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Multiparticle entanglement and graph states with cold
Rydberg atoms
We use long-range interactions of atoms excited to
Rydberg states for the generation of multi-particle entanglement and
the preparation of "cluster states". Using arrays of dipole traps
the geometrical arrangement of atoms can be made completely
arbitrary and symmetries in the spatial distribution can be broken.
We will investigate the possibility of mapping the geometrical
arrangement of the atoms directly onto the topology of the cluster
state, which could allow for arbitrary state preparation by global
addressing of the ensemble of atoms. Fundamental issues in our
understanding of the quantum phenomena lying at the heart of
multiparticle entanglement and measurement-based quantum computing
will be addressed.
The pursuit of schemes for quantum computation
(QC) and their experimental implementation are important both for
practical applications, which could lead to a revolution in
information technology, and for a deeper understanding of the nature
and behaviour of complex quantum systems. Recent years have seen
significant progress in quantum information, thanks to the
development of novel methods for controlling the state of quantum
objects. However, there are fundamental challenges that hinder the
successful implementation of prototypes, amongst these decoherence
and scalability issues.
Conventional quantum computation has
matured following processing schemes analogous to the classical
counterpart. A quantum register is an ensemble of quantum bits
(qubits) – a two-level quantum system, which can be found in the
states “0” and “1”, or in their superpositions. The quantum
algorithm is implemented using a sequence of single-qubit and
two-qubit logical gates. Following the processing stage, the quantum
state of the register must be measured (readout). The experimental
implementation of a prototype quantum computer, consisting of a
large number of qubits and capable of performing complicated
calculations, is extremely challenging. Recently, so called one-way
quantum protocols have been proposed, which promise more resilience
to quantum decoherence and therefore should relax the requirements
for experimental implementation. In one-way QC, all of the
register's qubits are entangled with each other, forming a
particular multi-particle entangled state called cluster state. A
particular cluster state is designed to perform a specific operation
through an algorithm that consists only of single-qubit
measurements.
We propose to use
atoms in highly excited states, known as Rydberg atoms,
to implement designer cluster states and demonstrate simple algorithms.
Atoms will be confined in space by laser
light, acting like arrays of microscopic tweezers with
arbitrary spatial arrangement. The excitation of atoms to high
Rydberg states switches on strong interactions between different tweezers, which
are used for entangling all nearest-neighbours. Ultimately a variety of
multiparticle entangled states can be obtained, by
varying the spatial geometry of the tweezers, their distance
and the interaction strength. Our goal is to demonstrate
multiparticle entanglement using arrays of microscopic tweezers mediated by Rydberg-Rydberg
interaction and a proof of principle one-way quantum
algorithm.
Testing the computational power of
discord **NEW**
At present, no single feature of
the quantum world has been identified as the source of the
computational enhancement, efficiency and speed-up of quantum
technology. Whilst entanglement is widely recognised as a key
resource in quantum technology, an exponential advantage over
classical technology can be achieved without it in the presence of
non-classical correlations. Furthermore for specific tasks separable
states with discord have been proved to be even more efficient than
entanglement.
The dynamics of entanglement and discord differ
considerably, with entanglement being extremely fragile towards
decoherence (even undergoing entanglement "sudden death" ) and
discord being much more robust. Since decoherence is a major hurdle
to the development of quantum technologies, the investigation of
discord is a promising route for progressing the
field.
Whilst discord has been proved to be a valuable
resource for speed up of specific computational tasks and for being
robust towards decoherence, the more general demonstration that
discord can provide computational enhancement for any computational
task has not been provided.This makes discord a
very
controversial asset for quantum information processing,
although it is widely recognised that if one day it could be made of
use for quantum computation, the impact would be truly
ground-breaking.
The goal of this project is to
experimentally investigate the physics and the computational power
of quantum discord in many-atom ensembles for a specific algorithm
that performs the normalized trace estimation. We will be using the
DQC1 model to compute sums over extremely large strings of numbers,
which make the computation classically intractable. As an
illustrative example, consider one hundred atoms trapped in an
optical dipole trap. A unitary operation on these atoms would be
described by a 2^100-by-2^100 matrix. Finding the normalised trace
of this matrix is equivalent to adding up 10^30 numbers, which is a
task that is classically intractable: modern supercomputers can
perform 10^12 operations per second and therefore it would take
about the age of the universe to have the same task. This is
potentially transformative because quantum discord has not yet been
studied in systems with large Hilbert spaces, and the successful
demonstration of the exponential speed-up of the computational
capability would be a major leap forward in the field. The ultimate
impact of this research idea would be to gain a variety of
experimental insights into, and thus a deeper understanding of, the
quantum correlations that would be present in all quantum
systems
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