Engineers are extending the capabilities of these highly sensitive detectors to the nanoscale with potential applications in quantum computing and biological sensing.

Quantum sensors, which detect the smallest fluctuations in magnetic or electric fields, have enabled precision measurements in materials science and fundamental physics. However, these sensors were only able to detect a few specific frequencies of these fields, limiting their usefulness. Now, researchers at MIT have developed a way for these sensors to detect any frequency without losing their ability to measure nanoscale features.

The new process, for which the team has already applied for patent protection, is described in the specialist journal Physical Examination Xin an article by graduate student Guoqing Wang, Professor of Nuclear Science and Engineering and Physics Paola Capellaro, and four others at MIT and Lincoln Laboratory.

Quantum sensors can take many forms; They are essentially systems in which certain particles are in such a delicate state of equilibrium that they are affected by even minute fluctuations in the fields to which they are exposed. These can take the form of neutral atoms, trapped ions, and solid-state spins, and research into such sensors has increased rapidly. For example, physicists use them to study exotic states of matter, including so-called time crystals and topological phases, while other researchers use them to characterize practical devices such as experimental quantum memories or computing devices. But many other interesting phenomena cover a much wider frequency range than current quantum sensors can detect.

The new system the team has developed, which they call a quantum mixer, uses a beam of microwaves to inject a second frequency into the detector. This converts the frequency of the field under study to another frequency – the difference between the original frequency and that of the added signal – that is tuned to the specific frequency to which the detector is most sensitive. This simple process allows the detector to tune to any desired frequency without losing the nanoscale spatial resolution of the sensor.

In their experiments, the team used a specific device based on an array of nitrogen vacancies in diamond, a widely used quantum detection system, and successfully demonstrated the detection of a signal with a frequency of 150 megahertz using a qubit detector with a frequency of 2 .2 gigahertz – a detection that would be impossible without the quantum multiplexer. They then performed detailed analyzes of the process by deriving a theoretical framework based on Floquet’s theory and testing the numerical predictions of that theory in a series of experiments.

Although their tests used this specific system, Wang says, “the same principle can be applied to any type of quantum sensor or device.” The system would be self-contained with the detector and second frequency source all housed in a single device.

According to Wang, for example, this system could be used to characterize the performance of a microwave antenna in detail. “It can characterize the distribution of the field [generated by the antenna] with nanoscale resolution, so it’s very promising in that direction,” he says.

There are other ways to change the frequency sensitivity of some quantum sensors, but these require the use of large devices and powerful magnetic fields that blur fine details and make it impossible to achieve the very high resolution that the new system offers. In such systems today, Wang says, “you have to use a strong magnetic field to tune the sensor, but that magnetic field can potentially affect the properties of the quantum material, which can affect the phenomena you want to measure.”

The system could open up new applications in biomedical fields, according to Cappellaro, as it can access a range of frequencies of electrical or magnetic activity at the level of a single cell. It would be very difficult to get any usable resolution out of these signals with current quantum detection systems, she says. It might be possible to use this system to capture, for example, the output signals of a single neuron in response to certain stimuli, which usually contain a lot of noise, making it difficult to isolate these signals.

The system could also be used to characterize in detail the behavior of exotic materials such as 2D materials, which are intensively studied for their electromagnetic, optical and physical properties.

In ongoing work, the team is exploring the possibility of finding ways to expand the system to study a range of frequencies simultaneously, rather than focusing the current system on a single frequency. They will also continue to define the system’s capabilities using more powerful quantum sensor devices at the Lincoln Laboratory, where some members of the research team are based.

The team included MIT’s Yi-Xiang Liu and Lincoln Laboratory’s Jennifer Schloss, Scott Alsid and Danielle Braje. The work was supported by the Defense Advanced Research Projects Agency (DARPA) and Q-Diamond.

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