59 research outputs found
The quantum Heisenberg antiferromagnet on the square lattice
The pure-quantum self-consistent harmonic approximation, a semiclassical
method based on the path-integral formulation of quantum statistical mechanics,
is applied to the study of the thermodynamic behaviour of the quantum
Heisenberg antiferromagnet on the square lattice (QHAF). Results for various
properties are obtained for different values of the spin and successfully
compared with experimental data.Comment: Proceedings of the Conference "Path Integrals from peV to TeV - 50
Years from Feynman's paper" (Florence, August 1998) -- 2 pages, ReVTeX, 2
figure
Determination of ground state properties in quantum spin systems by single qubit unitary operations and entanglement excitation energies
We introduce a method for analyzing ground state properties of quantum many
body systems, based on the characterization of separability and entanglement by
single subsystem unitary operations. We apply the method to the study of the
ground state structure of several interacting spin-1/2 models, described by
Hamiltonians with different degrees of symmetry. We show that the approach
based on single qubit unitary operations allows to introduce {\it
``entanglement excitation energies''}, a set of observables that can
characterize ground state properties, including the quantification of
single-site entanglement and the determination of quantum critical points. The
formalism allows to identify the existence and location of factorization
points, and a purely quantum {\it ``transition of entanglement''} that occurs
at the approach of factorization. This kind of quantum transition is
characterized by a diverging ratio of excitation energies associated to
single-qubit unitary operations.Comment: To appear in Phys. Rev.
Nonperturbative Entangling Gates between Distant Qubits Using Uniform Cold Atom Chains
We propose a new fast scalable method for achieving a two-qubit entangling gate between arbitrary distant qubits in a network by exploiting dispersionless propagation in uniform chains. This is achieved dynamically by switching on a strong interaction between the qubits and a bus formed by a nonengineered chain of interacting qubits. The quality of the gate scales very efficiently with qubit separations. Surprisingly, a sudden switching of the couplings is not necessary. Moreover, our gate mechanism works for multiple gate operations without resetting the bus. We propose a possible experimental realization in cold atoms trapped in optical lattices and near field Fresnel trapping potentials
Optimal dynamics for quantum-state and entanglement transfer through homogeneous quantum wires
It is shown that effective quantum-state and entanglement transfer can be
obtained by inducing a coherent dynamics in quantum wires with homogeneous
intrawire interactions. This goal is accomplished by tuning the coupling
between the wire endpoints and the two qubits there attached, to an optimal
value. A general procedure to determine such value is devised, and scaling laws
between the optimal coupling and the length of the wire are found. The
procedure is implemented in the case of a wire consisting of a spin-1/2 XY
chain: results for the time dependence of the quantities which characterize
quantum-state and entanglement transfer are found of extremely good quality and
almost independent of the wire length. The present approach does not require
`ad hoc' engineering of the intrawire interactions nor a specific initial pulse
shaping, and can be applied to a vast class of quantum channels.Comment: 5 pages, 5 figure
Using the J1-J2 Quantum Spin Chain as an Adiabatic Quantum Data Bus
This paper investigates numerically a phenomenon which can be used to
transport a single q-bit down a J1-J2 Heisenberg spin chain using a quantum
adiabatic process. The motivation for investigating such processes comes from
the idea that this method of transport could potentially be used as a means of
sending data to various parts of a quantum computer made of artificial spins,
and that this method could take advantage of the easily prepared ground state
at the so called Majumdar-Ghosh point. We examine several annealing protocols
for this process and find similar result for all of them. The annealing process
works well up to a critical frustration threshold.Comment: 14 pages, 13 figures (2 added), revisions made to add citations and
additional discussion at request of referee
Whenever a quantum environment emerges as a classical system, it behaves like a measuring apparatus
We study the dynamics of a quantum system Gamma with an environment Xi made of N elementary quantum components. We aim at answering the following questions: can the evolution of Gamma be characterized by some general features when N becomes very large, regardless of the specific form of its interaction with each and every component of Xi? In other terms: should we expect all quantum systems with a macroscopic environment to undergo a somehow similar evolution? And if yes, of what type? In order to answer these questions we use well established results from large-N quantum field theories, particularly referring to the conditions ensuring a large-N quantum model to be effectively described by a classical theory. We demonstrate that the fulfillment of these conditions, when properly imported into the framework of the open quantum systems dynamics, guarantees that the evolution of Gamma is always of the same type of that expected if Xi were a measuring apparatus, no matter the details of the actual interaction. On the other hand, such details are found to determine the specific basis w.r.t. which Gamma undergoes the decoherence dictated by the dynamical description of the quantum measurement process. This result wears two hats: on the one hand it clarifies the physical origin of the formal statement that, under certain conditions, any channel from rho(Gamma) to rho(Xi) takes the form of a measure-and-prepare map, as recently shown in Ref. [1]; on the other hand, it for-malizes the qualitative argument that the reason why we do not observe state superpositions is the continual measurement performed by the environment
Long quantum channels for high-quality entanglement transfer
High-quality quantum-state and entanglement transfer can be achieved in an
unmodulated spin bus operating in the ballistic regime, which occurs when the
endpoint qubits A and B are coupled to the chain by an exchange interaction
comparable with the intrachain exchange. Indeed, the transition amplitude
characterizing the transfer quality exhibits a maximum for a finite optimal
value , where is the channel length. We show that
scales as for large and that it ensures a
high-quality entanglement transfer even in the limit of arbitrarily long
channels, almost independently of the channel initialization. For instance, the
average quantum-state transmission fidelity exceeds 90% for any chain length.
We emphasize that, taking the reverse point of view, should be
experimentally constrained, high-quality transfer can still be obtained by
adjusting the channel length to its optimal value.Comment: 12 pages, 9 figure
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