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arXiv:2610.32292

High Resolution Hyperspectral Quantum Infrared Imaging with Undetected Photons

Aadi Suketu Bharatia, Vidya Prakash Muppalla, Ananya Asthana, Priyanka Kandankel, Tanmoy Chakraborty, Sreelaja Nair, Anshuman Kumar

quant-phphysics.opticsSubmitted Oct 23, 2026• Announced Oct 26, 2026• 0 citations
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Abstract

Quantum sensing in the infrared regime often demands the use of expensive and cryogenically operated detectors. Nonlinear interferometry has paved the way for information transfer from the infrared to the near--infrared or visible regime in various quantum technologies. A particularly interesting application of these interferometers involves spectrally--resolved quantum imaging using undetected photons, enabled by the use of temperature--tunable parametric amplifiers. In this work, we demonstrate spectrally resolved transmission imaging with undetected photons around 1550 nm using a spontaneous parametric down conversion based folded nonlinear interferometric scheme, registering the attenuation of a zebrafish larva as a biological feasibility test. We characterize and calibrate the interferometer, having achieved spectrally resolved interference fringes with a maximum baseline visibility of $\sim 74\%$ in the near--infrared regime. Phase--stepped readout of a grating spectrometer gives a resolving power of $\sim1460$ (from the spectrometer's point spread function) at 1550 nm, and $\sim 730$ from the coherence length, the finest reported for imaging with undetected photons, over an instantaneous spectral window of $\approx 51$ cm$^{-1}$.

AI Summary

The paper investigates spectrally resolved infrared transmission imaging with undetected photons, using visible/near-infrared detection to interrogate samples around $1550\,\mathrm{nm}$. It implements a folded, single-crystal nonlinear interferometer: a $532\,\mathrm{nm}$ continuous-wave pump traverses temperature-controlled PPLN twice, producing signal–idler pairs near $810/1550\,\mathrm{nm}$. The sample is traversed twice by the idler, while a grating spectrometer records signal interference as the idler mirror is phase-stepped. Transmission maps are reconstructed by raster scanning and normalizing wavelength-dependent fringe visibility to a substrate reference. Calibration gives a peak background-subtracted visibility of $74.4\%$. Fringe periods agree with energy conservation across a usable $1539.4$–$1551.6\,\mathrm{nm}$ idler window, approximately $51\,\mathrm{cm}^{-1}$, defined by visibility of at least $0.05$. The single-channel coherence envelope implies an instrument-limited spectral resolution of $4.4\,\mathrm{cm}^{-1}$, or resolving power $R\approx1460$, corresponding to about 12 independent spectral elements across that window. Separately, the measured SPDC bandwidth is $8.9\,\mathrm{cm}^{-1}$, corresponding to $R\approx730$. An estimated mean occupation below $3.7\times10^{-6}$ photons per mode supports operation in the spontaneous regime. Demonstrations image a printed carbon-ink target and a fixed zebrafish larva; acquisitions take approximately 12 and 8 hours, respectively. The stated spatial-resolution limit is the approximately $1\,\mathrm{mm}$ idler-beam FWHM, making the larva measurement sub-resolution. Other limitations include low visibility and unreliable background subtraction away from the spectral peak, chromatic mode mismatch, and wavelength-dependent image shifts of approximately $2\,\mathrm{mm}$. The biological measurement is presented as a feasibility test.

Little sign of AI-generated textIncrementalOverclaimsPossible error

Central claim may not hold: The claimed 4.4 cm−1 spectrometer resolution is inferred from an envelope containing both source-spectrum and channel-response effects, without the independent calibration or deconvolution needed to identify it with the spectrometer PSF. The resolving power of approximately 730 describes the full SPDC source bandwidth, not an independently measured imaging spectral resolution. Appendix C gives incorrect formulas for quadratic-extremum refinement and visibility uncertainty, leaving the quantitative image-processing procedure unreliable as documented.

Written by gpt-6.1-sol from the paper's full text. Automated: it can be wrong.

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Quantum sensing in the infrared regime often demands the use of expensive and cryogenically operated detectors. Nonlinear interferometry has paved the way for information transfer from the infrared to the near--infrared or visible regime in various quantum technologies. A particularly interesting application of these interferometers involves spectrally--resolved quantum imaging using undetected photons, enabled by the use of temperature--tunable parametric amplifiers. In this work, we demonstrate spectrally resolved transmission imaging with undetected photons around 1550 nm using a spontaneous parametric down conversion based folded nonlinear interferometric scheme, registering the attenuation of a zebrafish larva as a biological feasibility test. We characterize and calibrate the interferometer, having achieved spectrally resolved interference fringes with a maximum baseline visibility of ∼74%\sim 74\%∼74% in the near--infrared regime. Phase--stepped readout of a grating spectrometer gives a resolving power of ∼1460\sim1460∼1460 (from the spectrometer's point spread function) at 1550 nm, and ∼730\sim 730∼730 from the coherence length, the finest reported for imaging with undetected photons, over an instantaneous spectral window of ≈51\approx 51≈51 cm−1^{-1}−1.
The paper investigates spectrally resolved infrared transmission imaging with undetected photons, using visible/near-infrared detection to interrogate samples around 1550 nm1550\,\mathrm{nm}1550nm. It implements a folded, single-crystal nonlinear interferometer: a 532 nm532\,\mathrm{nm}532nm continuous-wave pump traverses temperature-controlled PPLN twice, producing signal–idler pairs near 810/1550 nm810/1550\,\mathrm{nm}810/1550nm. The sample is traversed twice by the idler, while a grating spectrometer records signal interference as the idler mirror is phase-stepped. Transmission maps are reconstructed by raster scanning and normalizing wavelength-dependent fringe visibility to a substrate reference.
Calibration gives a peak background-subtracted visibility of 74.4%74.4\%74.4%. Fringe periods agree with energy conservation across a usable 1539.41539.41539.4–1551.6 nm1551.6\,\mathrm{nm}1551.6nm idler window, approximately 51 cm−151\,\mathrm{cm}^{-1}51cm−1, defined by visibility of at least 0.050.050.05. The single-channel coherence envelope implies an instrument-limited spectral resolution of 4.4 cm−14.4\,\mathrm{cm}^{-1}4.4cm−1, or resolving power R≈1460R\approx1460R≈1460, corresponding to about 12 independent spectral elements across that window. Separately, the measured SPDC bandwidth is 8.9 cm−18.9\,\mathrm{cm}^{-1}8.9cm−1, corresponding to R≈730R\approx730R≈730. An estimated mean occupation below 3.7×10−63.7\times10^{-6}3.7×10−6 photons per mode supports operation in the spontaneous regime.
Demonstrations image a printed carbon-ink target and a fixed zebrafish larva; acquisitions take approximately 12 and 8 hours, respectively. The stated spatial-resolution limit is the approximately 1 mm1\,\mathrm{mm}1mm idler-beam FWHM, making the larva measurement sub-resolution. Other limitations include low visibility and unreliable background subtraction away from the spectral peak, chromatic mode mismatch, and wavelength-dependent image shifts of approximately 2 mm2\,\mathrm{mm}2mm. The biological measurement is presented as a feasibility test.
Metrology with undetected photons is an emerging technique that leverages quantum effects and photon correlations (entanglement) to retrieve valuable information in a target spectral range (e.g., mid-infrared, mid-IR) using measurements in an easily accessible domain (e.g., visible, near-IR). The underlying quantum process of spontaneous parametric down-conversion (SPDC) is utilized to generate non-degenerate correlated signal and idler photons to serve as detection and probing photons, respectively. Sensing with undetected photons enables important advantages, such as ultra-low probe powers, room-temperature operation, and shot-noise-limited detection. In this contribution, we apply a quantum nonlinear interferometer based on an SPDC source to perform applied mid-IR spectroscopy, mid-IR microscopy, and mid-IR optical coherence tomography (OCT) as among the most promising techniques for quantum-based routine non-destructive testing. Moreover, we characterize the system, benchmark it against classical systems, and provide a prospective outlook for this new technology.
Imaging via nonlinear interferometers make use of induced coherence and take advantage of photon pair correlations to image an object without detecting the photons that illuminate the object [1]. The object's information is transferred from the photon that illuminates the object to the interference pattern of its partner photon, which is detected [2]. Spontaneous parametric down conversion allows to utilize photon pairs with large spectral separation. This fact becomes relevant if we need to image at wavelengths with detection constraints, e.g., mid-infrared, hyperspectral, or terahertz wavelengths [3], [4].
We investigate quantum spectroscopy with undetected photons for protein detection in the mid-infrared spectral region. Classical Fourier-transform infrared spectroscopy of protein samples (bovine serum albumin and N-terminal pro-brain natriuretic peptide) is used as reference to define the sample's mid-infrared absorption, which is then embedded in a numerical model of a double-pass quantum interferometer. We analyse parameters that influence visibility of the interference pattern formed by the signal beams, including the length of nonlinear crystal, sample length and mirror-sample distance. This leads us to a practical quantum spectrometer design with optimal image contrast at the specific amide I-II spectral bands. The simulated visibility spectra reproduce nearly identically the protein absorption features in the mid-IR and reveal temperature-induced changes to the protein secondary structure. Overall, this provides practical design rules for future quantum bio-spectroscopy applications that use only visible wavelength sources and detectors.
Sensing with undetected photons has become a vibrant, application-driven research domain with a special focus on the mid-infrared (mid-IR) wavelength region. Since the mid-IR contains spectral bands with highly specific and strong molecular absorbance signatures, often referred to as fingerprints, a multitude of different samples and their compositions can be detected and quantified spectroscopically. Enhancing this spectroscopic method with imaging capabilities leads to a powerful technique for environmental monitoring and biomedical applications that enables automated diagnostics while omitting time-consuming and non-reversible labeling steps. To evade the shortcomings of state-of-the-art instruments for mid-IR hyperspectral microscopy—namely cost, complexity, power consumption, and performance, which stem from technological challenges in mid-IR detection and light sources—we construct a proof-of-concept nonlinear interferometer in a wide-field imaging arrangement. This nominally narrowband imaging technique is then expanded to acquire broadband spectral information through capturing images for varying interferometer displacement and applying a pixelwise Fourier-transform of the resulting interferograms, yielding high-resolution infrared spectra for each camera pixel. For the broadband range of 2300−3100cm −1 , covering the important CH-stretch band, we perform hyperspectral imaging that simultaneously resolves 3500 spatial modes, each with a spectral resolution of 10cm −1 , leveraging in total around 10 5 spatio-spectrally entangled photon modes. Our image acquisition uses a commercial sCMOS camera, while a medium-power and compact continuous-wave pump laser is the only necessary light source. For a moderate speed of 360 voxel/s, we obtain a predominantly shot-noise-influenced signal-to-noise ratio (SNR) of 50. We further demonstrate the practicality of our novel hyperspectral imaging technique for microplastics detection and bio-imaging tasks and outline engineering solutions to increase its speed by several orders of magnitude. This shows that our quantum imaging technique is highly promising for applications requiring compact, cost-effective label-free analyses.