Introduction
Quantum theory is one of the 20th century's major achievements. It revealed the structure, properties, and dynamics of matter in the microscopic domain, extending analysis from macroscopic systems to microscopic systems. A series of phenomena distinct from classical systems, such as entanglement, coherence, and uncertainty, were identified. Quantum theory and related methods have also been applied in chemistry, genetic engineering, atomic physics, and quantum information.
Recent advances in quantum information have made manipulation and control of microscopic quantum states increasingly important. The theory and methods for controlling quantum states have given rise to the field of quantum control.
Concept and Current Status of Quantum Sensors
Quantum control studies the control of quantum states in microscopic systems. Quantum sensors can be used to address measurement problems within quantum control.
In classical control, measurement is performed by instruments composed of sensors that convert, select, compare, and display signals. In quantum control, measurement likewise involves comparing the measured quantity with a reference, but the observables correspond to self-adjoint operators in quantum mechanics. Direct measurement of a quantum system's state is generally difficult; the measured quantity must usually be transformed into a more readily measurable physical quantity to enable indirect measurement of the quantum state. Quantum sensors perform this transformation.
Quantum sensors can be defined from two perspectives:
- Physical devices that perform transformations using quantum effects and algorithms designed for the corresponding quantum quantities.
- Transduction elements whose dimensions or behaviors are so fine that quantum effects must be taken into account to perform the required transformation.
From either perspective, quantum sensors must obey the laws of quantum mechanics. In essence, a quantum sensor is a physical device designed according to quantum mechanics that uses quantum effects to implement transformations of system observables.
Like biosensors, a quantum sensor consists of a sensitive element that generates signals and auxiliary instruments that process those signals. The sensitive element is the core and exploits quantum effects.
As quantum control research progresses, requirements for sensitive elements will increase. Sensors will trend toward miniaturization and quantum-specific designs, with quantum effects playing an inevitable and important role. Various quantum sensors are expected to see broad application in quantum control and state detection.
Performance Analysis of Quantum Sensors
Sensor performance is typically evaluated by accuracy, stability, and sensitivity. Considering the characteristics of quantum sensors, performance can be assessed from the following aspects.
1. Noninvasiveness
In quantum control, measurement can induce wavefunction collapse, and the sensor may itself perturb the system state. Therefore, the interaction between the quantum sensor and the system must be carefully considered. Because state detection in quantum control differs fundamentally from classical state detection, the collapse process implies that the measurement may destroy the state being measured. Noninvasiveness is thus a key consideration. In practice, the sensor can be regarded as part of the system or modeled as a perturbation by including the interaction Hamiltonian in the overall system evolution.
2. Real-time Performance
Measurements in quantum control often must keep pace with rapid state evolution, so real-time performance is an important metric. Real-time requirements mean the sensor output should closely reflect the system's current state and, where necessary, track quantum-state evolution. Sensor design must address measurement latency.
3. Sensitivity
The main function of a quantum sensor is to transform and measure quantities of microscopic objects, so it must detect minute changes. Sensor design must ensure that sensitivity meets practical requirements.
4. Stability
Quantum system states are susceptible to environmental influence. During detection, the sensor or the measured object may become unstable. Environmental engineering techniques, such as cooling traps or low-temperature enclosures, can be used to protect system and sensor stability.
5. Multifunctionality
Quantum systems are complex and subsystems can interact with each other and with sensors. To reduce human intervention and latency from multi-step measurements, it is desirable to integrate sampling, processing, and measurement functions into a single quantum sensor. Embedding appropriate control algorithms can yield intelligent, multifunctional quantum sensors.
In addition to converting quantum quantities that cannot be measured directly into measurable quantities, quantum sensor design should evaluate noninvasiveness, real-time performance, sensitivity, stability, and multifunctionality.
Market Applications of Quantum Sensors
For example, in the UK more than 73,000 people work in sensors and related equipment, contributing over 14 billion pounds annually. The value generated from sensor data services alone is significant.
The potential of quantum sensors extends further. Quantum magnetic sensors could substantially reduce the cost of magnetoencephalography and help broaden its adoption. Quantum sensors for gravity measurement could change conventional perceptions of time-consuming and complex underground surveying. In navigation, quantum sensors can enable inertial navigation in areas where satellite signals are unavailable.
1. Civil Engineering
Underground surveying is often expensive and time-consuming but necessary for major infrastructure projects such as high-speed rail or nuclear power. Many underground hazards, such as sewers, mines, and sinkholes, remain poorly characterized. Insufficient information leads to delays, cost overruns, and redesign. Existing radar, electronic detectors, and magnetometers have limited depth performance. Gravity sensing records tiny variations caused by buried objects, but traditional gravimeters are slow, inaccurate, and sensitive to ground vibration.
Quantum sensors for gravity measurement offer advantages: faster measurements, higher precision, greater depth reach, and reduced sensitivity to ground vibration. Broad adoption of this technology could significantly impact the civil engineering sector.
2. Natural Hazard Prevention
Quantum sensors can mark collapse risk areas and zones of excessive water accumulation on gravity maps, supporting real-time monitoring of risks such as subsidence and landslides. Quantum photonic sensors can also quickly identify hazards beneath the surface, such as fuel leaks, enabling routine inspections.
3. Resource Exploration
Determining extraction sites for oil and gas is a multibillion-dollar market. Seismic surveying is common, while gravity measurement is used in less-explored areas but is traditionally costly due to equipment setup. Quantum-enhanced MEMS sensors reduce calibration needs and speed measurements, lowering costs substantially.
4. Transport and Navigation
Transport systems increasingly require precise position and condition information for vehicles, leading to more sensors on cars, trains, and aircraft. Satellite navigation, radar, ultrasonic sensors, and optical sensors are becoming standard. Autonomous vehicles demand positioning and navigation accuracy within 10 centimeters, and next-generation driver assistance must detect local centimeter-scale hazards. Cold-atom quantum sensors can provide centimeter-level position accuracy and enable navigation where satellites cannot reach, such as underwater, underground, or in dense urban environments.
Other quantum sensors, for example operating in the terahertz band, can bring road assessment accuracy to the millimeter level. Laser-based microwave sources originally developed for atomic clocks can also enhance radar range and resolution at airports.
5. Gravity Measurement
Optical measurements are not suitable for all imaging tasks. Gravity measurements can reveal subtle local changes, such as inaccessible old mine workings, sinkholes, and deeply buried pipelines. This method facilitates oil and water resource exploration and monitoring.
Cold-atom-based gravity sensors and quantum-enhanced MEMS technology outperform earlier instruments and have significant commercial potential. Low-cost MEMS devices are envisaged to be tennis-ball sized with sensitivity up to a million times greater than motion sensors in smartphones. If realized, large-area gravity mapping would become possible.
MEMS sensors have already improved quantum imaging readout by several orders of magnitude. Researchers at the University of Glasgow and Qiaogang University developed a Wee-g detector that uses quantum light sources to enhance precision, enabling detection of smaller objects and supporting rescue efforts in avalanches and earthquakes.
Cold-atom sensors offer the highest precision and cost-effectiveness; no currently available technology surpasses them. The University of Birmingham is developing RSK and e2v cold-atom sensors for routine gravity measurement, which can help construction firms assess subterranean conditions, reduce delays caused by unexpected hazards, and decrease reliance on costly exploratory excavation.
In space applications, cold-atom sensors can detect gravitational waves and test theories of gravity, and precise gravity measurements can support Earth observation tasks such as monitoring groundwater storage and changes in glaciers and ice sheets. Researchers at the University of Glasgow are also developing spacecraft control technologies that use MEMS sensors for fine altitude control to enhance small-satellite capabilities.
6. Healthcare
Dementia: The global economic cost of dementia is large and growing. Current diagnostic methods based on patient questionnaires limit treatment options. Early diagnosis and intervention improve outcomes. Magnetoencephalography (MEG) shows promise for early diagnosis, but current MEG systems require magnetically shielded rooms and liquid helium cooling, making them costly. Quantum magnetometers offer higher sensitivity, minimal cooling and shielding requirements, and lower cost.
Cancer: Microwave tomography has been used for early breast-cancer detection, and quantum sensors can improve its sensitivity and resolution. Unlike X-rays, microwave imaging does not expose the breast to ionizing radiation.
Cellular studies: Diamond-based quantum sensors enable study of temperature and magnetic fields inside living cells at the atomic scale, providing new tools for biomedical research.
Cardiac disease: Arrhythmias are a leading cause of death in developed regions. Emerging magnetically based imaging techniques can diagnose and study atrial fibrillation; quantum magnetometers can enhance imaging for clinical applications, patient monitoring, and surgical planning.
Quantum sensors have broad application prospects. Current implementations emphasize high-sensitivity magnetic sensors. Building on existing quantum sensors, designers should consider combining laser advantages and optoelectronic conversion principles to develop quantum sensors based on laser coherence effects.
