A quantum-mechanical Maxwell's demon

A Maxwell's demon is a device that gets information and trades it in for thermodynamic advantage, in apparent (but not actual) contradiction to the second law of thermodynamics. Quantum-mechanical versions of Maxwell's demon exhibit features that classical versions do not: in particular, a device that gets information about a quantum system disturbs it in the process. In addition, the information produced by quantum measurement acts as an additional source of thermodynamic inefficiency. This paper investigates the properties of quantum-mechanical Maxwell's demons, and proposes experimentally realizable models of such devices.Comment: 13 pages, Te

The influence of Aluminium and Iron additions on Copper-Manganese-Zinc Alloys

MANY alloys in the ternary system Cu-Mn-Zn form the single phase a brass type structure and in particular, the alloys approximating to the composition 70 Cu-20 Mn-l0 Zn are sensibly white in colour.These latter alloys also possess mechanical properties and a corrosion resistance comparable with the brasses and nickel silvers. The potential of these alloys as subs-titutes for the white range of Cu-Ni alloys (nickel silvers) was originally investigated by R. S. Dean et al1 in 1945 but was not pursued commercially due, it is throught, to the limited output and excessive cost of high purity manganese at the time. Today, it is possible to purchase high purity electrolytic manganese at a price similar to copper. The potential of these alloys as substitutes nickel silver has thus become a more attractive proposition. The main aim of the present investigation was to evaluate some of the salient properties of the 70 Cu-20 Mn-10 Zn alloys and also to study the effects of separate additions of aluminium and iron as replacefnent elements for zinc

Scheme for direct measurement of a general two-qubit Hamiltonian

The construction of two-qubit gates appropriate for universal quantum computation is of enormous importance to quantum information processing. Building such gates is dependent on accurate knowledge of the interaction dynamics between two qubit systems. This letter will present a systematic method for reconstructing the full two-qubit interaction Hamiltonian through experimental measures of concurrence. This not only gives a convenient method for constructing two qubit quantum gates, but can also be used to experimentally determine various Hamiltonian parameters in physical systems. We show explicitly how this method can be employed to determine the first and second order spin-orbit corrections to the exchange coupling in quantum dots.Comment: 4 Pages, 1 Figur

Polarisation-sensitive terahertz detection by multicontact photoconductive receivers

We have developed a terahertz radiation detector that measures both the amplitude and polarization of the electric field as a function of time. The device is a three-contact photoconductive receiver designed so that two orthogonal electric-field components of an arbitrary polarized electromagnetic wave may be detected simultaneously. The detector was fabricated on Fe+ ion-implanted InP. Polarization-sensitive detection is demonstrated with an extinction ratio better than 100:1. This type of device will have immediate application in studies of birefringent and optically active materials in the far-infrared region of the spectrum.Comment: 3 pages, 3 figure

Negative entropy and information in quantum mechanics

A framework for a quantum mechanical information theory is introduced that is based entirely on density operators, and gives rise to a unified description of classical correlation and quantum entanglement. Unlike in classical (Shannon) information theory, quantum (von Neumann) conditional entropies can be negative when considering quantum entangled systems, a fact related to quantum non-separability. The possibility that negative (virtual) information can be carried by entangled particles suggests a consistent interpretation of quantum informational processes.Comment: 4 pages RevTeX, 2 figures. Expanded discussion of quantum teleportation and superdense coding, and minor corrections. To appear in Phys. Rev. Let