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arXiv:2604.17704

Quantum Spectroscopy with Undetected Photons for Biomolecular Sensing in the Mid-Infrared

Mahya Mohammadi, Meryem-Nur Duman, Isa Ahmadalidokht, Mohammad Sadraeian, Christopher G. Poulton, Alexander S. Solntsev, Irina V. Kabakova

quant-phSubmitted Apr 20, 2026• Announced Apr 21, 2026• 0 citations
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Abstract

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.

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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.
Abstract 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 enabled new, unconventional approaches to Fourier transform infrared spectroscopy (FT-IR). Leveraging properties of non-degenerate entangled photon pairs, mid-infrared (mid-IR) information can be accessed in the near-infrared (near-IR) spectral domain to perform mid-IR spectroscopy with silicon-based detection schemes. Here, we address practical aspects of vibrational spectroscopy with undetected photons using a quantum FT-IR (QFT-IR) implementation. The system operates in the spectral range from around 3000 cm −1 to 2380 cm −1 (detection at around 12 500 cm −1 ) and possesses only 68 pW of mid-IR probing power for spectroscopic measurements with a power-dependence of the signal-to-noise ratio of 1.5 × 10 5 mW −1/2 . We evaluate the system’s short- and long-term stability and experimentally compare it to a commercial FT-IR instrument using Allan–Werle plots to benchmark our QFT-IR implementation’s overall performance and stability. In addition, comparative qualitative spectroscopic measurements of polymer thin films are performed using the QFT-IR spectrometer and a commercial FT-IR with identical resolution and integration times. Our results show under which conditions QFT-IR can practically be competitive or potentially outperform conventional FT-IR technology.
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.
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.