7,078 research outputs found
Entanglement and the Quantum Brachistochrone Problem
Entanglement is closely related to some fundamental features of the dynamics
of composite quantum systems: quantum entanglement enhances the "speed" of
evolution of certain quantum states, as measured by the time required to reach
an orthogonal state. The concept of "speed" of quantum evolution constitutes an
important ingredient in any attempt to determine the fundamental limits that
basic physical laws impose on how fast a physical system can process or
transmit information. Here we explore the relationship between entanglement and
the speed of quantum evolution in the context of the quantum brachistochrone
problem. Given an initial and a final state of a composite system we consider
the amount of entanglement associated with the brachistochrone evolution
between those states, showing that entanglement is an essential resource to
achieve the alluded time-optimal quantum evolution.Comment: 6 pages, 3 figures. Corrected typos in Eqs. 1 and
The Mariner 5 flight path and its determination from tracking data
Mariner 5 flight path and its determination from tracking dat
Robustness of Highly Entangled Multi-Qubit States Under Decoherence
We investigate the decay of entanglement, due to decoherence, of multi-qubit
systems that are initially prepared in highly (in some cases maximally)
entangled states. We assume that during the decoherence processes each qubit of
the system interacts with its own, independent environment. We determine, for
systems with a small number of qubits and for various decoherence channels, the
initial states exhibiting the most robust entanglement. We also consider a
restricted version of this robustness optimization problem, only involving
states equivalent under local unitary transformations to the |GHZ> state.Comment: 16 pages, 3 figures. Changes in Sec.
Time-optimal CNOT between indirectly coupled qubits in a linear Ising chain
We give analytical solutions for the time-optimal synthesis of entangling
gates between indirectly coupled qubits 1 and 3 in a linear spin chain of three
qubits subject to an Ising Hamiltonian interaction with equal coupling plus
a local magnetic field acting on the intermediate qubit. The energy available
is fixed, but we relax the standard assumption of instantaneous unitary
operations acting on single qubits. The time required for performing an
entangling gate which is equivalent, modulo local unitary operations, to the
between the indirectly coupled qubits 1 and 3 is
, i.e. faster than a previous estimate based on a similar
Hamiltonian and the assumption of local unitaries with zero time cost.
Furthermore, performing a simple Walsh-Hadamard rotation in the Hlibert space
of qubit 3 shows that the time-optimal synthesis of the (which acts as the identity when the control qubit 1 is in the state
, while if the control qubit is in the state the target
qubit 3 is flipped as ) also requires the same
time .Comment: 9 pages; minor modification
Some features of the state-space trajectories followed by robust entangled four-qubit states during decoherence
In a recent work (Borras et al., Phys. Rev. A {\bf 79}, 022108 (2009)), we
have determined, for various decoherence channels, four-qubit initial states
exhibiting the most robust possible entanglement. Here we explore some
geometrical features of the trajectories in state space generated by the
decoherence process, connecting the initially robust pure state with the
completely decohered mixed state obtained at the end of the evolution. We
characterize these trajectories by recourse to the distance between the
concomitant time dependent mixed state and different reference states.Comment: 13 pages, 5 figures; new title, minor change
Multiqubit systems: highly entangled states and entanglement distribution
Texto completo arXiv:0803.3979v1.-- PACS: 03.67.Lx Quantum computation architectures and implementations
03.67.Mn Entanglement measures, witnesses, and other characterizations
03.65.Ud Entanglement and quantum nonlocality (e.g. EPR paradox, Bell's inequalities, GHZ states, etc.).-A comparison is made of various searching procedures, based upon different entanglement measures or entanglement indicators, for highly entangled multi-qubits states. In particular, our present results are compared with those recently reported by Brown et al. [J. Phys. A: Math. Gen. 38 (2005) 1119]. The statistical distribution of entanglement values for the aforementioned multi-qubit systems is also exploredThis work was partially supported by the MEC grant FIS2005-02796 (Spain) and FEDER (EU)
and by CONICET (Argentine Agency). The financial assistance of the National Research
Foundation (NRF; South African Agency) toward this research is hereby acknowledged.
Opinions expressed and conclusions arrived at, are those of the authors and are not necessarily
to be attributed to the NRF. A Borras acknowledges support from the FPU grant AP-2004- ´
2962 (MEC-Spain)Peer reviewe
Fragile Heterosexuality
Previous research demonstrates that membership of majority groups is often perceived as more fragile than membership of minority groups. Four studies (N1=90, N2=247, N3=500, N4=1176) investigated whether this was the case for heterosexual identity, relative to gay identity. Support for fragile heterosexuality was found using various methods: sexual orientation perceptions of a target who engaged in incongruent behaviour, free-responses concerning behaviours required to change someone’s mind about a target’s sexual orientation, agreement with statements about men/women’s sexual orientation and agreement with gender neutral statements about sexual orientation. Neither participant nor target gender eliminated or reversed this effect. Additionally, we investigated multiple explanations (moderators) of the perceived difference in fragility between heterosexual identity and gay identity and found that higher estimates of the gay/lesbian population decreased the difference between the (higher) perceived fragility of heterosexual identity and the (lower) perceived fragility of gay identity
Multi-Qubit Systems: Highly Entangled States and Entanglement Distribution
A comparison is made of various searching procedures, based upon different
entanglement measures or entanglement indicators, for highly entangled
multi-qubits states. In particular, our present results are compared with those
recently reported by Brown et al. [J. Phys. A: Math. Gen. 38 (2005) 1119]. The
statistical distribution of entanglement values for the aforementioned
multi-qubit systems is also explored.Comment: 24 pages, 3 figure
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