5 research outputs found
Implementation of a symmetric surface electrode ion trap with field compensation using a modulated Raman effect
We describe the fabrication and characterization of a new surface-electrode Paul ion trap designed for experiments in scalable quantum information processing with Ca+. A notable feature is a symmetric electrode pattern which allows rotation of the normal modes of ion motion, yielding efficient Doppler cooling with a single beam parallel to the planar surface. We propose and implement a technique for micromotion compensation in all directions using an infrared repumper laser beam directed into the trap plane. Finally, we employ an alternate repumping scheme that increases ion fluorescence and simplifies heating rate measurements obtained by time-resolved ion fluorescence during Doppler cooling
Dynamical decoupling and noise spectroscopy with a superconducting flux qubit
The characterization and mitigation of decoherence in natural and artificial
two-level systems (qubits) is fundamental to quantum information science and
its applications. Decoherence of a quantum superposition state arises from the
interaction between the constituent system and the uncontrolled degrees of
freedom in its environment. Within the standard Bloch-Redfield picture of
two-level system dynamics, qubit decoherence is characterized by two rates: a
longitudinal relaxation rate Gamma1 due to the exchange of energy with the
environment, and a transverse relaxation rate Gamma2 = Gamma1/2 + Gamma_phi
which contains the pure dephasing rate Gamma_phi. Irreversible energy
relaxation can only be mitigated by reducing the amount of environmental noise,
reducing the qubit's internal sensitivity to that noise, or through multi-qubit
encoding and error correction protocols (which already presume ultra-low error
rates). In contrast, dephasing is in principle reversible and can be refocused
dynamically through the application of coherent control pulse methods. In this
work we demonstrate how dynamical-decoupling techniques can moderate the
dephasing effects of low-frequency noise on a superconducting qubit with
energy-relaxation time T1 = 1/Gamma1 = 12 us. Using the CPMG sequence with up
to 200 pi-pulses, we demonstrate a 50-fold improvement in the transverse
relaxation time T2 over its baseline value. We observe relaxation-limited times
T2(CPMG) = 23 us = 2 T1 resulting from CPMG-mediated Gaussian pure-dephasing
times in apparent excess of 100 us. We leverage the filtering property of this
sequence in conjunction with Rabi and energy relaxation measurements to
facilitate the spectroscopy and reconstruction of the environmental noise power
spectral density.Comment: 21 pages (incl. 11-page appendix); 4 (+7) figure
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 π-pulses is identical to the Uhrig dynamic decoupling (UDD) sequence. We compare UDD and Carr-Purcell-Meiboom-Gill (CPMG) sequences applied to a single 43Ca+ trapped-ion qubit and find that they afford comparable protection in our ambient noise environment. © 2011 IOP Publishing Ltd
Optical Bloch equations with multiply connected states
The optical Bloch equations, which give the time evolution of the elements of the density matrix of an atomic system subject to radiation, are generalized so that they can be applied when transitions between pairs of states can proceed by more than one stimulated route. The case considered is that for which the time scale of interest in the problem is long compared with that set by the differences in detuning of the radiation fields stimulating via the different routes. It is shown that the Bloch equations then reduce to the standard form of linear differential equations with constant coefficients. The theory is applied to a two-state system driven by two lasers with different intensities and frequencies and to a three-state Λ-system with one laser driving one transition and two driving the second. It is also shown that the theory reproduces well the observed response of a cold 40Ca+ ion when subject to a single laser frequency driving the 4S1/2-4P 1/2 transition and a laser with two strong sidebands driving 3D 3/2-4P1/2. © 2008 IOP Publishing Ltd