Entangled Ions Have Been "Purified"

Entangled ions have been "purified" at record levels by researchers at the National Institute of Standards and Technology in Boulder, Colo., providing another tool that will be helpful in constructing real-world quantum computers. Entanglement is a quantum-mechanical property in which multiple particles, such as photons or atoms, become interlinked so that measuring a previously undetermined property in one particle instantly determines the property of the others.

Particles must be entangled for them to work together in a quantum computer. However, entanglement is a fragile property that is easily destroyed by outside disturbances, such as stray magnetic fields, which can demolish this special quantum property through a process known as decoherence. Even when particles become entangled, experimenters might not get the results they desire, especially if the entangled pairs are further manipulated. For example, if researchers would like entangled particles to have the same values of "spin" (e.g., spin-up) when they're finally measured, they're not going to get this desired "correlation" 100 percent of the time. This is troublesome, as quantum computers and other quantum devices depend on particles that are entangled in the intended fashion.

To combat this last entanglement complication, enter the idea of "purification," first proposed by Charles Bennett of IBM and his colleagues a decade ago (Bennett et al., Physical Review Letters, 29 January 1996) and first demonstrated with pairs of photons (Kwiat et al., Nature, 22 February 2001). Essentially a distillation process, purification improves the probabilities that the entangled particles will have the desired correlations (e.g., the same value of spin). In the process, researchers can verify that they have indeed purified the particles.

In the NIST demonstration, ultraviolet lasers first entangle two pairs of beryllium ions. The lasers then "cross-entangle" a member of the first pair with a member of the second. In the process the lasers perform a number of "purifying" operations that raise the chances that the ions will be properly correlated. Measuring the cross-entangled pair provides information on whether the other two entangled ions were purified. The researchers found that the purification process worked 30 percent of the time in entangled ions, much higher than previous demonstrations involving photons.

In a sense, purification is a kind of pre-emptive error correction tool, preventing undesirable mistakes in operations involving entangled particles. But it doesn't last for long. Left alone, purified particles will lose their correlations at the same rate as non-purified particles, as a result of decoherence. But according to NIST researcher Dietrich Leibfried (dil@boulder.nist.gov), one can purify the particles again and restore the entanglement to its initial high levels. In that sense one can slow down or even completely stall decoherence, Leibfried says.

Reichle et al., Nature, 19 October 2006,
Contact Dietrich Leibfried
National Institute of Standards and Technology
dil@boulder.nist.gov
For more information see the NIST press release