Build better quantum sensors

Usually, a flaw in a diamond is a bad thing. But for engineers, tiny dots in a diamond’s otherwise rigid crystal structure pave the way for ultra-sensitive quantum sensors that push the boundaries of today’s technologies. Now researchers at the University of Chicago’s Pritzker School of Molecular Engineering (PME) have developed a method to optimize these quantum sensors, which can, among other things, detect tiny disturbances in magnetic or electric fields.

Her new approach, published in PRX Quantumuses the way defects in diamonds or semiconductors behave like qubits – the smallest unit of quantum information.

“Researchers are already using this type of qubit to make really amazing sensors,” said Prof Aashish Clerk, senior author of the new work. “We found a better way to get the most information out of these qubits.”

Qubits light the way

A perfect diamond consists of carbon atoms arranged in a repeating lattice. Replace one of those atoms with something else — like a nitrogen atom — and the way the new, distinct atom sits within the diamond’s hard structure gives it unique quantum properties. Tiny changes in the environment, from temperature to electricity, change the way these “solid-state defects” spin and store energy.

Researchers discovered that they can shine a light on one of these qubits and then measure how light is deflected and released to study its quantum state. This way they can use it as a quantum sensor.

However, analyzing the information of a solid-state defect is difficult, especially when many such qubits are embedded in a sensor. As each qubit releases energy, that energy changes the behavior of nearby qubits.

“In the end, the qubits all correlate with each other in a weird way that doesn’t classically make sense,” Clerk said. “What a qubit does is closely related to what other qubits do.”

Additionally, if light shines on a qubit long enough, it reverts to its ground state, losing any information encoded in it.

amplify information

Clerk, along with colleagues including postdoc Martin Koppenhöfer, first author of the new paper, set out to ask a fundamental question in the physics of how qubits interact with each other. In the course of this research, they discovered a new trick to extract information from solid-state defect qubits.

When a network of solid-state defects releases energy in a burst of photons, researchers usually gloss over the exact nature of the qubits as that energy is released; they instead focus on the data before and after this sudden burst.

However, the Clerk group discovered that even more sensitive information about the qubits is encoded in this release of energy (dubbed “superradiant spin decay”).

“People had assumed that all qubits start out excited and end up relaxed, and it seems really boring,” he said. “But we found that there is this slight variation between qubits; they are not all fully excited and they are not all fully relaxed in sync.”

Clerk and his team focused on this long-ignored moment in the midst of superradiant spin decay, showing how the information stored in solid-state defects is amplified.

The future of quantum sensors

For engineers trying to develop quantum sensors that measure everything from magnetic fields – for better navigation or analysis of molecular structures – to temperature changes in living cells, the new approach offers a much-needed improvement in sensitivity.

“In the past, the very noisy final readout of qubits in these sensors really limited everything,” Clerk said. “Well, this mechanism gets you to a stage where you don’t care about that noisy ending display anymore; They focus on the more valuable data that was previously encoded.”

His team is now planning future research on how to improve the sensitivity of solid-state defects even further by distinguishing the data from each qubit, rather than getting a readout from the entire entanglement. They think their new approach will make that goal more achievable than in the past.