Classical measurement strategies in many areas are approaching their maximum resolution and sensitivity levels, but these levels often still fall far short of the ultimate limits allowed by the laws of physics. To go further, strategies must be adopted that take into account the quantum nature of the probe particles and that optimize their quantum states for the desired application. Here, we review some of these approaches, in which quantum entanglement, the orbital angular momentum of single photons, and quantum interferometry are used to produce optical measurements beyond the classical limit.
Classical measurement strategies in many areas are approaching their maximum resolution and sensitivity levels, but these levels often still fall far short of the ultimate limits allowed by the laws of physics. To go further, strategies must be adopted that take into account the quantum nature of the probe particles and that optimize their quantum states for the desired application. Here, we review some of these approaches, in which quantum entanglement, the orbital angular momentum of single photons, and quantum interferometry are used to produce optical measurements beyond the classical limit.
Quantum optical sensing is a technique that utilizes quantum mechanics to enhance, sensitivity and precision to a level considered unparalleled so far. This chapter attempts to introduce the reader to the basics of this fast-growing field along with various technologies and applications. It begins with an explanation of quantum mechanics, superposition, entanglement, and quantum coherence that allow sensors to go beyond classical performance. One of the central technologies is quantum entanglement – the phenomenon whereby several particles become instantaneous spouses and are measured with utmost precision, for instance, distance, time, and magnetic fields. Entanglement Interferometry is entanglement applied in the detection of gravitational waves and high-resolution imaging. Squeezed light, the other area of interest, is a method that removes some of the fluctuations in one parameter of light, to increase the interferometer sensor, to measure small movements and low-frequency gravitational waves. Single photon emitting devices and detectors enable high-sensitivity measurements in applications such as bioimaging and quantum-securing communication. The chapter also illustrates the usage of this technology in practice in healthcare, environment, and navigation, like the examination of biomarkers and pollutants. Last is the discussion of issues such as quantum coherence and issues of scalability and development of QTs, as well as the proposed further development and more potential advances for QTs as a medium for mass use.
A new perspective for photonic instrumentation is offered by quantum sensors. In the streamline of quantum computing, quantum sensors are based on some counter-intuitive properties of quantum optics, like squeezed states offering a measurement uncertainty below the normal quantum limit, and entangled states or bi-photons, pairing the result of measurements on photons propagated even very far away from each other. In the chapter, we describe at some length the squeezing of photons and its application to improved-sensitivity interferometry, and the basics of entangled states that potentially can also achieve performance at sub-quantum limit, even though till now they are limited to a speculative stage, although very promising.
High precision measurements are performed everyday through sensors already present in many modern technological applications. The challenge of reaching the maximum accuracy, building more powerful devices, can be done only using quantum effects [1]. The field that studies how to improve the sensitivity of measurements is Quantum Metrology. Quantum metrology techniques have been extensively adopted to perform static measurements in many scenarios, showing that multi-photon entangled states, may achieve the ultimate bound on sensitivity.
Entanglement-enhanced quantum metrology explores the utilization of quantum entanglement to enhance measurement precision. When particles in a probe are prepared into a suitable quantum entangled state, they may collectively accumulate information about the physical quantity to be measured, leading to an improvement in measurement precision beyond the standard quantum limit and approaching the Heisenberg limit. The rapid advancement of techniques for quantum manipulation and detection has enabled the generation, manipulation, and detection of multi-particle entangled states in synthetic quantum systems such as cold atoms and trapped ions. This article aims to review and illustrate the fundamental principles and experimental progresses that demonstrate multi-particle entanglement for quantum metrology, as well as discuss the potential applications of entanglement-enhanced quantum sensors.
Optical sensors have applications in a range of settings, from measuring mechanical vibrations in manufacturing processes to biometrics in security devices and the detection of proteins and viruses in biomedical research. Many sensing approaches exist and plasmonic systems in particular have been studied extensively due to their ability to confine light below the diffraction limit, which greatly enhances their sensitivity compared to conventional approaches. Recently, various quantum techniques have been discovered that can outperform classical sensing ones and provide sensitivity beyond the shot-noise limit. The use of these quantum techniques in plasmonics for improving sensing performance is currently being explored. We study the performance of a plasmonic sensor that exploits the quantum nature of light and its highly confined field. We consider specialized quantum states and by comparison we show how each can provide improved sensitivity beyond the shot-noise limit.
Quantum sensors and metrology harness quantum phenomena—such as superposition, entanglement, and squeezing—to achieve precision measurements beyond classical limits. This review presents an overview of the fundamental principles underlying quantum sensing, including quantum coherence, entanglement-assisted measurement, and quantum-enhanced interferometry. Various quantum sensor platforms are discussed, including atomic clocks, nitrogen-vacancy (NV) center magnetometers, superconducting quantum interference devices (SQUIDs), optomechanical sensors, and trapped-ion systems. We explore their operational mechanisms, performance metrics, and technological implementations. Comparative analyses highlight sensitivity, spatial resolution, bandwidth, and environmental robustness. Advances in materials, control techniques, and hybrid architectures are examined, and emerging applications in navigation, geophysics, medical imaging, and fundamental physics are reviewed. Finally, future prospects for scalable and robust quantum metrology are discussed.