509 research outputs found
Quantum communication between trapped ions through a dissipative environment
We study two trapped ions coupled to the axial phonon modes of a
one-dimensional Coulomb crystal. This system is formally equivalent to the "two
spin-boson" model. We propose a scheme to dynamically generate a maximally
entangled state of two ions within a decoherence-free subspace. Here the
phononic environment of the trapped ions, whatever its temperature and number
of modes, serves as the entangling bus. The efficient production of the pure
singlet state can be exploited to perform short-ranged quantum communication
which is essential in building up a large-scale quantum computer.Comment: 4 pages, 2 figure
Deterministic entanglement of ions in thermal states of motion
We give a detailed description of the implementation of a Molmer-Sorensen
gate entangling two Ca+ ions using a bichromatic laser beam near-resonant with
a quadrupole transition. By amplitude pulse shaping and compensation of
AC-Stark shifts we achieve a fast gate operation without compromising the error
rate. Subjecting different input states to concatenations of up to 21
individual gate operations reveals Bell state fidelities above 0.80. In
principle, the entangling gate does not require ground state cooling of the
ions as long as the Lamb-Dicke criterion is fulfilled. We present the first
experimental evidence for this claim and create Bell states with a fidelity of
0.974(1) for ions in a thermal state of motion with a mean phonon number of
=20(2) in the mode coupling to the ions' internal states.Comment: 18 pages, 9 figures (author name spelling corrected
Entanglement at the quantum phase transition in a harmonic lattice
The entanglement properties of the phase transition in a two dimensional
harmonic lattice, similar to the one observed in recent ion trap experiments,
are discussed both, for finite number of particles and thermodynamical limit.
We show that for the ground state at the critical value of the trapping
potential two entanglement measures, the negativity between two neighbouring
sites and the block entropy for blocks of size 1, 2 and 3, change abruptly.
Entanglement thus indicates quantum phase transitions in general; not only in
the finite dimensional case considered in [Phys. Rev. Lett. {\bf 93}, 250404
(2004)]. Finally, we consider the thermal state and compare its exact
entanglement with a temperature entanglement witness introduced in [Phys. Rev.
A {\bf 77} 062102 (2008)].Comment: extended published versio
Photon Shot Noise Dephasing in the Strong-Dispersive Limit of Circuit QED
We study the photon shot noise dephasing of a superconducting transmon qubit
in the strong-dispersive limit, due to the coupling of the qubit to its readout
cavity. As each random arrival or departure of a photon is expected to
completely dephase the qubit, we can control the rate at which the qubit
experiences dephasing events by varying \textit{in situ} the cavity mode
population and decay rate. This allows us to verify a pure dephasing mechanism
that matches theoretical predictions, and in fact explains the increased
dephasing seen in recent transmon experiments as a function of cryostat
temperature. We investigate photon dynamics in this limit and observe large
increases in coherence times as the cavity is decoupled from the environment.
Our experiments suggest that the intrinsic coherence of small Josephson
junctions, when corrected with a single Hahn echo, is greater than several
hundred microseconds.Comment: 5 pages, 4 figures; includes Supporting Online Material of 6 pages
with 5 figure
State-independent quantum violation of noncontextuality in four dimensional space using five observables and two settings
Recently, a striking experimental demonstration [G. Kirchmair \emph{et al.},
Nature, \textbf{460}, 494(2009)] of the state-independent quantum mechanical
violation of non-contextual realist models has been reported for any two-qubit
state using suitable choices of \emph{nine} product observables and \emph{six}
different measurement setups. In this report, a considerable simplification of
such a demonstration is achieved by formulating a scheme that requires only
\emph{five} product observables and \emph{two} different measurement setups. It
is also pointed out that the relevant empirical data already available in the
experiment by Kirchmair \emph{et al.} corroborate the violation of the NCR
models in accordance with our proof
Pentagrams and paradoxes
Klyachko and coworkers consider an orthogonality graph in the form of a
pentagram, and in this way derive a Kochen-Specker inequality for spin 1
systems. In some low-dimensional situations Hilbert spaces are naturally
organised, by a magical choice of basis, into SO(N) orbits. Combining these
ideas some very elegant results emerge. We give a careful discussion of the
pentagram operator, and then show how the pentagram underlies a number of other
quantum "paradoxes", such as that of Hardy.Comment: 14 pages, 4 figure
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