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Fundamental limitation to Quantum Computers
[July 7th 2005]
Dutch Theoretical physicist from Fundamental Research on Matter and Leiden University discovers a fundamental limitation to the storage
mechanism of Quantum Comuputers. Researchers published
the related article recently in the journal Physical
Review Letters.
Storage system of a
conceptualized Quantum Computers is based on Coherence
of the Quantum bits (Qubits). Coherence is tendency
of maintaining relationship between two quantum states,
this relationship is also called Quantum Entanglement.
Jasper van Wezel, Jeroen van den Brink (FOM) and Jan
Zaanen, all attached to the Lorentz Institute of Leiden
University claims that coherence disappears spontaneously
over course of the time. And this would be a major
bottleneck in developing storage mechanism for quantum
computers.
The 0 and 1 of a quantum
bit (called qubit) might be the ground and excited
states of an atom in a linear ion trap; they might
be polarizations of photons that interact in an optical
cavity; they might even be the excess of one nuclear
spin state over another in a liquid sample in an Nuclear
Magnetic Resonance (NMR) machine. As long as there
is a way to put the system in a quantum superposition
and interact multiple qubits, a system can potentially
be used as a quantum computer.
A quantum bit typically consists of a large number
of particles, with an unavoidably large number of
possibilities to be influenced by the environment
and thus be subjected to decoherence. Jasper van Wezel,
Jeroen van den Brink (FOM) and Jan Zaanen, all attached
to the Lorentz Institute of Leiden University have
now investigated whether it is possible to maintain
the coherence in an isolated qubit.
Much to their surprise
they discovered that the coherence tends to spontaneously
disappear, even without external influences. The degredation
process is linked to the occurrence of quantum mechanical
spontaneous symmetry breaking. In classical physics
an equivalent example of this process is spontaneous
crystallisation in a solution. At a certain position
a crystal is spontaneously formed, as a result of
which the fluid structure is broken.
According to the researchers'
predictions, the coherence in some highly promising
concepts for qubits will disappear after about a second.
Moreover, the smaller the qubits the faster that process
occurs. All of
this would seem to pose a fundamental limitation on
the development of qubits. Experimental research will
now have to demonstrate whether this phenomenon actually
occurs.
Original News can be found at
http://www.nwo.nl/nwohome.nsf/pages/NWOP_6DJ9XW_Eng
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