22 research outputs found
Long-lived qubit from three spin-1/2 atoms
A system of three spin-1/2 atoms allows the construction of a
reference-frame-free (RFF) qubit in the subspace with total angular momentum
. The RFF qubit stays coherent perfectly as long as the spins of the
three atoms are affected homogeneously. The inhomogeneous evolution of the
atoms causes decoherence, but this decoherence can be suppressed efficiently by
applying a bias magnetic field of modest strength perpendicular to the plane of
the atoms. The resulting lifetime of the RFF qubit can be many days, making RFF
qubits of this kind promising candidates for quantum information storage units.
Specifically, we examine the situation of three atoms trapped
in a -laser-generated optical lattice and find that, with
conservatively estimated parameters, a stored qubit maintains a fidelity of
0.9999 for two hours.Comment: 15 pages, 9 figures; version 2 reports a much improved analysis;
version 3 contains more details about the four-atom cas
Time-resolved measurement of Landau--Zener tunneling in different bases
A comprehensive study of the tunneling dynamics of a Bose--Einstein
condensate in a tilted periodic potential is presented. We report numerical and
experimental results on time-resolved measurements of the Landau--Zener
tunneling of ultracold atoms introduced by the tilt, which experimentally is
realized by accelerating the lattice. The use of different protocols enables us
to access the tunneling probability, numerically as well as experimentally, in
two different bases, namely, the adiabatic basis and the diabatic basis. The
adiabatic basis corresponds to the eigenstates of the lattice, and the diabatic
one to the free-particle momentum eigenstates. Our numerical and experimental
results are compared with existing two-state Landau--Zener models
Quantum Synchronization Blockade: Energy Quantization Hinders Synchronization of Identical Oscillators
Classically, the tendency towards spontaneous synchronization is strongest if the natural frequencies of the self-oscillators are as close as possible. We show that this wisdom fails in the deep quantum regime, where the uncertainty of amplitude narrows down to the level of single quanta. Under these circumstances identical self-oscillators cannot synchronize and detuning their frequencies can actually help synchronization. The effect can be understood in a simple picture: Interaction requires an exchange of energy. In the quantum regime, the possible quanta of energy are discrete. If the extractable energy of one oscillator does not exactly match the amount the second oscillator may absorb, interaction, and thereby synchronization, is blocked. We demonstrate this effect, which we coin quantum synchronization blockade, in the minimal example of two Kerr-type self-oscillators and predict consequences for small oscillator networks, where synchronization between blocked oscillators can be mediated via a detuned oscillator. We also propose concrete implementations with superconducting circuits and trapped ions. This paves the way for investigations of new quantum synchronization phenomena in oscillator networks both theoretically and experimentally.This work was financially supported by the Swiss SNF and the NCCR Quantum Science and Technology. A.N. holds a University Research Fellowship from the Royal Society and acknowledges support from the Winton Programme for the Physics of Sustainability
Wave Function Renormalization Effects in Resonantly Enhanced Tunneling
We study the time evolution of ultra-cold atoms in an accelerated optical
lattice. For a Bose- Einstein condensate with a narrow quasi-momentum
distribution in a shallow optical lattice the decay of the survival probability
in the ground band has a step-like structure. In this regime we establish a
connection between the wave function renormalization parameter Z introduced in
[Phys. Rev. Lett. 86, 2699 (2001)] to characterize non-exponential decay and
the phenomenon of resonantly enhanced tunneling, where the decay rate is peaked
for particular values of the lattice depth and the accelerating force.Comment: 12 page