Quantum Technology: Sensing and imaging

This blog is part of a series on Quantum Technology and future land warfare, put out by the Robotic and Autonomous Systems Implementation and Coordination Office (RICO), a part of Future Land Warfare Branch within Army HQ tasked with coordination and de-confliction for Army's Robotics and Autonomous Systems (RAS)/Artificial Intelligence (AI) and emerging disruptive technology.

Quantum sensors measure time, dynamics (i.e. forces, acceleration and rotation), and fields (i.e. gravitational, electromagnetic and mechanical) with unprecedented precision and stability. Imaging is an extension of quantum sensing where quantum sensors are combined with an imaging apparatus (e.g. a probe that scans the position of the sensor, an array of sensors or a beam of electromagnetic waves prepared in a quantum state) to perform high-resolution microscopy or macroscopy (e.g. radar) with unprecedented sensitivity.

Quantum sensing technologies are incredibly diverse, with a great variety of different methods implemented using different quantum systems. Some technologies are already available on the market,[i] some are in advanced stages of development,[ii] and others are in the laboratory. Owing to this diversity and their unprecedented capabilities, quantum sensors have a great variety of applications across science, industry and defence. The current challenge being the identification of the most promising technologies and applications.

In this post, I aim to expand upon my previous article on this technology. I first outline the most promising defence applications of quantum sensors. I then introduce their key operating principles, performance metrics and limitations, concluding with several examples of their relevance to Defence applications.Defence applications of quantum sensors

The most promising defence applications are:[iii]

• Enhanced positioning, navigation and timing. Quantum accelerometers, magnetometers, gyroscopes and clocks promise the long-term stability and precision required for accurate inertial navigation in the absence of GPS. Furthermore, advanced atomic clocks promise the precision timing that is key to accurate positioning as well as reducing the errors and increasing the speed of digital communications.
• Enhanced situational awareness. Quantum gravimeters and magnetometers promise new capabilities in geospatial mapping and anomaly detection. Quantum microwave spectrometers promise high sensitivity and unbroken detection bands for monitoring the electromagnetic spectrum. Quantum radar has the potential to increase radar sensitivity to detect weakly scattering objects or to reduce its own detection signature.
• Enhanced human-machine interfacing. Quantum magnetometers may enable wearable high-resolution magnetoencephalography for brain activity imaging.
• Enhanced defence science and industry. Quantum microscopes, spectrometers and nanosensors promise to drive innovation in materials science and nano-, bio- and medical technology.

Operating principles, performance metrics and limitations

The general operating principle of quantum sensors is depicted above. Quantum sensors exploit quantum superposition to apply interferometry techniques to detect small changes in a qubit’s state by the passage of time, dynamics or interactions with fields.[iv] Quantum entanglement between multiple qubits may be exploited to further enhance precision. Stability is achieved through the qubits having fixed and universal susceptibilities (e.g. electron gyromagnetic ratio and atomic mass).

A key performance metric of quantum sensors is sensitivity, which defines the smallest detectable change in a measurand after a given exposure time. Mathematically, sensitivity is expressed as η∝1/αδNTc  in terms of the qubit properties: α  - accuracy of the qubit’s initialisation and readout, δ  - susceptibility of the qubit’s state to the measurand, Tc  - lifetime of the qubit’s state before decoherence; and N  – the number of independent uncorrelated qubits in the sensor. Thus, the greater each of these qubit properties and the larger the number of qubits, the smaller the detectable change in a measurand after a given exposure time. Entanglement and other correlations can improve the sensitivity by up to a factor of 1/N  until the ultimate – Heisenberg – limit of sensitivity is obtained.

The limitations of quantum sensors are primarily dynamic range, speed and environmental exposure. Although highly sensitive, quantum sensors are often limited to a small range of measurand values and will saturate if there are large variations. This means that quantum sensors can take longer to perform measurements than other technologies, and so cannot provide the same update rate. In addition, the environment can bring unwanted noise that deteriorates the qubit properties and so the qubits need to be shielded from the elements of the environment other than the measurand. This is the classic packaging problem experienced by all sensors. The first two limitations can be ameliorated by integrating quantum sensors with other sensors that are faster and have larger dynamic ranges. The third limitation is a difficult problem, but one where these is an established wealth of knowledge to draw upon, and thus reason to be optimistic that a workable solution can be found.

Quantum Sensors in Action

Example 1: Gravimetry and accelerometry

By taking full advantage of atom-light interactions and the universal properties of atoms, atom-based quantum sensors are capable of precision measurements of gravity, gravity gradients, accelerations, magnetics, magnetic gradients, and time. Most notably, the quantum sensors operate without the need for moving mechanical parts and so deliver extremely good long-term stability.

The technology has an array of defence applications, including quantum augmented inertial navigation (a backup for GPS in contested environments), gravitational anomaly mapping to augment inertial navigation,  surveying of littoral environments and coastal protection, gravitational anomaly detection for the mapping of underground structures. The technology is dual use with civilian applications that promise significant economic impact including the mapping and monitoring of underground water resources, mineral exploration, space exploration and satellite-based Earth observation.

Example 2: Magnetometry

Contributed by Andre Luiten, Institute for Photonics and Advanced Sensing, University of Adelaide.

The development of submarines spawned a technological race in methods aimed at detecting them. There are many potential sensing approaches including passive acoustic, SONAR, radar and even chemical approaches: but one of the hardest to evade is the sensing of the magnetic field that is associated with a submarine’s presence. 

Conventional magnetic anomaly detection (MAD) is conducted using specially-prepared atomic vapours in which the individual atoms respond to the local magnetic field with a precision of a few picotesla (a change of just a few billionths in the earth’s magnetic field). The Institute of Photonics and Advanced Sensing is working closely with the the Defence Science and Technology Group (DSTG) to develop a quantum sensor that can deliver much higher accuracy and precision than conventional approaches.

The brilliant thing about a quantum sensor of this type is that the rules which govern how fast this quantum flipping occurs are set by the unchanging rules of quantum mechanics.  This means that such a sensor has no drift, and needs no calibration, and thus could be remotely operated and still deliver measurements that have a high degree of confidence. 

Example 3: Chip-scale chemical analysis

Contributed by David Simpson and Liam Hall, School of Physics, University of Melbourne.

NMR spectrometers are ubiquitous in chemical laboratories around the world, and are frequently used to characterise and quantify both known and unknown chemical samples. This is due to being the most effective analytic technique to quantify the content, purity, and molecular structures of the components comprising a given chemical sample.

Despite its utility, NMR spectroscopy is yet to find any appreciable uptake outside of high-end research and quality control laboratories, due primarily to the facts that NMR spectrometers:

• need space, typically occupying over 10 m2 and weighing hundreds of kilograms,
• are costly and require significant maintenance/consumables,
• need to be operated by highly trained specialists, and
• have significant electrical power requirements.

The recent advent of quantum sensors promises to revolutionise the NMR chemical detection industry by dramatically reducing the size, cost, maintenance, and complexity of its instrumentation without compromising on chemical sensitivity and specificity. This is made possible thanks to atomic defects which can be engineered into synthetic diamond chips. These atomic scale quantum sensors act as extremely sensitive magnetometers and can report the magnetic field from molecular nuclei present in chemicals applied to the chip’s surface.

The University of Melbourne is currently developing this chip-scale NMR technology to man-portable form factors with potential Defence applications in chemical trace detection analysis, drug discovery, and industrial chemical monitoring applications.