55 research outputs found
A photon loss tolerant Zeno CSIGN gate
We model an optical implementation of a CSIGN gate that makes use of the
Quantum Zeno effect [1,2] in the presence of photon loss. The raw operation of
the gate is severely affected by this type of loss. However, we show that by
using the same photon loss codes that have been proposed for linear optical
quantum computation (LOQC), the performance is greatly enhanced and such gates
can outperform LOQC equivalents. The technique can be applied to other types of
nonlinearities, making the implementation of nonlinear optical gates much more
attractive
Quantum walks with encrypted data
In the setting of networked computation, data security can be a significant
concern. Here we consider the problem of allowing a server to remotely
manipulate client supplied data, in such a way that both the information
obtained by the client about the server's operation and the information
obtained by the server about the client's data are significantly limited. We
present a protocol for achieving such functionality in two closely related
models of restricted quantum computation -- the Boson sampling and quantum walk
models. Due to the limited technological requirements of the Boson scattering
model, small scale implementations of this technique are feasible with
present-day technology.Comment: 4 pages, 2 figure
Optimal tracking for pairs of qubit states
In classical control theory, tracking refers to the ability to perform
measurements and feedback on a classical system in order to enforce some
desired dynamics. In this paper we investigate a simple version of quantum
tracking, namely, we look at how to optimally transform the state of a single
qubit into a given target state, when the system can be prepared in two
different ways, and the target state depends on the choice of preparation. We
propose a tracking strategy that is proved to be optimal for any input and
target states. Applications in the context of state discrimination, state
purification, state stabilization and state-dependent quantum cloning are
presented, where existing optimality results are recovered and extended.Comment: 15 pages, 8 figures. Extensive revision of text, optimality results
extended, other physical applications include
Generating optical nonlinearity using trapped atoms
We describe a scheme for producing an optical nonlinearity using an
interaction with one or more ancilla two-level atomic systems. The
nonlinearity, which can be implemented using high efficiency fluorescence
shelving measurements, together with general linear transformations is
sufficient for simulating arbitrary Hamiltonian evolution on a Fock state
qudit. We give two examples of the application of this nonlinearity, one for
the creation of nonlinear phase shifts on optical fields as required in single
photon quantum computation schemes, and the other for the preparation of
optical Schrodinger cat states.Comment: Substantially extended from quant-ph/020815
Efficient Parity Encoded Optical Quantum Computing
We present a linear optics quantum computation scheme with a greatly reduced
cost in resources compared to KLM. The scheme makes use of elements from
cluster state computation and achieves comparable resource usage to those
schemes while retaining the circuit based approach of KLM
Encoding qubits into oscillators with atomic ensembles and squeezed light
The Gottesman-Kitaev-Preskill (GKP) encoding of a qubit within an oscillator
provides a number of advantages when used in a fault-tolerant architecture for
quantum computing, most notably that Gaussian operations suffice to implement
all single- and two-qubit Clifford gates. The main drawback of the encoding is
that the logical states themselves are challenging to produce. Here we present
a method for generating optical GKP-encoded qubits by coupling an atomic
ensemble to a squeezed state of light. Particular outcomes of a subsequent spin
measurement of the ensemble herald successful generation of the resource state
in the optical mode. We analyze the method in terms of the resources required
(total spin and amount of squeezing) and the probability of success. We propose
a physical implementation using a Faraday-based quantum non-demolition
interaction.Comment: (v2) consistent with published version; (v1) 16 pages, 5 figure
Scalable boson-sampling with time-bin encoding using a loop-based architecture
We present an architecture for arbitrarily scalable boson-sampling using two
nested fiber loops. The architecture has fixed experimental complexity,
irrespective of the size of the desired interferometer, whose scale is limited
only by fiber and switch loss rates. The architecture employs time-bin
encoding, whereby the incident photons form a pulse train, which enters the
loops. Dynamically controlled loop coupling ratios allow the construction of
the arbitrary linear optics interferometers required for boson-sampling. The
architecture employs only a single point of interference and may thus be easier
to stabilize than other approaches. The scheme has polynomial complexity and
could be realized using demonstrated present-day technologies.Comment: 7 pages, 7 figure
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