Spectroscopy is used to study the interaction of light with matter, providing valuable insights into the structure, composition, and properties of substances. It plays a crucial role in diverse scientific fields, including chemistry, physics, and biology. In technology, spectroscopy is vital for applications like material analysis, environmental monitoring, and medical diagnostics.1,2
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Conventional spectroscopy uses classical light to analyze matter through frequency or time variations. Time-domain spectroscopy, with ultra-short laser pulses, allows precise measurements of material composition over time. However, its reliance on classical light introduces "shot noise" that limits resolution and sensitivity as noise overpowers the signal beyond a certain point, hindering further compositional analysis.1,2
Quantum light improves spectroscopy using the quantum state of light as novel control knobs and through photon statistics variation. This approach reduces noise significantly compared to classical light, enhancing sensitivity and enabling more precise measurements beyond classical limitations.1,2
Understanding Quantum Light
Quantum light refers to light that exhibits quantum mechanical properties like superposition, entanglement, and quantum coherence. It includes phenomena such as squeezed and entangled light, where photons are manipulated in ways beyond classical physics. It explores the wave-particle duality of light at a deeper level.2-4
Photon entanglement occurs when two beams of light share a quantum state that cannot be described individually. Common entanglement types are their orbital angular momentum, position and momentum, polarization, and time and energy.2
Entangled-photon pairs are key in tests of quantum mechanics, such as Bell's inequality violations, and are useful in quantum-enhanced applications, offering advantages through quantum correlations.2
The quantum theory states that energy exchange between physical systems is quantized, leading to quantum noise that limits measurement sensitivity. Heisenberg's Uncertainty Principle introduces measurement back-action for a system’s observables when they are measured repeatedly.4
Squeezed states of light reduce uncertainty/noise in one property, improving measurement sensitivity beyond typical quantum noise limits, and can be generated using nonlinear optics.4
Single-photon sources are crucial for quantum light applications, offering high precision in both detection and measurement. In spectroscopy, these sources enable highly sensitive and accurate analysis of materials by allowing the detection of individual photons. This level of precision enhances the resolution and reliability of spectroscopic techniques.2,3
Quantum Light-enhanced Spectroscopy
Quantum light exhibits properties of both particles and waves that enable it to interact with matter in a probabilistic manner. It can be more easily manipulated to create highly sensitive measurements.2
Quantum light’s unusual intensity scaling, demonstrated in atomic and molecular samples, allows nonlinear spectroscopy at lower photon fluxes. Unlike classical heterodyne signals that scale quadratically, two-photon absorption of entangled photons scales linearly, enabling stronger signal detection at lower intensities. This has practical applications, particularly in imaging delicate biological samples.2
Quantum properties of light, specifically time-frequency entanglement, offer a new way to shape and control exciton dynamics in molecular aggregates in a way that cannot be realized using shaped classical pulses. This is because the light-material system becomes entangled between the two absorption events in entangled two-photon absorption. The entanglement time provides temporal correlations, while the frequency anticorrelation between entangled photons is created by the pump bandwidth.2
By adjusting entanglement time and pump bandwidth, specific biexciton states can be selected in the optical signal. Unlike classical light, entangled photons overcome the trade-off between spectral and temporal resolution, allowing for high simultaneous temporal and spectral resolution in probing matter, revealing detailed information not possible classically.2
Classical light is constrained by the frequency-time uncertainty principle, which imposes a trade-off between the precision of frequency and time measurements. In contrast, entangled photons can exhibit correlations that allow for measurements exceeding this classical limit, enabling simultaneous high precision in both time and frequency determinations.2
Various quantum light techniques like quantum-enhanced absorption spectroscopy, sub-shot noise spectroscopy, entangled-photon spectroscopy, and ghost imaging spectroscopy can improve spectroscopy.2,5-7
Quantum-enhanced Absorption Spectroscopy: This technique utilizes the unique properties of quantum light like entanglement and squeezed states to detect extremely small changes in materials with exceptional sensitivity.5
A work recently published in Physics Review Letters introduced a quantum-enhanced absorption spectroscopy method where a bright squeezed frequency comb was used to probe gases. The approach combined frequency modulation spectroscopy with noise reduction from squeezing the probe state.5
A homodyne detection scheme enabled simultaneous measurement across several frequencies, overcoming dispersion effects. With 10 dB squeezing, the signal-to-noise ratio improved by an order of magnitude, allowing the detection of environmental gases at much lower concentrations compared to classical methods.5
Sub-shot Noise Spectroscopy: This method leverages squeezed light to reduce quantum noise, achieving measurement precision beyond classical limits. By manipulating the uncertainty in light's amplitude and phase, squeezed light allows for more accurate detection of weak signals, even in the presence of noise.2,6
A study demonstrated sub-shot-noise absorption spectroscopy using wavelength-correlated and tunable photon pairs. The researchers measured the absorption spectra of spectrally similar biological samples, carboxyhemoglobin, and oxyhemoglobin. The results showed that achieving the same precision in resolution required fewer heralded single probe photons compared to using an ideal laser, offering a more efficient approach to absorption spectroscopy.6
Entangled-photon Spectroscopy: This technique utilizes quantum correlations between pairs of entangled photons to enhance both resolution and selectivity in measurements. The entanglement allows for the precise manipulation of photon properties, leading to improved signal-to-noise ratios and the ability to detect fine details in complex systems.2
Ghost Imaging Spectroscopy: It involves using one photon from an entangled pair to probe a sample, while its twin photon is detected at a separate location. The correlation between the photons allows for the reconstruction of detailed images or spectra of the sample, even without directly interacting with it.7
A work recently published in IEEE Photonics Journal demonstrated ghost imaging in the frequency domain using frequency speckle patterns. The method, effective for ultrafast signals, studied reconstruction resolution based on signal periodicity and speckle size. Experimental results agreed with numerical calculations, offering a simple, broadband approach for ultrafast spectral measurements.7
Key Applications
Spectroscopy using quantum light has revolutionized various fields with its exceptional sensitivity and precision. In biomedical imaging, it enables the detection of minute biochemical changes, offering insights into cellular and molecular processes that are often undetectable with classical methods. For environmental monitoring, quantum light techniques allow ultra-sensitive detection of pollutants and greenhouse gases at incredibly low concentrations, contributing to better environmental protection.1-7
In material science, spectroscopy using quantum light probes nanoscale structures with unprecedented detail, advancing the development of new materials with unique properties. Furthermore, quantum light plays a crucial role in enhancing optical measurements for quantum computing and communication, helping to push the boundaries of emerging quantum technologies and their practical applications.1-7
Challenges and Future Prospects
Despite the potential of quantum light sources in spectroscopy, challenges remain in their implementation. Low-intensity fields require longer signal collection times compared to classical techniques with higher photon flux. Implementing quantum spectroscopy in biological systems also demands expertise in both biology and quantum optics.2
Additionally, research on quantum light sources has largely focused on their use in quantum information applications, with limited attention given to their potential in spectroscopy. There is no comprehensive understanding of the ideal quantum light source, nor clarity on the conditions or quantum states best suited for enhancing spectroscopy in specific tasks.2
Time-resolved detection of mid to far-infrared electric fields holds great potential for fields like security, quality control, and medical diagnostics. However, the sensitivity of standard methods is limited by shot noise.1
A latest study in Science Advances demonstrated that using a two-mode squeezed state can overcome this limitation, enhancing far-infrared detection sensitivity. This breakthrough improves noise reduction and opens the door to quantum-enhanced time-resolved electric field spectroscopy, surpassing classical sensitivity limits.1
These developments promise breakthroughs in ultra-sensitive, high-resolution spectroscopy. Future prospects of quantum light spectroscopy rely on advances in quantum optics and photonic engineering to enable broader adoption.
Conclusion
Quantum light-enhanced spectroscopy holds immense potential for revolutionizing various fields by offering unprecedented sensitivity and precision. Despite existing challenges, advancements in quantum optics promise to push the boundaries of spectroscopy, enabling more accurate and detailed analyses.
References and Further Reading
- Adamou, D. et al. (2025). Quantum-enhanced time-domain spectroscopy. Science Advances. DOI: 10.1126/sciadv.adt2187, https://www.science.org/doi/10.1126/sciadv.adt2187
- Mukamel, S. et al. (2020). Roadmap on quantum light spectroscopy. Journal of Physics B: Atomic, Molecular and Optical Physics, 53(7), 072002. DOI 10.1088/1361-6455/ab69a8, https://iopscience.iop.org/article/10.1088/1361-6455/ab69a8/meta
- Walmsley, I. A. (2015). Light: A very short introduction. DOI: 10.1093/actrade/9780199682690.001.0001, https://www.researchgate.net/publication/364873316_Light_A_Very_Short_Introduction
- Squeezed light [Online] Available at https://www.physik.uni-hamburg.de/en/iqp/ag-schnabel/forschung/gequetschtes-licht.html (Accessed on 26 February 2025)
- Belsley, A. (2023). Quantum-enhanced absorption spectroscopy with bright squeezed frequency combs. Physical Review Letters, 130(13), 133602. DOI: 10.1103/PhysRevLett.130.133602, https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.133602
- Whittaker, R. et al. (2015). Quantum-enhanced absorption spectroscopy. ArXiv. DOI: 10.1088/1367-2630/aa5512, https://arxiv.org/abs/1508.00849
- Rabi, S. et al. (2022). Spectral ghost imaging for ultrafast spectroscopy. IEEE Photonics Journal, 14(1), 1-4. DOI: 10.1109/JPHOT.2021.3138689, https://ieeexplore.ieee.org/abstract/document/9667190
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