18,314 research outputs found
Multi-Party Pseudo-Telepathy
Quantum entanglement, perhaps the most non-classical manifestation of quantum
information theory, cannot be used to transmit information between remote
parties. Yet, it can be used to reduce the amount of communication required to
process a variety of distributed computational tasks. We speak of
pseudo-telepathy when quantum entanglement serves to eliminate the classical
need to communicate. In earlier examples of pseudo-telepathy, classical
protocols could succeed with high probability unless the inputs were very
large. Here we present a simple multi-party distributed problem for which the
inputs and outputs consist of a single bit per player, and we present a perfect
quantum protocol for it. We prove that no classical protocol can succeed with a
probability that differs from 1/2 by more than a fraction that is exponentially
small in the number of players. This could be used to circumvent the detection
loophole in experimental tests of nonlocality.Comment: 11 pages. To be appear in WADS 2003 proceeding
Quantum amplification and purification of noisy coherent states
Quantum-limited amplifiers increase the amplitude of quantum signals at the
price of introducing additional noise. Quantum purification protocols operate
in the reverse way, by reducing the noise while attenuating the signal. Here we
investigate a scenario that interpolates between these two extremes. We search
for the optimal physical process that generates approximate copies of pure
and amplified coherent state, starting from copies of a noisy coherent
state with Gaussian modulation. We prove that the optimal deterministic
processes are always Gaussian, whereas non-Gaussianity powers up probabilistic
advantages in suitable parameter regimes. The optimal processes are
experimentally feasible, both in the deterministic and in the probabilistic
scenario. In view of this fact, we provide benchmarks that can be used to
certify the experimental demonstration of the quantum-enhanced amplification
and purification of coherent states.Comment: 10 page
Optimal design and quantum benchmarks for coherent state amplifiers
We establish the ultimate quantum limits to the amplification of an unknown
coherent state, both in the deterministic and probabilistic case, investigating
the realistic scenario where the expected photon number is finite. In addition,
we provide the benchmark that experimental realizations have to surpass in
order to beat all classical amplification strategies and to demonstrate genuine
quantum amplification. Our result guarantees that a successful demonstration is
in principle possible for every finite value of the expected photon number.Comment: 5 + 8 pages, published versio
Entangling unitary gates on distant qubits with ancilla feedback
By using an ancilla qubit as a mediator, two distant qubits can undergo a
non-local entangling unitary operation. This is desirable for when attempting
to scale up or distribute quantum computation by combining fixed static local
sets of qubits with ballistic mediators. Using a model driven by measurements
on the ancilla, it is possible to generate a maximally entangling CZ gate while
only having access to a less entangling gate between the pair qubits and the
ancilla. However this results in a stochastic process of generating control
phase rotation gates where the expected time for success does not correlate
with the entangling power of the connection gate. We explore how one can use
feedback into the preparation and measurement parameters of the ancilla to
speed up the expected time to generate a CZ gate between a pair of separated
qubits and to leverage stronger coupling strengths for faster times.
Surprisingly, by choosing an appropriate strategy, control of a binary discrete
parameter achieves comparable speed up to full continuous control of all
degrees of freedom of the ancilla.Comment: 8 pages, 11 figure
Adaptive strategies for graph state growth in the presence of monitored errors
Graph states (or cluster states) are the entanglement resource that enables
one-way quantum computing. They can be grown by projective measurements on the
component qubits. Such measurements typically carry a significant failure
probability. Moreover, they may generate imperfect entanglement. Here we
describe strategies to adapt growth operations in order to cancel incurred
errors. Nascent states that initially deviate from the ideal graph states
evolve toward the desired high fidelity resource without impractical overheads.
Our analysis extends the diagrammatic language of graph states to include
characteristics such as tilted vertices, weighted edges, and partial fusion,
which arise from experimental imperfections. The strategies we present are
relevant to parity projection schemes such as optical `path erasure' with
distributed matter qubits.Comment: 4 pages, 4 figures. Typos corrected, nicer figures, neater notation
and better rea
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