311 research outputs found

### Reply to "Comment on "Some implications of the quantum nature of laser fields for quantum computations''''

In this revised reply to quant-ph/0211165, I address the question of the
validity of my results in greater detail, by comparing my predictions to those
of the Silberfarb-Deutsch model, and I deal at greater length with the beam
area paradox. As before, I conclude that my previous results are an
(order-of-magnitude) accurate estimate of the error probability introduced in
quantum logical operations by the quantum nature of the laser field. While this
error will typically (for a paraxial beam) be smaller than the total error due
to spontaneous emission, a unified treatment of both effects reveals that they
lead to formally similar constraints on the minimum number of photons per pulse
required to perform an operation with a given accuracy; these constraints agree
with those I have derived elsewhere.Comment: A reply to quant-ph/0211165. Added more calculations and discussion,
removed some flippanc

### Oscillator tunneling dynamics in the Rabi model

The familiar Rabi model, comprising a two-level system coupled to a quantum
harmonic oscillator, continues to produce rich and surprising physics when the
coupling strength becomes comparable to the individual subsystem frequencies.
We construct approximate solutions for the regime in which the oscillator
frequency is small compared to that of the two-level system and the coupling
strength matches or exceeds the oscillator frequency. Relating our fully
quantum calculation to a previous semi-classical approximation, we find that
the dynamics of the oscillator can be considered to a good approximation as
that of a particle tunneling in a classical double-well potential, despite the
fundamentally entangled nature of the joint system. We assess the prospects for
observation of oscillator tunneling in the context of nano- or micro-mechanical
experiments and find that it should be possible if suitably high coupling
strengths can be engineered.Comment: 25 pages, 5 figures, preprint forma

### Minimum-energy pulses for quantum logic cannot be shared

We show that if an electromagnetic energy pulse with average photon number
is used to carry out the same quantum logical operation on a set of N
atoms, either simultaneously or sequentially, the overall error probability in
the worst case scenario (i.e., maximized over all the possible initial atomic
states) scales as N^2/. This means that in order to keep the error
probability bounded by N\epsilon, with \epsilon ~ 1/, one needs to use
N/\epsilon photons, or equivalently N separate "minimum-energy'' pulses: in
this sense the pulses cannot, in general, be shared. The origin for this
phenomenon is found in atom-field entanglement. These results may have
important consequences for quantum logic and, in particular, for large-scale
quantum computation.Comment: To appear in Phys. Rev. A, Rapid Communication

### "Modes of the universe" study of two-photon deterministic, passive quantum logical gates

We use the "modes of the universe" approach to study a cavity-mediated
two-photon logical gate recently proposed by Koshino, Ishizaka and Nakamura. We
clarify the relationship between the more commonly used input-output formalism,
and that of Koshino et al., and show that some elements of this gate had been
anticipated by other authors. We conclude that their proposed gate can work
both in the good and bad cavity limits, provided only that the pulses are long
enough. Our formalism allows us to estimate analytically the size of the
various error terms, and to follow the spectral evolution of the field + cavity
system in the course of the interaction.Comment: 9 pages, 8 figure

### Gate fidelity of arbitrary single-qubit gates constrained by conservation laws

Recent investigations show that conservation laws limit the accuracy of gate
operations in quantum computing. The inevitable error under the angular
momentum conservation law has been evaluated so far for the CNOT, Hadamard, and
NOT gates for spin 1/2 qubits, while the SWAP gate has no constraint. Here, we
extend the above results to general single-qubit gates. We obtain an upper
bound of the gate fidelity of arbitrary single-qubit gates implemented under
arbitrary conservation laws, determined by the geometry of the conservation law
and the gate operation on the Bloch sphere as well as the size of the ancilla.Comment: Title changed; to appear in J. Phys. A: Math. Theor.; 19 pages, 2
figure

### Approximate programmable quantum processors

A quantum processor is a programmable quantum circuit in which both the data
and the program, which specifies the operation that is carried out on the data,
are quantum states. We study the situation in which we want to use such a
processor to approximate a set of unitary operators to a specified level of
precision. We measure how well an operation is performed by the process
fidelity between the desired operation and the operation produced by the
processor. We show how to find the program for a given processor that produces
the best approximation of a particular unitary operation. We also place bounds
on the dimension of the program space that is necessary to approximate a set of
unitary operators to a specified level of precision.Comment: 8 page

### Quantum limits in interferometric measurements

Quantum noise limits the sensitivity of interferometric measurements. It is
generally admitted that it leads to an ultimate sensitivity, the ``standard
quantum limit''. Using a semi-classical analysis of quantum noise, we show that
a judicious use of squeezed states allows one in principle to push the
sensitivity beyond this limit. This general method could be applied to large
scale interferometers designed for gravitational wave detection.Comment: 4 page

### Steady State Entanglement in Cavity QED

We investigate steady state entanglement in an open quantum system,
specifically a single atom in a driven optical cavity with cavity loss and
spontaneous emission. The system reaches a steady pure state when driven very
weakly. Under these conditions, there is an optimal value for atom-field
coupling to maximize entanglement, as larger coupling favors a loss port due to
the cavity enhanced spontaneous emission. We address ways to implement
measurements of entanglement witnesses and find that normalized
cross-correlation functions are indicators of the entanglement in the system.
The magnitude of the equal time intensity-field cross correlation between the
transmitted field of the cavity and the fluorescence intensity is proportional
to the concurrence for weak driving fields.Comment: enhanced discussion, corrected formulas, title change, 1 added figur

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