492,465 research outputs found
Strong Converse for Identification via Quantum Channels
In this paper we present a simple proof of the strong converse for
identification via discrete memoryless quantum channels, based on a novel
covering lemma. The new method is a generalization to quantum communication
channels of Ahlswede's recently discovered appoach to classical channels. It
involves a development of explicit large deviation estimates to the case of
random variables taking values in selfadjoint operators on a Hilbert space.
This theory is presented separately in an appendix, and we illustrate it by
showing its application to quantum generalizations of classical hypergraph
covering problems.Comment: 11 pages, LaTeX2e, requires IEEEtran2e.cls. Some errors and omissions
corrected, references update
Quantum and Classical Message Identification via Quantum Channels
We discuss concepts of message identification in the sense of Ahlswede and
Dueck via general quantum channels, extending investigations for classical
channels, initial work for classical-quantum (cq) channels and "quantum
fingerprinting".
We show that the identification capacity of a discrete memoryless quantum
channel for classical information can be larger than that for transmission;
this is in contrast to all previously considered models, where it turns out to
equal the common randomness capacity (equals transmission capacity in our
case): in particular, for a noiseless qubit, we show the identification
capacity to be 2, while transmission and common randomness capacity are 1.
Then we turn to a natural concept of identification of quantum messages (i.e.
a notion of "fingerprint" for quantum states). This is much closer to quantum
information transmission than its classical counterpart (for one thing, the
code length grows only exponentially, compared to double exponentially for
classical identification). Indeed, we show how the problem exhibits a nice
connection to visible quantum coding. Astonishingly, for the noiseless qubit
channel this capacity turns out to be 2: in other words, one can compress two
qubits into one and this is optimal. In general however, we conjecture quantum
identification capacity to be different from classical identification capacity.Comment: 18 pages, requires Rinton-P9x6.cls. On the occasion of Alexander
Holevo's 60th birthday. Version 2 has a few theorems knocked off: Y Steinberg
has pointed out a crucial error in my statements on simultaneous ID codes.
They are all gone and replaced by a speculative remark. The central results
of the paper are all unharmed. In v3: proof of Proposition 17 corrected,
without change of its statemen
Identification via Quantum Channels in the Presence of Prior Correlation and Feedback
Continuing our earlier work (quant-ph/0401060), we give two alternative
proofs of the result that a noiseless qubit channel has identification capacity
2: the first is direct by a "maximal code with random extension" argument, the
second is by showing that 1 bit of entanglement (which can be generated by
transmitting 1 qubit) and negligible (quantum) communication has identification
capacity 2.
This generalises a random hashing construction of Ahlswede and Dueck: that 1
shared random bit together with negligible communication has identification
capacity 1.
We then apply these results to prove capacity formulas for various quantum
feedback channels: passive classical feedback for quantum-classical channels, a
feedback model for classical-quantum channels, and "coherent feedback" for
general channels.Comment: 19 pages. Requires Rinton-P9x6.cls. v2 has some minor errors/typoes
corrected and the claims of remark 22 toned down (proofs are not so easy
after all). v3 has references to simultaneous ID coding removed: there were
necessary changes in quant-ph/0401060. v4 (final form) has minor correction
Formation of black-hole X-ray binaries in globular clusters
Inspired by the recent identification of the first candidate BH-WD X-ray
binaries, where the compact accretors may be stellar-mass black hole candidates
in extragalactic globular clusters, we explore how such binaries could be
formed in a dynamical environment. We provide analyses of the formation rates
via well known formation channels like binary exchange and physical collisions
and propose that the only possibility to form BH-WD binaries is via coupling
these usual formation channels with subsequent hardening and/or triple
formation. Indeed, we find that the most important mechanism to make a BH-WD
X-ray binary from an initially dynamically formed BH-WD binary is triple
induced mass transfer via the Kozai mechanism. Even using the most optimistic
estimates for the formation rates, we cannot match the observationally inferred
production rates if black holes undergo significant evaporation from the
cluster or form a completely detached subcluster of black holes. We estimate
that at least 1% of all formed black holes, or presumably 10% of the black
holes present in the core now, must be involved in interactions with the rest
of the core stellar population.Comment: 10 pages, 2 figures, submitted to Ap
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