1,383 research outputs found
Overcoming a limitation of deterministic dense coding with a non-maximally entangled initial state
Under two-party deterministic dense-coding, Alice communicates (perfectly
distinguishable) messages to Bob via a qudit from a pair of entangled qudits in
pure state |Psi>. If |Psi> represents a maximally entangled state (i.e., each
of its Schmidt coefficients is sqrt(1/d)), then Alice can convey to Bob one of
d^2 distinct messages. If |Psi> is not maximally entangled, then Ji et al.
[Phys. Rev. A 73, 034307 (2006)] have shown that under the original
deterministic dense-coding protocol, in which messages are encoded by unitary
operations performed on Alice's qudit, it is impossible to encode d^2-1
messages. Encoding d^2-2 is possible; see, e.g., the numerical studies by Mozes
et al. [Phys. Rev. A 71, 012311 (2005)]. Answering a question raised by Wu et
al. [Phys. Rev. A 73, 042311 (2006)], we show that when |Psi> is not maximally
entangled, the communications limit of d^2-2 messages persists even when the
requirement that Alice encode by unitary operations on her qudit is weakened to
allow encoding by more general quantum operators. We then describe a
dense-coding protocol that can overcome this limitation with high probability,
assuming the largest Schmidt coefficient of |Psi> is sufficiently close to
sqrt(1/d). In this protocol, d^2-2 of the messages are encoded via unitary
operations on Alice's qudit, and the final (d^2-1)-th message is encoded via a
(non-trace-preserving) quantum operation.Comment: 18 pages, published versio
Deterministic dense coding and entanglement entropy
We present an analytical study of the standard two-party deterministic
dense-coding protocol, under which communication of perfectly distinguishable
messages takes place via a qudit from a pair of non-maximally entangled qudits
in pure state |S>. Our results include the following: (i) We prove that it is
possible for a state |S> with lower entanglement entropy to support the sending
of a greater number of perfectly distinguishable messages than one with higher
entanglement entropy, confirming a result suggested via numerical analysis in
Mozes et al. [Phys. Rev. A 71 012311 (2005)]. (ii) By explicit construction of
families of local unitary operators, we verify, for dimensions d = 3 and d=4, a
conjecture of Mozes et al. about the minimum entanglement entropy that supports
the sending of d + j messages, j = 2, ..., d-1; moreover, we show that the j=2
and j= d-1 cases of the conjecture are valid in all dimensions. (iii) Given
that |S> allows the sending of K messages and has the square roof of c as its
largest Schmidt coefficient, we show that the inequality c <= d/K, established
by Wu et al. [ Phys. Rev. A 73, 042311 (2006)], must actually take the form c <
d/K if K = d+1, while our constructions of local unitaries show that equality
can be realized if K = d+2 or K = 2d-1.Comment: 19 pages, 2 figures. Published versio
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The augmented message-matrix approach to deterministic dense coding theory
A method is presented for producing analytical results applicable to the
standard two-party deterministic dense coding protocol, wherein communication
of K perfectly distinguishable messages is attainable with the aid of K
selected local unitary operations on one qudit from a pair of entangled qudits
of equal dimension d in a pure state. The method utilizes the properties of a
(d^2)x(d^2) unitary matrix whose initial columns represent message states of
the system used for communication, augmented by sufficiently many additional
orthonormal column vectors so that the resulting matrix is unitary. Using the
unitarity properties of this augmented message-matrix, we produce simple proofs
of previously established results including (i) an upper bound on the value of
the square of the largest Schmidt coefficient, given by d/K, and (ii) the
impossibility of finding a pure state that can enable transmission of K=d^2-1
messages but not d^2. Additional results obtained using the method include
proofs that when K=d+1 the upper bound on the square of the largest Schmidt
coefficient (i) always reduces to at least (1/2)[1+sqrt{(d-2)/(d+2)}], and (ii)
reduces to (d-1)/d in the special case that the identity and shift operators
are two of the selected local unitaries.Comment: 16 page
A hypercyclic finite rank perturbation of a unitary operator
A unitary operator and a rank operator acting on a Hilbert space
\H are constructed such that is hypercyclic. This answers affirmatively
a question of Salas whether a finite rank perturbation of a hyponormal operator
can be supercyclic.Comment: published in Mathematische Annale
Deciphering metal-induced oxidative damages on glycated albumin structure and function
Background: Metal ions such as copper or zinc are involved in the development of neurodegenerative pathologies and metabolic diseases such as diabetes mellitus. Albumin structure and functions are impaired following metal- and glucose-mediated oxidative alterations. The aim of this study was to elucidate effects of Cu(II) and Zn(II) ions on glucose-induced modifications in albumin by focusing on glycation, aggregation, oxidation and functional aspects. Methods: Aggregation and conformational changes in albumin were monitored by spectroscopy, fluorescence and microscopy techniques. Biochemical assays such as carbonyl, thiol groups, albumin-bound Cu, fructosamine and amine group measurements were used. Cellular assays were used to gain functional information concerning antioxidant activity of oxidized albumins. Results: Both metals promoted inhibition of albumin glycation associated with an enhanced aggregation and oxidation process. Metal ions gave rise to the formation of β-amyloid type aggregates in albumin exhibiting impaired antioxidant properties and toxic activity to murine microglia cells (BV2). The differential efficiency of both metal ions to inhibit albumin glycation, to promote aggregation and to affect cellular physiology is compared. Conclusions and general significance: Considering the key role of oxidized protein in pathology complications, glycation-mediated and metal ion-induced impairment of albumin properties might be important parameters to be followed and fought. © 2013 Elsevier B.V
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