Physicists Measure Ultra-Small Motion by Supersizing “Quantum Squeezing”

By harnessing the phenomenon of “quantum squeezing,” physicists at the National Institute of Standards and Technology (NIST) have successfully amplified and determined trillionths-of-a-meter motions of a single trapped magnesium ion, that is, electrically charged atom.

Diagram of NIST’s ion trap used for reversible “quantum squeezing” to amplify and measure ion motion. The ion (white ball) is confined 30 micrometers above the trap surface by voltages applied to the eight gold electrodes and the two red electrodes. Squeezing—which reduces the uncertainty of motion measurements—is achieved by applying a specific signal to the red electrodes. The ion is moved by applying another type of signal to one of the gold electrodes. Then the squeezing is reversed, and the blue electrodes generate magnetic fields used to decode the amplified motion measurements. (Image credit: S. Burd/NIST)

The fast, reversible squeezing technique developed by the NIST team has been described in the June 21st issue of Science. The method could improve the sensing of very weak electric fields in surface science applications, for instance, or it could also be used for detecting the absorption of extremely slight amounts of light in such devices like atomic clocks. In addition, the method may accelerate operations in a quantum computer.

By using squeezing, we can measure with greater sensitivity than could be achieved without quantum effects,” stated lead author Shaun Burd.

We demonstrate one of the highest levels of quantum squeezing ever reported and use it to amplify small mechanical motions. We are 7.3 times more sensitive to these motions than would be possible without the use of this technique.

Daniel Slichter, Physicist, National Institute of Standards and Technology

Quantum squeezing, unlike orange squeezing which can create a juicy mess, is an extremely accurate process, which shifts measurement uncertainty from place to place. Imagining holding a long balloon and the air present inside this balloon indicates uncertainty.

Quantum squeezing is similar to pinching one end of the balloon to force air into the other end. Uncertainty can be moved from a place where one needs more accurate measurements, to another place where one can live with less accuracy, while maintaining the system’s total uncertainty the same.

With regards to the magnesium ion, measurements of its movement are usually restricted by what is known as quantum fluctuations in the momentum and position of the ion, which take place all the time, even when the ion contains the lowest possible energy. These fluctuations are manipulated by squeezing, for instance, by forcing uncertainty from the position to the momentum when there is a need for enhanced position sensitivity.

In the NIST technique, one ion is held in space of 30 µm (millionths of a meter) above a flat sapphire chip enclosed with gold electrodes used for trapping and regulating the ion. Following this, microwave and laser pulses are applied to keep the electrons and motion of the ion to their lowest-energy states. Subsequently, the motion is squeezed by wiggling the voltage on specific electrodes at double the natural frequency of the back-and-forth motion of the ion. It takes only a few microseconds for this process to last.

Post the squeezing, the ion receives a tiny, oscillating electric field “test signal”, allowing it to move slightly in three-dimensional (3D) space. To amplify this additional motion, it is important that it remains “in sync” with the squeezing. In the end, the squeezing step is again performed, but this time making sure that the electrode voltages are precisely out of sync with the original squeezing voltages. While this out-of-sync squeezing reverses the first squeezing, it simultaneously amplifies the slight motion induced by the test signal.

Upon completing this step, the uncertainty in the motion of the ion comes back to its original value; however, the ion’s back-and-forth motion is larger than in case the test signal had been applied without using any of the squeezing steps.

In order to achieve the results, an oscillating magnetic field is applied to encode or map the motion of the ion onto its electronic “spin” state, which is subsequently determined by illuminating a laser on the ion and watching whether it fluoresces.

The use of a test signal enabled the NIST team to determine the level of amplification provided by their method. In an actual sensing application, the test signal would be substituted by the real signal to be amplified and determined.

In addition, the NIST technique both amplifies and rapidly determines the motions of ions in just 50 picometers (trillionths of a meter), which is approximately one-hundredth the size of the unsqueezed quantum fluctuations and roughly one-tenth the size of the tiniest atom (hydrogen).

By repeating the experiment more times and then averaging the results, even smaller motions can be determined. Moreover, the squeezing-based amplification method makes it possible to sense motions of a specified size with 53 times fewer measurements than would otherwise be required.

In the past, squeezing has been realized in a wide range of physical systems, including ions; however, the NIST outcome indicates one among the largest squeezing-based sensing improvements to be ever reported.

The new squeezing technique developed by the NIST researchers can increase measurement sensitivity in quantum sensors and can perhaps be applied to produce entanglement more quickly, which connects the properties of quantum particles, thereby boosting up operations related to quantum computing and quantum simulation. In addition, the techniques may be used to create unusual motional states. The amplification technique is relevant to several other vibrating mechanical objects as well as other charged particles like electrons.

The study was partly supported by the Army Research Office and the Office of Naval Research.

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