1,638 research outputs found
Coherent control of single photon states
We define a class of multi-mode single photon states suitable for quantum
information applications. We show how standard amplitude modulation techniques
may be used to control the pulse shape of single photon states.Comment: Lecture given at the workshop on "Theoretical and Experimental
Foundations of Recent Quantum Technologies" Durban, July 2006. Submited to
Journal de Physique IV - Proceeding
Quantum stochastic processes in mesoscopic conductors
We show an equivalence between the approach of Buttiker and the Fermi quantum
stochastic calculus for mesoscopic systems. To illustrate the method we derive
the current fluctuations in a two terminal mesoscopic circuit with two tunnel
barriers containing a single quasi bound state on the well. The method enables
us to focus on either the incoming/outgoing Fermi fields in the leads, or on
the irreversible dynamics of the well state itself. The quantum stochastic
calculus we use is the Fermi analogue of the input/output methods of quantum
optics.Comment: 17 pages, 1 figur
Nonlinear quantum optical computing via measurement
We show how the measurement induced model of quantum computation proposed by
Raussendorf and Briegel [Phys. Rev. Letts. 86, 5188 (2001)] can be adapted to a
nonlinear optical interaction. This optical implementation requires a Kerr
nonlinearity, a single photon source, a single photon detector and fast feed
forward. Although nondeterministic optical quantum information proposals such
as that suggested by KLM [Nature 409, 46 (2001)] do not require a Kerr
nonlinearity they do require complex reconfigurable optical networks. The
proposal in this paper has the benefit of a single static optical layout with
fixed device parameters, where the algorithm is defined by the final
measurement procedure.Comment: 14 pages, 4 figures, 4 table
Quantum Phase Transitions in a Linear Ion Trap
We show that the quantum phase transition of the Tavis-Cummings model can be
realised in a linear ion trap of the kind proposed for quantum computation. The
Tavis-Cummings model describes the interaction between a bosonic degree of
freedom and a collective spin. In an ion trap, the collective spin system is a
symmetrised state of the internal electronic states of N ions, while the
bosonic system is the vibrational degree of freedom of the centre of mass mode
for the ions.Comment: 6 pages and 2 figures. submitted to Dan Walls Memorial Volume, edited
by H. Carmichael, R. Glauber, and M. Scully, to be published by Springe
Decoherence and fidelity in ion traps with fluctuating trap parameters
We consider two different kinds of fluctuations in an ion trap potential:
external fluctuating electrical fields, which cause statistical movement
(``wobbling'') of the ion relative to the center of the trap, and fluctuations
of the spring constant, which are due to fluctuations of the ac-component of
the potential applied in the Paul trap for ions. We write down master equations
for both cases and, averaging out the noise, obtain expressions for the heating
of the ion. We compare our results to previous results for far-off resonance
optical traps and heating in ion traps. The effect of fluctuating external
electrical fields for a quantum gate operation (controlled-NOT) is determined
and the fidelity for that operation derived.Comment: 11 pages, 4 figure
Entanglement in the Dicke model
We show how an ion trap, configured for the coherent manipulation of external
and internal quantum states, can be used to simulate the irreversible dynamics
of a collective angular momentum model known as the Dicke model. In the special
case of two ions, we show that entanglement is created in the coherently driven
steady state with linear driving. For the case of more than two ions we
calculate the entanglement between two ions in the steady state of the Dicke
model by tracing over all the other ions. The entanglement in the steady state
is a maximum for the parameter values corresponding roughly to a bifurcation of
a fixed point in the corresponding semiclassical dynamics. We conjecture that
this is a general mechanism for entanglement creation in driven dissipative
quantum systems.Comment: Minor changes: Reference added and references correcte
Homodyne Measurements on a Bose-Einstein Condensate
We investigate a non-destructive measurement technique to monitor
Josephson-like oscillations between two spatially separated neutral atom
Bose-Einstein condensates. One condensate is placed in an optical cavity, which
is strongly driven by a coherent optical field. The cavity output field is
monitored using a homodyne detection scheme. The cavity field is well detuned
from an atomic resonance, and experiences a dispersive phase shift proportional
to the number of atoms in the cavity. The detected current is modulated by the
coherent tunneling oscillations of the condensate. Even when there is an equal
number of atoms in each well initially, a phase is established by the
measurement process and Josephson-like oscillations develop due to measurement
back-action noise alone.Comment: 8 pages, 12 figures to appear in PR
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