Photon sensors have a lot of applications in our productive life, from inspection of steel building structures to early disease screening in preventive medicine. However, the precision of these sensors is limited by the shot-noise limit (SNL). Fortunately, previous studies have shown that quantum technology not only has better resolution than traditional sensing methods but also can achieve ultrahigh precision that breaks the SNL. Here, a new fiber salinity sensing method with quantum-limited precision is demonstrated by applying the single-photon interferometer. A novel detection method is designed and implemented by using the Fisher information analysis. Also, the precision is defined as the estimation error of the single-photon transmittance. The salinity sensitivity of the single-photon interferometer is about −0.00053/‰ with a good linearity. The salinity resolution of the proposed fiber salinity sensor is 0.062‰. Also, the detection precision is beyond the SNL. This work demonstrates that the introduction of quantum resources has opened a new door for high-performance detection technology.
Photon sensors have a lot of applications in our productive life, from inspection of steel building structures to early disease screening in preventive medicine. However, the precision of these sensors is limited by the shot-noise limit (SNL). Fortunately, previous studies have shown that quantum technology not only has better resolution than traditional sensing methods but also can achieve ultrahigh precision that breaks the SNL. Here, a new fiber salinity sensing method with quantum-limited precision is demonstrated by applying the single-photon interferometer. A novel detection method is designed and implemented by using the Fisher information analysis. Also, the precision is defined as the estimation error of the single-photon transmittance. The salinity sensitivity of the single-photon interferometer is about −0.00053/‰ with a good linearity. The salinity resolution of the proposed fiber salinity sensor is 0.062‰. Also, the detection precision is beyond the SNL. This work demonstrates that the introduction of quantum resources has opened a new door for high-performance detection technology.
Quantum entanglement can help to increase the precision of optical phase measurements beyond the shot noise limit (SNL) to the ultimate Heisenberg limit. However, the N-photon parity measurements required to achieve this optimal sensitivity are extremely difficult to realize with current photon detection technologies, requiring high-fidelity resolution of N + 1 different photon distributions between the output ports. Recent experimental demonstrations of precision beyond the SNL have therefore used only one or two photon-number detection patterns instead of parity measurements. Here we investigate the achievable phase sensitivity of the simple and efficient single interference fringe detection technique. We show that the maximally-entangled "NOON" state does not achieve optimal phase sensitivity when N > 4, rather, we show that the Holland-Burnett state is optimal. We experimentally demonstrate this enhanced sensitivity using a single photon-counted fringe of the six-photon Holland-Burnett state. Specifically, our single-fringe six-photon measurement achieves a phase variance three times below the SNL.
Recent advances in quantum photonics have enabled increasingly robust protocols in optical phase estimation, achieving precisions beyond the standard quantum limit and approaching the Heisenberg limit. While intrinsic losses hinder the realization of unconditional super-sensitivity, reaching quantum advantage, defined as sensitivity surpassing that of any classical counterpart with identical resources, remains achievable. Here we experimentally demonstrate such an advantage using a fully fibered Mach-Zehnder-type interferometer operating at telecom wavelengths, free of post-selection. The scheme relies on the conversion of polarization-entangled photon pairs, a degree of freedom commonly favored for experimental convenience, into energy-time entanglement, which is particularly well suited for scalable fiber-based sensors. All system imperfections, including asymmetric losses and detector inefficiencies, are accounted for in the Fisher information analysis, yielding a measured quantum advantage of 10%. This result highlights the practicality of compact, alignment-free quantum interferometers for real-world sensing applications.
High-sensitivity interferometers are one of the basic tools for precision measurement, and their sensitivity is limited by their shot noise limit (SNL), which is determined by vacuum fluctuations of the probe field. The quantum interferometer with novel structures can break the SNL and measure the weak signals, such as the direct observation of gravity wave signal. Combining classical interferometers and the optical parametric amplifier (OPA) can enhance the signal; meanwhile, the quantum noise is kept at the vacuum level, so that the sensitivity of the nonlinear interferometer beyond the SNL can be achieved. By analyzing in detail the influence of system parameters on the precision of quantum metrology, including the intensity of optical fields for phase sensing, the gain factor of OPA, and the losses inside and outside the interferometers, the application conditions of high-sensitivity nonlinear quantum interferometers are obtained. Quantum interferometer-based OPAs provide the direct references for the practical development of quantum precise measurement.
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.
The classically defined minimum uncertainty of the optical phase is known as the standard quantum limit or shot-noise limit (SNL), originating in the uncertainty principle of quantum mechanics. Based on the SNL, the phase sensitivity is inversely proportional to K, where K is the number of interfering photons or statistically measured events. Thus, using a high-power laser is advantageous to enhance sensitivity due to the K gain in the signal-to-noise ratio. In a typical interferometer, however, the resolution remains in the diffraction limit of the K = 1 case unless the interfering photons are resolved as in quantum sensing. Here, a projection measurement method in quantum sensing is adapted for classical sensing to achieve an additional K gain in the resolution. To understand the projection measurements, several types of conventional interferometers based on N-wave interference are coherently analyzed as a classical reference and numerically compared with the proposed method. As a result, the Kth-order intensity product applied to the N-wave spectrometer exceeds the diffraction limit in classical sensing and the Heisenberg limit in quantum sensing, where the classical N-slit system inherently satisfies the Heisenberg limit of π/N in resolution.
Background: High-precision sensing is vital for modern technology, yet conventional sensors are fundamentally constrained by classical noise limits. Quantum sensing leverages principles of quantum mechanics, such as superposition and entanglement, to overcome these barriers, offering unprecedented sensitivity. This review comprehensively analyzes the current state of quantum sensor technology, focusing on its underlying physical principles, emerging applications, and the pathway toward industrial maturation.Methods: We synthesize findings from key research areas, detailing the foundational quantum systems like Nitrogen-Vacancy (NV) centers in diamond and advanced optical techniques. The paper reviews the theoretical methods used to achieve sub-shot-noise performance and discusses the system-level engineering challenges necessary for practical deployment, particularly concerning microwave control and optical readout. We also assess the current landscape of standardization efforts for quantum technologies.Results: Significant breakthroughs are highlighted in both biomedical and environmental sensing. Applications range from tracking cellular redox-status using electron-paramagnetic resonance to ultra-sensitive salinity measurements. The analysis confirms that quantum sensing provides superior resolution compared to classical counterparts, unlocking new capabilities in in-vivo and real-time monitoring.Discussion: While offering transformative potential, widespread adoption faces hurdles, including the maintenance of quantum coherence in ambient environments and the high cost and complexity of current instrumentation. Future efforts must focus on miniaturization, integration with classical systems, and standardized fabrication protocols. Quantum sensing represents the next-generation technological paradigm for metrology.