Seeing the invisible with quantum sensing

Alberto Marino hopes to help the hunt for dark matter, the mysterious substance that keeps galaxies together. Dark matter is invisible to the eye as well as current cameras, photodetectors, and radio telescopes. As group leader for quantum sensing and computing at Oak Ridge National Laboratory, Marino is collaborating with scientists from several institutions who are searching for a way to spot dark matter.

If all goes as planned, Marino’s group will, in the coming years, deploy large arrays of micro-electromechanical systems (MEMs) full of membranes millimeters in diameter that will move miniscule amounts due to interactions with dark matter. To track those tiny movements, the researchers will use a still-being-developed sensing method, one that utilizes the quantum properties of light to enhance sensor performance.

For this and other applications where current sensors fall short, sensing technologies that exploit the quantum nature of light and matter offer an advantage. “It allows you to see things that you’re not able to detect otherwise,” Marino says.

The benefit of quantum sensing over classical sensing varies by application. The improvement could be orders of magnitude in instrument sensitivity, according to a 2024 report by McKinsey & Company. They pegged the market for quantum sensors to be between $0.7 and $1.0 billion by 2030 and as much as $6.0 billion by 2040.

Quantum sensors can measure magnetic fields, electric fields, temperature, pressure, rotation, frequency, and time. They can develop magnetic and gravitational maps that reveal terrestrial faults, oil and other mineral deposits, and buried objects. Other applications involve the life sciences, where quantum sensors would allow room temperature diagnostics at a distance or via small wearable devices. Another potential application involves navigation, with quantum sensors making this possible underwater, underground, and in other locations where GPS is not available.

Quantum sensors already exist and are gathering information from the ground below to the skies above. Emerging quantum sensors that are on the verge of commercialization use novel materials, potentially enabling advances in medical care, materials research, computer-chip fabrication, navigation, and more. On the horizon, and still confined to labs like Marino’s, lie quantum sensors that exploit the exotic quantum properties of superposition and entanglement, promising still further improvements in sensor performance

An example of existing quantum technology comes from LI-COR Environmental. The company’s decades-old quantum sensor counts photons that fall in the photosynthetically active 400-700 nm range, a critically important piece of information when investigating plant activity.

“You always want to measure photosynthetically active radiation because that’s a primary driver of it,” says Jason Hupp, director of science, research, and applications. He notes that the sensor uses a photodiode with a cut-off filter to get rid of any light outside the target spectrum.

An assembled diamond magnetometer that works by using the quantum properties of an NV-center diamond. Photo credit: SBQuantum.

In its latest instrument, LI-COR paired the photon-counting sensor with one that measures carbon and water vapor as they flow in and out of a field of plants. A measurement of light going into an ecosystem and the resulting flow of materials creates a more complete picture of an ecosystem.

A second current quantum technology example is the superconducting edge sensor. It is used by researchers at the US National Institute of Standards and Technology (NIST) to make devices of great sensitivity for astronomical applications like studying the cosmic background radiation, says Logan Howe, a physicist in NIST’s quantum sensor division.

In an edge sensor, researchers set up a superconducting film so that it sits just below the transition point from superconductivity. When a photon of the right wavelength—and therefore energy—arrives, the material goes from no to some resistance, which makes it possible to count photons. The edge sensor approach makes for an extremely sensitive detector because a high-quality film will have a very sharp transition from superconductivity to regular conductivity, according to Howe. Hence, the film will have a stronger and more easily detected response to incoming photons, improving the ability to count them.

A third and final example of current quantum sensing technology comes from the Laser Interferometer Gravitational-wave Observatory, or LIGO. It detects gravitational waves, such as those that arise when black holes collide or other cosmic events occur.

Researchers running LIGO use squeezed light to reduce noise, making it possible to extract fainter gravitational wave signals. Squeezing light diminishes noise in one attribute of light, like intensity or amplitude, while increasing it in a complementary characteristic, like phase. If amplitude is the measured variable, the result will be an improvement in instrument sensitivity. Tests have shown that using squeezed light can lead to a significant improvement in sensor performance.

Emerging quantum sensors that are on the verge of commercial use promise even better performance than what is possible with current technology. These novel sensors exploit the properties of neutral atoms, superconducting devices, trapped ions, and diamonds.

For instance, one area of great activity is quantum diamond sensors. After decades of research, there are now products under development that will soon be commercially available. These devices contain an artificial diamond, which, like a natural diamond, consists of carbon atoms in a matrix. Replacing some of those carbon atoms with nitrogen and getting rid of an adjacent carbon atom leads to diamonds with nitrogen-vacancy (NV) centers.

“It is an excellent material because the additional nitrogen atoms provide extra electrons that act as tiny magnetic field-sensing elements,” says Katrin Kobe, CEO of Bosch Quantum Sensing. The startup has a quantum sensor prototype that’s the size of a cell phone, the smallest with its level of measurement accuracy and much more advanced sensors will come soon, she says.

Kobe predicts that one application of the technology could be a mobile magneto-cardiogram, thereby making it possible to measure the heart’s natural magnetic field and enabling straightforward, contactless long-term measurements. Kobe notes that, by providing this capability, magnetic field quantum sensors would generate far more data than today’s electrocardiograph.

Readout of a quantum diamond sensor used in a magnetometer involves illuminating the NV-center diamond with green light, causing it to emit red light. The emission reveals the strength and direction of the ambient magnetic field, which is a combination of the Earth’s field and that of any nearby object. Quantum diamond sensor uses are emerging in a variety of areas ranging from biomedicine to research, to mapping, and to navigation, Kobe says.

The theoretical sensitivity of quantum diamond sensors with a dense ensemble of nitrogen-vacancy centers is in the pico-Tesla range, according to startup Qnami. For comparison, Earth’s magnetic field is about 50 micro-Tesla, a million times as strong, and a standard fridge magnet is about a milli-Tesla, nine orders of magnitude greater.

Along with offering quantum diamond foundry services, Qnami makes a scanning nano-magnetometry microscope that uses a quantum diamond sensor to probe materials. The instrument offers a three- to four-order-of-magnitude-greater-sensitivity than instruments based on the magneto-optical Kerr (MOKE) effect or using magnetic force microscopy, traditionally used magnetic microscope scanning technologies. The quantum-based approach can map magnetic fields with a resolution measured in nanometers, a far better spatial resolution than traditional approaches. The combination of sensitivity and resolution makes quantum diamond sensors useful when doing failure analysis on semiconductors or for materials research.

Felipe Favaro de Oliveira, Qnami’s chief technology officer, notes that the capabilities of the microscope are critical when dealing with devices that measure in nanometers. He recalls one instance in which magnetic nanowires did not perform as they should. However, traditional measurement techniques found no problem with the nanowires.

Quantum sensing using nitrogen vacancy diamond as part of a minerals magnetic survey. Photo credit: SBQuantum.

Using the company’s microscope, Qnami researchers examined the nanowires and found atom-sized issues with their construction. “We saw they were full of defects. Those defects were at the atomic scale. We’re talking variations of a few atoms,” de Oliveira says.

SBQuantum, a third startup, is developing quantum diamond sensors that can look for minerals underground, submarines in the ocean, or spot otherwise invisible metallic objects, notes CEO David Roy-Guay. He adds that the quantum diamond approach offers a critical benefit because the sensor itself is small.

“We’re actually probing a cylinder of a few hundred microns in the diamond. With that volume, we can extract vector readings with high accuracy,” Roy-Guay says.

The small size, light weight, and low-power requirements of his company’s diamond sensor make it useful for drone applications, he adds. Flying the sensor in a drone will enable faster magnetic mapping of an extended area.

To prove the technology’s worth, SBQuantum entered the US National Geospatial-Intelligence Agency and NASA-sponsored MagQuest Challenge. The startup is a finalist in the contest, which will entail mapping and monitoring the Earth’s magnetic field from space.

Finally, there are sensor improvements from quantum approaches that are still in the lab but that could lead to even greater performance gains. For example, in the case of Marino and his collaborators in the hunt for a better dark-matter detector, further enhancement is possible using entanglement, the quantum process by which the quantum states of two or more particles or systems become connected. In the case of the work being done by Marino and his collaborators, the photons that probe the MEMs and do the readout of the membrane movement will be entangled.

Entanglement benefits sensors by improving the signal-to-noise ratio (SNR) through correlations. The greater the SNR, the easier it is to extract meaningful information from a sensor. This entanglement effect only arises for arrays of sensors and only for those variables that are global.

The advantage of entanglement can be large. With classical approaches, improving SNR by a factor of two requires four times as many sensors. With entanglement, four sensors would, in theory, quadruple the SNR—double what is possible with a classical method. The gain from entanglement goes up with the number of sensors involved.

However, realizing the quantum benefit is not easy. It’s only possible to entangle sensors if losses along the connection paths are low along with meeting other exacting criteria. Today, researchers are doing proof-of-principle demonstrations with a small number of entangled sensors, hoping to eventually create instruments that could consist of a vast array of entangled elements for such tasks as sensing dark matter.

A quantum sensor that counts photons in the photosynthetically important 400-700 nm range is combined with a light sensor to track carbon flux in an ecosystem. Photo credit: LI-COR Environmental 

While research into future advances such as entanglement are underway, progress in quantum sensors will benefit from the deployment of more advanced photonics. For instance, Qnami’s de Oliveira notes a need for an inexpensive laser emitting in the 515-530 nm range, which would be useful as a light source in the company’s quantum diamond sensors.

Marino points to a requirement for both better photon sources and detectors. He would like to see sources of light that would emit photons on demand, with quantum properties tailored to specific applications. He also wants to have more efficient detectors.

Miles Padgett, a University of Glasgow professor who studies quantum imaging, is the co-director of science for the recently formed QuSIT, the UK hub for Quantum Sensing, Imaging, and Timing. He notes that the border between photonic sensing and quantum sensing is fuzzy.

“Both depend upon and are enabled by improved photonics components, particularly detector technology both in the visible region of the spectrum and ever more importantly the IR,” Padgett notes.

Quantum sensing offers significant improvements over traditional technology, he says. Padgett predicts that the quantum approach will become an integral part of the sensing toolkit going forward.

Thus, the technology’s use in specific, but not all, instances make sense. One area where quantum sensing might not prevail, for example, is in the magnetometers in smartphones. These sensors must be inexpensive yet capable enough for the intended use. For this application and others like it, classical technology is likely to remain the preferred choice in the immediate future and perhaps forever.

As for the quest for a better dark matter detector, Marino notes that creating the sensors needed will be a challenge, one not likely solved soon. “Scaling to the point where we can have the number of sensors that are needed to achieve the sensitivity to see dark matter, that’s going to be a long-term goal,” Marino says.

Hank Hogan is a freelance science and technology writer.