6,952 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
Jensen Shannon divergence as a measure of the degree of entanglement
The notion of distance in Hilbert space is relevant in many scenarios. In
particular, distances between quantum states play a central role in quantum
information theory. An appropriate measure of distance is the quantum Jensen
Shannon divergence (QJSD) between quantum states. Here we study this distance
as a geometrical measure of entanglement and apply it to different families of
states.Comment: 5 pages, 2 figures, to appear in the special issue of IJQI "Noise,
Information and Complexity at Quantum Scale", eds. S. Mancini and F.
Marcheson
Detecting entanglement of two electron spin qubits with witness operators
We propose a scheme for detecting entanglement between two electron spin
qubits in a double quantum dot using an entanglement witness operator. We first
calculate the optimal configuration of the two electron spins, defined as the
position in the energy level spectrum where, averaged over the nuclear spin
distribution, 1) the probability to have two separated electrons, and 2) the
degree of entanglement of the quantum state quantified by the concurrence are
both large. Using a density matrix approach, we then calculate the evolution of
the expectation value of the witness operator for the two-spin singlet state,
taking into account the effect of decoherence due to quantum charge
fluctuations modeled as a boson bath. We find that, for large interdot
coupling, it is possible to obtain a highly entangled and robust ground state.Comment: 4 pages, 3 figure
Internal Business Control Systems in Small Puerto Rican Retail Businesses
Small businesses\u27 vulnerability to financial losses brought about by employee theft is higher than for larger organizations. Small business owners are concerned about occupational fraud, as the annual average loss from fraud for small business owners in the United States was $150,000 per incident. Grounded in the fraud triangle theory and COSO Internal Control Integrated Framework, the purpose of this quantitative correlational study was to examine the relationship between the degree of segregation of duties, attitude toward internal controls, and the internal control system in small gas and convenience stores in Puerto Rico. Data were collected from 129 small business owners and managers of gas and convenience stores located in Puerto Rico. The results of the multiple regression analysis were significant, F(2, 126) = 49.158, p \u3c. 01, R2 =. 438, indicating the degree of segregation of duties and the attitude toward internal controls, significantly predicted the internal control system. The implications for positive social change included the potential for a better understanding of the existing relationship between the degree of segregation of duties, attitude towards internal control, and internal controls. Thus, small business owners and managers could become interested in engaging in a low-budget internal control system for their business to reach their goals and diminish the occurrence of occupational fraud
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.
Efficient generation of random multipartite entangled states using time optimal unitary operations
We review the generation of random pure states using a protocol of repeated
two qubit gates. We study the dependence of the convergence to states with Haar
multipartite entanglement distribution. We investigate the optimal generation
of such states in terms of the physical (real) time needed to apply the
protocol, instead of the gate complexity point of view used in other works.
This physical time can be obtained, for a given Hamiltonian, within the
theoretical framework offered by the quantum brachistochrone formalism. Using
an anisotropic Heisenberg Hamiltonian as an example, we find that different
optimal quantum gates arise according to the optimality point of view used in
each case. We also study how the convergence to random entangled states depends
on different entanglement measures.Comment: 5 pages, 2 figures. New title, improved explanation of the algorithm.
To appear in Phys. Rev.
Microwave-assisted synthesis and electrochemical evaluation of VO2 (B) nanostructures
Understanding how intercalation materials change during electrochemical operation is paramount to optimizing their behaviour and function and in situ characterization methods allow us to observe these changes without sample destruction. Here we first report the improved intercalation properties of bronze phase vanadium dioxide VO2 (B) prepared by a microwave-assisted route which exhibits a larger electrochemical capacity (232 mAh g-1) compared with VO2 (B) prepared by a solvothermal route (197 mAh g-1). These electrochemical differences have also been followed using in situ X-ray absorption spectroscopy allowing us to follow oxidation state changes as they occur during battery operation
Fundamental Limits on the Speed of Evolution of Quantum States
This paper reports on some new inequalities of
Margolus-Levitin-Mandelstam-Tamm-type involving the speed of quantum evolution
between two orthogonal pure states. The clear determinant of the qualitative
behavior of this time scale is the statistics of the energy spectrum. An
often-overlooked correspondence between the real-time behavior of a quantum
system and the statistical mechanics of a transformed (imaginary-time)
thermodynamic system appears promising as a source of qualitative insights into
the quantum dynamics.Comment: 6 pages, 1 eps figur
The Mariner 5 flight path and its determination from tracking data
Mariner 5 flight path and its determination from tracking dat
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
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