September 2011

Abstracts of the QSIT Lunch Seminar, September 1, 2011

The thermodynamic meaning of negative entropy

Johan Aarberg, Institute for Theoretical Physics, ETH Zurich

Landauer’s principle states that the erasure of data stored in a system has an inherent work cost and therefore dissipates heat. However, this standard formulation does not take into account the general case where an observer may have quantum information about the system to be erased, for instance by means of a quantum memory entangled with the system. Our main result is that the work cost of erasure is determined by the entropy of the system, conditioned on the quantum information the observer has about it. This gives a direct thermodynamic significance to conditional entropies, originally introduced in information theory. Since conditional entropies can become negative in the quantum case, an observer who is strongly correlated with a system may gain work while erasing it, thereby cooling the environment.

Low-Temperature Nuclear Spin Relaxation in an All-Electrical Lateral Spin Transport Device

Dominikus Kölbl, Quantum Coherence Lab, University of Basel

Using all-electrical Fe/GaAs lateral spin-injection devices operated in a non-local geometry, we study nuclear spin relaxation in a one micron thick n-doped GaAs epilayer as a function of temperature in a dilution refrigerator, investigating unprecedented temperatures well below 1 K. We create a dynamic nuclear spin polarization via the hyperfine coupling using spin polarized electrons injected from surface Fe bars. Nuclear spin signatures in non-local electron transport include a depolarization peak centred around zero magnetic field when the field is applied along the Fe injector bars. Further, in the Hanle geometry satellite peaks appear away from zero field. Using these signatures, we investigate the nuclear spin relaxation time as a function of temperature and compare it with the Korringa law which is known to be applicable in metallic systems. In addition,we observe features originating from inhomogeneities of both the electron- and nuclear-spin polarization.