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 a few specific frequencies of these fields, limiting their usefulness. Now, researchers at MIT have developed a method that allows these sensors to detect any arbitrary frequency, without losing their ability to measure nanoscale features.
The new method, for which the team has already applied for patent protection, is described in the journal Physical Review X, in a document by graduate student Guoqing Wang, professor of nuclear science 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 certain particles are in such a delicate state of equilibrium that they are affected by even minute 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 current quantum sensors can detect.
The new system the team developed, 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 to 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 an array of nitrogen vacancy centers 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 – 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, says Wang, “the same principle can also be applied to any kind of quantum sensors or devices.” The system would be self-contained, with the detector and second frequency source all 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 which blur fine detail and make it impossible to achieve the very high resolution offered. by the new system. In such systems today, says Wang, “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 influence the phenomena you want to measure.”
The system may open up new applications in biomedical fields, according to Cappellaro, because it can make accessible a range of frequencies of electrical or magnetic activity at the level of a single cell. It would be very difficult to get any useful resolution out of these signals using current quantum detection systems, she says. It may be possible to use this system to detect the output signals of a single neuron in response to certain stimuli, for example, which usually include a lot of noise, which makes 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 intensely 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 probe a range of frequencies at a time, 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 Lincoln Laboratory, where some members of the research team are based.
The team included Yi-Xiang Liu from MIT and Jennifer Schloss, Scott Alsid and Danielle Braje from Lincoln Laboratory. The work was supported by the Defense Advanced Research Projects Agency (DARPA) and Q-Diamond.
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