8,222 research outputs found
Quantum and Classical Binomial Distributions for the Charge Transmitted through Coherent Conductor
We discuss controversial results for the statistics of charge transport
through coherent conductors. Two distribution functions for the charge
transmitted was obtained previously, first by L.Levitov and G.Lesovik, [JETP
Letters Vol.55 p.555 (1992)] and the other initially by the same authors [ibid.
Vol.58 p.230 (1993)], and later the result was reproduced by several authors.
The latter distribution functions actually coincides with classical binomial
distribution (though obtained purely quantum mechanically) former (result of
1992) is different and we call it here quantum binomial distribution. The two
distribution function represent two opposite universal limits - one is purely
quantum, where interference is important, and the other is semiclassical, where
interference is smeared out. We show, that high order charge correlators,
determined by the either distribution functions, can all be measured in
different setups. The high order current correlators, starting the third order,
reveal (missed in previous studies) special oscillating frequency dependence on
the scale of the inverted time flight from the obstacle to the measuring point.
Depending on setup, the oscillating terms give substantially different
contributions.Comment: 4 pages; english versio
Quantum measurement of coherence in coupled quantum dots
We describe the conditional and unconditional dynamics of two coupled quantum
dots when one dot is subjected to a measurement of its occupation number using
a single electron transistor (SET). The measurement is made when the bare
tunneling rate though the SET is changed by the occupation number of one of the
dots. We show that there is a difference between the time scale for the
measurement-induced decoherence between the localized states of the dots and
the time scale on which the system becomes localized due to the measurement. A
comparison between theory and current experiments is made.Comment: 12 pages, 7 figure
Elementary analysis of the special relativistic combination of velocities, Wigner rotation, and Thomas precession
The purpose of this paper is to provide an elementary introduction to the
qualitative and quantitative results of velocity combination in special
relativity, including the Wigner rotation and Thomas precession. We utilize
only the most familiar tools of special relativity, in arguments presented at
three differing levels: (1) utterly elementary, which will suit a first course
in relativity; (2) intermediate, to suit a second course; and (3) advanced, to
suit higher level students. We then give a summary of useful results, and
suggest further reading in this often obscure field.Comment: V1: 25 pages, 6 figures; V2: 22 pages, 5 figures. The revised version
is shortened and the arguments streamlined. Minor changes in notation and
figures. This version matches the published versio
Massive Neutrinos and (Heterotic) String Theory
String theories in principle address the origin and values of the quark and
lepton masses. Perhaps the small values of neutrino masses could be explained
generically in string theory even if it is more difficult to calculate
individual values, or perhaps some string constructions could be favored by
generating small neutrino masses. We examine this issue in the context of the
well-known three-family standard-like Z_3 heterotic orbifolds, where the theory
is well enough known to construct the corresponding operators allowed by string
selection rules, and analyze the D- and F-flatness conditions. Surprisingly, we
find that a simple see-saw mechanism does not arise. It is not clear whether
this is a property of this construction, or of orbifolds more generally, or of
string theory itself. Extended see-saw mechanisms may be allowed; more analysis
will be needed to settle that issue. We briefly speculate on their form if
allowed and on the possibility of alternatives, such as small Dirac masses and
triplet see-saws. The smallness of neutrino masses may be a powerful probe of
string constructions in general. We also find further evidence that there are
only 20 inequivalent models in this class, which affects the counting of string
vacua.Comment: 18 pages in RevTeX format. Single-column postscript version available
at http://sage.hep.upenn.edu/~bnelson/singpre.p
Many-body spin related phenomena in ultra-low-disorder quantum wires
Zero length quantum wires (or point contacts) exhibit unexplained conductance
structure close to 0.7 X 2e^2/h in the absence of an applied magnetic field. We
have studied the density- and temperature-dependent conductance of
ultra-low-disorder GaAs/AlGaAs quantum wires with nominal lengths l=0 and 2 mu
m, fabricated from structures free of the disorder associated with modulation
doping. In a direct comparison we observe structure near 0.7 X 2e^2/h for l=0
whereas the l=2 mu m wires show structure evolving with increasing electron
density to 0.5 X 2e^2/h in zero magnetic field, the value expected for an ideal
spin-split sub-band. Our results suggest the dominant mechanism through which
electrons interact can be strongly affected by the length of the 1D region.Comment: 5 Pages, 4 figure
Is there a renormalization of the 1D conductance in Luttinger Liquid model?
Properties of 1D transport strongly depend on the proper choice of boundary
conditions. It has been frequently stated that the Luttinger Liquid (LL)
conductance is renormalized by the interaction as . To
contest this result I develop a model of 1D LL wire with the interaction
switching off at the infinities. Its solution shows that there is no
renormalization of the universal conductance while the electrons have a free
behavior in the source and drain reservoirs.Comment: 5 pages, RevTex 2.0, attempted repair of tex error
Topological Insulators
Topological insulators are electronic materials that have a bulk band gap
like an ordinary insulator, but have protected conducting states on their edge
or surface. The 2D topological insulator is a quantum spin Hall insulator,
which is a close cousin of the integer quantum Hall state. A 3D topological
insulator supports novel spin polarized 2D Dirac fermions on its surface. In
this Colloquium article we will review the theoretical foundation for these
electronic states and describe recent experiments in which their signatures
have been observed. We will describe transport experiments on HgCdTe quantum
wells that demonstrate the existence of the edge states predicted for the
quantum spin Hall insulator. We will then discuss experiments on Bi_{1-x}Sb_x,
Bi_2 Se_3, Bi_2 Te_3 and Sb_2 Te_3 that establish these materials as 3D
topological insulators and directly probe the topology of their surface states.
We will then describe exotic states that can occur at the surface of a 3D
topological insulator due to an induced energy gap. A magnetic gap leads to a
novel quantum Hall state that gives rise to a topological magnetoelectric
effect. A superconducting energy gap leads to a state that supports Majorana
fermions, and may provide a new venue for realizing proposals for topological
quantum computation. We will close by discussing prospects for observing these
exotic states, a well as other potential device applications of topological
insulators.Comment: 23 pages, 20 figures, Published versio
Error Rate of the Kane Quantum Computer CNOT Gate in the Presence of Dephasing
We study the error rate of CNOT operations in the Kane solid state quantum
computer architecture. A spin Hamiltonian is used to describe the system.
Dephasing is included as exponential decay of the off diagonal elements of the
system's density matrix. Using available spin echo decay data, the CNOT error
rate is estimated at approsimately 10^{-3}.Comment: New version includes substantial additional data and merges two old
figures into one. (12 pages, 6 figures
Charge qubits in semiconductor quantum computer architectures: Tunnel coupling and decoherence
We consider charge qubits based on shallow donor electron states in silicon
and coupled quantum dots in GaAs. Specifically, we study the feasibility of
P charge qubits in Si, focusing on single qubit properties in terms of
tunnel coupling between the two phosphorus donors and qubit decoherence caused
by electron-phonon interaction. By taking into consideration the multi-valley
structure of the Si conduction band, we show that inter-valley quantum
interference has important consequences for single-qubit operations of P
charge qubits. In particular, the valley interference leads to a
tunnel-coupling strength distribution centered around zero. On the other hand,
we find that the Si bandstructure does not dramatically affect the
electron-phonon coupling and consequently, qubit coherence. We also critically
compare charge qubit properties for Si:P and GaAs double quantum dot
quantum computer architectures.Comment: 10 pages, 3 figure
- …