Quantum sensor can detect electromagnetic signals of any frequency: Engineers are expanding the capabilities of these ultra-sensitive nanoscale detectors, with potential applications for quantum computing and biological sensing.

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 have only been able to detect a few specific frequencies of these fields, limiting their usefulness. Now, researchers at MIT have developed a method that allows such sensors to detect any frequency without losing their ability to measure features at the nanometer scale.

The new method, for which the team has already filed for patent protection, is described in the journal Physical Assessment Xin a paper by graduate student Guoqing Wang, professor of nuclear and engineering and physics Paola Cappellaro, and four others at MIT and Lincoln Laboratory.

Quantum sensors can take many forms; they are essentially systems in which some particles are in such a delicately balanced state that they are affected by even small variations in the fields they are exposed to. These can take the form of neutral atoms, trapped ions and solid-state spins, and research with such sensors has grown rapidly. For example, physicists use them to investigate 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 span a much wider frequency range than current quantum sensors can detect.

The new system the team devised, which they call a quantum mixer, injects a second frequency into the detector using a beam of microwaves. This converts the frequency of the field being studied into another frequency – the difference between the original frequency and that of the added signal – tuned to the specific frequency to which the detector is most sensitive. This simple process allows the detector to focus at any desired frequency without losing the spatial resolution of the nanoscale sensor.

In their experiments, the team used a specific device based on a series of nitrogen void centers in diamond, a widely used quantum detection system, and successfully demonstrated the detection of a signal at 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 did detailed analyzes of the process by deriving a theoretical framework based on Floquet theory and testing that theory’s numerical predictions in a series of experiments.

While their tests used this particular system, Wang says, “the same principle can also be applied to all kinds of sensors or quantum devices.” The system would be self-contained, with the detector and the second frequency source all packed into 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 shows promise in that direction,” he says.

There are other ways to change the frequency sensitivity of some quantum sensors, but this requires large devices and strong magnetic fields that blur the fine 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 could potentially break the properties of the quantum material, affecting the phenomena you want to measure.”

The system could enable 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 a usable resolution of such signals with current quantum detection systems, she says. It may be possible to use this system to detect outputs from a single neuron in response to a particular stimulus, for example, which is typically noisy, making such signals difficult to isolate.

The system can 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 to be able to examine a range of frequencies at once, rather than the single frequency targeting of the current system. They will also continue to define the system’s capabilities using more powerful quantum sensing devices at the Lincoln Laboratory, where some 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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