The establishment of quantum theory is one of the most brilliant achievements of the 20th century. It revealed the structure, properties, and laws of motion of matter in the microscopic realm, shifting our perspective from the macroscopic to microscopic systems. A series of phenomena distinct from classical systems, such as quantum entanglement, quantum coherence, and uncertainty, were discovered. Furthermore, quantum theory and quantum methods have been applied to fields such as chemical reactions, genetic engineering, atomic physics, and quantum information.
The development of quantum information science in recent years has made the manipulation and control of the quantum states of microscopic objects increasingly important. The theory and methods of quantum control have led to the development of quantum cybernetics.
Quantum cybernetics is a discipline that studies the control of quantum states in microscopic systems, and quantum sensors can be used to solve detection problems in quantum control.
The concept and current status of quantum sensors
In classical control, the measurement process is accomplished by various measuring instruments, with the transformation process typically handled by corresponding measuring sensors. Measuring instruments can consist of several sensors connected in a suitable manner, working together to perform transformation, selection, comparison, and display functions. Similar to classical control, the key to measurement in quantum control is also the comparison between the measurand and a standard quantity. However, observable measurements in quantum control correspond to the corresponding self-conjugate operators in quantum mechanics. Direct measurement of the state of a quantum system is generally difficult to achieve; it is necessary to transform the measurand into a measurable physical quantity according to certain rules, thereby achieving indirect measurement of the quantum state. This process can be accomplished using quantum sensors.
A quantum sensor can be defined in two ways:
(1) A physical device designed to perform transformation functions using quantum effects and based on corresponding quantum algorithms;
(2) In order to transform the measured quantity, some parts are so fine that their quantum effects must be taken into account.
Regardless of the definition, quantum sensors must adhere to the laws of quantum mechanics. In essence, a quantum sensor is a physical device designed based on the laws of quantum mechanics and utilizing quantum effects to perform transformations on the measured quantity of a system.
Like the rapidly developing biosensors, quantum sensors should consist of two parts: a sensitive element that generates signals and an auxiliary instrument that processes the signals. The sensitive element is the core of the sensor, and it utilizes quantum effects.
As quantum control research deepens, the requirements for sensitive elements will become increasingly stringent, and the development of sensors themselves will trend towards miniaturization and quantum-based technologies. Quantum effects will inevitably play an important role in sensors, and various quantum sensors will be widely used in quantum control, state detection, and other fields.
Performance analysis of quantum sensors
The performance of a sensor is primarily evaluated based on its accuracy, stability, and sensitivity. Considering the unique characteristics of quantum sensors, their performance can be assessed from the following aspects:
(1) Non-destructive:
In quantum control, measurement can induce wavefunction reduction in the measured system, and sensors can also cause changes in the system state. Therefore, the interaction between the quantum sensor and the system must be fully considered during measurement. Because state detection in quantum control differs fundamentally from that in classical control, the wavefunction reduction process induced by measurement implies that the measurement of the state has already destroyed the state itself. Therefore, non-destructiveness is one of the key considerations for quantum sensors. In practical detection, the quantum sensor can be considered as part of the system, or as a perturbation to the system, and the Hamiltonian of the interaction between the sensor and the measured object can be included in the evolution of the entire system state.
(2) Real-time performance:
Given the characteristics of measurements in quantum control, particularly the rapid state evolution, real-time performance has become a crucial indicator for evaluating the quality of quantum sensors. Real-time performance requires that the measurement results of the quantum sensor closely match the current state of the measured object, and, when necessary, track the quantum state evolution of the measured object. Therefore, when designing quantum sensors, it is essential to consider how to address the measurement lag problem.
(3) Sensitivity:
Since the main function of quantum sensors is to realize the transformation of the measured microscopic object, it is required that even tiny changes in the object can be captured. Therefore, when designing quantum sensors, it is necessary to consider that their sensitivity can meet the practical requirements.
(4) Stability:
In quantum control, the state of the controlled object is easily affected by the environment. When quantum sensors detect the quantum state of the object, they may also cause instability in the state of the object or the sensor itself. The solution is to introduce the concept of environmental engineering and consider using methods such as cooling traps and cryogenic holders for protection.
(5) Multifunctionality:
Quantum systems are inherently complex systems, and interactions easily occur between subsystems or between sensors and systems. In practical applications, it is always desirable to reduce human influence and the lag caused by multi-step measurements. Therefore, multiple functions, such as sampling, processing, and measurement, can be integrated into the same quantum sensor, and appropriate intelligent control algorithms can be incorporated into it to design intelligent, multifunctional quantum sensors .
Quantum sensors possess many properties that classical sensors do not have. When designing quantum sensors, in addition to focusing on transforming quantities that cannot be directly measured in the quantum realm into measurable quantities, the performance of quantum sensors should also be evaluated from aspects such as non-destructiveness, real-time performance, sensitivity, stability, and multifunctionality.
Market Applications of Quantum Sensors
In the UK, for example, the sensor and related equipment sector employs over 73,000 people and contributes significantly to the economy annually, exceeding £14 billion. The value derived from a single sensor data service is already astronomical.
However, the imagination surrounding quantum sensors doesn't stop there: the development of quantum magnetic sensors will significantly reduce the cost of magnetoencephalography (MEG), which will help promote the technology; quantum sensors for measuring gravity are expected to change people's impression of traditional underground surveying as cumbersome and time-consuming; even in the field of navigation, areas that navigation satellites often cannot search are where quantum sensors can provide inertial navigation.
1. Civil Engineering
Underground exploration is typically extremely expensive and time-consuming, but it is essential for building new infrastructure, especially large projects like high-speed rail and nuclear power plants before construction begins. In fact, many geologically unexplored underground environments pose hazards such as sewers, mines, and sinkholes.
The cost of insufficient information is often very high, with project delays, cost overruns, and replanning becoming commonplace. The UK's approach to infrastructure maintenance involves spending £5 billion annually to dig 4 million holes in roads, simply because people are unaware of the exact locations of underground infrastructure.
In people's general perception, any inspection should be conducted on the ground, without the need to dig pits. However, the performance of existing radar, electronic detectors, and magnetometers is not ideal, and objects deeper than a few meters underground are difficult to detect.
In such cases, the usual solution is to use gravity sensing technology, because even minute changes in the gravity of any buried object can be recorded and plotted as a gravity map. However, traditional gravimeters suffer from inaccurate readings, are time-consuming, and are susceptible to ground vibrations.
However, using quantum sensors for gravity measurements offers significant advantages: faster speed, more accurate readings, deeper penetration, and immunity to ground vibrations. The widespread application of this technology will undoubtedly greatly promote the civil engineering industry.
2. Prevention of natural hazards
In the UK, over 5 million homes are located in areas at risk of collapse and subsidence; the UK rail service also needs to monitor water levels around railway tracks in real time to prevent landslides. Quantum sensors can effectively mark areas of potential collapse risk and areas with excessive water accumulation on gravity maps.
Furthermore, quantum photon sensors can quickly identify hazards underground, such as oil spills. This is all based on the rapid scanning capabilities of quantum sensors, which makes routine inspections possible.
3. Resource exploration
The key to acquiring natural resources such as oil and gas lies in identifying extraction sites, which represents a massive $3 billion market in the United States. Currently, seismic exploration is the most prevalent method, offering superior results, but the more expensive gravity measurement method is only used in areas with limited knowledge.
However, a large part of the high cost of gravity measurement actually comes from adjusting the equipment. But now, the emergence of quantum-enhanced MEMS sensors has reduced the need for equipment adjustments, allowing the entire measurement process to proceed more quickly, and even reducing the cost to one-tenth of what it used to be.
4. Transportation and Navigation
The more developed transportation becomes, the more necessary it is to understand the accurate location and status of various means of transportation. This places demands on the number of sensors carried by cars, trains, and airplanes. Satellite navigation equipment, radar sensors, ultrasonic sensors, optical sensors, and other similar devices will gradually become standard equipment.
However, these are far from enough, and the development of sensor technology will face new challenges. The positioning and navigation accuracy of autonomous vehicles and trains is strictly required to be within 10 centimeters; next-generation driver assistance systems must be able to monitor local dangerous road conditions at the centimeter level in real time. Using cold atom-based quantum sensors, navigation systems must not only be able to provide location information accurate to the centimeter, but also be able to operate in places inaccessible to navigation satellites, such as underwater, underground, and within building complexes.
Meanwhile, other types of quantum sensors are also under development (such as sensors operating in the terahertz band), which can improve road assessment accuracy to the millimeter level. Furthermore, laser-based microwave sources, originally developed for atomic clocks, can also enhance the operating range and accuracy of airport radar systems.
5. Gravity measurement
While light measurements are not suitable for all imaging tasks, gravity measurements, as a new and complementary method, can effectively reflect subtle changes in a location, such as inaccessible old mines, pits, and deeply buried water and gas pipes. This method also makes oil exploration and water level monitoring exceptionally easy.
The novel gravity sensors and quantum-enhanced MEMS (microelectromechanical systems) technologies developed using quantum cold atoms have higher performance than previous devices and will have more important commercial applications.
Low-cost MEMS devices are also being envisioned, expected to be only the size of a tennis ball and a million times more sensitive than motion sensors used in smartphones. Once this technology matures, it will make it possible to map large areas of gravity fields.
MEMS sensors have made at least several orders of magnitude progress in quantum imaging readout. Researchers from the University of Glasgow and the University of Bridgeport have developed a Wee-g detector that can use quantum light sources to improve the accuracy of the device, allowing even smaller objects to be detected—potentially aiding rescue operations in avalanche and earthquake disasters.
Cold atom sensors will offer the highest accuracy and unparalleled cost-effectiveness; currently, no more advanced technology can surpass them. The University of Birmingham is currently developing RSK and e2v cold atom sensors for everyday gravity measurements. For example, they could help the construction industry determine detailed underground conditions, reduce project delays due to unforeseen hazards, and reduce reliance on expensive exploration and excavation.
In space, cold atom sensors can achieve new scientific breakthroughs by detecting gravitational waves and verifying Einstein's theories. Of course, routine Earth remote sensing observations can also be achieved through precise gravity measurements, monitoring areas including groundwater reserves, changes in glaciers and ice sheets.
At the University of Glasgow, researchers are also creating a new and transformative space technology that uses MEMS sensors to precisely control the altitude of spacecraft, which will help enhance the competitiveness of British small satellite technology worldwide.
6. Healthcare
Dementia: According to the Alzheimer's Association, the global economic loss due to dementia is estimated at approximately £500 billion annually, and this figure is constantly increasing. Current diagnostic methods based on patient questionnaires often severely limit the options for treatment; only early diagnosis and intervention can lead to better outcomes.
Researchers are exploring the potential of a technique called magnetoencephalography (MEG) for early diagnosis. However, the current requirement for a magnetically shielded chamber and liquid helium cooling makes its widespread adoption extremely expensive. Quantum magnetometers, on the other hand, can effectively overcome these limitations, offering higher sensitivity, requiring virtually no cooling or shielding, and, crucially, at a lower cost.
Cancer: A technique called microwave tomography has been used for the early detection of breast cancer for many years, and quantum sensors help improve the sensitivity and display resolution of this technique. Unlike traditional X-rays, microwave imaging does not directly expose the breast to ionizing radiation.
Furthermore, diamond-based quantum sensors have made it possible to study temperature and magnetic fields within living cells at the atomic level, providing new tools for medical research.
Heart disease: Cardiac arrhythmias are generally considered the leading cause of death in developed countries, and the pathological characteristic of this condition is an irregular heart rate that varies in speed. Magnetic induction computed tomography (MRI), currently under development, is seen as a tool for diagnosing fibrillation and studying its formation mechanisms. The advent of quantum magnetometers will greatly enhance the application of this technology, providing significant benefits in clinical imaging, patient monitoring, and surgical planning.
Quantum sensors have broad application prospects. Currently, quantum sensors are mainly high-sensitivity magnetic sensors. Based on in-depth research on existing quantum sensors, we should consider combining the advantages of lasers and using the photoelectric conversion principle to design a quantum sensor based on the laser coherence effect.
