94 research outputs found
Pulsed force sequences for fast phase-insensitive quantum gates in trapped ions
We show how to create quantum gates of arbitrary speed between trapped ions,
using a laser walking wave, with complete insensitivity to drift of the optical
phase, and requiring cooling only to the Lamb-Dicke regime. We present pulse
sequences that satisfy the requirements and are easy to produce in the
laboratory.Comment: 11 pages, 3 figure
Experimental recovery of a qubit from partial collapse
We describe and implement a method to restore the state of a single qubit, in
principle perfectly, after it has partially collapsed. The method resembles the
classical Hahn spin-echo, but works on a wider class of relaxation processes,
in which the quantum state partially leaves the computational Hilbert space. It
is not guaranteed to work every time, but successful outcomes are heralded. We
demonstrate using a single trapped ion better performance from this recovery
method than can be obtained employing projection and post-selection alone. The
demonstration features a novel qubit implementation that permits both partial
collapse and coherent manipulations with high fidelity.Comment: 5 pages, 3 figure
High-fidelity readout of trapped-ion qubits
We demonstrate single-shot qubit readout with fidelity sufficient for
fault-tolerant quantum computation, for two types of qubit stored in single
trapped calcium ions. For an optical qubit stored in the (4S_1/2, 3D_5/2)
levels of 40Ca+ we achieve 99.991(1)% average readout fidelity in one million
trials, using time-resolved photon counting. An adaptive measurement technique
allows 99.99% fidelity to be reached in 145us average detection time. For a
hyperfine qubit stored in the long-lived 4S_1/2 (F=3, F=4) sub-levels of 43Ca+
we propose and implement a simple and robust optical pumping scheme to transfer
the hyperfine qubit to the optical qubit, capable of a theoretical fidelity
99.95% in 10us. Experimentally we achieve 99.77(3)% net readout fidelity,
inferring at least 99.87(4)% fidelity for the transfer operation.Comment: 4 pages, 3 figures; improved readout fidelity (numerical results
changed
Long-lived mesoscopic entanglement outside the Lamb-Dicke regime
We create entangled states of the spin and motion of a single Ca
ion in a linear ion trap. The motional part consists of coherent states of
large separation and long coherence time. The states are created by driving the
motion using counterpropagating laser beams. We theoretically study and
experimentally observe the behaviour outside the Lamb-Dicke regime, where the
trajectory in phase space is modified and the coherent states become squeezed.
We directly observe the modification of the return time of the trajectory, and
infer the squeezing. The mesoscopic entanglement is observed up to with coherence time 170 microseconds and mean phonon excitation
\nbar = 16.Comment: 5 pages, 3 figures. Revised version after editor comment
Deterministic entanglement and tomography of ion spin qubits
We have implemented a universal quantum logic gate between qubits stored in
the spin state of a pair of trapped calcium 40 ions. An initial product state
was driven to a maximally entangled state deterministically, with 83% fidelity.
We present a general approach to quantum state tomography which achieves good
robustness to experimental noise and drift, and use it to measure the spin
state of the ions. We find the entanglement of formation is 0.54.Comment: 3 figures, 4 pages, footnotes fixe
Time-separated entangled light pulses from a single-atom emitter
The controlled interaction between a single, trapped, laser-driven atom and
the mode of a high-finesse optical cavity allows for the generation of
temporally separated, entangled light pulses. Entanglement between the
photon-number fluctuations of the pulses is created and mediated via the atomic
center-of-mass motion, which is interfaced with light through the mechanical
effect of atom-photon interaction. By means of a quantum noise analysis we
determine the correlation matrix which characterizes the entanglement, as a
function of the system parameters. The scheme is feasible in experimentally
accessible parameter regimes. It may be easily extended to the generation of
entangled pulses at different frequencies, even at vastly different
wavelengths.Comment: 17 pages, 5 figures. Modified version, to appear in the New Journal
of Physic
Keeping a Single Qubit Alive by Experimental Dynamic Decoupling
We demonstrate the use of dynamic decoupling techniques to extend the
coherence time of a single memory qubit by nearly two orders of magnitude. By
extending the Hahn spin-echo technique to correct for unknown, arbitrary
polynomial variations in the qubit precession frequency, we show analytically
that the required sequence of pi-pulses is identical to the Uhrig dynamic
decoupling (UDD) sequence. We compare UDD and CPMG sequences applied to a
single Ca-43 trapped-ion qubit and find that they afford comparable protection
in our ambient noise environment.Comment: 5 pages, 5 figure
Fabrication and heating rate study of microscopic surface electrode ion traps
We report heating rate measurements in a microfabricated gold-on-sapphire
surface electrode ion trap with trapping height of approximately 240 micron.
Using the Doppler recooling method, we characterize the trap heating rates over
an extended region of the trap. The noise spectral density of the trap falls in
the range of noise spectra reported in ion traps at room temperature. We find
that during the first months of operation the heating rates increase by
approximately one order of magnitude. The increase in heating rates is largest
in the ion loading region of the trap, providing a strong hint that surface
contamination plays a major role for excessive heating rates. We discuss data
found in the literature and possible relation of anomalous heating to sources
of noise and dissipation in other systems, namely impurity atoms adsorbed on
metal surfaces and amorphous dielectrics.Comment: 17 pages, 5 figure
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