475 research outputs found
Spin state readout by quantum jump technique: for the purpose of quantum computing
Utilizing the Pauli-blocking mechanism we show that shining circular
polarized light on a singly-charged quantum dot induces spin dependent
fluorescence. Employing the quantum-jump technique we demonstrate that this
resonance luminescence, due to a spin dependent optical excitation, serves as
an excellent readout mechanism for measuring the spin state of a single
electron confined to a quantum dot.Comment: 11 pages, 4 eps figure
Holstein model and Peierls instability in 1D boson-fermion lattice gases
We study an ultracold bose-fermi mixture in a one dimensional optical
lattice. When boson atoms are heavier then fermion atoms the system is
described by an adiabatic Holstein model, exhibiting a Peierls instability for
commensurate fermion filling factors. A Bosonic density wave with a wavenumber
of twice the Fermi wavenumber will appear in the quasi one-dimensional system.Comment: 5 pages, 4 figure
Monte Carlo Simulation of 2-D Quantum Gravity as Open Dynamically Triangulate Random Surfaces
We describe a Monte Carlo procedure for the simulation of dynamically
triangulate random surfaces with a boundary (topology of a disk). The algorithm
keeps the total number of triangles fixed, while the length of the boundary is
allowed to fluctuate. The algorithm works in the presence of matter fields. We
here present results for the pure gravity case. The algorithm reproduces the
theoretical expectations.Comment: LaTeX file, 16 pages, 7 LaTeX figures, preprints CERN-TH.7028/93,
MS-TPI-93-0
High-finesse optical quantum gates for electron spins in artificial molecules
A doped semiconductor double-quantum-dot molecule is proposed as a qubit
realization. The quantum information is encoded in the electron spin, thus
benefiting from the long relevant decoherence times; the enhanced flexibility
of the molecular structure allows to map the spin degrees of freedom onto the
orbital ones and vice versa, and opens the possibility for high-finesse
(conditional and unconditional) quantum gates by means of stimulated Raman
adiabatic passage.Comment: To appear in Phys. Rev. Let
On the conversion efficiency of ultracold fermionic atoms to bosonic molecules via Feshbach resonances
We explain why the experimental efficiency observed in the conversion of
ultracold Fermi gases of K and Li atoms into diatomic Bose gases
is limited to 0.5 when the Feshbach resonance sweep rate is sufficiently slow
to pass adiabatically through the Landau Zener transition but faster than ``the
collision rate'' in the gas, and increases beyond 0.5 when it is slower. The
0.5 efficiency limit is due to the preparation of a statistical mixture of two
spin-states, required to enable s-wave scattering. By constructing the
many-body state of the system we show that this preparation yields a mixture of
even and odd parity pair-states, where only even parity can produce molecules.
The odd parity spin-symmetric states must decorrelate before the constituent
atoms can further Feshbach scatter thereby increasing the conversion
efficiency; ``the collision rate'' is the pair decorrelation rate.Comment: 4 pages, 3 figures, final version accepted to Phys. Rev. Let
Quantum particle statistics on the holographic screen leads to Modified Newtonian Dynamics (MOND)
Employing a thermodynamic interpretation of gravity based on the holographic
principle and assuming underlying particle statistics, fermionic or bosonic,
for the excitations of the holographic screen leads to Modified Newtonian
Dynamics (MOND). A connection between the acceleration scale appearing in
MOND and the Fermi energy of the holographic fermionic degrees of freedom is
obtained. In this formulation the physics of MOND results from the
quantum-classical crossover in the fermionic specific heat. However, due to the
dimensionality of the screen, the formalism is general and applies to two
dimensional bosonic excitations as well. It is shown that replacing the
assumption of the equipartition of energy on the holographic screen by a
standard quantum-statistical-mechanics description wherein some of the degrees
of freedom are frozen out at low temperatures is the physical basis for the
MOND interpolating function . The interpolating function is calculated within the statistical mechanical formalism and compared to
the leading phenomenological interpolating functions, most commonly used. Based
on the statistical mechanical view of MOND, its cosmological implications are
re-interpreted: the connection between and the Hubble constant is
described as a quantum uncertainty relation; and the relationship between
and the cosmological constant is better understood physically
Many-body effects on adiabatic passage through Feshbach resonances
We theoretically study the dynamics of an adiabatic sweep through a Feshbach
resonance, thereby converting a degenerate quantum gas of fermionic atoms into
a degenerate quantum gas of bosonic dimers. Our analysis relies on a zero
temperature mean-field theory which accurately accounts for initial molecular
quantum fluctuations, triggering the association process. The structure of the
resulting semiclassical phase space is investigated, highlighting the dynamical
instability of the system towards association, for sufficiently small detuning
from resonance. It is shown that this instability significantly modifies the
finite-rate efficiency of the sweep, transforming the single-pair exponential
Landau-Zener behavior of the remnant fraction of atoms Gamma on sweep rate
alpha, into a power-law dependence as the number of atoms increases. The
obtained nonadiabaticity is determined from the interplay of characteristic
time scales for the motion of adiabatic eigenstates and for fast periodic
motion around them. Critical slowing-down of these precessions near the
instability leads to the power-law dependence. A linear power law is obtained when the initial molecular fraction is smaller than the 1/N
quantum fluctuations, and a cubic-root power law is
attained when it is larger. Our mean-field analysis is confirmed by exact
calculations, using Fock-space expansions. Finally, we fit experimental low
temperature Feshbach sweep data with a power-law dependence. While the
agreement with the experimental data is well within experimental error bars,
similar accuracy can be obtained with an exponential fit, making additional
data highly desirable.Comment: 9 pages, 9 figure
Storage qubits and their potential implementation through a semiconnductor double quantum dot
In the context of a semiconductor-based implementation of a quantum computer the idea of a quantum storage bit is presented and a possible implementation using a double-quantum-dot structure is considered. A measurement scheme using a stimulated Raman adiabatic passage is discussed
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