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Quantum sensor can detect electromagnetic signals of any frequency: Engineers are expanding the capacity of these ultra-sensitive nanoscale detectors, with potential uses 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 basic physics. However, these sensors have only been able to detect a few specific frequencies of these fields, which limits their usefulness. Now, researchers at MIT have developed a method to enable such sensors to detect any frequency, without losing their ability to measure nanometer-scale functions.

The new method, for which the team has already applied for patent protection, is described in the journal Physical examination Xin an essay by doctoral student Guoqing Wang, professor of nuclear science and technology and in 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 sensitive balanced state that they are affected by even small 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 scientists use them to characterize practical units such as experimental quantum memory or computational units. But many other phenomena of interest span a much wider frequency range than today’s 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 beam of microwaves. This converts the frequency of the field being studied to another 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 enter at any frequency, without losing the nanoscale spatial resolution of the sensor.

In their experiments, the team used a specific device based on a series of diamond nitrogen vacancy centers, a commonly used quantum sensing 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 made detailed analyzes of the process by deriving a theoretical framework, based on the Floquet theory, and testing the numerical predictions of that theory in a series of experiments.

While their tests used this specific system, Wang says, “the same principle can also be applied to all types of sensors or quantum units.” The system would be standalone, with the detector and the source of the second frequency all packaged in a single unit.

Wang says that this system could be used, for example, to characterize in detail the performance of a microwave antenna. “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 units and strong magnetic fields that blur the 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 set up the sensor, but that magnetic field can potentially break the properties of the quantum material, which can affect the phenomena you want to measure.”

The system can 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 useful resolution of such signals with the current quantum sensing system, she says. It may be possible to use this system to detect output signals from an individual neuron in response to any stimulus, for example, which usually includes a lot of noise, making such signals difficult to isolate.

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

In the ongoing work, the team is investigating the possibility of finding ways to expand the system in order to be able to examine a number of frequencies at the same time, rather than the current system’s unique frequency orientation. They will also continue to define the system’s capabilities using more powerful quantum sensors at the Lincoln Laboratory, where some members of the research team are based.

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

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