New Frontier for Quantum Sensing

MARIE FREEBODY, CONTRIBUTING EDITOR, marie.freebody@photonics.com

Quantum physics is a peculiar field. In the subatomic domain, the usual physical laws no longer apply. Instead, nonintuitive behaviors reign and new laws govern. A case in point is the Heisenberg uncertainty principle, which tells us that it is not possible to know both the exact position and the exact momentum of an object at any one time.

Quantum mechanics — an idea that started with Planck, Einstein and Heisenberg more than 100 years ago — gave us a better understanding of the nature of matter and light. It resulted in the first quantum revolution, which yielded the laser and the transistor.

Since then, devices such as smartphones, laptops and digital cameras — even the internet — have completely transformed every aspect of society, with endless future implications to continue this rapid technological advancement.

Today, experts predict that we are on the verge of a second quantum revolution where technologies that rely on quantum superposition and quantum entanglement will emerge. Thanks to smaller components, including lasers, photonics and optics components, new possibilities have been unlocked for quantum sensing.

“Harnessing the quantum effects and creating miniaturized, affordable devices are probably the most important considerations being made in the development of quantum-sensing technologies,” said Nils Hempler, head of innovation at M Squared in Glasgow, Scotland. “Teams in laboratories across the world can study the effects of quantum mechanics without relying on extraordinary and bulky equipment, and it opens the way for commercial and industrial applications.”

An engineer assembles “Ice Bloc” instrumentation at M Squared’s headquarters in Glasgow. Courtesy of Nick Callaghan.

M Squared has been working in quantum technology since 2006, when it first developed SolsTiS, a laser for scientists working in quantum technologies. Sol-sTiS has been used in many of the world’s most accurate clocks and is currently powering a number of other quantum technologies the company hopes to bring to market.

One example comes from the company’s partnership with the U.K.’s University of Birmingham in the form of a recently demonstrated quantum gravimeter. The system uses quantum technology to detect gravitational fields of hidden objects and is one of the first commercial/industrial demonstrations of its kind.

M Squared designs and manufactures complementary electronics as well as photonics and quantum technology. Courtesy of Nick Callaghan.

Gravitational measurements are not just of interest to space scientists; here on Earth, tiny variations in gravity can reveal the presence of valuable resources. Applications include detecting oil and gas deposits, surveying underground infrastructures, monitoring the water table and even providing clues to predict natural disasters such as avalanches, volcanic eruptions and tsunamis.

The quantum gravimeter uses novel lasers, control electronics and vacuum systems to apply atom interferometry techniques on clouds of atoms at ultracold temperatures. This results in the ability to detect tiny variations in gravity — an ability the company says is ready for application.

M Squared’s quantum gravimeter, the first commercial device of its kind in the U.K., uses the interference of clouds of atoms at ultracold temperatures to detect tiny variations in the gravitational field. Courtesy of Nick Callaghan.

In a related technology, M Squared is helping develop the U.K.’s first commercial three-axis quantum accelerometer with partners at Imperial College London. Quantum accelerometers can sense very slight changes in acceleration; they are related to gravimeters, which sense changes in gravitational acceleration.

“Combining this with quantum gyroscopes and clocks will see the creation of a navigation system that is highly accurate and completely independent of satellite navigation systems,” Hempler said. “Such a technology will enable submarine navigation to improve from 2-kilometer accuracy over a 24-hour period to 100 meters over several months.”

Since the technology measures acceleration using atom interferometry, it has the potential to pave the way for high-precision satellite-free navigation, which, unlike GPS, will be unhindered by dead areas or the effects of terrain and weather.

Diamond defects

“I would say that the areas in which future quantum technologies could be meaningfully applied haven’t changed dramatically over the past few years,” said Warwick Bowen, director of the University of Queensland Precision Sensing Initiative in Queensland, Australia. “What has changed is our technological capabilities to achieve these applications.”

The first example lies in quantum nanomechanics. Nanomechanical resonators are widely used in applications ranging from the accelerometers, clocks and filters in mobile electronics to biochemical sensing and medical diagnostics. Progress in manufacturing has had a big impact on the ability to apply quantum techniques into sensing.

“Recently, advances in micro- and nanofabrication now allow us to use the radiation pressure from light to control, cool and read out these resonators with precision down to the quantum level — the level where the Heisenberg uncertainty principle becomes apparent,” Bowen said.

Quantum microscopy is another area that has seen progress. Recently developed techniques can be used to achieve quantum-noise-limited (or quantum-noise-surpassing) performance in gravitational wave interferometers in microscopy and biophysics. This has the potential to allow biophysical motor molecules — the nanoscale machines of life — and other biophysical systems to be studied with exquisite precision in their native state, without need for labels or high light intensities.

Perhaps the most important advance comes from exploiting quantum defects in diamond. Today, one of the most popular quantum sensors of choice is based on the nitrogen-vacancy (NV) color center in diamond. The NV center is a defect formed in diamond by replacing two neighboring carbon atoms in the diamond’s crystal structure with one nitrogen atom and an adjacent vacancy. The NV forms a ground state spin triplet that can be controlled coherently at room temperature using electromagnetic fields.

Crucially, the resulting quantum-coherent properties allow for high-performance sensing applications, such as single-electron spin detection, even at room temperature.

NV centers are a primary focus at Thales Research and Technology in Palaiseau, France. These centers have enabled investigation into several applications in the field of magnetism. Some include measuring nanoscale magnetic fields in the write heads of storage disks, studying the physics and nature of walls between the magnetic domains at the nanoscale, and quantitative nanoscale vortex imaging using a cryogenic quantum magnetometer.

A nitrogen-vacancy-center-based magnetometer for antiferromagnetic materials imaging developed within the DIADEMS project. Courtesy of Quantum Sensing Group and Qnami, University of Basel.

“The most advanced application of NV centers in diamond is magnetic field measurement,” said Thierry Debuisschert, a scientist at Thales Research & Technology and coordinator of the DIADEMS (diamond devices enabled metrology and sensing) project (www.diadems.eu). “The sensitivity has been improved, allowing the detection of small magnetic fields down to the picotesla at room temperature.”

Such feats of sensitivity have been made possible by several technological advances. For example, large diamond samples of millimeter size can now be produced with a perfect crystalline structure and almost no impurity. This enables very long measurements to be performed with a single NV center, which directly improves the magnetic field sensitivity and has allowed the detection of a single spin.

The processing of diamond crystal has improved considerably, allowing fabrication of single-crystal-diamond nanostructures with ultrahigh aspect ratio. Diamond samples with nanometer thickness NV layers can be produced either by ion beam implantation or with in situ doping during growth. The yield of the NV production can then be increased by helium post-irradiation.

A conceptual image of a matter-wave interferometer based on laser control of the flow of superfluid helium (a form of quantum liquid) around devices fabricated on a silicon chip. Such interferometers could, in the future, be used to build gyroscopes with unprecedented precision — miniaturizing 100-m-scale current laser gyros to millimeter scale. Professor Warwick Bowen and colleagues are working toward building such gyros, supported by the Australian Centre of Excellence for Engineered Quantum Systems and the U.S. Army Research Office. Courtesy of Dr. Christopher Baker.

“In a standard diamond crystal, the NV center can exist in four possible orientations. For several applications it is desirable to control this orientation,” Debuisschert said. “We are now able to control the growth conditions to produce samples with perfect preferential orientation of the NV centers along one direction.”

Although impressive progress has been made, many challenges remain. Fabricating artificial atoms in solids, such as NV centers, has improved, but few defects can be operated at room temperature. This is a problem for many applications in biology where cryogenic cooling is not appropriate.

“In some cases the atomic structure needs to be better understood to fully exploit all the potential of those defects,” Debuisschert said. “An ultimate goal would be to realize a quantum register with a few tens of single defects located in controlled distance one from another, ideally with atomic precision and with deterministic implantation. This goal would require the best of present technology and is a strong motivation to push further those techniques and invent new tools.”

Quantum sensing has gained some impressive funding in recent times, and major industrial players such as Bosch are investing to commercialize the technology, reflecting a general shift from fundamental research to applied technology.

In 2013, the U.K. government identified quantum technology as an area of huge potential with an initial public investment of £270 million. This was followed by a £30 million investment from the Defence Science and Technology Laboratory, and there has been a concerted effort in the U.K. to bring quantum technologies to market.

“Quantum sensing, imaging and computing technologies are forming a key part of the future industries in the government-led industrial strategy,” said Nils Hempler, head of innovation at M Squared in Glasgow, Scotland. “This has had a huge influence on our own business strategy, enabling us to collaborate more closely with world-leading quantum research teams based at universities such as Oxford, Birmingham, Glasgow and Imperial.”

Marrying research with industry combines reliable tools such as single-frequency lasers, nitrogen-vacancy (NV) centers in diamond and quantum dots with the know-how to better prepare and manipulate quantum systems. “This at the end leads to increased resolution and/or more stable operation of quantum-sensing devices, and by that to a broader range of applications,” Hempler said.

Real-World Applications of Quantum Sensing

Quantum sensing is emerging in numerous fields, bringing with it new levels of precision and accuracy:

• In the field of biological applications, it is possible to measure and identify single molecules among different species. The increase in sensitivity makes possible the detection of single electron spin.

• In microelectronics, NV magnetometers have been used to monitor the current density in electronic circuits, using either single NV centers or ensembles of them.

• New applications have been identified, such as the use of NV centers, to realize a microwave spectrum analyzer with instantaneous measurement of the entire spectrum. NV centers have been used for nanoscale imaging of the microwave field produced by an electronic circuit.

• A prominent example is the measurement of time with optical atom clocks. In laboratory environments, they now achieve stabilities and accuracy below 10^-18.

• Measurements of magnetic field, gravitational potential, acceleration, rotation, molecular content in gases and air fluctuations/wind.

• Magnetocardiography is used to measure the tiny magnetic fields produced by electrical activity in the heart.

• Magnetoencephalography is a neuroimaging technique for mapping brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain.

• Defense and automotive industry for satellite-free navigation.

• Geophysics for mapping the gravitational surface of Earth to reveal volcanism and search for natural resources such as oil and water.

• Civil construction to find holes within ground.

• Isotope tracing with atom trap trace analysis (ATTA), which uses magneto-optical traps as detectors.

• Water dating for determining the age of groundwater and therefore flow of groundwater and ice dating.

• Monitoring or detecting nuclear reactions such as weapon tests.

• Fast telecommunication wavelength detectors with high fidelity.

• Nanoscale, biologically compatible temperature sensors.