Physicists boost 'entanglement' of atom pairs

Oct 18, 2006
NIST Process for Purifying Entangled Ions
The NIST process for "purifying" an unusual property of quantum physics called entanglement involves illuminating two pairs of beryllium ions (charged atoms) with a series of ultraviolet laser pulses. Credit: Bill Pietsch, Astronaut 3 Media Group Inc.











Background: Purifying Entangled Ions

Just as a magnet has a north and south pole, an ion has a “magnetic moment,” which for quantum computing purposes is called the spin (a spinning electrical charge is one way to create a magnetic moment). The values 0 and 1 are represented by the direction of the spin: Spin up is 0, and spin down is 1. In beryllium ions, the spin down state fluoresces when illuminated by a laser beam with a certain wavelength, whereas spin up does not.

Ions can exist in both 0 and 1 states at the same time, a quantum condition known as superposition. A superposition may be closer to 0, about equally positioned between 0 and 1, or closer to 1. When the ion is measured, the superposition “collapses” to either 0 or 1, depending on the superposition at the time. In an equal superposition, the ion would have a 50 percent chance of being measured as 0 and a 50 percent chance of being measured as a 1.

The NIST scientists adapted a theoretical method developed by another research group for purifying entangled particles, as follows:

Step 1—All four ions are prepared spin down (fluorescing).

Step 2—A laser pulse rotates all the ions to place them in equal superpositions of all possible spin states.

Step 3—Two ultraviolet laser beams, positioned at right angles and overlapping the ions, apply an oscillating force to the four ions, to entangle them in two pairs. The lasers are tuned so the difference between their frequencies is very close to the frequency of one of the ions’ natural vibrations, a motion of swinging back and forth. The ions are brought into a superposition of standing still while also swinging left and right in unison. The superposition of different motions leaves an imprint on the spin states that has the effect of entangling pairs of ions in a controlled way.

Step 4—A laser pulse undoes the rotation in Step 2. The four ions are now entangled in pairs.

Entangled particles do not necessarily have identical properties, just properties that are linked in predictable ways. The NIST team can establish different forms of entanglement based on the spacing between the ions, the type of ion motion targeted, and the duration of the laser pulses. The purification process was designed so that each pair of entangled ions has a superposition of spin states that are not identical, but rather are mirror images of each other: ↑↓ and ↓↑. This means that, if ion 1 were measured as spin up, for example, then its entangled partner, ion 2, would be certain to be spin down. The individual states of ions 3 and 4 at this point would be unknown.

Step 5—A laser operation similar to Steps 1-4 is used to entangle each ion of the first pair with its counterpart in the second pair, so ions 1 and 3 are entangled and ions 2 and 4 are entangled. The properties of all four ions are now linked.

Step 6—Ions 1 and 2 are measured by illuminating them with a laser pulse and evaluating their overall fluorescence. Very high or low fluorescence levels indicate the two ions are in the same spin state; a mid-range value indicates they are in different states. If ions 1 and 2 have identical values, then an error has occurred somewhere. (They were entangled in a way that should make these results different.) This means that ions 3 and 4 probably are not entangled with higher fidelity. But, if ions 1 and 2 have opposing spins or values, then ions 3 and 4 (unmeasured) have been purified and are now more likely to be entangled with mirror-image superpositions of spin states than before. (That is, ions 3 and 4 are more likely to be in an entangled state of ↑↓ and ↓↑). Ions 3 and 4 would thus be available for use in a quantum computer or communications system.

In a final step to verify their success (this would not be done in a computer or other real device), the NIST scientists used lasers to rotate ions 3 and 4 by various amounts, and measured them at each angle by illuminating them with a laser pulse. The fluorescence resulting from the different rotation angles was compared to known patterns produced by perfectly entangled pairs.

Citation: R. Reichle, D. Leibfried, E. Knill, J. Britton, R.B. Blakestad, J.D. Jost, C. Langer, R. Ozeri, S. Seidelin, and D.J. Wineland. Experimental purification of two-atom entanglement. Nature. Oct. 19, 2006.

Source: NIST


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