171 research outputs found
Detection of a persistent-current qubit by resonant activation
We present the implementation of a new scheme to detect the quantum state of
a persistent-current qubit. It relies on the dependency of the measuring
Superconducting Quantum Interference Device (SQUID) plasma frequency on the
qubit state, which we detect by resonant activation. With a measurement pulse
of only 5ns, we observed Rabi oscillations with high visibility (65%).Comment: 4 pages, 4 figures, submitted to PRB Rapid Co
Parametric coupling for superconducting qubits
We propose a scheme to couple two superconducting charge or flux qubits
biased at their symmetry points with unequal energy splittings. Modulating the
coupling constant between two qubits at the sum or difference of their two
frequencies allows to bring them into resonance in the rotating frame.
Switching on and off the modulation amounts to switching on and off the
coupling which can be realized at nanosecond speed. We discuss various physical
implementations of this idea, and find that our scheme can lead to rapid
operation of a two-qubit gate.Comment: 6 page
Dephasing of a superconducting qubit induced by photon noise
We have studied the dephasing of a superconducting flux-qubit coupled to a
DC-SQUID based oscillator. By varying the bias conditions of both circuits we
were able to tune their effective coupling strength. This allowed us to measure
the effect of such a controllable and well-characterized environment on the
qubit coherence. We can quantitatively account for our data with a simple model
in which thermal fluctuations of the photon number in the oscillator are the
limiting factor. In particular, we observe a strong reduction of the dephasing
rate whenever the coupling is tuned to zero. At the optimal point we find a
large spin-echo decay time of .Comment: New version of earlier paper arXiv/0507290 after in-depth rewritin
Asymmetry and decoherence in a double-layer persistent-current qubit
Superconducting circuits fabricated using the widely used shadow evaporation
technique can contain unintended junctions which change their quantum dynamics.
We discuss a superconducting flux qubit design that exploits the symmetries of
a circuit to protect the qubit from unwanted coupling to the noisy environment,
in which the unintended junctions can spoil the quantum coherence. We present a
theoretical model based on a recently developed circuit theory for
superconducting qubits and calculate relaxation and decoherence times that can
be compared with existing experiments. Furthermore, the coupling of the qubit
to a circuit resonance (plasmon mode) is explained in terms of the asymmetry of
the circuit. Finally, possibilities for prolonging the relaxation and
decoherence times of the studied superconducting qubit are proposed on the
basis of the obtained results.Comment: v.2: published version; 8 pages, 12 figures; added comparison with
experiment, improved discussion of T_ph
Crossover from weak to strong coupling regime in dispersive circuit QED
We study the decoherence of a superconducting qubit due to the dispersive
coupling to a damped harmonic oscillator. We go beyond the weak
qubit-oscillator coupling, which we associate with a phase Purcell effect, and
enter into a strong coupling regime, with qualitatively different behavior of
the dephasing rate. We identify and give a physicaly intuitive discussion of
both decoherence mechanisms. Our results can be applied, with small
adaptations, to a large variety of other physical systems, e. g. trapped ions
and cavity QED, boosting theoretical and experimental decoherence studies.Comment: Published versio
Reaching the quantum limit of sensitivity in electron spin resonance
We report pulsed electron-spin resonance (ESR) measurements on an ensemble of
Bismuth donors in Silicon cooled at 10mK in a dilution refrigerator. Using a
Josephson parametric microwave amplifier combined with high-quality factor
superconducting micro-resonators cooled at millikelvin temperatures, we improve
the state-of-the-art sensitivity of inductive ESR detection by nearly 4 orders
of magnitude. We demonstrate the detection of 1700 bismuth donor spins in
silicon within a single Hahn echo with unit signal-to-noise (SNR) ratio,
reduced to just 150 spins by averaging a single Carr-Purcell-Meiboom-Gill
sequence. This unprecedented sensitivity reaches the limit set by quantum
fluctuations of the electromagnetic field instead of thermal or technical
noise, which constitutes a novel regime for magnetic resonance.Comment: Main text : 10 pages, 4 figures. Supplementary text : 16 pages, 8
figure
One- and two-photon spectroscopy of a flux qubit coupled to a microscopic defect
We observed the dynamics of a superconducting flux qubit coupled to an
extrinsic quantum system (EQS). The presence of the EQS is revealed by an
anticrossing in the spectroscopy of the qubit. The excitation of a two-photon
transition to the third excited state of the qubit-EQS system allows us to
extract detailed information about the energy level structure and the coupling
of the EQS. We deduce that the EQS is a two-level system, with a transverse
coupling to the qubit. The transition frequency and the coupling of the EQS
changed during experiments, which supports the idea that the EQS is a two-level
system of microscopic origin.Comment: accepted in Physical Review
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