251 research outputs found
Relaxation of Josephson qubits due to strong coupling to two-level systems
We investigate the energy relaxation (T1) process of a qubit coupled to a
bath of dissipative two-level fluctuators (TLF). We consider the fluctuators
strongly coupled to the qubit both in the limit of spectrally separated single
TLF's as well as in the limit of spectrally dense TLF's. We conclude that the
avoided level crossings, usually attributed to very strongly coupled single
TLF's, could also be caused by many weakly coupled spectrally dense
fluctuators.Comment: 11+ pages, 10 figures, citations added, discussion extende
Topological surface states in three-dimensional magnetic insulators
An electron moving in a magnetically ordered background feels an effective
magnetic field that can be both stronger and more rapidly varying than typical
externally applied fields. One consequence is that insulating magnetic
materials in three dimensions can have topologically nontrivial properties of
the effective band structure. For the simplest case of two bands, these "Hopf
insulators" are characterized by a topological invariant as in quantum Hall
states and Z_2 topological insulators, but instead of a Chern number or parity,
the underlying invariant is the Hopf invariant that classifies maps from the
3-sphere to the 2-sphere. This paper gives an efficient algorithm to compute
whether a given magnetic band structure has nontrivial Hopf invariant, a
double-exchange-like tight-binding model that realizes the nontrivial case, and
a numerical study of the surface states of this model.Comment: 4 pages, 2 figures; published versio
Quantum logic operations and creation of entanglement in a scalable superconducting quantum computer with long-range constant interaction between qubits
We consider a one-dimensional chain of many superconducting quantum
interference devices (SQUIDs), serving as charge qubits. Each SQUID is coupled
to its nearest neighbors through constant capacitances. We study the quantum
logic operations and implementation of entanglement in this system.
Arrays with two and three qubits are considered in detail. We show that the
creation of entanglement with an arbitrary number of qubits can be implemented,
without systematic errors, even when the coupling between qubits is not small.
A relatively large coupling constant allows one to increase the clock speed of
the quantum computer. We analytically and numerically demonstrate the creation
of the entanglement for this case, which can be a good test for the
experimental implementation of a relatively simple quantum protocol with many
qubits. We discuss a possible application of our approach for implementing
universal quantum logic for more complex algorithms by decreasing the coupling
constant and, correspondingly, decreasing the clock speed. The errors
introduced by the long-range interaction for the universal logic gates are
estimated analytically and calculated numerically. Our results can be useful
for experimental implementation of quantum algorithms using controlled magnetic
fluxes and gate voltages applied to the SQUIDs. The algorithms discussed in
this paper can be implemented using already existing technologies in
superconducting systems with constant inter-qubit coupling.Comment: 24 page
Non-adiabatically detecting the geometric phase of the macroscopic quantum state with symmetric SQUID
We give a simple way to detect the geometric phase shift and the conditional
geometric phase shift with Josephson junction system. Comparing with the
previous work(Falcl G, Fazio R, Palma G.M., Siewert J and Verdal V, {\it
Nature} {\bf 407}, 355(2000)), our scheme has two advantages. We use the
non-adiabatic operation, thus the detection is less affected by the
decoherence. Also, we take the time evolution on zero dynamic phase loop, we
need not take any extra operation to cancel the dynamic phase.Comment: 8 pages, 4 figure
Connecting Berry's phase and the pumped charge in a Cooper pair pump
The properties of the tunnelling-charging Hamiltonian of a Cooper pair pump
are well understood in the regime of weak and intermediate Josephson coupling,
i.e. when . It is also known that
Berry's phase is related to the pumped charge induced by the adiabatical
variation of the eigenstates. We show explicitly that pumped charge in Cooper
pair pump can be understood as a partial derivative of Berry's phase with
respect to the phase difference across the array. The phase fluctuations
always present in real experiments can also be taken into account, although
only approximately. Thus the measurement of the pumped current gives reliable,
yet indirect, information on Berry's phase. As closing remarks, we give the
differential relation between Berry's phase and the pumped charge, and state
that the mathematical results are valid for any observable expressible as a
partial derivative of the Hamiltonian.Comment: 5 pages, 5 figures, RevTeX, Presentation has been clarifie
Topological Objects in Two-component Bose-Einstein Condensates
We study the topological objects in two-component Bose-Einstein condensates.
We compare two competing theories of two-component Bose-Einstein condensate,
the popular Gross-Pitaevskii theory and the recently proposed gauge theory of
two-component Bose-Einstein condensate which has an induced vorticity
interaction. We show that two theories produce very similar topological
objects, in spite of the obvious differences in dynamics. Furthermore we show
that the gauge theory of two-component Bose-Einstein condensate, with the U(1)
gauge symmetry, is remarkably similar to the Skyrme theory. Just like the
Skyrme theory the theory admits the non-Abelian vortex, the helical vortex, and
the vorticity knot. We construct the lightest knot solution in two-component
Bose-Einstein condensate numerically, and discuss how the knot can be
constructed in the spin-1/2 condensate of atoms.Comment: 18 pages, 15 figures, Phys. Rev. A in pres
Quantum information processing using Josephson junctions coupled through cavities
Josephson junctions have been shown to be a promising solid-state system for
implementation of quantum computation. The significant two-qubit gates are
generally realized by the capacitive coupling between the nearest neighbour
qubits. We propose an effective Hamiltonian to describe charge qubits coupled
through the cavity. We find that nontrivial two-qubit gates may be achieved by
this coupling. The ability to interconvert localized charge qubits and flying
qubits in the proposed scheme implies that quantum network can be constructed
using this large scalable solid-state system.Comment: 5 pages, to appear in Phys Rev A; typos corrected, solutions in last
eqs. correcte
A nonlinear mechanism of charge qubit decoherence in a lossy cavity: the quasi normal mode approach
In the viewpoint of quasi normal modes, we describe a novel decoherence
mechanism of charge qubit of Josephson Junctions (JJ) in a lossy micro-cavity,
which can appear in the realistic experiment for quantum computation based on
JJ qubit. We show that the nonlinear coupling of a charge qubit to quantum
cavity field can result in an additional dissipation of resonant mode due to
its effective interaction between those non-resonant modes and a resonant mode,
which is induced by the charge qubit itself. We calculate the characterized
time of the novel decoherence by making use of the system plus bath method.Comment: 6 pages, 2 figur
Perturbation Theory for Quantum Computation with Large Number of Qubits
We describe a new and consistent perturbation theory for solid-state quantum
computation with many qubits. The errors in the implementation of simple
quantum logic operations caused by non-resonant transitions are estimated. We
verify our perturbation approach using exact numerical solution for relatively
small (L=10) number of qubits. A preferred range of parameters is found in
which the errors in processing quantum information are small. Our results are
needed for experimental testing of scalable solid-state quantum computers.Comment: 8 pages RevTex including 2 figure
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