31,394 research outputs found
Spectral implementation of some quantum algorithms by one- and two-dimensional nuclear magnetic resonance
Quantum information processing has been effectively demonstrated on a small
number of qubits by nuclear magnetic resonance. An important subroutine in any
computing is the readout of the output. ``Spectral implementation'' originally
suggested by Z.L. Madi, R. Bruschweiler and R.R. Ernst,
[J. Chem. Phys. 109, 10603 (1999)], provides an elegant method of readout
with the use of an extra `observer' qubit. At the end of computation, detection
of the observer qubit provides the output via the multiplet structure of its
spectrum. In "spectral implementation" by two-dimensional experiment the
observer qubit retains the memory of input state during computation, thereby
providing correlated information on input and output, in the same spectrum.
"Spectral implementation" of Grover's search algorithm, approximate quantum
counting, a modified version of Berstein-Vazirani problem, and Hogg's algorithm
is demonstrated here in three and four-qubit systems.Comment: 39 pages,11 figure
Boson Sampling from Gaussian States
We pose a generalized Boson Sampling problem. Strong evidence exists that
such a problem becomes intractable on a classical computer as a function of the
number of Bosons. We describe a quantum optical processor that can solve this
problem efficiently based on Gaussian input states, a linear optical network
and non-adaptive photon counting measurements. All the elements required to
build such a processor currently exist. The demonstration of such a device
would provide the first empirical evidence that quantum computers can indeed
outperform classical computers and could lead to applications
Non-Gaussian states for continuous variable quantum computation via Gaussian maps
We investigate non-Gaussian states of light as ancillary inputs for
generating nonlinear transformations required for quantum computing with
continuous variables. We consider a recent proposal for preparing a cubic phase
state, find the exact form of the prepared state and perform a detailed
comparison to the ideal cubic phase state. We thereby identify the main
challenges to preparing an ideal cubic phase state and describe the gates
implemented with the non-ideal prepared state. We also find the general form of
operations that can be implemented with ancilla Fock states, together with
Gaussian input states, linear optics and squeezing transformations, and
homodyne detection with feed forward, and discuss the feasibility of continuous
variable quantum computing using ancilla Fock states.Comment: 8 pages, 6 figure
On the Complexity of Random Quantum Computations and the Jones Polynomial
There is a natural relationship between Jones polynomials and quantum
computation. We use this relationship to show that the complexity of evaluating
relative-error approximations of Jones polynomials can be used to bound the
classical complexity of approximately simulating random quantum computations.
We prove that random quantum computations cannot be classically simulated up to
a constant total variation distance, under the assumption that (1) the
Polynomial Hierarchy does not collapse and (2) the average-case complexity of
relative-error approximations of the Jones polynomial matches the worst-case
complexity over a constant fraction of random links. Our results provide a
straightforward relationship between the approximation of Jones polynomials and
the complexity of random quantum computations.Comment: 8 pages, 4 figure
On Quantum Algorithms
Quantum computers use the quantum interference of different computational
paths to enhance correct outcomes and suppress erroneous outcomes of
computations. In effect, they follow the same logical paradigm as
(multi-particle) interferometers. We show how most known quantum algorithms,
including quantum algorithms for factorising and counting, may be cast in this
manner. Quantum searching is described as inducing a desired relative phase
between two eigenvectors to yield constructive interference on the sought
elements and destructive interference on the remaining terms.Comment: 15 pages, 8 figure
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