As military technology continues to advance, the need for improved sensing capabilities remains a priority. One of the most promising frontiers in this field is quantum sensing, with a particular focus on the use of squeezed states of light, which pushes the boundaries of sensor performance far beyond what classical physics allows. This cutting-edge research has the potential to significantly enhance the precision and sensitivity of sensors across a variety of applications, from imaging systems to accelerometers, and even gravimeters[1], leading to competitive edge in the navigation and situational awareness. In Occasional Paper 43 Quantum Sensing Using Squeezed States, Dr Ria Rushin Joseph explores how squeezing the properties of light is paving the way for next generation optomechanical sensors that could transform military technologies.
Before looking into the military applications, it is crucial to understand what squeezed states are and why they are so important. In classical physics, light is typically described as electromagnetic waves with a certain amplitude and phase. However, in quantum mechanics, light can exhibit unusual behaviours due to the so-called uncertainty principle[2]. Squeezed states refer to a specific quantum state of light in which one property (such as the amplitude or phase of the light wave) is squeezed or reduced in uncertainty, at the expense of increasing the uncertainty in the conjugate property. This reduction in uncertainty can improve signal-to-noise ratio[3], lead to measurements with unprecedented precision.
Optomechanical sensors are devices that integrate mechanical systems with optical techniques to measure physical quantities. In these sensors, light interacts with mechanical components, (like mirrors or oscillators) to detect changes in physical quantities (such as displacement and acceleration). These sensors rely heavily on the interaction between the light field and the mechanical element, whose sensitivity can benefit immensely from squeezed states. By utilising squeezed light to probe the mechanical system, it is possible to measure very small changes in mechanical properties with far greater precision than traditional techniques allow. In essence, squeezed light helps break through the standard quantum limit inherent in classical sensing systems. This enhancement is particularly beneficial for military applications where precision and sensitivity are crucial.
Optomechanical sensors are instrumental in advancing both quantum imaging and quantum sensing technologies. By using squeezed states of light, these sensors achieve great precision and sensitivity. Quantum imaging extends the capabilities of quantum sensing by combining quantum sensors with imaging apparatuses, enabling us to "see the unseen." Techniques like quantum illumination exploit the principle of quantum entanglement[4] by using entangled photon pairs to illuminate a target, with the received signals detected at high sensitivity. This method significantly enhances the signal-to-noise ratio, allowing for the detection of objects obscured by smoke, mist, or other scattering media.
In the paper, Dr Joseph considers how optomechanical sensors are being developed to use intracavity squeezing to enhance measurement precision beyond classical limits. Such sensors reduce quantum noise through the generation and manipulation of squeezed light within an optical cavity[5]. Central to its design is a high-finesse cavity featuring a movable end mirror acting as a mechanical oscillator (such as a microfabricated cantilever or membrane) that responds to external forces. A nonlinear medium (either a Kerr material [6] or an optical parametric amplifier (OPA)[7]) is embedded within the cavity to enable the generation of squeezed states of light via nonlinear interactions. A strong pump laser drives the cavity, with its frequency precisely detuned from resonance to optimise optomechanical coupling and maximise squeezing.
Unlike external squeezing methods, intracavity squeezing reduces losses and ensures strong interaction with the mechanical oscillator. The squeezed light interacts with the oscillator through radiation pressure, with external forces causing minute displacements that modulate the cavity’s length and alter the phase of the intracavity field. These phase changes measured using a homodyne detector[8], allow for precise detection of quadrature components, while advanced signal processing enhances sensitivity and reduces noise. Surpassing the 3 dB squeezing limit, the sensor achieves sensitivity beyond the standard quantum limit.
Optimisation involves selecting materials with strong Kerr nonlinearity, engineering the oscillator’s properties, and maintaining precise laser detuning. Challenges include minimising environmental noise, ensuring phase stability, and maintaining high-quality optical components. The sensor’s compact, integrated design reduces complexity, minimises losses, and enhances robustness, making it suitable for deployment in demanding environments such as military and aerospace applications. By achieving unprecedented noise suppression, the sensor enables high-precision measurements of weak signals, with transformative potential for inertial navigation, gravitational wave detection, and secure communications, marking a significant advancement in quantum sensing technology.
The Path Forward
While the potential applications of quantum sensing using squeezed states are exciting, there are still challenges to overcome. In this regard, Dr Joseph observes that one of the primary difficulties is to generate and manipulate squeezed states in real-world environments. Maintaining the quantum properties of light in noisy environments can be difficult. However, ongoing research in quantum optics and nanotechnology is already addressing many of these challenges. As researchers continue to refine techniques for generating and manipulating squeezed states, it is likely that these obstacles will be progressively overcome, paving the way for practical and deployable quantum sensing technologies.
Using squeezed states of light in quantum sensing marks a ground-breaking step forward in military technology. This approach boosts precision and sensitivity to levels never before possible, significantly improving the performance of sensors in various applications. These advancements strengthen military systems, enhancing everything from navigation to situational awareness. For example, quantum accelerometers offer reliable navigation in GPS-denied environments, while quantum imaging (powered by quantum entanglement) excels in detecting hidden threats even in difficult conditions. Together, these technologies provide a decisive edge in modern defence scenarios.
Ultimately, Dr Joseph concludes that the future of quantum sensing using squeezed states is bright, with the potential to revolutionise military technology and go beyond. Continued investment in research and development will be crucial to unlocking the full capabilities of these cutting-edge quantum sensors. Such investment offers significant strategic advantages and can help ensure that the Australian Army stays ahead in modern warfare, particularly in complex and contested environments where existing systems may be less effective.
Endnotes
[1] Gravimeters are instruments used to measure tiny variations in the Earth's gravitational field, often for applications like studying geology, detecting underground structures, or monitoring environmental changes.
[2] The uncertainty principle states that it is impossible to precisely measure both the position and momentum of a particle at the same time, as improving the accuracy of one inherently makes the other less certain.
[3] Signal-to-noise ratio (SNR) is a measure of how much a desired signal stands out from background noise, with a higher ratio indicating a clearer and more distinguishable signal.
[4] Quantum entanglement is a phenomenon where two or more particles become linked such that the state of one particle instantly influences the state of the other, no matter how far apart they are.
[5] An optical cavity is a set of mirrors arranged to form a closed or semi-closed path that traps light, allowing it to bounce back and forth, enhancing interactions with materials inside and creating resonant conditions for various optical processes.
[6] A Kerr material is a nonlinear optical medium whose refractive index changes in response to the intensity of light passing through it, enabling effects like self-focusing, optical switching, and the generation of squeezed light.
[7] An Optical Parametric Amplifier (OPA) is a device that amplifies a weak optical signal by using a nonlinear crystal and a strong pump laser, splitting photons from the pump into two lower-energy photons (signal and idler) while preserving energy and momentum.
[8] A homodyne detector is a device used in optical measurements to analyse the phase and amplitude of a light signal by mixing it with a reference beam (local oscillator) of the same frequency and detecting the resulting interference.
