Exploding electron bubbles.

Electron bubbles, used in laboratories throughout the world for probing the unusual properties of liquid helium, can be made to explode by the application of negative pressure, according to investigations by Classen et al. published last month

Decoherence in Crystals of Quantum Molecular Magnets

Decoherence in Nature has become one of the most pressing problems in physics. Many applications, including quantum information processing, depend on understanding it; and fundamental theories going beyond quantum mechanics have been suggested [1-3], where the breakdown of quantum theory appears as an 'intrinsic decoherence', mimicking environmental decoherence [4]. Such theories cannot be tested until we have a handle on ordinary environmental decoherence processes. Here we show that the theory for insulating electronic spin systems can make accurate predictions for environmental decoherence in molecular-based quantum magnets [5]. Experimental understanding of decoherence in molecular magnets has been limited by short decoherence times, which make coherent spin manipulation extremely difficult [6-9]. Here we reduce the decoherence by applying a strong magnetic field. The theory predicts the contributions to the decoherence from phonons, nuclear spins, and intermolecular dipolar interactions, for a single crystal of the Fe8 molecular magnet. In experiments we find that the decoherence time varies strongly as a function of temperature and magnetic field. The theoretical predictions are fully verified experimentally - there are no other visible decoherence sources. Our investigation suggests that the decoherence time is ultimately limited by nuclear spins, and can be extended up to about 500 microseconds, by optimizing the temperature, magnetic field, and nuclear isotopic concentrations.Comment: Submitted version including 11 pages, 3 figures and online supporting materials. Appeared on Nature Advance Online Publication (AOP) on July 20th, 2011. (http://www.nature.com/nature/journal/vaop/ncurrent/full/nature10314.html

Quantum nanomagnets and nuclear spins: an overview

This mini-review presents a simple and accessible summary on the fascinating physics of quantum nanomagnets coupled to a nuclear spin bath. These chemically synthesized systems are an ideal test ground for the theories of decoherence in mesoscopic quantum degrees of freedom, when the coupling to the environment is local and not small. We shall focus here on the most striking quantum phenomenon that occurs in such nanomagnets, namely the tunneling of their giant spin through a high anisotropy barrier. It will be shown that perturbative treatments must be discarded, and replaced by a more sophisticated formalism where the dynamics of the nanomagnet and the nuclei that couple to it are treated together from the beginning. After a critical review of the theoretical predictions and their experimental verification, we continue with a set of experimental results that challenge our present understanding, and outline the importance of filling also this last gap in the theory.Comment: 14 pages, 3 figures. Chapter in the Proceedings of the 2006 Les Houches summer school "Quantum Magnetism", ed. B. Barbara & Y. Imry, Springer (2007

Rare-earth solid-state qubits

Quantum bits (qubits) are the basic building blocks of any quantum computer. Superconducting qubits have been created with a 'top-down' approach that integrates superconducting devices into macroscopic electrical circuits [1-3], whereas electron-spin qubits have been demonstrated in quantum dots [4-6]. The phase coherence time (Tau2) and the single qubit figure of merit (QM) of superconducting and electron-spin qubits are similar -- Tau2 ~ microseconds and QM ~10-1000 below 100mK -- and it should be possible to scale-up these systems, which is essential for the development of any useful quantum computer. Bottom-up approaches based on dilute ensembles of spins have achieved much larger values of tau2 (up to tens of ms) [7, 8], but these systems cannot be scaled up, although some proposals for qubits based on 2D nanostructures should be scalable [9-11]. Here we report that a new family of spin qubits based on rare-earth ions demonstrates values of Tau2 (~ 50microseconds) and QM (~1400) at 2.5 K, which suggests that rare-earth qubits may, in principle, be suitable for scalable quantum information processing at 4He temperatures

MEAN-FIELD AND SPIN-ROTATION PHENOMENA IN FERMI SYSTEMS - THE RELATION BETWEEN THE LEGGETT-RICE AND LHUILLIER-LALOE EFFECTS

The term in the Boltzmann equation collision integral causing identical-particle spin rotation in the Lhuillier-Laloë (LL) theory of a non-degenerate quantum gas is shown to be equivalent in the dilute limit to the molecular field precessional term giving rise to the Leggett-Rice (LR) effect in a degenerate Fermi system. This equivalence is shown (a) by considering the spin-rotation term at low temperatures and (b) by deriving hydrodynamic equations valid for all temperatures from the Landau-Silin equation in the s-wave approximation. The spin-rotation factor μ resulting from (b) is found to agree with the LR value at low temperatures and with the LL value at high temperatures. The diffusion constant Do that results has the proper low temperature behaviour, including the standard mean-field correction factor ; at high temperatures Do has the LL form times a the mean-field correction factor. The importance of the missing mean-field term is illustrated by showing that it gives rise to second virial corrections to the pressure

MEAN-FIELD AND SPIN-ROTATION PHENOMENA IN FERMI SYSTEMS - THE RELATION BETWEEN THE LEGGETT-RICE AND LHUILLIER-LALOE EFFECTS

The term in the Boltzmann equation collision integral causing identical-particle spin rotation in the Lhuillier-Laloë (LL) theory of a non-degenerate quantum gas is shown to be equivalent in the dilute limit to the molecular field precessional term giving rise to the Leggett-Rice (LR) effect in a degenerate Fermi system. This equivalence is shown (a) by considering the spin-rotation term at low temperatures and (b) by deriving hydrodynamic equations valid for all temperatures from the Landau-Silin equation in the s-wave approximation. The spin-rotation factor μ resulting from (b) is found to agree with the LR value at low temperatures and with the LL value at high temperatures. The diffusion constant Do that results has the proper low temperature behaviour, including the standard mean-field correction factor ; at high temperatures Do has the LL form times a the mean-field correction factor. The importance of the missing mean-field term is illustrated by showing that it gives rise to second virial corrections to the pressure