9,230 research outputs found
High-fidelity ion-trap quantum computing with hyperfine clock states
We propose the implementation of a geometric-phase gate on
magnetic-field-insensitive qubits with -dependent forces for
trapped ion quantum computing. The force is exerted by two laser beams in a
Raman configuration. Qubit-state dependency is achieved by a small frequency
detuning from the virtually-excited state. Ion species with excited states of
long radiative lifetimes are used to reduce the chance of a spontaneous photon
emission to less than 10 per gate-run. This eliminates the main source
of gate infidelity of previous implementations. With this scheme it seems
possible to reach the fault tolerant threshold.Comment: 4 pages, 1 figur
Nonlinear coupling of continuous variables at the single quantum level
We experimentally investigate nonlinear couplings between vibrational modes
of strings of cold ions stored in linear ion traps. The nonlinearity is caused
by the ions' Coulomb interaction and gives rise to a Kerr-type interaction
Hamiltonian H = n_r*n_s, where n_r,n_s are phonon number operators of two
interacting vibrational modes. We precisely measure the resulting oscillation
frequency shift and observe a collapse and revival of the contrast in a Ramsey
experiment. Implications for ion trap experiments aiming at high-fidelity
quantum gate operations are discussed
Weighted Bergman kernels and virtual Bergman kernels
We introduce the notion of "virtual Bergman kernel" and apply it to the
computation of the Bergman kernel of "domains inflated by Hermitian balls", in
particular when the base domain is a bounded symmetric domain.Comment: 12 pages. One-hour lecture for graduate students, SCV 2004, August
2004, Beijing, P.R. China. V2: typo correcte
Irreversible nucleation in molecular beam epitaxy: From theory to experiments
Recently, the nucleation rate on top of a terrace during the irreversible
growth of a crystal surface by MBE has been determined exactly. In this paper
we go beyond the standard model usually employed to study the nucleation
process, and we analyze the qualitative and quantitative consequences of two
important additional physical ingredients: the nonuniformity of the
Ehrlich-Schwoebel barrier at the step-edge, because of the existence of kinks,
and the steering effects, due to the interaction between the atoms of the flux
and the substrate. We apply our results to typical experiments of second layer
nucleation.Comment: 11 pages. Table I corrected and one appendix added. To be published
in Phys. Rev. B (scheduled issue: 15 February 2003
Geometric phase gate on an optical transition for ion trap quantum computation
We propose a geometric phase gate of two ion qubits that are encoded in two
levels linked by an optical dipole-forbidden transition. Compared to hyperfine
geometric phase gates mediated by electric dipole transitions, the gate has
many interesting properties, such as very low spontaneous emission rates,
applicability to magnetic field insensitive states, and use of a co-propagating
laser beam geometry. We estimate that current technology allows for
infidelities of around 10.Comment: 4 pages, 2 figure
Impact of storage conditions on profilometry of eroded dental hard tissue
The aim of the present study was to analyze in how far drying of eroded dentin and enamel surfaces influence the results of profilometrical determinations with a stylus profilometer. Each five dentin and enamel samples were eroded with HCl (pH 2.6, 2min). Surface profiles of the samples were recorded with a stylus profilometer in three series. In series 1, the samples were measured while stored in water and in series 2, under ambient conditions (21°C, 35% humidity). In series 3, samples were completely desiccated and then rewetted. Profilometry was conducted at various time intervals for a period of up to 181min (series 1 and 2) and 72h (series 3). Only the dentin samples were affected by the storage conditions. Stable profilometrical readings for the eroded dentin samples were only feasible when the specimens were stored in water during the complete period of the experiment, including the profilometrical measurement. Thus, for erosion experiments using profilometrical analysis with a stylus profilometer, it is advised to store and measure dentin samples under wet condition
Process tomography of ion trap quantum gates
A crucial building block for quantum information processing with trapped ions
is a controlled-NOT quantum gate. In this paper, two different sequences of
laser pulses implementing such a gate operation are analyzed using quantum
process tomography. Fidelities of up to 92.6(6)% are achieved for single gate
operations and up to 83.4(8)% for two concatenated gate operations. By process
tomography we assess the performance of the gates for different experimental
realizations and demonstrate the advantage of amplitude--shaped laser pulses
over simple square pulses. We also investigate whether the performance of
concatenated gates can be inferred from the analysis of the single gates
Generalized spin squeezing inequalities in qubit systems: theory and experiment
We present detailed derivations, various improvements and application to
concrete experimental data of spin squeezing inequalities formulated recently
by some of us [Phys. Rev. Lett. {\bf 95}, 120502 (2005)]. These inequalities
generalize the concept of the spin squeezing parameter, and provide necessary
and sufficient conditions for genuine 2-, or 3- qubit entanglement for
symmetric states, and sufficient entanglement condition for general -qubit
states. We apply our method to theoretical study of Dicke states, and, in
particular, to -states of qubits. Then, we analyze the recently
experimentally generated 7- and 8-ion -states [Nature {\bf 438}, 643
(2005)]. We also present some novel details concerning this experiment.
Finally, we improve criteria for detection of genuine tripartite entanglement
based on entanglement witnesses.Comment: Final versio
Precision spectroscopy with two correlated atoms
We discuss techniques that allow for long coherence times in laser
spectroscopy experiments with two trapped ions. We show that for this purpose
not only entangled ions prepared in decoherence-free subspaces can be used but
also a pair of ions that are not entangled but subject to the same kind of
phase noise. We apply this technique to a measurement of the electric
quadrupole moment of the 3d D5/2 state of 40Ca+ and to a measurement of the
linewidth of an ultrastable laser exciting a pair of 40Ca+ ions
Frequency metrology on the 4s 2S1/2 - 4p 2P1/2 transition in the calcium ion for a comparison with quasar data
High accuracy frequency metrology on the 4s 2S1/2 - 4p 2P1/2 transition in
calcium ions is performed using laser cooled and crystallized ions in a linear
Paul trap. Calibration is performed with a frequency comb laser, resulting in a
transition frequency of f=755222766.2(1.7) MHz. The accuracy presents an
improvement of more than one order of magnitude, and will facilitate a
comparison with quasar data in a search for a possible change of the fine
structure constant on a cosmological time scale.Comment: Corrected typos (including one on the axis of figure 6
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