318 research outputs found
Exploring frustrated spin systems using projected entangled pair states
We study the nature of the ground state of the frustrated J(1)-J(2) model and the J(1)-J(3) model using a variational algorithm based on projected entangled pair states. By investigating spin-spin correlation functions, we observe a separation in parameter regions with long- and short-range order. A direct comparison with exact diagonalizations in the subspace of short-range valence bond singlets reveals that the system is well described by states within this subset in the short-range order regions. We discuss the question whether the system forms a spin liquid, a plaquette valence bond crystal, or a columnar dimer crystal in these parameter regions
Matrix product operator representations
We show how to construct relevant families of matrix product operators (MPOs) in one and higher dimensions. These form the building blocks for the numerical simulation methods based on matrix product states and projected entangled pair states. In particular, we construct translationally invariant MPOs suitable for time evolution, and show how such descriptions are possible for Hamiltonians with long-range interactions. We show how these tools can be exploited for constructing new algorithms for simulating quantum spin systems
Implementation of a Quantum Search Algorithm on a Nuclear Magnetic Resonance Quantum Computer
We demonstrate an implementation of a quantum search algorithm on a two qubit
NMR quantum computer based on cytosine.Comment: Six pages, three figure
Transport dynamics of single ions in segmented microstructured Paul trap arrays
It was recently proposed to use small groups of trapped ions as qubit
carriers in miniaturized electrode arrays that comprise a large number of
individual trapping zones, between which ions could be moved. This approach
might be scalable for quantum information processing with a large numbers of
qubits. Processing of quantum information is achieved by transporting ions to
and from separate memory and qubit manipulation zones in between quantum logic
operations. The transport of ion groups in this scheme plays a major role and
requires precise experimental control and fast transport. In this paper we
introduce a theoretical framework to study ion transport in external potentials
that might be created by typical miniaturized Paul trap electrode arrays. In
particular we discuss the relationship between classical and quantum
descriptions of the transport and study the energy transfer to the oscillatory
motion during near-adiabatic transport. Based on our findings we suggest a
numerical method to find electrode potentials as a function of time to optimize
the local potential an ion experiences during transport. We demonstrate this
method for one specific electrode geometry that should closely represent the
situation encountered in realistic trap arrays.Comment: 20 pages, 5 figure
Damagnetization cooling of a gas
We demonstrate demagnetization cooling of a gas of ultracold Cr atoms.
Demagnetization is driven by inelastic dipolar collisions which couple the
motional degrees of freedom to the spin degree. By that kinetic energy is
converted into magnetic work with a consequent temperature reduction of the
gas. Optical pumping is used to magnetize the system and drive continuous
demagnetization cooling. Applying this technique, we can increase the phase
space density of our sample by one order of magnitude, with nearly no atom
loss. This method can be in principle extended to every dipolar system and
could be used to achieve quantum degeneracy via optical means.Comment: 10 pages, 5 figure
Experimental demonstration of quantum memory for light
The information carrier of today's communications, a weak pulse of light, is
an intrinsically quantum object. As a consequence, complete information about
the pulse cannot, even in principle, be perfectly recorded in a classical
memory. In the field of quantum information this has led to a long standing
challenge: how to achieve a high-fidelity transfer of an independently prepared
quantum state of light onto the atomic quantum state? Here we propose and
experimentally demonstrate a protocol for such quantum memory based on atomic
ensembles. We demonstrate for the first time a recording of an externally
provided quantum state of light onto the atomic quantum memory with a fidelity
up to 70%, significantly higher than that for the classical recording. Quantum
storage of light is achieved in three steps: an interaction of light with
atoms, the subsequent measurement on the transmitted light, and the feedback
onto the atoms conditioned on the measurement result. Density of recorded
states 33% higher than that for the best classical recording of light on atoms
is achieved. A quantum memory lifetime of up to 4 msec is demonstrated.Comment: 22 pages (double line spacing) incl. supplementary information, 4
figures, accepted for publication in Natur
Entanglement Percolation in Quantum Networks
Quantum networks are composed of nodes which can send and receive quantum
states by exchanging photons. Their goal is to facilitate quantum communication
between any nodes, something which can be used to send secret messages in a
secure way, and to communicate more efficiently than in classical networks.
These goals can be achieved, for instance, via teleportation. Here we show that
the design of efficient quantum communication protocols in quantum networks
involves intriguing quantum phenomena, depending both on the way the nodes are
displayed, and the entanglement between them. These phenomena can be employed
to design protocols which overcome the exponential decrease of signals with the
number of nodes. We relate the problem of establishing maximally entangled
states between nodes to classical percolation in statistical mechanics, and
demonstrate that quantum phase transitions can be used to optimize the
operation of quantum networks.Comment: Accepted for publication in Nature Physics. This is the original
submitted versio
Long-distance quantum communication with atomic ensembles and linear optics
Quantum communication holds a promise for absolutely secure transmission of
secret messages and faithful transfer of unknown quantum states. Photonic
channels appear to be very attractive for physical implementation of quantum
communication. However, due to losses and decoherence in the channel, the
communication fidelity decreases exponentially with the channel length. We
describe a scheme that allows to implement robust quantum communication over
long lossy channels. The scheme involves laser manipulation of atomic
ensembles, beam splitters, and single-photon detectors with moderate
efficiencies, and therefore well fits the status of the current experimental
technology. We show that the communication efficiency scale polynomially with
the channel length thereby facilitating scalability to very long distances.Comment: 2 tex files (Main text + Supplement), 4 figure
Quantum Simulation of Tunneling in Small Systems
A number of quantum algorithms have been performed on small quantum
computers; these include Shor's prime factorization algorithm, error
correction, Grover's search algorithm and a number of analog and digital
quantum simulations. Because of the number of gates and qubits necessary,
however, digital quantum particle simulations remain untested. A contributing
factor to the system size required is the number of ancillary qubits needed to
implement matrix exponentials of the potential operator. Here, we show that a
set of tunneling problems may be investigated with no ancillary qubits and a
cost of one single-qubit operator per time step for the potential evolution. We
show that physically interesting simulations of tunneling using 2 qubits (i.e.
on 4 lattice point grids) may be performed with 40 single and two-qubit gates.
Approximately 70 to 140 gates are needed to see interesting tunneling dynamics
in three-qubit (8 lattice point) simulations.Comment: 4 pages, 2 figure
Two-Photon Scattering by a Cavity-Coupled Two-Level Emitter in a One-Dimensional Waveguide
We show that two-photon transport can be modulated by a two-level emitter
coupled to a cavity in a one-dimensional waveguide. In the ordinary case, the
transmitted light has a wider frequency spectrum than the situation without the
cavity because it is reflected and scattered many times. But when the two
photons are resonant with the cavity resonance reflection frequency, the
frequency spectrum of the transmitted light becomes narrower than that without
the cavity. This means that properly tuning the cavity resonance frequency can
improve the photon-photon interaction. In addition, we show that the two-photon
intensity correlation functions are nearly opposite to each other at the two
sides of the emitter transition frequency rather than be the same, which is
exactly the Fano resonance line shape for two photons. Such an effect is
important for lowering the power threshold in optical bistable devices and for
sensing applications. When the emitter transition frequency equals to the
cavity resonance frequency for a high-Q cavity, our results agree with the
recent experiments and theories.Comment: 12 pages, 16 figure
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