7,749 research outputs found
Relation between classical communication capacity and entanglement capability for two-qubit unitary operations
Two-qubit operations may be characterized by their capacities for
communication, both with and without free entanglement, and their capacity for
creating entanglement. We establish a set of inequalities that give an ordering
to the capacities of two-qubit unitary operations. Specifically, we show that
the capacities for entanglement creation and bidirectional communication
without entanglement assistance are at least as great as half the bidirectional
communication capacity with entanglement assistance. In addition, we show that
the bidirectional communication that can be performed using an ensemble may be
increased via a two-qubit unitary operation by twice the operation's capacity
for entanglement.Comment: 12 pages, published version plus minor correction
Interconvertibility of single-rail optical qubits
We show how to convert between partially coherent superpositions of a single
photon with the vacuum using linear optics and postselection based on homodyne
measurements. We introduce a generalized quantum efficiency for such states and
show that any conversion that decreases this quantity is possible. We also
prove that our scheme is optimal by showing that no linear optical scheme with
generalized conditional measurements, and with one single-rail qubit input can
improve the generalized efficiency.Comment: 3 pages, 2 figure
Efficiency limits for linear optical processing of single photons and single-rail qubits
We analyze the problem of increasing the efficiency of single-photon sources
or single-rail photonic qubits via linear optical processing and destructive
conditional measurements. In contrast to previous work we allow for the use of
coherent states and do not limit to photon-counting measurements. We conjecture
that it is not possible to increase the efficiency, prove this conjecture for
several important special cases, and provide extensive numerical results for
the general case.Comment: 10 pages, 4 figure
Simulating Quantum Dynamics On A Quantum Computer
We present efficient quantum algorithms for simulating time-dependent
Hamiltonian evolution of general input states using an oracular model of a
quantum computer. Our algorithms use either constant or adaptively chosen time
steps and are significant because they are the first to have time-complexities
that are comparable to the best known methods for simulating time-independent
Hamiltonian evolution, given appropriate smoothness criteria on the Hamiltonian
are satisfied. We provide a thorough cost analysis of these algorithms that
considers discretizion errors in both the time and the representation of the
Hamiltonian. In addition, we provide the first upper bounds for the error in
Lie-Trotter-Suzuki approximations to unitary evolution operators, that use
adaptively chosen time steps.Comment: Paper modified from previous version to enhance clarity. Comments are
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Dudley Knox Library, Naval Postgraduate School Postcard (NPS student heads home from the library, with a stack of books, ca. 1971)
Some things never change! A student heads home from the library, ca. 1971. Today's students use a rich collection of electronic resources, including ejournals, databases, and...books
Equivalence between two-mode spin squeezed states and pure entangled states with equal spin
We prove that a pure entangled state of two subsystems with equal spin is
equivalent to a two-mode spin-squeezed state under local operations except for
a set of bipartite states with measure zero, and we provide a counterexample to
the generalization of this result to two subsystems of unequal spin.Comment: 6 pages, no figure
Solovay-Kitaev Decomposition Strategy for Single-Qubit Channels
Inspired by the Solovay-Kitaev decomposition for approximating unitary
operations as a sequence of operations selected from a universal quantum
computing gate set, we introduce a method for approximating any single-qubit
channel using single-qubit gates and the controlled-NOT (CNOT). Our approach
uses the decomposition of the single-qubit channel into a convex combination of
"quasiextreme" channels. Previous techniques for simulating general
single-qubit channels would require as many as 20 CNOT gates, whereas ours only
needs one, bringing it within the range of current experiments
Stating Appointment Costs in SMS Reminders Reduces Missed Hospital Appointments: Findings from Two Randomised Controlled Trials
BACKGROUND:
Missed hospital appointments are a major cause of inefficiency worldwide. Healthcare providers are increasingly using Short Message Service reminders to reduce 'Did Not Attend' (DNA) rates. Systematic reviews show that sending such reminders is effective, but there is no evidence on whether their impact is affected by their content. Accordingly, we undertook two randomised controlled trials that tested the impact of rephrasing appointment reminders on DNA rates in the United Kingdom.
TRIAL METHODS:
Participants were outpatients with a valid mobile telephone number and an outpatient appointment between November 2013 and January 2014 (Trial One, 10,111 participants) or March and May 2014 (Trial Two, 9,848 participants). Appointments were randomly allocated to one of four reminder messages, which were issued five days in advance. Message assignment was then compared against appointment outcomes (appointment attendance, DNA, cancellation by patient).
RESULTS:
In Trial One, a message including the cost of a missed appointment to the health system produced a DNA rate of 8.4%, compared to 11.1% for the existing message (OR 0.74, 95% CI 0.61-0.89, P<0.01). Trial Two replicated this effect (DNA rate 8.2%), but also found that expressing the same concept in general terms was significantly less effective (DNA rate 9.9%, OR 1.22, 95% CI 1.00-1.48, P<0.05). Moving from the existing reminder to the more effective costs message would result in 5,800 fewer missed appointments per year in the National Health Service Trust in question, at no additional cost. The study's main limitations are that it took place in a single location in England, and that it required accurate phone records, which were only obtained for 20% of eligible patients. We conclude that missed appointments can be reduced, for no additional cost, by introducing persuasive messages to appointment reminders. Future studies could examine the impact of varying reminder messages in other health systems.
TRIAL REGISTRATION:
Controlled-Trials.com 49432571
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