Quantum Imaging and Nonlinear Spectroscopy Techniques

Quantum imaging and nonlinear spectroscopy unite quantum optics and nonlinear photonics to overcome classical limits in spatial resolution, sensitivity and spectral range. By exploiting quantum correlations between photons—often generated via nonlinear processes such as parametric down-conversion—researchers can reconstruct images or extract spectral signatures without directly detecting all interacting photons. Techniques involving undetected photons leverage entanglement or indistinguishability to probe samples in challenging spectral regions, notably the mid-infrared, while performing detection in more convenient visible or near-infrared bands. Nonlinear interferometry schemes, including SU(1,1) interferometers, amplify weak signals through high parametric gain, enabling label-free molecular identification and low-dose imaging. Applications span from biological tissue mapping and chemical sensing to materials characterisation and environmental monitoring. Recent advances weave deep-learning algorithms into quantum holography, integrate waveguide-based sources for on-chip interferometry, and extend entangled-photon bandwidths, collectively enhancing speed, robustness against noise and practical deployment of quantum-enhanced imaging and spectroscopy platforms.

Research from Nature Portfolio

Recent studies have demonstrated control of multiphoton non-local interference without requiring intrinsic entanglement of photon properties, harnessing superposition of photon-generation origins to induce constructive and destructive interference across four-photon product states. One photon, never detected, serves as a non-local control, opening new avenues for foundational tests of quantum mechanics and potential quantum sensing applications. In parallel, high-parametric-gain SU(1,1) interferometry has been advanced for Fourier-transform infrared spectroscopy using undetected mid-infrared photons. By increasing photon flux at the interferometer output and reducing sample exposure, this approach achieves improved interference contrast and a broadened spectral range via aperiodic poling in the gain medium. These developments underscore the potential for highly sensitive, compact spectrometers that operate without conventional mid-infrared detectors.

Research from all publishers

Ultra-broadband quantum infrared spectroscopy has been realised with entangled visible–infrared photon pairs spanning 2 to 5 µm, generated in a specially engineered chirped-poling non-linear crystal. A nonlinear interferometer then enables mid-infrared molecular spectroscopy using a silicon-based visible detector, achieving ultra-broadband coverage and paving the way for compact, high-sensitivity IR spectrometers. Separately, integrated two-colour broadband SU(1,1) interferometers fabricated in titanium-doped lithium niobate waveguides have been employed to measure ultrashort biphoton correlation times below 100 fs, offering direct insight into dispersion effects and temporal resolution critical for quantum spectroscopy. Additionally, a quantum ghost imaging spectrometer exploits spatial and spectral correlations in parametric down-conversion to remotely reconstruct the frequency-dependent absorption profile of composite samples, providing model-independent spectral mapping in low-count regimes with potential for remote sensing and materials analysis.

Quantum Imaging and Nonlinear Spectroscopy Techniques publication trend

The graph below shows the total number of articles in quantum imaging and nonlinear spectroscopy techniques across all publications each year (not limited to Nature Index journals).

Technical terms

Parametric down-conversion: A nonlinear optical process in which a high-energy photon is converted into a pair of lower-energy photons that share quantum correlations.

Quantum entanglement: A phenomenon in which two or more particles share a joint quantum state such that the measurement of one instantaneously influences the state of the other, regardless of separation.

SU(1,1) interferometer: A nonlinear interferometric configuration that uses parametric amplifiers at its input and output ports to enhance phase sensitivity beyond classical limits.

Undetected photon: A photon whose presence or phase influences an interferometric measurement despite being neither directly measured nor interacting with the detector.

Biphoton correlation time: The temporal uncertainty in the arrival times of two photons generated as a pair, indicative of the degree of temporal entanglement and relevant for high-resolution spectroscopy.

Nonlinear interferometry: Interferometric techniques that employ nonlinear optical processes, such as parametric amplification or frequency mixing, to generate and manipulate quantum states for enhanced measurement performance.

References

1. Deep learning enhanced quantum holography with undetected photons. PhotoniX (2024).
2. Multiphoton non-local quantum interference controlled by an undetected photon. Nature Communications (2023).
3. Fourier-transform infrared spectroscopy with undetected photons from high-gain spontaneous parametric down-conversion. Communications Physics (2024).
4. Ultra-broadband quantum infrared spectroscopy. Optica (2024).
5. Measurement of Ultrashort Biphoton Correlation Times with an Integrated Two-Color Broadband SU(1,1)-Interferometer. PRX Quantum (2024).
6. Quantum Ghost Imaging Spectrometer. ACS Photonics (2023).