Squeezing to amplify tiny displacements of a trapped ion

Short answer: Squeezing an oscillator's motion in one direction, letting a small displacement act, and then squeezing in the orthogonal direction amplifies the displacement without adding quantum noise, in principle. A trapped-ion experiment reported an increase in displacement sensitivity of up to a factor of 7.3, quoted in a mirrored version as 17.5 decibels. A 2020 theory paper proposed a related gate for trapped ions. The experiment is a laboratory demonstration.

Why is displacement sensing limited?

The Science abstract says detection of the weakest forces in nature is aided by increasingly sensitive measurements of the motion of mechanical oscillators, but that what can be known about the motion is limited by quantum fluctuations that exist even when the oscillator is in its lowest energy state. The introduction of the paper lists precision sensing uses of mechanical oscillators, including gravitational wave detection, atomic force microscopy, cavity optomechanics and the measurement of weak electric fields.

What did the experiment show?

The authors demonstrate a technique for amplifying coherent displacements of a mechanical oscillator whose initial magnitudes are well below the zero-point fluctuations. They apply two orthogonal squeezing interactions, one before and one after a small displacement, and the displacement is amplified, ideally with no added quantum noise. They implement the protocol with a trapped-ion mechanical oscillator, meaning the ion's motion in the trap. The Science abstract reports an increase by a factor of up to 7.3 plus or minus 0.3 in sensitivity to small displacements. A mirrored copy of the abstract on the exa.ai library page, whose venue is not confirmed there, quotes the increase as up to 17.5 plus or minus 0.3 decibels, which is a different way of expressing a gain of about this size. This post reports both as the authors gave them.

What did the theory paper propose?

A 2020 Physical Review A paper shows that the motion of a cold trapped ion can be squeezed by modulating the intensity of a phase-stable optical lattice placed inside the trap. The authors say the method is reversible and state selective, so it effectively implements a controlled-squeeze gate. They suggest it could be useful for continuous-variable quantum information processing, and show it can prepare coherent superpositions of states squeezed along complementary quadratures, which they call X states. They show these states are highly sensitive to small displacements along two complementary quadratures, which they say makes them useful for quantum metrology. This is a proposal and a calculation, and no experiment is reported in the abstract.

How to read this

The experimental paper and the mirrored abstract describe the same work, so the post covers one experiment and one theory proposal. A factor of 7.3 and 17.5 decibels are two ways of stating a gain, and the conversion depends on convention, so the post does not convert between them. All numbers are the authors' own. This post is based on abstracts and visible page text, and full texts were not checked. It makes no claim about products. For related ion-based sensing, see our post on trapped-ion force sensing.

FAQ

How does the amplification work?

Squeezing before and after a small displacement amplifies it, ideally with no added quantum noise, per the abstract.

What gain was reported?

A factor of up to 7.3, quoted elsewhere as 17.5 decibels.

What did the theory paper add?

A proposed controlled-squeeze gate for trapped ions that prepares states sensitive to displacements along two quadratures.

Is this a commercial sensor?

No. The papers describe a physics experiment and a proposal.