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A Cirac-Zoller
controlled-NOT quantum gate with two ions
Quantum
computing and entangled states in Innsbruck
Inside a computer, data are stored as bits which take the value 0 or 1.
Electronic transistors process the chosen program and the user's input
by interconnecing these bits until they get the desired result of the
task. Like today's ordinary computer, a future quantum computing
machine will carry out computational tasks. Some problems, however, can
be solved much more efficiently on a quantum computer than on an
ordinary classical maschine. In a quantum computer, the information is
stored in qubits (quantum bits of information) which can take values of
0 and 1 like their classical couterparts, but also any coherent superposition of 0 and
1. Quantum circuits, which consist of logic operations between the
qubits, replace the electric circuits of a classical computer.
Now a new experiment in Innsbruck shows that trapped ions, manipulated
by well-tailored laser pulses, may serve as a central processor for a
future quantum computer. The information is stored in the quantum
states
of single atoms.
The story dates back to 1995, when theorists I. Cirac and P. Zoller
from Innsbruck proposed to implement a quantum computer on a string of
trapped ions. In this scheme, each ion's electronic state represents a
qubit, and a quantum gate between any pair of ions is realised by
coupling them through their collective motion. A major experimental
step
towards a Cirac-Zoller type computer has now been realized by
implementing the controlled-NOT (CNOT) gate operation between two
individual ions. The CNOT quantum logical gate corresponds to the XOR
gate operation of classical logic which flips the state of a target bit
conditioned on the state of a control bit.
Perhaps the strangest and most puzzling features of the quantum world
are the notions of a superposition (a
single qubit is in "both" states, 0 and 1) and of entanglement, which was introduced
by the Austrian Nobel prize laureate Erwin Schrödinger about 75
years ago. The CNOT gate operation can be used to entangle two atoms:
while we do not have any certainty about the individual properties of
either qubit (i.e. whether it is in 0 or 1), we know with certainty
about their correlations. Thus, if one qubit is 0, the other is also in
0. This "nonlocal" property of quantum mechanics can now be studied
more
deeply in the new experiment.
For details of the experiment, please see the Slide-presentation
of results in English, pdf format,
Download
"Realization of the Cirac–Zoller controlled-NOT quantum
gate", F.
Schmidt-Kaler,
H. Häffner, M. Riebe, S. Gulde, G. P. T. Lancaster, T. Deuschle,
C.
Becher, C. F. Roos, J. Eschner & R. Blatt, Nature 422, 408-411 (2003). Download
We are financially supported
by Universität Innsbruck,
the Austrian
research fund (FWF) within the program
"Control and Measurement of
Coherent Quantum Systems", the European networks
QGATES and
CONQUEST, IQI, ARO and
the
Austrian academy of sciences.
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