238 research outputs found
Orbital entanglement and violation of Bell inequalities in mesoscopic conductors
We propose a spin-independent scheme to generate and detect two-particle
entanglement in a mesoscopic normal-superconductor system. A superconductor,
weakly coupled to the normal conductor, generates an orbitally entangled state
by injecting pairs of electrons into different leads of the normal conductor.
The entanglement is detected via violation of a Bell inequality, formulated in
terms of zero-frequency current cross-correlators. It is shown that the Bell
inequality can be violated for arbitrary strong dephasing in the normal
conductor.Comment: 4 pages, 2 figure
Quantum Noise in Multipixel Image Processing
We consider the general problem of the quantum noise in a multipixel
measurement of an optical image. We first give a precise criterium in order to
characterize intrinsic single mode and multimode light. Then, using a
transverse mode decomposition, for each type of possible linear combination of
the pixels' outputs we give the exact expression of the detection mode, i.e.
the mode carrying the noise. We give also the only way to reduce the noise in
one or several simultaneous measurements.Comment: 8 pages and 1 figur
Symmetric qubits from cavity states
Two-mode cavities can be prepared in quantum states which represent symmetric
multi-qubit states. However, the qubits are impossible to address individually
and as such cannot be independently measured or otherwise manipulated. We
propose two related schemes to coherently transfer the qubits which the cavity
state represents onto individual atoms, so that the qubits can then be
processed individually. In particular, our scheme can be combined with the
quantum cloning scheme of Simon and coworkers [C. Simon et al, PRL 84, 2993
(2000)] to allow the optimal clones which their scheme produces to be spatially
separated and individually utilized.Comment: 8 pages, 4 figures, minor typographical errors correcte
NNLO corrections to top-pair production at hadron colliders: the all-fermionic scattering channels
This is a second paper in our ongoing calculation of the
next-to-next-to-leading order (NNLO) QCD correction to the total inclusive
top-pair production cross-section at hadron colliders. In this paper we
calculate the reaction which was not considered
in our previous work on due to its phenomenologically
negligible size. We also calculate all remaining fermion-pair-initiated
partonic channels and that contribute to top-pair
production starting from NNLO. The contributions of these reactions to the
total cross-section for top-pair production at the Tevatron and LHC are small,
at the permil level. The most interesting feature of these reactions is their
characteristic logarithmic rise in the high energy limit. We compute the
constant term in the leading power behavior in this limit, and achieve
precision that is an order of magnitude better than the precision of a recent
theoretical prediction for this constant. All four partonic reactions computed
in this paper are included in our numerical program Top++. The calculation of
the NNLO corrections to the two remaining partonic reactions,
and , is ongoing.Comment: 1+16 pages; 3 figure
Optimum Small Optical Beam Displacement Measurement
We derive the quantum noise limit for the optical beam displacement of a
TEM00 mode. Using a multimodal analysis, we show that the conventional split
detection scheme for measuring beam displacement is non-optimal with 80%
efficiency. We propose a new displacement measurement scheme that is optimal
for small beam displacement. This scheme utilises a homodyne detection setup
that has a TEM10 mode local oscillator. We show that although the quantum noise
limit to displacement measurement can be surpassed using squeezed light in
appropriate spatial modes for both schemes, the TEM10 homodyning scheme
out-performs split detection for all values of squeezing.Comment: 13 pages, 7 figure
Electron Entanglement via a Quantum Dot
This Letter presents a method of electron entanglement generation. The system
under consideration is a single-level quantum dot with one input and two output
leads. The leads are arranged such that the dot is empty, single electron
tunneling is suppressed by energy conservation, and two-electron virtual
co-tunneling is allowed. This yields a pure, non-local spin-singlet state at
the output leads. Coulomb interaction is the nonlinearity essential for
entanglement generation, and, in its absence, the singlet state vanishes. This
type of electron entanglement is a four-wave mixing process analogous to the
photon entanglement generated by a Chi-3 parametric amplifier.Comment: 4 page
Hopf algebras, coproducts and symbols: an application to Higgs boson amplitudes
We show how the Hopf algebra structure of multiple polylogarithms can be used
to simplify complicated expressions for multi-loop amplitudes in perturbative
quantum field theory and we argue that, unlike the recently popularized
symbol-based approach, the coproduct incorporates information about the zeta
values. We illustrate our approach by rewriting the two-loop helicity
amplitudes for a Higgs boson plus three gluons in a simplified and compact form
involving only classical polylogarithms.Comment: 46 page
Electrical current noise of a beam splitter as a test of spin-entanglement
We investigate the spin entanglement in the superconductor-quantum dot system
proposed by Recher, Sukhorukov and Loss, coupling it to an electronic
beam-splitter. The superconductor-quantum dot entangler and the beam-splitter
are treated within a unified framework and the entanglement is detected via
current correlations. The state emitted by the entangler is found to be a
linear superposition of non-local spin-singlets at different energies, a
spin-entangled two-particle wavepacket. Colliding the two electrons in the
beam-splitter, the singlet spin-state gives rise to a bunching behavior,
detectable via the current correlators. The amount of bunching depends on the
relative positions of the single particle levels in the quantum dots and the
scattering amplitudes of the beam-splitter. The singlet spin entanglement,
insensitive to orbital dephasing but suppressed by spin dephasing, is
conveniently quantified via the Fano factors. It is found that the
entanglement-dependent contribution to the Fano factor is of the same magnitude
as the non-entangled, making an experimental detection feasible. A detailed
comparison between the current correlations of the non-local spin-singlet state
and other states, possibly emitted by the entangler, is performed. This
provides conditions for an unambiguous identification of the non-local singlet
spin entanglement.Comment: 13 pages, 8 figures, section on quantification of entanglement adde
NNLO phase space master integrals for two-to-one inclusive cross sections in dimensional regularization
We evaluate all phase space master integrals which are required for the total
cross section of generic 2 -> 1 processes at NNLO as a series expansion in the
dimensional regulator epsilon. Away from the limit of threshold production, our
expansion includes one order higher than what has been available in the
literature. At threshold, we provide expressions which are valid to all orders
in terms of Gamma functions and hypergeometric functions. These results are a
necessary ingredient for the renormalization and mass factorization of
singularities in 2 -> 1 inclusive cross sections at NNNLO in QCD.Comment: 37 pages, plus 3 ancillary files containing analytic expressions in
Maple forma
Single-valued harmonic polylogarithms and the multi-Regge limit
We argue that the natural functions for describing the multi-Regge limit of
six-gluon scattering in planar N=4 super Yang-Mills theory are the
single-valued harmonic polylogarithmic functions introduced by Brown. These
functions depend on a single complex variable and its conjugate, (w,w*). Using
these functions, and formulas due to Fadin, Lipatov and Prygarin, we determine
the six-gluon MHV remainder function in the leading-logarithmic approximation
(LLA) in this limit through ten loops, and the next-to-LLA (NLLA) terms through
nine loops. In separate work, we have determined the symbol of the four-loop
remainder function for general kinematics, up to 113 constants. Taking its
multi-Regge limit and matching to our four-loop LLA and NLLA results, we fix
all but one of the constants that survive in this limit. The multi-Regge limit
factorizes in the variables (\nu,n) which are related to (w,w*) by a
Fourier-Mellin transform. We can transform the single-valued harmonic
polylogarithms to functions of (\nu,n) that incorporate harmonic sums,
systematically through transcendental weight six. Combining this information
with the four-loop results, we determine the eigenvalues of the BFKL kernel in
the adjoint representation to NNLLA accuracy, and the MHV product of impact
factors to NNNLLA accuracy, up to constants representing beyond-the-symbol
terms and the one symbol-level constant. Remarkably, only derivatives of the
polygamma function enter these results. Finally, the LLA approximation to the
six-gluon NMHV amplitude is evaluated through ten loops.Comment: 71 pages, 2 figures, plus 10 ancillary files containing analytic
expressions in Mathematica format. V2: Typos corrected and references added.
V3: Typos corrected; assumption about single-Reggeon exchange made explici
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