4,752 research outputs found
Quantum Speed Limit for Perfect State Transfer in One Dimension
The basic idea of spin chain engineering for perfect quantum state transfer
(QST) is to find a set of coupling constants in the Hamiltonian, such that a
particular state initially encoded on one site will evolve freely to the
opposite site without any dynamical controls. The minimal possible evolution
time represents a speed limit for QST. We prove that the optimal solution is
the one simulating the precession of a spin in a static magnetic field. We also
argue that, at least for solid-state systems where interactions are local, it
is more realistic to characterize the computation power by the couplings than
the initial energy.Comment: 5 pages, no figure; improved versio
Conclusive and arbitrarily perfect quantum state transfer using parallel spin chain channels
We suggest a protocol for perfect quantum communication through spin chain
channels. By combining a dual-rail encoding with measurements only at the
receiving end, we can get conclusively perfect state transfer, whose
probability of success can be made arbitrarily close to unity. As an example of
such an amplitude delaying channel, we show how two parallel Heisenberg spin
chains can be used as quantum wires. Perfect state transfer with a probability
of failure lower than P in a Heisenberg chain of N spin-1/2 particles can be
achieved in a timescale of the order of N^1.7|ln(P)|. We demonstrate that our
scheme is more robust to decoherence and non-optimal timing than any scheme
using single spin chains.Comment: 6 pages, 4 figures ; expanded version inluding discussion of
transmission tim
Perfect State Transfer, Effective Gates and Entanglement Generation in Engineered Bosonic and Fermionic Networks
We show how to achieve perfect quantum state transfer and construct effective
two-qubit gates between distant sites in engineered bosonic and fermionic
networks. The Hamiltonian for the system can be determined by choosing an
eigenvalue spectrum satisfying a certain condition, which is shown to be both
sufficient and necessary in mirror-symmetrical networks. The natures of the
effective two-qubit gates depend on the exchange symmetry for fermions and
bosons. For fermionic networks, the gates are entangling (and thus universal
for quantum computation). For bosonic networks, though the gates are not
entangling, they allow two-way simultaneous communications. Protocols of
entanglement generation in both bosonic and fermionic engineered networks are
discussed.Comment: RevTeX4, 6 pages, 1 figure; replaced with a more general example and
clarified the sufficient and necessary condition for perfect state transfe
Optimal 1->M universal quantum cloning via spin networks
We present a scheme that transform 1 qubit to M identical copies with optimal
fidedelity via free dynamical evolution of spin star networks. We show that the
Heisenberg XXZ coupling can fulfill the challenge. The initial state of the
copying machine and the parameters of the spin Hamiltonian are discussed in
detail. Furthermore we have proposed a feasible method to prepare the initial
state of the copying machine.Comment: 4 pages, 2 figure
Role of interference in quantum state transfer through spin chains
We examine the role that interference plays in quantum state transfer through
several types of finite spin chains, including chains with isotropic Heisenberg
interaction between nearest neighbors, chains with reduced coupling constants
to the spins at the end of the chain, and chains with anisotropic coupling
constants. We evaluate quantitatively both the interference corresponding to
the propagation of the entire chain, and the interference in the effective
propagation of the first and last spins only, treating the rest of the chain as
black box. We show that perfect quantum state transfer is possible without
quantum interference, and provide evidence that the spin chains examined
realize interference-free quantum state transfer to a good approximation.Comment: 10 figure
Quantum Communication Through a Spin-Ring with Twisted Boundary Conditions
We investigate quantum communication between the sites of a spin-ring with
twisted boundary conditions. Such boundary conditions can be achieved by a flux
through the ring. We find that a non-zero twist can improve communication
through finite odd numbered rings and enable high fidelity multi-party quantum
communication through spin rings (working near perfectly for rings of 5 and 7
spins). We show that in certain cases, the twist results in the complete
blockage of quantum information flow to a certain site of the ring. This effect
can be exploited to interface and entangle a flux qubit and a spin qubit
without embedding the latter in a magnetic field.Comment: four pages two figure
Spin Star as Switch for Quantum Networks
Quantum state transfer is an important task in quantum information
processing. It is known that one can engineer the couplings of a
one-dimensional spin chain to achieve the goal of perfect state transfer. To
leverage the value of these spin chains, a spin star is potentially useful for
connecting different parts of a quantum network. In this work, we extend the
spin-chain engineering problem to the problems with a topology of a star
network. We show that a permanently coupled spin star can function as a network
switch for transferring quantum states selectively from one node to another by
varying the local potentials only. Together with one-dimensional chains, this
result allows applications of quantum state transfer be applied to more general
quantum networks.Comment: 10 pages, 2 figur
Quantum Communication through Spin Chain Dynamics: an Introductory Overview
We present an introductory overview of the use of spin chains as quantum
wires, which has recently developed into a topic of lively interest. The
principal motivation is in connecting quantum registers without resorting to
optics. A spin chain is a permanently coupled 1D system of spins. When one
places a quantum state on one end of it, the state will be dynamically
transmitted to the other end with some efficiency if the spins are coupled by
an exchange interaction. No external modulations or measurements on the body of
the chain, except perhaps at the very ends, is required for this purpose. For
the simplest (uniformly coupled) chain and the simplest encoding (single qubit
encoding), however, dispersion reduces the quality of transfer. We present a
variety of alternatives proposed by various groups to achieve perfect quantum
state transfer through spin chains. We conclude with a brief discussion of the
various directions in which the topic is developing.Comment: Material covered till Dec 200
Direct versus measurement assisted bipartite entanglement in multi-qubit systems and their dynamical generation in spin systems
We consider multi-qubit systems and relate quantitatively the problems of
generating cluster states with high value of concurrence of assistance, and
that of generating states with maximal bipartite entanglement. We prove an
upper bound for the concurrence of assistance. We consider dynamics of spin-1/2
systems that model qubits, with different couplings and possible presence of
magnetic field to investigate the appearance of the discussed entanglement
properties. We find that states with maximal bipartite entanglement can be
generated by an XY Hamiltonian, and their generation can be controlled by the
initial state of one of the spins. The same Hamiltonian is capable of creating
states with high concurrence of assistance with suitably chosen initial state.
We show that the production of graph states using the Ising Hamiltonian is
controllable via a single-qubit rotation of one spin-1/2 subsystem in the
initial multi-qubit state. We shown that the property of Ising dynamics to
convert a product state basis into a special maximally entangled basis is
temporally enhanced by the application of a suitable magnetic field. Similar
basis transformations are found to be feasible in the case of isotropic XY
couplings with magnetic field.Comment: (14 pages, 7 figures, RevTeX4
Quantum communication via a continuously monitored dual spin chain
We analyze a recent protocol for the transmission of quantum states via a
dual spin chain [Burgarth and Bose, Phys. Rev. A 71, 052315 (2005)] under the
constraint that the receiver's measurement strength is finite. That is, we
consider the channel where the ideal, instantaneous and complete von Neumann
measurements are replaced with a more realistic continuous measurement. We show
that for optimal performance the measurement strength must be "tuned" to the
channel spin-spin coupling, and once this is done, one is able to achieve a
similar transmission rate to that obtained with ideal measurements. The spin
chain protocol thus remains effective under measurement constraints.Comment: 5 pages, revtex 4, 3 eps figure
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