Go further · Chapter 9 · 9b · A step deeper
How do you hold a single atom still?
The short answer (tap to open)
To use single atoms as qubits you must hold them still in a vacuum. Charged atoms (ions) sit in electric fields that flip back and forth millions of times a second. Neutral atoms sit in the focus of laser beams called optical tweezers. Lasers also cool them almost to a standstill first. Three Nobel Prizes cover these tools.
Why can't you just use a fixed electric field for ions?
A rule of physics called Earnshaw's theorem says no fixed arrangement of electric charges can hold a charged particle still: push it in from one side and it escapes from another. Wolfgang Paul's fix, from the 1950s, is to make the field flip direction millions of times a second. On average the ion feels pushed back towards the centre from every side. Paul shared the 1989 Nobel Prize in Physics for it.
The ball rolls off.
The saddle, in words
- Still, the saddle tips the ball off along its downhill direction.
- Spun slowly, the ball still rolls off before the downhill direction turns away.
- Spun fast enough, the downhill direction keeps turning into uphill before the ball gets far, and on average it's pushed back to the middle.
That's how a Paul trap holds an ion.
How do lasers cool atoms?
Atoms at room temperature fly around at hundreds of metres per second. Shine a laser tuned just below an atom's colour (chapter 1) from every direction. An atom moving towards a beam sees its light slightly bluer, absorbs it, and gets a tiny push backwards. Thousands of pushes slow it almost to a stop, a millionth of a degree above absolute zero. Steven Chu, Claude Cohen-Tannoudji and William Phillips shared the 1997 Nobel Prize for laser cooling.
What are optical tweezers?
A tightly focused laser beam draws a small particle towards its brightest point. Arthur Ashkin invented these optical tweezers in the 1980s, first for tiny beads and living cells, and shared the 2018 Nobel Prize. Neutral-atom computers use hundreds or thousands of tweezers, steered by fast-switching optics, to hold and move individual atoms.
How do atoms talk to each other?
Ions push on each other through their charge and share a wobble (chapter 9). Neutral atoms don't, so a laser excites them into a Rydberg state, where the outer electron orbits so far out that the atom becomes hundreds of times larger. Two Rydberg atoms within a few micrometres block each other: if a laser excites one, it can't excite the other. That "Rydberg blockade" makes the two-qubit gate.
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Fields that flip, light that grips.
Three Nobel-winning tools hold single atoms still enough to compute with.