862 research outputs found
Simple Phase Bias for Superconducting Circuits
A phase-bias tool, based on a trapped fluxoid in a ring, is proposed and
demonstrated. It can provide arbitrary phase values and is simple to fabricate.
The phase bias has been realized in two superconducting quantum interference
devices, where the critical current versus magnetic flux is shown to be shifted
by a \pi/2 and \pi.Comment: 5 pages, including 4 figures. Submitted to AP
Modelling reversibility of Central European mountain lakes from acidification: Part I - the Bohemian forest
International audienceA dynamic, process-based acidification model, MAGIC7, has been applied to three small, strongly acidified lakes in the Bohemian Forest, the Czech Republic. The model was calibrated for a set of experimental records on lake water composition over the 1984?2000 period, and produced hindcast concentrations that compared well, even with older (40-year) irregular determinations of nitrate, chloride and pH. Water and soil chemistry forecasts up to 2050 were based on reductions in S and N emissions presupposed by the Gothenburg Protocol. Modelled sulphate and chloride concentrations were predicted to decrease to the levels at the beginning of the 20th century by 2050. The lake water carbonate buffering system is predicted to be re-established in only two lakes (Cerné and Ple?né), with current soil base saturations of 12-15%. Concentrations of ionic aluminium species decreased sharply, from 110 ?eq l-1 in the mid-1980s to the current ~40 ?eq l-1, and were predicted to decrease below 10 ?eq l-1 in the 2020s. Diatom-inferred pH in pre-industrial times was substantially lower than modelled pH. It is suggested that the diatom pH, based almost entirely on non-planktonic species, is biased by inwash of diatoms from more acidic tributaries into the sediment of these small lakes. Generally significant results can be summarised as follows: (1) Simulated sulphate levels agree well with observations during acidification progress and retreat only for values of soil SO42- adsorption capacity three to six times (20 to 40 ?eq kg-1) higher than those found experimentally. This implies a further mechanism of S retention and release in addition to physical sulphate adsorption to Fe and Al oxides of soils. (2) The catchments' ability to retain deposited N appeared to decline after ~1950 but this was not connected with a sufficient change in the C:N ratio of the soils. Agreement between modelled and observed concentrations of nitrate was therefore achieved by empirical restriction of N retention in the soils. Based on their current ability to retain N, the catchments will remain N-saturated and could, temporarily, produce more inorganic N than they receive due to additional nitrate production from soil N-organic pools. This situation has occurred already in the Cerné Lake catchment. (3) Differences in responses of individual lakes can be attributed to different land usages over the past several centuries as well as to differences in geology and primary production. Keywords: MAGIC, atmospheric deposition, N retention, diatom-inferred pH, sulphate, nitrate, base cations, aluminium, Czech Republi
Quantum state transfer in arrays of flux qubits
In this work, we describe a possible experimental realization of Bose's idea
to use spin chains for short distance quantum communication [S. Bose, {\it
Phys. Rev. Lett.} {\bf 91} 207901]. Josephson arrays have been proposed and
analyzed as transmission channels for systems of superconducting charge qubits.
Here, we consider a chain of persistent current qubits, that is appropriate for
state transfer with high fidelity in systems containing flux qubits. We
calculate the fidelity of state transfer for this system. In general, the
Hamiltonian of this system is not of XXZ-type, and we analyze the magnitude and
the effect of the terms that don't conserve the z-component of the total spin.Comment: 10 pages, 8 figure
Multi-band quantum ratchets
We investigate directed motion in non-adiabatically rocked ratchet systems
sustaining few bands below the barrier. Upon restricting the dynamics to the
lowest M bands, the total system-plus-bath Hamiltonian is mapped onto a
discrete tight-binding model containing all the information both on the intra-
and inter-well tunneling motion. A closed form for the current in the
incoherent tunneling regime is obtained. In effective single-band ratchets, no
current rectification occurs. We apply our theory to describe rectification
effects in vortex quantum ratchets devices. Current reversals upon variation of
the ac-field amplitude or frequency are predicted.Comment: Accepted for publication in Physical Review Letter
Spectroscopy on two coupled flux qubits
We have performed spectroscopy measurements on two coupled flux qubits. The
qubits are coupled inductively, which results in a
interaction. By applying microwave radiation, we observe resonances due to
transitions from the ground state to the first two excited states. From the
position of these resonances as a function of the magnetic field applied we
observe the coupling of the qubits. The coupling strength agrees well with
calculations of the mutual inductance
Strong magnetic coupling of an ultracold gas to a superconducting waveguide cavity
Placing an ensemble of ultracold atoms in the near field of a
superconducting coplanar waveguide resonator (CPWR) with one can
achieve strong coupling between a single microwave photon in the CPWR and a
collective hyperfine qubit state in the ensemble with kHz larger than the cavity line width of
kHz. Integrated on an atomchip such a system constitutes a hybrid quantum
device, which also can be used to interconnect solid-state and atomic qubits,
to study and control atomic motion via the microwave field, observe microwave
super-radiance, build an integrated micro maser or even cool the resonator
field via the atoms
Coupling Superconducting Qubits via a Cavity Bus
Superconducting circuits are promising candidates for constructing quantum
bits (qubits) in a quantum computer; single-qubit operations are now routine,
and several examples of two qubit interactions and gates having been
demonstrated. These experiments show that two nearby qubits can be readily
coupled with local interactions. Performing gates between an arbitrary pair of
distant qubits is highly desirable for any quantum computer architecture, but
has not yet been demonstrated. An efficient way to achieve this goal is to
couple the qubits to a quantum bus, which distributes quantum information among
the qubits. Here we show the implementation of such a quantum bus, using
microwave photons confined in a transmission line cavity, to couple two
superconducting qubits on opposite sides of a chip. The interaction is mediated
by the exchange of virtual rather than real photons, avoiding cavity induced
loss. Using fast control of the qubits to switch the coupling effectively on
and off, we demonstrate coherent transfer of quantum states between the qubits.
The cavity is also used to perform multiplexed control and measurement of the
qubit states. This approach can be expanded to more than two qubits, and is an
attractive architecture for quantum information processing on a chip.Comment: 6 pages, 4 figures, to be published in Natur
Qubit-photon interactions in a cavity: Measurement induced dephasing and number splitting
We theoretically study measurement induced-dephasing of a superconducting
qubit in the circuit QED architecture and compare the results to those obtained
experimentally by Schuster {\it et al.}, [Phys. Rev. Lett. 94, 123602 (2005)].
Strong coupling of the qubit to the resonator leads to a significant ac-Stark
shift of the qubit transition frequency. As a result, quantum fluctuations in
the photon number populating the resonator cause dephasing of the qubit. We
find good agreement between the predicted line shape of the qubit spectrum and
the experimental results. Furthermore, in the strong dispersive limit, where
the Stark shift per photon is large compared to the cavity decay rate and the
qubit linewidth, we predict that the qubit spectrum will be split into multiple
peaks, with each peak corresponding to a different number of photons in the
cavity.Comment: 15 pages and 10 figures. Section IV revised. Author and references
added. Version with high resolution figures available at available at
http://www.physique.usherbrooke.ca/~ablais/publications.ht
Fast geometric gate operation of superconducting charge qubits in circuit QED
A scheme for coupling superconducting charge qubits via a one-dimensional
superconducting transmission line resonator is proposed. The qubits are working
at their optimal points, where they are immune to the charge noise and possess
long decoherence time. Analysis on the dynamical time evolution of the
interaction is presented, which is shown to be insensitive to the initial state
of the resonator field. This scheme enables fast gate operation and is readily
scalable to multiqubit scenario
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