37 research outputs found
Non-Abelian off-diagonal geometric phases in nano-engineered four-qubit systems
The concept of off-diagonal geometric phase (GP) has been introduced in order
to recover interference information about the geometry of quantal evolution
where the standard GPs are not well-defined. In this Letter, we propose a
physical setting for realizing non-Abelian off-diagonal GPs. The proposed
non-Abelian off-diagonal GPs can be implemented in a cyclic chain of four
qubits with controllable nearest-neighbor interactions. Our proposal seems to
be within reach in various nano-engineered systems and therefore opens up for
first experimental test of the non-Abelian off-diagonal GP.Comment: Some changes, journal reference adde
Non-Abelian quantum holonomy of hydrogen-like atoms
We study the Uhlmann holonomy [Rep. Math. Phys. 24, 229 (1986)] of quantum
states for hydrogen-like atoms where the intrinsic spin and orbital angular
momentum are coupled by the spin-orbit interaction and subject to a slowly
varying magnetic field. We show that the holonomy for the orbital angular
momentum and spin subsystems is non-Abelian, while the holonomy of the whole
system is Abelian. Quantum entanglement in the states of the whole system is
crucially related to the non-Abelian gauge structure of the subsystems. We
analyze the phase of the Wilson loop variable associated with the Uhlmann
holonomy, and find a relation between the phase of the whole system with
corresponding marginal phases. Based on the result for the model system we
provide evidence that the phase of the Wilson loop variable and the mixed-state
geometric phase [E. Sj\"oqvist {\it et al.} Phys. Rev. Lett. 85, 2845 (2000)]
are in general inequivalent.Comment: Shortened version; journal reference adde
Transition-metal dimers and physical limits on magnetic anisotropy
Recent advances in nanoscience have raised interest in the minimum bit size
required for classical information storage, i.e. for bistability with
suppressed quantum tunnelling and energy barriers that exceed ambient
temperatures. In the case of magnetic information storage much attention has
centred on molecular magnets[1] with bits consisting of ~ 100 atoms, magnetic
uniaxial anisotropy energy barriers ~ 50 K, and very slow relaxation at low
temperatures. In this article we draw attention to the remarkable magnetic
properties of some transition metal dimers which have energy barriers
approaching ~ 500 K with only two atoms. The spin dynamics of these ultra small
nanomagnets is strongly affected by a Berry phase which arises from
quasi-degeneracies at the electronic Highest Occupied Molecular Orbital (HOMO)
energy. In a giant spin-approximation, this Berry phase makes the effective
reversal barrier thicker. [1] Gatteschi, D., Sessoli, R. & Villain, J.
Molecular Nanomagnets. (Oxford, New York 2006).Comment: 14 pages, 1 figur
Measuring the free fall of antihydrogen
After the first production of cold antihydrogen by the ATHENA and ATRAP experiments ten years ago, new second-generation experiments are aimed at measuring the fundamental properties of this anti-atom. The goal of AEGIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) is to test the weak equivalence principle by studying the gravitational interaction between matter and antimatter with a pulsed, cold antihydrogen beam. The experiment is currently being assembled at CERN's Antiproton Decelerator. In AEGIS, antihydrogen will be produced by charge exchange of cold antiprotons with positronium excited to a high Rydberg state (n > 20). An antihydrogen beam will be produced by controlled acceleration in an electric-field gradient (Stark acceleration). The deflection of the horizontal beam due to its free fall in the gravitational field of the earth will be measured with a moire deflectometer. Initially, the gravitational acceleration will be determined to a precision of 1%, requiring the detection of about 105 antihydrogen atoms. In this paper, after a general description, the present status of the experiment will be reviewed
Exploring the WEP with a pulsed cold beam of antihydrogen
The AEGIS experiment, currently being set up at the Antiproton Decelerator at CERN, has the objective of studying the free fall of antimatter in the Earth's gravitational field by means of a pulsed cold atomic beam of antihydrogen atoms. Both duration of free fall and vertical displacement of the horizontally emitted atoms will be measured, allowing a first test of the WEP with antimatter