Quantum crystal with “time inversion” could be a new dark matter sensor

Physicists at the National Institute of Standards and Technology (NIST) have linked, or “entangled,” the mechanical motion and electronic properties of a tiny blue crystal, giving it a quantum advantage in measuring electric fields with a record sensitivity that can improve understanding of the universe.

The quantum sensor is made up of 150 beryllium ions (electrically charged atoms) confined in a magnetic field, so that they self-organize into a 2D flat crystal only 200 millionths of a meter in diameter. Quantum sensors like this have the potential to detect signals from dark matter – a mysterious substance that could turn out to be, among other theories, subatomic particles that interact with normal matter through a weak field. electromagnetic. The presence of dark matter could cause the crystal to move in a telltale way, revealed by collective changes among the ions in the crystal in one of their electronic properties, known as spin.

As described in the August 6, 2021 issue of Science, researchers can measure the vibrational excitement of the crystal – the flat plane moving up and down like the head of a drum – by monitoring changes in collective rotation. The measurement of the spin indicates the extent of the vibrational excitation, called displacement.

This sensor can measure external electric fields that have the same vibrating frequency as the crystal with more than 10 times the sensitivity of any atomic sensor previously demonstrated. (Technically, the sensor can measure 240 nanovolts per meter in one second.) In the experiments, researchers apply a weak electric field to excite and test the crystal sensor. A dark matter search would look for such a signal.

“Ion crystals could detect certain types of dark matter – for example axions and hidden photons – that interact with normal matter through a weak electric field,” NIST lead author John Bollinger said. “Dark matter forms a background signal with an oscillation frequency that depends on the mass of the dark matter particle. Experiments looking for this type of dark matter have been going on for more than a decade with superconducting circuits. The movement of the trapped ions provides sensitivity over a different range of frequencies.

Bollinger’s group has been working with ionic crystal for over a decade. What is new is the use of a specific type of laser light to entangle the movement and collective spins of large numbers of ions, as well as what the researchers call a “time inversion” strategy. To detect the results.

The experiment benefited from a collaboration with NIST theorist Ana Maria Rey, who works at JILA, a joint institute of NIST and the University of Colorado Boulder. Theoretical work was essential to understand the limitations of the laboratory setup, offered a new model for understanding the experiment that is valid for a large number of trapped ions, and demonstrated that the quantum advantage comes from the entanglement of the spin and movement, said Bollinger.

Rey noted that entanglement is beneficial in canceling the intrinsic quantum noise of ions. However, it is difficult to measure the entangled quantum state without destroying the information shared between spin and motion.

“To avoid this problem, John is able to reverse the dynamics and disentangle the rotation and motion after the displacement is applied,” Rey said. “This time reversal decouples rotation and motion, and now the collective rotation itself contains the displacement information, and when we measure the rotations, we can determine the displacement very precisely.

The researchers used microwaves to produce the desired values ​​of the spins. Ions can be facing up (often thought of as an arrow pointing up), down, or at other angles, including both at the same time, a special quantum state. In this experiment, the ions all had the same spin – first upwards and then horizontally – so when excited, they spun together in a pattern characteristic of spinning tops.

Crossed laser beams, with a frequency difference almost identical to that of the movement, were used to entangle the collective rotation with the movement. The crystal was then excited vibrator. The same lasers and microwaves were used to undo the tangle. To determine how far the crystal moved, the researchers measured the level of fluorescence from the spin of the ions (the spin up scatters light, the spin down is dark).

In the future, increasing the number of ions to 100,000 by making 3D crystals is expected to increase detection capacity by a factor of thirty. In addition, the stability of the excited movement of the crystal could be improved, which would improve the time inversion process and the accuracy of the results.

“If we are able to improve on this aspect, this experiment can become a fundamental resource for detecting dark matter,” Rey said. “We know that 85% of the matter in the universe is dark matter, but to date, we don’t know what dark matter is. This experience could allow us in the future to unveil this mystery.

Co-authors included researchers from the University of Oklahoma. This work is supported in part by the United States Department of Energy, Air Force Office of Scientific Research, Defense Advanced Research Projects Agency, Army Research Office and the National Science Foundation.

Reference: “Enhanced Quantum Detection of Movements and Electric Fields with Two-Dimensional Trapped Ion Crystals” by KA Gilmore, M. Affolter, RJ Lewis-Swan, D. Barberena, E. Jordan, AM Rey, and JJ Bollinger. , August 5, 2021, Science.
DOI: 10.1126 / science.abi5226