18,934 research outputs found
Efficient Quantum Computation with Probabilistic Quantum Gates
With a combination of the quantum repeater and the cluster state approaches, we show that efficient quantum computation can be constructed even if all the entangling quantum gates only succeed with an arbitrarily small probability p. The required computational overhead scales efficiently both with 1/p and n, where n is the number of qubits in the computation. This approach provides an efficient way to combat noise in a class of quantum computation implementation schemes, where the dominant noise leads to probabilistic signaled errors with an error probability 1-p far beyond any threshold requirement
Trapped ion quantum computation with transverse phonon modes
We propose a scheme to implement quantum gates on any pair of trapped ions
immersed in a large linear crystal, using interaction mediated by the
transverse phonon modes. Compared with the conventional approaches based on the
longitudinal phonon modes, this scheme is much less sensitive to ion heating
and thermal motion outside of the Lamb-Dicke limit thanks to the stronger
confinement in the transverse direction. The cost for such a gain is only a
moderate increase of the laser power to achieve the same gate speed. We also
show how to realize arbitrary-speed quantum gates with transverse phonon modes
based on simple shaping of the laser pulses.Comment: 5 page
Efficient engineering of multi-atom entanglement through single-photon detections
We propose an efficient scheme to engineer multi-atom entanglement by
detecting cavity decay through single-photon detectors. In the special case of
two atoms, this scheme is much more efficient than previous probabilistic
schemes, and insensitive to randomness in the atom's position. More generally,
the scheme can be used to prepare arbitrary superpositions of multi-atom Dicke
states without the requirements of high-efficiency detection and separate
addressing of different atoms.Comment: 5 pages, 2 figure
Superfluidity of fermions with repulsive on-site interaction in an anisotropic optical lattice near a Feshbach resonance
We present a numerical study on ground state properties of a one-dimensional
(1D) general Hubbard model (GHM) with particle-assisted tunnelling rates and
repulsive on-site interaction (positive-U), which describes fermionic atoms in
an anisotropic optical lattice near a wide Feshbach resonance. For our
calculation, we utilize the time evolving block decimation (TEBD) algorithm,
which is an extension of the density matrix renormalization group and provides
a well-controlled method for 1D systems. We show that the positive-U GHM, when
hole-doped from half-filling, exhibits a phase with coexistence of
quasi-long-range superfluid and charge-density-wave orders. This feature is
different from the property of the conventional Hubbard model with positive-U,
indicating the particle-assisted tunnelling mechanism in GHM brings in
qualitatively new physics.Comment: updated with published version
Level crossing in the three-body problem for strongly interacting fermions in a harmonic trap
We present a solution of the three-fermion problem in a harmonic potential
across a Feshbach resonance. We compare the spectrum with that of the two-body
problem and show that it is energetically unfavorable for the three fermions to
occupy one lattice site rather than two. We also demonstrate the existence of
an energy level crossing in the ground state with a symmetry change of its wave
function, suggesting the possibility of a phase transition for the
corresponding many-body case.Comment: 5 pages, 6 figures, typos corrected, references adde
BCS-BEC crossover and quantum phase transition for 6Li and 40K atoms across Feshbach resonance
We systematically study the BCS-BEC crossover and the quantum phase
transition in ultracold 6Li and 40K atoms across a wide Feshbach resonance. The
background scattering lengths for 6Li and 40K have opposite signs, which lead
to very different behaviors for these two types of atoms. For 40K, both the
two-body and the many-body calculations show that the system always has two
branches of solutions: one corresponds to a deeply bound molecule state; and
the other, the one accessed by the current experiments, corresponds to a weakly
bound state with population always dominantly in the open channel. For 6Li,
there is only a unique solution with the standard crossover from the weakly
bound Cooper pairs to the deeply bound molecules as one sweeps the magnetic
field through the crossover region. Because of this difference, for the
experimentally accessible state of 40K, there is a quantum phase transition at
zero temperature from the superfluid to the normal fermi gas at the positive
detuning of the magnetic field where the s-wave scattering length passes its
zero point. For 6Li, however, the system changes continuously across the zero
point of the scattering length. For both types of atoms, we also give detailed
comparison between the results from the two-channel and the single-channel
model over the whole region of the magnetic field detuning.Comment: 7 pages, 6 figure
Three-dimensional theory for interaction between atomic ensembles and free-space light
Atomic ensembles have shown to be a promising candidate for implementations
of quantum information processing by many recently-discovered schemes. All
these schemes are based on the interaction between optical beams and atomic
ensembles. For description of these interactions, one assumed either a
cavity-QED model or a one-dimensional light propagation model, which is still
inadequate for a full prediction and understanding of most of the current
experimental efforts which are actually taken in the three-dimensional free
space. Here, we propose a perturbative theory to describe the three-dimensional
effects in interaction between atomic ensembles and free-space light with a
level configuration important for several applications. The calculations reveal
some significant effects which are not known before from the other approaches,
such as the inherent mode-mismatching noise and the optimal mode-matching
conditions. The three-dimensional theory confirms the collective enhancement of
the signal-to-noise ratio which is believed to be one of the main advantage of
the ensemble-based quantum information processing schemes, however, it also
shows that this enhancement need to be understood in a more subtle way with an
appropriate mode matching method.Comment: 16 pages, 9 figure
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