2,245 research outputs found
Superpositions of SU(3) coherent states via a nonlinear evolution
We show that a nonlinear Hamiltonian evolution can transform an SU(3)
coherent state into a superposition of distinct SU(3) coherent states, with a
superposition of two SU(2) coherent states presented as a special case. A phase
space representation is depicted by projecting the multi-dimensional -symbol
for the state to a spherical subdomain of the coset space. We discuss
realizations of this nonlinear evolution in the contexts of nonlinear optics
and Bose--Einstein condensates
Weak non-linearities and cluster states
We propose a scalable approach to building cluster states of matter qubits
using coherent states of light. Recent work on the subject relies on the use of
single photonic qubits in the measurement process. These schemes have a low
initial success probability and low detector efficiencies cause a serious
blowup in resources. In contrast, our approach uses continuous variables and
highly efficient measurements. We present a two-qubit scheme, with a simple
homodyne measurement system yielding an entangling operation with success
probability 1/2. Then we extend this to a three-qubit interaction, increasing
this probability to 3/4. We discuss the important issues of the overhead cost
and the time scaling, showing how these can be vastly improved with access to
this new probability range.Comment: 5 pages, to appear in Phys. Rev.
Study of a model for the folding of a small protein
We describe the results obtained from an improved model for protein folding.
We find that a good agreement with the native structure of a 46 residue long,
five-letter protein segment is obtained by carefully tuning the parameters of
the self-avoiding energy. In particular we find an improved free-energy
profile. We also compare the efficiency of the multidimensional replica
exchange method with the widely used parallel tempering.Comment: typos corrected, one figure adde
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
Entanglement detection from interference fringes in atom-photon systems
A measurement scheme of atomic qubits pinned at given positions is studied by
analyzing the interference pattern obtained when they emit photons
spontaneously. In the case of two qubits, a well-known relation is revisited,
in which the interference visibility is equal to the concurrence of the state
in the infinite spatial separation limit of the qubits. By taking into account
the super-radiant and sub-radiant effects, it is shown that a state tomography
is possible when the qubit spatial separation is comparable to the wavelength
of the atomic transition. In the case of three qubits, the relations between
various entanglement measures and the interference visibility are studied,
where the visibility is defined from the two-qubit case. A qualitative
correspondence among these entanglement relations is discussed. In particular,
it is shown that the interference visibility is directly related to the maximal
bipartite negativity.Comment: 12 pages, 2 figures, published versio
Practical effects in the preparation of cluster states using weak non-linearities
We discuss experimental effects in the implementation of a recent scheme for
performing bus mediated entangling operations between qubits. Here a bus mode,
a strong coherent state, successively undergoes weak Kerr-type non-linear
interactions with qubits. A quadrature measurement on the bus then projects the
qubits into an entangled state. This approach has the benefit that entangling
gates are non-destructive, may be performed non-locally, and there is no need
for efficient single photon detection. In this paper we examine practical
issues affecting its experimental implementation. In particular, we analyze the
effects of post-selection errors, qubit loss, bus loss, mismatched coupling
rates and mode-mismatch. We derive error models for these effects and relate
them to realistic fault-tolerant thresholds, providing insight into realistic
experimental requirements.Comment: 8 pages, 5 figure
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
Tunneling and rattling in clathrate crystal
We present tunneling and rattling motions of an off-center guest atom in cage
referring to a clathrate crystal La_3Pd_{20}Ge_6. The elastic constant C_{44}
of La_3Pd_{20}Ge_6 shows a Debye-type dispersion around 20 K obeying a
relaxation time tau = tau_0exp(E/k_{B}T) with an attempt time tau_0 =
2.0*10^{-12} sec and an activation energy E = 197 K. At low temperatures below
3 K down to 20 mK, the C_{44} shows a softening of C_{44} =
C_{44}^0(T-T_C^0)/(T-Theta) with T_C^0 = -337.970 mK and Theta = -338.044 mK.
These facts are attributed to two different types of the off-center motions
with Gamma_5 symmetry in 4a-site cage of La_3Pd_{20}Ge_6, a thermally activated
rattling motion over the potential hill and a tunneling motion through the
potential hill at low temperatures.Comment: 5 pages, 4 figures, to be published Phys. Rev.
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