More accurate than Heisenberg allows?

A quantum particle is hard to grasp, because one cannot determine all its properties precisely at the same time. Measurements of certain parameter pairs such as position and momentum remain inaccurate to a degree given by Heisenberg's Uncertainty Principle. This is important for the security of quantum cryptography, where information is transmitted in the form of quantum states such as the polarization of particles of light. A group of scientists from LMU and the ETH in Zurich, including Professor Matthias Christandl, has now shown that position and momentum can be predicted more precisely than Heisenberg's Uncertainty Principle would lead one to expect, if the recipient makes use of a quantum memory that employs ions or atoms. The results show that the magnitude of the uncertainty depends on the degree of correlation ("entanglement") between the quantum memory and the quantum particle. "The result not only enhances our understanding of quantum memories, it also provides us with a method for determining the degree of correlation between two quantum particles", says Christandl. "Moreover, the effect we have observed could yield a means of testing the security of quantum cryptographic systems." (Nature Physics online, July 25, 2010)

Unlike classical computers, quantum computers operate not with bits, but with quantum bits or qubits, quantum mechanical states of particles. The crucial feature of qubits is that they can exist in different states at once, not just 0 or 1, but also as a superposition of 0 and 1. The ability to exploit superposition states is what makes quantum computers potentially so powerful. "The goal of our research is to work out how quantum memories, i.e. memory systems for qubits, might be utilized in the future and how they affect the transmission of quantum bits", explains Christandl, who left LMU Munich in June 2010 to take up a position in the Institute of Theoretical Physics at the ETH in Zurich. ...

via More accurate than Heisenberg allows?.

In quantum computing, a qubit (pronounced /ˈkʲuːbɪt/) or quantum bit is a unit of quantum information —the quantum analogue of the classical bit —with additional dimensions associated to the quantum properties of a physical atom. The physical construction of a quantum computer is itself an arrangement of entangled atoms, and the qubit represents both the state memory and the state of entanglement in a system. A quantum computation is performed by initializing a system of qubits with a quantum algorithm —"initialization" here referring to some advanced physical process that puts the system into an entangled state.

The qubit is described by a state vector in a two-level quantum-mechanical system, which is formally equivalent to a two-dimensional vector space over the complex numbers. - wiki