179 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
Multiplexed Readout of Transmon Qubits with Josephson Bifurcation Amplifiers
Achieving individual qubit readout is a major challenge in the development of
scalable superconducting quantum processors. We have implemented the
multiplexed readout of a four transmon qubit circuit using non-linear
resonators operated as Josephson bifurcation amplifiers. We demonstrate the
simultaneous measurement of Rabi oscillations of the four transmons. We find
that multiplexed Josephson bifurcation is a high-fidelity readout method, the
scalability of which is not limited by the need of a large bandwidth nearly
quantum-limited amplifier as is the case with linear readout resonators.Comment: 7 pages, 6 figures, and 31 reference
Relaxation and Dephasing in a Flux-qubit
We report detailed measurements of the relaxation and dephasing time in a
flux-qubit measured by a switching DC SQUID. We studied their dependence on the
two important circuit bias parameters: the externally applied magnetic flux and
the bias current through the SQUID in two samples. We demonstrate two
complementary strategies to protect the qubit from these decoherence sources.
One consists in biasing the qubit so that its resonance frequency is stationary
with respect to the control parameters ({\it optimal point}) ; the second
consists in {\it decoupling} the qubit from current noise by chosing a proper
bias current through the SQUID. At the decoupled optimal point, we measured
long spin-echo decay times of up to .Comment: 4 pages, 4 figures, submitted to Phys. Rev. Letter
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
Generation algorithm of a concept lattice with limited object access
Abstract. Classical algorithms for generating the concept lattice (C, ≤ ) of a binary table (O, I, R) have a complexity in O(|C| * |I| 2 * |O|). Although the number of concepts is exponential in the size of the table in the worst case, the generation of a concept is output polynomial. In practice, the number of concepts is often polynomial in the size of the table. However, the cost of generating a concept remains high when the table is composed of a large number of objects. We propose in this paper an algorithm for generating the lattice with limited object access, which can improve the computation time. Experiments were conducted with Joomla!, a content management system based on relational algebra, and located on a MySQL database
On implicational bases of closure systems with unique critical sets
We show that every optimum basis of a finite closure system, in D.Maier's
sense, is also right-side optimum, which is a parameter of a minimum CNF
representation of a Horn Boolean function. New parameters for the size of the
binary part are also established. We introduce a K-basis of a general closure
system, which is a refinement of the canonical basis of Duquenne and Guigues,
and discuss a polynomial algorithm to obtain it. We study closure systems with
the unique criticals and some of its subclasses, where the K-basis is unique. A
further refinement in the form of the E-basis is possible for closure systems
without D-cycles. There is a polynomial algorithm to recognize the D-relation
from a K-basis. Thus, closure systems without D-cycles can be effectively
recognized. While E-basis achieves an optimum in one of its parts, the
optimization of the others is an NP-complete problem.Comment: Presented on International Symposium of Artificial Intelligence and
Mathematics (ISAIM-2012), Ft. Lauderdale, FL, USA Results are included into
plenary talk on conference Universal Algebra and Lattice Theory, June 2012,
Szeged, Hungary 29 pages and 2 figure
Concept lattices : a tool for primitives selection ?
In this paper, we present the problem of noisy images recognition and in particular the stage of primitives selection in a
classification process. This selection stage appears after segmentation and statistical describers extraction on
documentary images are realized. We describe precisely the use of decision tree in order to harmonize and compare it
with another less studied method based on a concept lattice.Dans ce papier, nous présentons la problématique de la reconnaissance d'images détériorées et plus
particulièrement l'étape de sélection de primitives au sein d'un traitement de classification supervisée. Cette
étape de sélection a lieu après que la segmentation et l'extraction des descripteurs statistiques sur des
images documentaires aient été réalisées. Nous exposons en détail l'utilisation d'un arbre de décision, afin de
l'harmoniser puis la comparer avec une approche moins étudiée utilisant un treillis de Galois
Storage and retrieval of microwave fields at the single-photon level in a spin ensemble
We report the storage of microwave pulses at the single-photon level in a
spin-ensemble memory consisting of NV centers in a diamond crystal
coupled to a superconducting LC resonator. The energy of the signal, retrieved
later by spin-echo techniques, reaches of the
energy absorbed by the spins, and this storage efficiency is quantitatively
accounted for by simulations. This figure of merit is sufficient to envision
first implementations of a quantum memory for superconducting qubits.Comment: 6 page
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
- …