47 research outputs found
Causality - Complexity - Consistency: Can Space-Time Be Based on Logic and Computation?
The difficulty of explaining non-local correlations in a fixed causal
structure sheds new light on the old debate on whether space and time are to be
seen as fundamental. Refraining from assuming space-time as given a priori has
a number of consequences. First, the usual definitions of randomness depend on
a causal structure and turn meaningless. So motivated, we propose an intrinsic,
physically motivated measure for the randomness of a string of bits: its length
minus its normalized work value, a quantity we closely relate to its Kolmogorov
complexity (the length of the shortest program making a universal Turing
machine output this string). We test this alternative concept of randomness for
the example of non-local correlations, and we end up with a reasoning that
leads to similar conclusions as in, but is conceptually more direct than, the
probabilistic view since only the outcomes of measurements that can actually
all be carried out together are put into relation to each other. In the same
context-free spirit, we connect the logical reversibility of an evolution to
the second law of thermodynamics and the arrow of time. Refining this, we end
up with a speculation on the emergence of a space-time structure on bit strings
in terms of data-compressibility relations. Finally, we show that logical
consistency, by which we replace the abandoned causality, it strictly weaker a
constraint than the latter in the multi-party case.Comment: 17 pages, 16 figures, small correction
Geometric Phase: a Diagnostic Tool for Entanglement
Using a kinematic approach we show that the non-adiabatic, non-cyclic,
geometric phase corresponding to the radiation emitted by a three level cascade
system provides a sensitive diagnostic tool for determining the entanglement
properties of the two modes of radiation. The nonunitary, noncyclic path in the
state space may be realized through the same control parameters which control
the purity/mixedness and entanglement. We show analytically that the geometric
phase is related to concurrence in certain region of the parameter space. We
further show that the rate of change of the geometric phase reveals its
resilience to fluctuations only for pure Bell type states. Lastly, the
derivative of the geometric phase carries information on both purity/mixedness
and entanglement/separability.Comment: 13 pages 6 figure
Distinguishability measures between ensembles of quantum states
A quantum ensemble is a set of quantum states each
occurring randomly with a given probability. Quantum ensembles are necessary to
describe situations with incomplete a priori information, such as the output of
a stochastic quantum channel (generalized measurement), and play a central role
in quantum communication. In this paper, we propose measures of distance and
fidelity between two quantum ensembles. We consider two approaches: the first
one is based on the ability to mimic one ensemble given the other one as a
resource and is closely related to the Monge-Kantorovich optimal transportation
problem, while the second one uses the idea of extended-Hilbert-space (EHS)
representations which introduce auxiliary pointer (or flag) states. Both types
of measures enjoy a number of desirable properties. The Kantorovich measures,
albeit monotonic under deterministic quantum operations, are not monotonic
under generalized measurements. In contrast, the EHS measures are. We present
operational interpretations for both types of measures. We also show that the
EHS fidelity between ensembles provides a novel interpretation of the fidelity
between mixed states--the latter is equal to the maximum of the fidelity
between all pure-state ensembles whose averages are equal to the mixed states
being compared. We finally use the new measures to define distance and fidelity
for stochastic quantum channels and positive operator-valued measures (POVMs).
These quantities may be useful in the context of tomography of stochastic
quantum channels and quantum detectors.Comment: 31 pages, typos correcte
Generation of 40ns laser pulses by a diode-pumped passively Q-switched Tm-Ho:YLF laser
We demonstrate for the first time generation of short pulses from a repetitively passively Q-switched Ho-laser that is diode-pumped near 800 nm by codoping with Tm-ions. The laser material used is the well-known fluoride crystal Tm,Ho:LiYF4 and polycrystalline Cr:ZnSe was employed as a saturable absorber. The maximum peak power achieved was ~640 W and the emission wavelength was ~2050 nm
Majorization relations and entanglement generation in a beam splitter
We prove that a beam splitter, one of the most common optical components,
fulfills several classes of majorization relations, which govern the amount of
quantum entanglement that it can generate. First, we show that the state
resulting from k photons impinging on a beam splitter majorizes the
corresponding state with any larger photon number k'>k, implying that the
entanglement monotonically grows with k. Then, we examine parametric
infinitesimal majorization relations as a function of the beam-splitter
transmittance, and find that there exists a parameter region where majorization
is again fulfilled, implying a monotonic increase of entanglement by moving
towards a balanced beam splitter. We also identify regions with a majorization
default, where the output states become incomparable. In this latter situation,
we find examples where catalysis may nevertheless be used in order to recover
majorization. The catalyst states can be as simple as a path-entangled
single-photon state or a two-mode vacuum squeezed state