2,954 research outputs found
SU(N)-symmetric quasi-probability distribution functions
We present a set of N-dimensional functions, based on generalized
SU(N)-symmetric coherent states, that represent finite-dimensional Wigner
functions, Q-functions, and P-functions. We then show the fundamental
properties of these functions and discuss their usefulness for analyzing
N-dimensional pure and mixed quantum states.Comment: 16 pages, 2 figures. Updated text to reflect referee comment
Precision Measurements Using Squeezed Spin States via Two-axis Counter-twisting Interactions
We show that the two-axis counter twisting interaction squeezes a coherent
spin state into three states of interest in quantum information, namely, the
twin-Fock state, the equally-weighted superposition state, and the state that
achieves the Heisenberg limit of optimal sensitivity defined by the Cramer-Rao
inequality in addition to the well-known Heisenberg-limited state of spin
fluctuations.Comment: 5 pages, 3 figure
Quantum error correction via robust probe modes
We propose a new scheme for quantum error correction using robust continuous
variable probe modes, rather than fragile ancilla qubits, to detect errors
without destroying data qubits. The use of such probe modes reduces the
required number of expensive qubits in error correction and allows efficient
encoding, error detection and error correction. Moreover, the elimination of
the need for direct qubit interactions significantly simplifies the
construction of quantum circuits. We will illustrate how the approach
implements three existing quantum error correcting codes: the 3-qubit bit-flip
(phase-flip) code, the Shor code, and an erasure code.Comment: 5 pages, 3 figure
Modified Thouless-Anderson-Palmer equations for the Sherrington-Kirkpatrick spin glass: Numerical solutions
For large but finite systems the static properties of the infinite ranged
Sherrington-Kirkpatrick model are numerically investigated in the entire the
glass regime. The approach is based on the modified Thouless-Anderson-Palmer
equations in combination with a phenomenological relaxational dynamics used as
a numerical tool. For all temperatures and all bond configurations stable and
meta stable states are found. Following a discussion of the finite size
effects, the static properties of the state of lowest free energy are presented
in the presence of a homogeneous magnetic field for all temperatures below the
spin glass temperature. Moreover some characteristic features of the meta
stable states are presented. These states exist in finite temperature intervals
and disappear via local saddle node bifurcations. Numerical evidence is found
that the excess free energy of the meta stable states remains finite in the
thermodynamic limit. This implies a the `multi-valley' structure of the free
energy on a sub-extensive scale.Comment: Revtex 10 pages 13 figures included, submitted to Phys.Rev.B.
Shortend and improved version with additional numerical dat
Renormalized Finite Temperature phi^4 theory from the 2PI Effective Action
We present an analytical and numerical study of scalar phi^4 theory at finite
temperature with a renormalized 2-loop truncation of the 2PI effective action.Comment: 4 pages. Presented at International Conference on Strong and
Electroweak Matter (SEWM 2006), Upton, New York, 10-13 May 200
Qudit Quantum State Tomography
Recently quantum tomography has been proposed as a fundamental tool for
prototyping a few qubit quantum device. It allows the complete reconstruction
of the state produced from a given input into the device. From this
reconstructed density matrix, relevant quantum information quantities such as
the degree of entanglement and entropy can be calculated. Generally orthogonal
measurements have been discussed for this tomographic reconstruction. In this
paper, we extend the tomographic reconstruction technique to two new regimes.
First we show how non-orthogonal measurement allow the reconstruction of the
state of the system provided the measurements span the Hilbert space. We then
detail how quantum state tomography can be performed for multi qudits with a
specific example illustrating how to achieve this in one and two qutrit
systems.Comment: 6 pages, 4 figures, submitted to PR
Adiabatic quantum computation along quasienergies
The parametric deformations of quasienergies and eigenvectors of unitary
operators are applied to the design of quantum adiabatic algorithms. The
conventional, standard adiabatic quantum computation proceeds along
eigenenergies of parameter-dependent Hamiltonians. By contrast, discrete
adiabatic computation utilizes adiabatic passage along the quasienergies of
parameter-dependent unitary operators. For example, such computation can be
realized by a concatenation of parameterized quantum circuits, with an
adiabatic though inevitably discrete change of the parameter. A design
principle of adiabatic passage along quasienergy is recently proposed: Cheon's
quasienergy and eigenspace anholonomies on unitary operators is available to
realize anholonomic adiabatic algorithms [Tanaka and Miyamoto, Phys. Rev. Lett.
98, 160407 (2007)], which compose a nontrivial family of discrete adiabatic
algorithms. It is straightforward to port a standard adiabatic algorithm to an
anholonomic adiabatic one, except an introduction of a parameter |v>, which is
available to adjust the gaps of the quasienergies to control the running time
steps. In Grover's database search problem, the costs to prepare |v> for the
qualitatively different, i.e., power or exponential, running time steps are
shown to be qualitatively different. Curiously, in establishing the equivalence
between the standard quantum computation based on the circuit model and the
anholonomic adiabatic quantum computation model, it is shown that the cost for
|v> to enlarge the gaps of the eigenvalue is qualitatively negligible.Comment: 11 pages, 2 figure
Modified TAP equations for the SK spin glass
The stability of the TAP mean field equations is reanalyzed with the
conclusion that the exclusive reason for the breakdown at the spin glass
instability is an inconsistency for the value of the local susceptibility. A
new alternative approach leads to modified equations which are in complete
agreement with the original ones above the instability. Essentially altered
results below the instability are presented and the consequences for the
dynamical mean field equations are discussed.Comment: 7 pages, 2 figures, final revised version to appear in Europhys. Let
- …