21,156 research outputs found
Quantum parallelism of the controlled-NOT operation: an experimental criterion for the evaluation of device performance
It is shown that a quantum controlled-NOT gate simultaneously performs the
logical functions of three distinct conditional local operations. Each of these
local operations can be verified by measuring a corresponding truth table of
four local inputs and four local outputs. The quantum parallelism of the gate
can then be observed directly in a set of three simple experimental tests, each
of which has a clear intuitive interpretation in terms of classical logical
operations. Specifically, quantum parallelism is achieved if the average
fidelity of the three classical operations exceeds 2/3. It is thus possible to
evaluate the essential quantum parallelism of an experimental controlled-NOT
gate by testing only three characteristic classical operations performed by the
gate.Comment: 6 pages, no figures, added references and discussio
Reconceptualising Personas Across Cultures: Archetypes, Stereotypes & Collective Personas in Pastoral Namibia
The paucity of projects where persona is the research foci and a lack of consensus on this artefact keep many reticent about its purpose and value. Besides crafting personas is expected to differ across cultures, which contrasts the advancements in Western theory with studies and progress in other sites. We postulate User-Created Personas reveal specific characteristics of situated contexts by allowing laypeople to design persona artefacts in their own terms. Hence analysing four persona sessions with an ethnic group in pastoral Namibia –ovaHerero– brought up a set of fundamental questions around the persona artefact regarding stereotypes, archetypes, and collective persona representations: (1) to what extent user depictions are stereotypical or archetypal? If stereotypes prime (2) to what degree are current personas a useful method to represent end-users in technology design? And, (3) how can we ultimately read accounts not conforming to mainstream individual persona descriptions but to collectives
A study of separability criteria for mixed three-qubit states
We study the noisy GHZ-W mixture. We demonstrate some necessary but not
sufficient criteria for different classes of separability of these states. It
turns out that the partial transposition criterion of Peres and the criteria of
G\"uhne and Seevinck dealing with matrix elements are the strongest ones for
different separability classes of this 2 parameter state. As a new result we
determine a set of entangled states of positive partial transpose.Comment: 18 pages, 10 figures, PRA styl
Superconducting Spin Qubits
We propose and theoretically investigate spin superconducting qubits. Spin
superconducting qubit consists of a single spin confined in a Josephson
junction. We show that owing to spin-orbit interaction, superconducting
difference across the junction can polarize this spin. We demonstrate that this
enables single qubit operations and more complicated quantum gates, where spins
of different qubits interact via a mutual inductance of superconducting loop
where the junctions are embedded. Recent experimental realizations of Josephson
junctions made of semiconductor quantum dots in contact with superconducting
leads have shown that the number of electrons in the quantum dot can be tuned
by a gate voltage. Spin superconducting qubit is realized when the number of
electrons is odd. We discuss the qubit properties at phenomenological level. We
present a microscopic theory that enables us to make accurate estimations of
the qubit parameters by evaluating the spin-dependent Josephson energy in the
framework of fourth-order perturbation theory.Comment: 11 pages, 8 figure
Quantum Phase Transitions and Bipartite Entanglement
We develop a general theory of the relation between quantum phase transitions
(QPTs) characterized by nonanalyticities in the energy and bipartite
entanglement. We derive a functional relation between the matrix elements of
two-particle reduced density matrices and the eigenvalues of general two-body
Hamiltonians of -level systems. The ground state energy eigenvalue and its
derivatives, whose non-analyticity characterizes a QPT, are directly tied to
bipartite entanglement measures. We show that first-order QPTs are signalled by
density matrix elements themselves and second-order QPTs by the first
derivative of density matrix elements. Our general conclusions are illustrated
via several quantum spin models.Comment: 5 pages, incl. 2 figures. v3: The version published in PRL, including
a few extra comments and clarifications for which there was no space in the
PR
Quantum gate characterization in an extended Hilbert space
We describe an approach for characterizing the process of quantum gates using
quantum process tomography, by first modeling them in an extended Hilbert
space, which includes non-qubit degrees of freedom. To prevent unphysical
processes from being predicted, present quantum process tomography procedures
incorporate mathematical constraints, which make no assumptions as to the
actual physical nature of the system being described. By contrast, the
procedure presented here ensures physicality by placing physical constraints on
the nature of quantum processes. This allows quantum process tomography to be
performed using a smaller experimental data set, and produces parameters with a
direct physical interpretation. The approach is demonstrated by example of
mode-matching in an all-optical controlled-NOT gate. The techniques described
are non-specific and could be applied to other optical circuits or quantum
computing architectures.Comment: 4 pages, 2 figures, REVTeX (published version
One-loop corrections to the instanton transition in the two-dimensional Abelian Higgs model
We present an evaluation of the fluctuation determinant which appears as a
prefactor in the instanton transition rate for the two-dimensional Abelian
Higgs model. The corrections are found to change the rate at most by a factor
of 2 for 0.4 < M_W/M_H < 2.0.Comment: DO-TH-94/17, 20 pages, 4 figures appended as uucompressed .eps files,
LaTeX, needs epsfig.st
Quantum phase transition in the Dicke model with critical and non-critical entanglement
We study the quantum phase transition of the Dicke model in the classical
oscillator limit, where it occurs already for finite spin length. In contrast
to the classical spin limit, for which spin-oscillator entanglement diverges at
the transition, entanglement in the classical oscillator limit remains small.
We derive the quantum phase transition with identical critical behavior in the
two classical limits and explain the differences with respect to quantum
fluctuations around the mean-field ground state through an effective model for
the oscillator degrees of freedom. With numerical data for the full quantum
model we study convergence to the classical limits. We contrast the classical
oscillator limit with the dual limit of a high frequency oscillator, where the
spin degrees of freedom are described by the Lipkin-Meshkov-Glick model. An
alternative limit can be defined for the Rabi case of spin length one-half, in
which spin frequency renormalization replaces the quantum phase transition.Comment: 1o pages, 10 figures, published versio
Decoherence-free preparation of Dicke states of trapped ions by collective stimulated Raman adiabatic passage
We propose a simple technique for the generation of arbitrary-sized Dicke
states in a chain of trapped ions. The method uses global addressing of the
entire chain by two pairs of delayed but partially overlapping laser pulses to
engineer a collective adiabatic passage along a multi-ion dark state. Our
technique, which is a many-particle generalization of stimulated Raman
adiabatic passage (STIRAP), is decoherence-free with respect to spontaneous
emission and robust against moderate fluctuations in the experimental
parameters. Furthermore, because the process is very rapid, the effects of
heating are almost negligible under realistic experimental conditions. We
predict that the overall fidelity of synthesis of a Dicke state involving ten
ions sharing two excitations should approach 98% with currently achievable
experimental parameters.Comment: 14 pages, 8 figure
Completely positive covariant two-qubit quantum processes and optimal quantum NOT operations for entangled qubit pairs
The structure of all completely positive quantum operations is investigated
which transform pure two-qubit input states of a given degree of entanglement
in a covariant way. Special cases thereof are quantum NOT operations which
transform entangled pure two-qubit input states of a given degree of
entanglement into orthogonal states in an optimal way. Based on our general
analysis all covariant optimal two-qubit quantum NOT operations are determined.
In particular, it is demonstrated that only in the case of maximally entangled
input states these quantum NOT operations can be performed perfectly.Comment: 14 pages, 2 figure
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