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Engineers are extending the capabilities of these ultra-sensitive detectors to the nanoscale, with potential uses for quantum computing and biological sensing.

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

The new method, for which the team has already applied for patent protection, is described in the journal Physical examination Xin an article by graduate student Guoqing Wang, professor of nuclear sciences, engineering, and physics Paola Capellaro, and four others at MIT and the Lincoln Laboratory.

Quantum sensors can take many forms; they are essentially systems in which some particles are in such a delicate state of equilibrium that they are affected by even minimal variations in the fields to which they are exposed. These can take the form of neutral atoms, trapped ions and solid-state spins, and research using such sensors has grown 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 memory or computing devices. But many other interesting phenomena cover a much wider range of frequencies than what current quantum sensors can detect.

The new system developed by the team, which they call a quantum mixer, injects a second frequency into the detector using a microwave beam. This converts the frequency of the field being studied into a different frequency – the difference between the original frequency and that of the added signal – which 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 sensor’s nanoscale spatial resolution.

In their experiments, the team used a specific device based on a series of nitrogen vacation centers in the diamond, a widely used quantum sensing system, and successfully demonstrated detection of a signal with a frequency of 150 megahertz, using a qubit detector with a frequency of 2.2 gigahertz – detection that would be impossible without the quantum multiplexer. They then carried out detailed analyzes of the process by deriving a theoretical framework, based on Floquet’s theory, and testing the numerical predictions of this theory in a series of experiments.

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

Wang says this system could be used, for example, 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 alter the frequency sensitivity of some quantum sensors, but these require the use of large devices and strong magnetic fields that obscure the finest details and make it impossible to achieve the very high resolution offered by the new system. In such systems today, Wang says, “you have to use a strong magnetic field to tune the sensor, but that magnetic field can potentially break down the properties of the quantum material, which can affect the phenomena you want to measure.”

The system could open up new applications in the biomedical field, according to Cappellaro, because it can make a range of frequencies of electrical or magnetic activity accessible at the level of a single cell. It would be very difficult to get useful resolution from these signals using current quantum sensing systems, he says. It may be possible to use this system to detect a single neuron’s output signals in response to certain stimuli, for example, which usually include a lot of noise, making these signals difficult to isolate.

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

In ongoing work, the team is exploring the possibility of finding ways to extend the system so that it can probe one range of frequencies at a time, rather than the single-frequency targeting of the current system. They will also continue to define the capabilities of the system using more powerful quantum sensing devices at the Lincoln Laboratory, where members of the research team are based.

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

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