31 research outputs found
Toward Spin Squeezing with Trapped Ions
Building robust instruments capable of making interferometric measurements
with precision beyond the standard quantum limit remains an important goal in
many metrology laboratories. We describe here the basic concepts underlying
spin squeezing experiments that allow one to surpass this limit. In priniciple
it is possible to reach the so-called Heisenberg limit, which constitutes an
improvement in precision by a factor , where is the number of
particles on which the measurement is carried out. In particular, we focus on
recent progress toward implementing spin squeezing with a cloud of beryllium
ions in a Penning ion trap, via the geometric phase gate used more commonly for
performing two-qubit entangling operations in quantum computing experiments.Comment: 18 pages, 9 figures, Contribution to Quantum Africa 2010 conference
proceeding
High-order noise filtering in nontrivial quantum logic gates
Treating the effects of a time-dependent classical dephasing environment
during quantum logic operations poses a theoretical challenge, as the
application of non-commuting control operations gives rise to both dephasing
and depolarization errors that must be accounted for in order to understand
total average error rates. We develop a treatment based on effective
Hamiltonian theory that allows us to efficiently model the effect of classical
noise on nontrivial single-bit quantum logic operations composed of arbitrary
control sequences. We present a general method to calculate the
ensemble-averaged entanglement fidelity to arbitrary order in terms of noise
filter functions, and provide explicit expressions to fourth order in the noise
strength. In the weak noise limit we derive explicit filter functions for a
broad class of piecewise-constant control sequences, and use them to study the
performance of dynamically corrected gates, yielding good agreement with
brute-force numerics.Comment: Revised and expanded to include filter function terms beyond first
order in the Magnus expansion. Related manuscripts available from
http://www.physics.usyd.edu.au/~mbiercu
Process tomography via sequential measurements on a single quantum system
We utilize a discrete (sequential) measurement protocol to investigate
quantum process tomography of a single two-level quantum system, with an
unknown initial state, undergoing Rabi oscillations. The ignorance of the
dynamical parameters is encoded into a continuous-variable classical system
which is coupled to the two-level quantum system via a generalized Hamiltonian.
This combined estimate of the quantum state and dynamical parameters is updated
by using the information obtained from sequential measurements on the quantum
system and, after a sufficient waiting period, faithful state monitoring and
parameter determination is obtained. Numerical evidence is used to demonstrate
the convergence of the state estimate to the true state of the hybrid system.Comment: 7 pages, 2 figure
Matter-Wave Decoherence due to a Gas Environment in an Atom Interferometer
Decoherence due to scattering from background gas particles is observed for
the first time in a Mach-Zehnder atom interferometer, and compared with
decoherence due to scattering photons. A single theory is shown to describe
decoherence due to scattering either atoms or photons. Predictions from this
theory are tested by experiments with different species of background gas, and
also by experiments with different collimation restrictions on an atom beam
interferometer.Comment: 4 pages, 3 figures, accepted to PR
Experimental Uhrig Dynamical Decoupling using Trapped Ions
We present a detailed experimental study of the Uhrig Dynamical Decoupling
(UDD) sequence in a variety of noise environments. Our qubit system consists of
a crystalline array of Be ions confined in a Penning trap. We use
an electron-spin-flip transition as our qubit manifold and drive qubit
rotations using a 124 GHz microwave system. We study the effect of the UDD
sequence in mitigating phase errors and compare against the well known
CPMG-style multipulse spin echo as a function of pulse number, rotation axis,
noise spectrum, and noise strength. Our results agree well with theoretical
predictions for qubit decoherence in the presence of classical phase noise,
accounting for the effect of finite-duration pulses. Finally, we
demonstrate that the Uhrig sequence is more robust against systematic
over/underrotation and detuning errors than is multipulse spin echo, despite
the precise prescription for pulse-timing in UDD