20 research outputs found
A simple scheme for expanding photonic cluster states for quantum information
We show how an entangled cluster state encoded in the polarization of single
photons can be straightforwardly expanded by deterministically entangling
additional qubits encoded in the path degree of freedom of the constituent
photons. This can be achieved using a polarization--path controlled-phase gate.
We experimentally demonstrate a practical and stable realization of this
approach by using a Sagnac interferometer to entangle a path qubit and
polarization qubit on a single photon. We demonstrate precise control over
phase of the path qubit to change the measurement basis and experimentally
demonstrate properties of measurement-based quantum computing using a 2 photon,
3 qubit cluster state
Testing sequential quantum measurements: how can maximal knowledge be extracted?
The extraction of information from a quantum system unavoidably implies a
modification of the measured system itself. It has been demonstrated recently
that partial measurements can be carried out in order to extract only a portion
of the information encoded in a quantum system, at the cost of inducing a
limited amount of disturbance. Here we analyze experimentally the dynamics of
sequential partial measurements carried out on a quantum system, focusing on
the trade-off between the maximal information extractable and the disturbance.
In particular we consider two different regimes of measurement, demonstrating
that, by exploiting an adaptive strategy, an optimal trade-off between the two
quantities can be found, as observed in a single measurement process. Such
experimental result, achieved for two sequential measurements, can be extended
to N measurement processes.Comment: 5 pages, 3 figure
Calculating Unknown Eigenvalues with a Quantum Algorithm
Quantum algorithms are able to solve particular problems exponentially faster
than conventional algorithms, when implemented on a quantum computer. However,
all demonstrations to date have required already knowing the answer to
construct the algorithm. We have implemented the complete quantum phase
estimation algorithm for a single qubit unitary in which the answer is
calculated by the algorithm. We use a new approach to implementing the
controlled-unitary operations that lie at the heart of the majority of quantum
algorithms that is more efficient and does not require the eigenvalues of the
unitary to be known. These results point the way to efficient quantum
simulations and quantum metrology applications in the near term, and to
factoring large numbers in the longer term. This approach is architecture
independent and thus can be used in other physical implementations
Experimental measurement-based quantum computing beyond the cluster-state model
The paradigm of measurement-based quantum computation opens new experimental
avenues to realize a quantum computer and deepens our understanding of quantum
physics. Measurement-based quantum computation starts from a highly entangled
universal resource state. For years, clusters states have been the only known
universal resources. Surprisingly, a novel framework namely quantum computation
in correlation space has opened new routes to implement measurement-based
quantum computation based on quantum states possessing entanglement properties
different from cluster states. Here we report an experimental demonstration of
every building block of such a model. With a four-qubit and a six-qubit state
as distinct from cluster states, we have realized a universal set of
single-qubit rotations, two-qubit entangling gates and further Deutsch's
algorithm. Besides being of fundamental interest, our experiment proves
in-principle the feasibility of universal measurement-based quantum computation
without using cluster states, which represents a new approach towards the
realization of a quantum computer.Comment: 26 pages, final version, comments welcom