148 research outputs found
Ionization rates in a Bose-Einstein condensate of metastable Helium
We have studied ionizing collisions in a BEC of He*. Measurements of the ion
production rate combined with measurements of the density and number of atoms
for the same sample allow us to estimate both the 2 and 3-body contributions to
this rate. A comparison with the decay of the number of condensed atoms in our
magnetic trap, in the presence of an rf-shield, indicates that ionizing
collisions are largely or wholly responsible for the loss. Quantum depletion
makes a substantial correction to the 3-body rate constant.Comment: 4 pages, 3 figure
Getting the elastic scattering length by observing inelastic collisions in ultracold metastable helium atoms
We report an experiment measuring simultaneously the temperatureand the flux
of ions produced by a cloud of triplet metastablehelium atoms at the
Bose-Einstein critical temperature. The onsetof condensation is revealed by a
sharp increase of the ion fluxduring evaporative cooling. Combining our
measurements withprevious measurements of ionization in a pure BEC,we extract
an improved value of the scattering length nm. The analysis
includes corrections takinginto accountthe effect of atomic interactions on the
criticaltemperature, and thus an independent measurement of the
scatteringlength would allow a new test of these calculations
Randomized Benchmarking of Quantum Gates
A key requirement for scalable quantum computing is that elementary quantum
gates can be implemented with sufficiently low error. One method for
determining the error behavior of a gate implementation is to perform process
tomography. However, standard process tomography is limited by errors in state
preparation, measurement and one-qubit gates. It suffers from inefficient
scaling with number of qubits and does not detect adverse error-compounding
when gates are composed in long sequences. An additional problem is due to the
fact that desirable error probabilities for scalable quantum computing are of
the order of 0.0001 or lower. Experimentally proving such low errors is
challenging. We describe a randomized benchmarking method that yields estimates
of the computationally relevant errors without relying on accurate state
preparation and measurement. Since it involves long sequences of randomly
chosen gates, it also verifies that error behavior is stable when used in long
computations. We implemented randomized benchmarking on trapped atomic ion
qubits, establishing a one-qubit error probability per randomized pi/2 pulse of
0.00482(17) in a particular experiment. We expect this error probability to be
readily improved with straightforward technical modifications.Comment: 13 page
Using ion production to monitor the birth and death of a metastable helium Bose-Einstein condensate
We discuss observations of the ion flux from a cloud of trapped metastable
helium atoms. Both Bose-Einstein condensates and thermal clouds were
investigated. The ion flux is compared to time-of-flight observations of the
expanded cloud. We show data concerning BEC formation and decay, as well as
measurements of two- and three-body ionization rate constants. We also discuss
possible improvements and extensions of our results.Comment: 14 pages, 9 figures, submitted to Journal of Optics B (special issue,
cold quantum gases
Prospects for measurement and control of the scattering length of metastable helium using photoassociation techniques
A numerical investigation of two-laser photoassociation (PA) spectroscopy on
spin-polarized metastable helium (He*) atoms is presented within the context of
experimental observation of the least-bound energy level in the scattering
potential and subsequent determination of the s-wave scattering length.
Starting out from the model developed by Bohn and Julienne [Phys. Rev. A
\textbf{60}, (1999) 414], PA rate coefficients are obtained as a function of
the parameters of the two lasers. The rate coefficients are used to simulate
one- and two-laser PA spectra. The results demonstrate the feasibility of a
spectroscopic determination of the binding energy of the least-bound level. The
simulated spectra may be used as a guideline when designing such an experiment,
whereas the model may also be employed for fitting experimentally obtained PA
spectra. In addition, the prospects for substantial modification of the He*
scattering length by means of optical Feshbach resonances are considered.
Several experimental issues relating to the numerical investigation presented
here are discussed.Comment: 9 pages, 7 figure
Scalable ion traps for quantum information processing
We report on the design, fabrication, and preliminary testing of a 150 zone
array built in a `surface-electrode' geometry microfabricated on a single
substrate. We demonstrate transport of atomic ions between legs of a `Y'-type
junction and measure the in-situ heating rates for the ions. The trap design
demonstrates use of a basic component design library that can be quickly
assembled to form structures optimized for a particular experiment
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