2,253 research outputs found
Quantum slow motion
We simulate the center of mass motion of cold atoms in a standing, amplitude
modulated, laser field as an example of a system that has a classical mixed
phase-space. We show a simple model to explain the momentum distribution of the
atoms taken after any distinct number of modulation cycles. The peaks
corresponding to a classical resonance move towards smaller velocities in
comparison to the velocities of the classical resonances. We explain this by
showing that, for a wave packet on the classical resonances, we can replace the
complicated dynamics in the quantum Liouville equation in phase-space by the
classical dynamics in a modified potential. Therefore we can describe the
quantum mechanical motion of a wave packet on a classical resonance by a purely
classical motion
Bounds on quantum communication via Newtonian gravity
Newtonian gravity yields specific observable consequences, the most striking
of which is the emergence of a force. In so far as communication can
arise via such interactions between distant particles, we can ask what would be
expected for a theory of gravity that only allows classical communication. Many
heuristic suggestions for gravity-induced decoherence have this restriction
implicitly or explicitly in their construction. Here we show that communication
via a force has a minimum noise induced in the system when the
communication cannot convey quantum information, in a continuous time analogue
to Bell's inequalities. Our derived noise bounds provide tight constraints from
current experimental results on any theory of gravity that does not allow
quantum communication.Comment: 13 pages, 1 figur
Universal state inversion and concurrence in arbitrary dimensions
Wootters [Phys. Rev. Lett. 80, 2245 (1998)] has given an explicit formula for
the entanglement of formation of two qubits in terms of what he calls the
concurrence of the joint density operator. Wootters's concurrence is defined
with the help of the superoperator that flips the spin of a qubit. We
generalize the spin-flip superoperator to a "universal inverter," which acts on
quantum systems of arbitrary dimension, and we introduce the corresponding
concurrence for joint pure states of (D1 X D2) bipartite quantum systems. The
universal inverter, which is a positive, but not completely positive
superoperator, is closely related to the completely positive universal-NOT
superoperator, the quantum analogue of a classical NOT gate. We present a
physical realization of the universal-NOT superoperator.Comment: Revtex, 25 page
Fast and robust two-qubit gates for scalable ion trap quantum computing
We propose a new concept for a two-qubit gate operating on a pair of trapped
ions based on laser coherent control techniques. The gate is insensitive to the
temperature of the ions, works also outside the Lamb-Dicke regime, requires no
individual addressing by lasers, and can be orders of magnitude faster than the
trap period
Generalized uncertainty relations: Theory, examples, and Lorentz invariance
The quantum-mechanical framework in which observables are associated with
Hermitian operators is too narrow to discuss measurements of such important
physical quantities as elapsed time or harmonic-oscillator phase. We introduce
a broader framework that allows us to derive quantum-mechanical limits on the
precision to which a parameter---e.g., elapsed time---may be determined via
arbitrary data analysis of arbitrary measurements on identically prepared
quantum systems. The limits are expressed as generalized Mandelstam-Tamm
uncertainty relations, which involve the operator that generates displacements
of the parameter---e.g., the Hamiltonian operator in the case of elapsed time.
This approach avoids entirely the problem of associating a Hermitian operator
with the parameter. We illustrate the general formalism, first, with
nonrelativistic uncertainty relations for spatial displacement and momentum,
harmonic-oscillator phase and number of quanta, and time and energy and,
second, with Lorentz-invariant uncertainty relations involving the displacement
and Lorentz-rotation parameters of the Poincar\'e group.Comment: 39 pages of text plus one figure; text formatted in LaTe
Mesoscopic one-way channels for quantum state transfer via the Quantum Hall Effect
We show that the one-way channel formalism of quantum optics has a physical
realisation in electronic systems. In particular, we show that magnetic edge
states form unidirectional quantum channels capable of coherently transporting
electronic quantum information. Using the equivalence between one-way photonic
channels and magnetic edge states, we adapt a proposal for quantum state
transfer to mesoscopic systems using edge states as a quantum channel, and show
that it is feasible with reasonable experimental parameters. We discuss how
this protocol may be used to transfer information encoded in number, charge or
spin states of quantum dots, so it may prove useful for transferring quantum
information between parts of a solid-state quantum computer.Comment: 4 pages, 3 figure
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