement.
We believe that our results set the basis for definition of new quantum
rotation sensors. At the same time, our scheme can be employed to explore
fundamental questions involving dynamics of bosonic condensates.Comment: Figures are enlarged. Section IV is added. Journal reference adde
Coherent cavity networks with complete connectivity
When cavity photons couple to an optical fiber with a continuum of modes,
they usually leak out within a finite amount of time. However, if the fiber is
about one meter long and linked to a mirror, photons bounce back and forth
within the fiber on a much faster time scale. As a result, {\em dynamical
decoupling} prevents the cavity photons from entering the fiber. In this paper
we use the simultaneous dynamical decoupling of a large number of distant
cavities from the fiber modes of linear optics networks to mediate effective
cavity-cavity interactions in a huge variety of configurations. Coherent cavity
networks with complete connectivity can be created with potential applications
in quantum computing and simulation of the complex interaction Hamiltonians of
biological systems.Comment: revised version, improved analysis, 4 pages and 4 figure
Scalable quantum memory in the ultrastrong coupling regime
Circuit quantum electrodynamics, consisting of superconducting artificial
atoms coupled to on-chip resonators, represents a prime candidate to implement
the scalable quantum computing architecture because of the presence of good
tunability and controllability. Furthermore, recent advances have pushed the
technology towards the ultrastrong coupling regime of light-matter interaction,
where the qubit-resonator coupling strength reaches a considerable fraction of
the resonator frequency. Here, we propose a qubit-resonator system operating in
that regime, as a quantum memory device and study the storage and retrieval of
quantum information in and from the Z2 parity-protected quantum memory, within
experimentally feasible schemes. We are also convinced that our proposal might
pave a way to realize a scalable quantum random-access memory due to its fast
storage and readout performances.Comment: We have updated the title, abstract and included a new section on the
open-system dynamic
Robust-fidelity atom-photon entangling gates in the weak-coupling regime
We describe a simple entangling principle based on the scattering of photons
off single emitters in one-dimensional waveguides (or extremely-lossy
cavities). The scheme can be applied to photonic qubits encoded in polarization
or time-bin, and features a filtering mechanism that works effectively as a
built-in error-correction directive. This automatically maps imperfections from
weak couplings, atomic decay into undesired modes, frequency mismatches, or
finite bandwidths of the incident photonic pulses, into heralded losses instead
of infidelities. The scheme is thus adequate for high-fidelity maximally
entangling gates even in the weak-coupling regime. These, in turn, can be
directly applied to store and retrieve photonic-qubit states, thereby
completing an atom-photon interface toolbox, or to sequential measurement-based
quantum computations with atomic memories.Comment: 5 pages, 2 figure
Bell inequalities for three particles
We present tight Bell inequalities expressed by probabilities for three four-
and five-dimensional systems. The tight structure of Bell inequalities for
three -dimensional systems (qudits) is proposed. Some interesting Bell
inequalities of three qubits reduced from those of three qudits are also
studied.Comment: 8 pages, 3 figures. Accepted for publication in Phys. Rev.
Kinematic approach to off-diagonal geometric phases of nondegenerate and degenerate mixed states
Off-diagonal geometric phases have been developed in order to provide
information of the geometry of paths that connect noninterfering quantal
states. We propose a kinematic approach to off-diagonal geometric phases for
pure and mixed states. We further extend the mixed state concept proposed in
[Phys. Rev. Lett. {\bf 90}, 050403 (2003)] to degenerate density operators. The
first and second order off-diagonal geometric phases are analyzed for unitarily
evolving pairs of pseudopure states.Comment: New section IV, new figure, journal ref adde
Multipartite entanglement in quantum spin chains
We study the occurrence of multipartite entanglement in spin chains. We show
that certain genuine multipartite entangled states, namely W states, can be
obtained as ground states of simple XX type ferromagnetic spin chains in a
transverse magnetic field, for any number of sites. Moreover, multipartite
entanglement is proven to exist even at finite temperatures. A transition from
a product state to a multipartite entangled state occurs when decreasing the
magnetic field to a critical value. Adiabatic passage through this point can
thus lead to the generation of multipartite entanglement.Comment: 4 pages, 1 figur
Kinematic approach to the mixed state geometric phase in nonunitary evolution
A kinematic approach to the geometric phase for mixed quantal states in
nonunitary evolution is proposed. This phase is manifestly gauge invariant and
can be experimentally tested in interferometry. It leads to well-known results
when the evolution is unitary.Comment: Minor changes; journal reference adde
Learning from Examples with Unspecified Attribute Values
We introduce the UAV learning model in which some of the attributes in the examples are unspecified. In our model, an example x is classified positive (resp., negative) if all possible assignments for the unspecified attributes result in a positive (resp., negative) classification. Otherwise the classificatoin given to x is ? (for unknown). Given an example x in which some attributes are unspecified, the oracle UAV-MQ responds with the classification of x. Given a hypothesis h, the oracle UAV-EQ returns an example x (that could have unspecified attributes) for which h(x) is incorrect. We show that any class learnable in the exact model using the MQ and EQ oracles is also learnable in the UAV model using the MQ and UAV-EQ oracles as long as the counterexamples provided by the UAV-EQ oracle have a logarithmic number of unspecified attributes. We also show that any class learnable in the exact model using the MQ and EQ oracles is also learnable in the UAV model using the UAV-MQ and UAV-EQ oracles as well as an oracle to evaluate a given boolean formula on an example with unspecified attributes. (For some hypothesis classes such as decision trees and unate formulas the evaluation can be done in polynomial time without an oracle.) We also study the learnability of a universal class of decision trees under the UAV model and of DNF formulas under a representation-dependent variation of the UAV model
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