2,443 research outputs found
Designing Robust Unitary Gates: Application to Concatenated Composite Pulse
We propose a simple formalism to design unitary gates robust against given
systematic errors. This formalism generalizes our previous observation [Y.
Kondo and M. Bando, J. Phys. Soc. Jpn. 80, 054002 (2011)] that vanishing
dynamical phase in some composite gates is essential to suppress amplitude
errors. By employing our formalism, we naturally derive a new composite unitary
gate which can be seen as a concatenation of two known composite unitary
operations. The obtained unitary gate has high fidelity over a wider range of
the error strengths compared to existing composite gates.Comment: 7 pages, 4 figures. Major revision: improved presentation in Sec. 3,
references and appendix adde
Distillation of Bell states in open systems
In this work we review the entire classification of 2x2 distillable states
for protocols with a finite numbers of copies. We show a distillation protocol
that allows to distill Bell states with non zero probability at any time for an
initial singlet in vacuum. It is shown that the same protocol used in non zero
thermal baths yields a considerable recovering of entanglement.Comment: 10 pages, 3 figure
Quantization and Periodicity of the Axion Action in Topological Insulators
The Lagrangian describing the bulk electromagnetic response of a
three-dimensional strong topological insulator contains a topological `axion'
term of the form '\theta E dot B'. It is often stated (without proof) that the
corresponding action is quantized on periodic space-time and therefore
invariant under '\theta -> \theta +2\pi'. Here we provide a simple, physically
motivated proof of the axion action quantization on the periodic space-time,
assuming only that the vector potential is consistent with single-valuedness of
the electron wavefunctions in the underlying insulator.Comment: 4 pages, 1 figure, version2 (section on axion action quantization of
non-periodic systems added
Recursive Encoding and Decoding of Noiseless Subsystem and Decoherence Free Subspace
When the environmental disturbace to a quantum system has a wavelength much
larger than the system size, all qubits localized within a small area are under
action of the same error operators. Noiseless subsystem and decoherence free
subspace are known to correct such collective errors. We construct simple
quantum circuits, which implement these collective error correction codes, for
a small number of physical qubits. A single logical qubit is encoded with
and , while two logical qubits are encoded with . The recursive
relations among the subspaces employed in noiseless subsystem and decoherence
free subspace play essential r\^oles in our implementation. The recursive
relations also show that the number of gates required to encode logical
qubits increases linearly in .Comment: 9 pages, 3 figure
Experimental determination of the Berry phase in a superconducting charge pump
We present the first measurements of the Berry phase in a superconducting
Cooper pair pump. A fixed amount of Berry phase is accumulated to the
quantum-mechanical ground state in each adiabatic pumping cycle, which is
determined by measuring the charge passing through the device. The dynamic and
geometric phases are identified and measured quantitatively from their
different response when pumping in opposite directions. Our observations, in
particular, the dependencies of the dynamic and geometric effects on the
superconducting phase bias across the pump, agree with the basic theoretical
model of coherent Cooper pair pumping.Comment: 4 pages, 3 figure
Geometric phase contribution to quantum non-equilibrium many-body dynamics
We study the influence of geometry of quantum systems underlying space of
states on its quantum many-body dynamics. We observe an interplay between
dynamical and topological ingredients of quantum non-equilibrium dynamics
revealed by the geometrical structure of the quantum space of states. As a
primary example we use the anisotropic XY ring in a transverse magnetic field
with an additional time-dependent flux. In particular, if the flux insertion is
slow, non-adiabatic transitions in the dynamics are dominated by the dynamical
phase. In the opposite limit geometric phase strongly affects transition
probabilities. We show that this interplay can lead to a non-equilibrium phase
transition between these two regimes. We also analyze the effect of geometric
phase on defect generation during crossing a quantum critical point.Comment: 4 pages, 3 figures. Added an appendix with supplementary informatio
Saddle index properties, singular topology, and its relation to thermodynamical singularities for a phi^4 mean field model
We investigate the potential energy surface of a phi^4 model with infinite
range interactions. All stationary points can be uniquely characterized by
three real numbers $\alpha_+, alpha_0, alpha_- with alpha_+ + alpha_0 + alpha_-
= 1, provided that the interaction strength mu is smaller than a critical
value. The saddle index n_s is equal to alpha_0 and its distribution function
has a maximum at n_s^max = 1/3. The density p(e) of stationary points with
energy per particle e, as well as the Euler characteristic chi(e), are singular
at a critical energy e_c(mu), if the external field H is zero. However, e_c(mu)
\neq upsilon_c(mu), where upsilon_c(mu) is the mean potential energy per
particle at the thermodynamic phase transition point T_c. This proves that
previous claims that the topological and thermodynamic transition points
coincide is not valid, in general. Both types of singularities disappear for H
\neq 0. The average saddle index bar{n}_s as function of e decreases
monotonically with e and vanishes at the ground state energy, only. In
contrast, the saddle index n_s as function of the average energy bar{e}(n_s) is
given by n_s(bar{e}) = 1+4bar{e} (for H=0) that vanishes at bar{e} = -1/4 >
upsilon_0, the ground state energy.Comment: 9 PR pages, 6 figure
Torsion induces Gravity
In this work the Poincare-Chern Simons and Anti de Sitter Chern Simons
gravities are studied. For both a solution that can be casted as a black hole
with manifest torsion is found. Those solutions resemble Schwarzschild and
Schwarzschild-AdS solutions respectively.Comment: 4 pages, RevTe
Effects of mechanical rotation on spin currents
We study the Pauli--Schr\"odinger equation in a uniformly rotating frame of
reference to describe a coupling of spins and mechanical rotations. The
explicit form of the spin-orbit interaction (SOI) with the inertial effects due
to the mechanical rotation is presented. We derive equations of motion for a
wavepacket of electrons in two-dimensional planes subject to the SOI. The
solution is a superposition of two cyclotron motions with different frequencies
and a circular spin current is created by the mechanical rotation.Comment: 4 pages, 2 figure
Translations and dynamics
We analyze the role played by local translational symmetry in the context of
gauge theories of fundamental interactions. Translational connections and
fields are introduced, with special attention being paid to their universal
coupling to other variables, as well as to their contributions to field
equations and to conserved quantities.Comment: 22 Revtex pages, no figures. Published version with minor correction
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