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Quantum Squeezing Lets Researchers Measure Sub-Nanoscale Motion

June 24, 2019
Juggling uncertainty gives close-up look at very small objects.

Researchers at the National Institute of Standards and Technology (NIST) have harnessed the phenomenon of “quantum squeezing” to amplify and measure the motion of a magnesium ion as it travelled a few trillionths of a meter. The phenomenon takes advantage of the Heisenberg Uncertainty Principle and trades off accuracy in measuring momentum for more accuracy measuring position.

NIST’s rapid, reversible squeezing method could improve the detection of extremely weak electric fields and observations of the absorption of slight amounts of light in devices such as atomic clocks. The technique could also speed up operations in quantum computers.

The NIST physicists describe quantum squeezing using a metaphor involving a partially inflated sausage-shaped balloon. The air inside represents the uncertainty in the system. Quantum squeezing is squeezing the balloon at one end so most of the air goes to the other end and inflates it a bit more. The total amount of uncertainty remains the same, but you’ve moved it where you want it. So, the researchers devised a way to move uncertainty from a place where they wanted more precise measurements to one having less precision while keeping the total uncertainty of the system the same.

This diagram represents 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 done 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. (Courtesy: Burd/NIST)

In the case of the magnesium ion, measurements of its motion are normally limited by so-called quantum fluctuations in the ion’s position and momentum, which occur all the time, even when the ion has the lowest possible energy. Squeezing manipulates these fluctuations by pushing uncertainty from the position to the momentum when they want more accuracy as to its position.

In NIST’s method, a single ion is held in space 30 micrometers (millionths of a meter) above a flat sapphire chip covered with gold electrodes. These electrodes trap and control the ion. Laser and microwave pulses “calm” the ion’s electrons and reduce their motion to their lowest-energy states. Their motion is then “squeezed” by varying the voltage on certain electrodes at twice the natural frequency of the ion’s back-and-forth motion for just a few microseconds.

After the squeezing, a small, oscillating electric field “test signal” is applied to the ion to make it move a little bit in three-dimensional space. To be amplified, this extra motion needs to be “in sync” with the squeezing.

Finally, the squeezing step is repeated, but now with the electrode voltages exactly out of sync with the original squeezing voltages. This out-of-sync squeezing reverses the initial squeezing; however, at the same time it amplifies the small motion caused by the test signal. When this step is complete, the uncertainty in the ion motion is back to its original value, but the back-and-forth motion of the ion is larger than if the test signal had been applied without any of the squeezing steps.

To get the results, an oscillating magnetic field is applied to map or encode the ion’s motion onto its electronic “spin” state, which is then measured by shining a laser on the ion and observing whether it fluoresces.

This animation demonstrates the concept of using quantum squeezing to measure ion motion. (Courtesy D. Slichter, S. Burd/NIST)

Using a test signal lets researchers measure how much amplification their technique provides. In a real sensing application, the test signal would be replaced by the actual signal that needs to be amplified and measured.

The NIST method can amplify and quickly measure ion motions of just 50 picometers (trillionths of a meter), which is about one-tenth the size of the smallest atom (hydrogen) and about one-hundredth the size of the unsqueezed quantum fluctuations. Even smaller motions can be measured by repeating the experiment and averaging the results. The squeezing-based amplification technique lets motions of a given size to be sensed with 53 times fewer measurements than would otherwise be needed.

Squeezing has previously been achieved in a variety of physical systems, including ions, but the NIST result represents one of the largest squeezing-based sensing improvements ever reported.

NIST’s new squeezing method can boost measurement sensitivity in quantum sensors and could be used to more rapidly create entanglement, which links properties of quantum particles, thus speeding up quantum simulation and quantum computing operations. The amplification method is applicable to other vibrating mechanical objects and charged particles such as electrons.

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