220 research outputs found
Generation of high-fidelity controlled-not logic gates by coupled superconducting qubits
Building on the previous results of the Weyl chamber steering method, we
demonstrate how to generate high-fidelity controlled-not by direct application
of certain, physically relevant Hamiltonians with fixed coupling constants
containing Rabi terms. Such Hamiltonians are often used to describe two
superconducting qubits driven by local rf-pulses.
It is found that in order to achieve 100% fidelity in a system with
capacitive coupling of strength g one Rabi term suffices. We give the exact
values of the physical parameters needed to implement such CNOT. The gate time
and all possible Rabi frequencies are found to be t=pi/(2g) and Omega_1/g =
sqrt(64n^2-1), n=1,2,3,... .
Generation of a perfect CNOT in a system with inductive coupling,
characterized by additional constant k, requires the presence of both Rabi
terms. The gate time is again t = pi/(2g), but now there is an infinite number
of solutions, each of which is valid in a certain range of k and is
characterized by a pair of positive integers (n,m). We distinguish two cases,
depending on the sign of the coupling constant:
(1) the antiferromagnetic case (k>=0);
(2) the ferromagnetic case (k<=0).
The paper concludes with consideration of fidelity degradation by switching
to resonance. Simulation of time-evolution based on the 4th order Magnus
expansion reveals characteristics of the gate similar to those found in the
exact case, with slightly shorter gate time and shifted values of Rabi
frequencies.Comment: 14 pages, 1 figure (select PostScript to view); minor corrections;
added Appendix: Derivation of the Hamiltonian for inductively coupled flux
qubit
Quantum collisions of finite-size ultrarelativistic nuclei
We show that the boost variable, the conjugate to the coordinate rapidity,
which is associated with the center-of-mass motion, encodes the information
about the finite size of colliding nuclei in a Lorentz-invariant way. The
quasi-elastic forward color-changing scattering between the quantum boost
states rapidly grows with the total energy of the collision and leads to an
active breakdown of the color coherence at the earliest moments of the
collision. The possible physical implications of this result are discussed.Comment: 23 pages, RevTeX. New references and two figures added. Final version
accepted for publication in Physical Review
Quasiparticle scattering by quantum phase slips in one-dimensional superfluids
Quantum phase slips (QPS) in narrow superfluid channels generate momentum by
unwinding the supercurrent. In a uniform Bose gas, this momentum needs to be
absorbed by quasiparticles (phonons). We show that this requirement results in
an additional exponential suppression of the QPS rate (compared to the rate of
QPS induced by a sharply localized perturbation). In BCS-paired fluids,
momentum can be transferred to fermionic quasiparticles, and we find an
interesting interplay between quasiparticle scattering on QPS and on disorder.Comment: 4 pages, revtex, no figures; to be published in Phys. Rev. Letter
Quantum Zeno effect in the Cooper-pair transport through a double-island Josephson system
Motivated by recent experiments, we analyze transport of Cooper pairs through
a double-island Josephson qubit. At low bias in a certain range of gate
voltages coherent superpositions of charge states play a crucial role. Analysis
of the evolution of the density matrix allows us to cover a wide range of
parameters, incl. situations with degenerate levels, when dissipation strongly
affects the coherent eigenstates. At high noise levels the so-called Zeno
effect can be observed, which slows down the transport. Our analysis explains
certain features of the I-V curves, in particular the visibility and shape of
resonant peaks and lines
Dephasing of qubits by transverse low-frequency noise
We analyze the dissipative dynamics of a two-level quantum system subject to
low-frequency, e.g. 1/f noise, motivated by recent experiments with
superconducting quantum circuits. We show that the effect of transverse linear
coupling of the system to low-frequency noise is equivalent to that of
quadratic longitudinal coupling. We further find the decay law of quantum
coherent oscillations under the influence of both low- and high-frequency
fluctuations, in particular, for the case of comparable rates of relaxation and
pure dephasing
Information entropic superconducting microcooler
We consider a design for a cyclic microrefrigerator using a superconducting
flux qubit. Adiabatic modulation of the flux combined with thermalization can
be used to transfer energy from a lower temperature normal metal thin film
resistor to another one at higher temperature. The frequency selectivity of
photonic heat conduction is achieved by including the hot resistor as part of a
high frequency LC resonator and the cold one as part of a low-frequency
oscillator while keeping both circuits in the underdamped regime. We discuss
the performance of the device in an experimentally realistic setting. This
device illustrates the complementarity of information and thermodynamic entropy
as the erasure of the quantum bit directly relates to the cooling of the
resistor.Comment: 4 pages, 3 figure
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