Is quantum computer correlated with overlay

Quantum measurement: more precise than Heisenberg allowed?

The position and momentum of a quantum particle can be predicted better than would be expected from Heisenberg's uncertainty principle. As researchers have now found out, this works when the receiver uses a quantum memory made up of ions or atoms. Your study, published in “nature Physics”, demonstrates for the first time that the uncertainty depends on the strength of the correlation between the quantum memory and the quantum particle.

The special thing about quantum particles is that they can assume several states at the same time, i.e. 0 or 1 or an overlay of 0 and 1. The possibility of overlaying opens up enormous new computing potential for the quantum computer. But quantum particles are difficult to grasp because not all of their properties can be measured exactly at the same time. For certain parameter pairs - for example position and momentum - there remains a residual inaccuracy, determined by Heisenberg's uncertainty relation. It defines how precisely a quantum state can be determined.

Conversely, quantum mechanics says that the choice of measurement method can change the state of the quantum particle. As soon as one size is measured exactly, the particle falls into the maximally indeterminate state for the other parameter. Quantum cryptography makes use of this principle to encrypt data. Among other things, it uses quantum particles whose state is correlated in such a way that the probability that the measurement of one particle will produce a certain result depends on the state of the other particle. An attempt at eavesdropping would be uncovered because the measurement changes the state of the "eavesdropped" particle.

Quantum memory reduces blurring

A group of scientists from LMU Munich and ETH Zurich, including Professor Matthias Christandl, are now showing that the measurement result of a quantum particle can be better predicted if information about the particle is available in a quantum memory. A quantum memory can be made up of ions or atoms, for example. "With our research we want to find out how quantum memories, i.e. memories for quantum bits, can be used in the future and how they influence the transmission of quantum bits," explains Christandl.

This was the first time that a formulation of Heisenberg's uncertainty principle was derived, which takes the influence of a quantum memory into account. In the case of very strongly correlated, entangled particles, the blurring can even disappear completely. Christandl draws a comparison: “You could say that the disorder or indeterminacy of the particle depends on the information contained in the quantum memory. It's similar to papers on a desk: they often only show order for the person who placed them there. "

More security for quantum cryptographic systems

"Our result not only contributes to a better understanding of quantum memories, but also allows the correlation of two quantum particles to be determined," says Christandl. “The connection could also help to check the security of quantum cryptographic systems.” This is conceivable in the context of a game, when player B sends a particle to player A. The measurement by A creates a blurring. "B can now also measure, but will only match the value determined by A up to the Heisenberg limit," says the physicist. "If he uses a quantum memory, he will hit the desired value and win the game."

(Ludwig Maximilians University Munich, 07/27/2010 - NPO)

July 27, 2010