Ultra-broadband quantum infrared spectroscopy extending to 20 {\textmu}m

Yu Mukai; Ryo Okamoto; Shigeki Takeuchi

AI summary · arXiv: 2611.00561v1 · quant-ph

Ultra-broadband quantum infrared spectroscopy extending to 20 {\textmu}m

Yu Mukai, Ryo Okamoto, Shigeki Takeuchi

Original paper Submitted to arXiv on 31 Oct. 2026 Submitted to Optica Quantum on 6 July 2026 License: NONEXCLUSIVE-DISTRIB 1.0 Also in: fr

How to read this summary? Illustrated guide

The essentials

The authors demonstrate quantum infrared spectroscopy from 8.6 to 20 µm using a GaSe photon-pair source and measure characteristic absorption bands in polystyrene.

Question

Can quantum infrared spectroscopy be extended beyond 10 µm into the 10–20 µm molecular fingerprint region, where conventional infrared measurements face thermal-background limitations? The study tests whether a GaSe-based photon-pair source can enable this extension and measure a sample’s spectrum.

Method

The authors used a 100-µm-thick GaSe crystal pumped by a 785-nm continuous-wave laser to generate angle-tunable photon pairs, and built a Michelson-type nonlinear interferometer for quantum Fourier-transform infrared spectroscopy. They measured a 30-µm polystyrene film over 8.6–20 µm at 4 cm⁻¹ resolution, averaging 20 reference–sample pairs at each of three pump incidence angles; the 120 interferograms required approximately 42 hours in total.

Findings

Result Changing the pump incidence angle produced signal spectra corresponding to idler wavelengths spanning approximately 8–20 µm.
See the passage in the paper
Specifically, signal emission was observed between 815 nm and 870 nm, implying that the correlated idler photons were generated across a broad FIR range from 8 µm to 20 µm.
Result The interferometer produced quantum interference across the long-wavelength infrared range, with retrieved idler spectra distributed from 8 to 20 µm.
See the passage in the paper
For each condition, the emission bandwidth exceeded $200\text{\,}{\mathrm{cm}}^{-1}$, with the infrared idler spectra distributed over 8 µm to 20 µm.
Result The measured polystyrene spectrum showed characteristic absorption bands, including features associated with phenyl-group vibrations.
See the passage in the paper
Notably, strong absorption peaks appeared at around 760 and 700 cm-1, which are characteristic of out-of-plane C–H deformation modes of the monosubstituted benzene ring.
Result Across most of the measured spectral range, the QFTIR results agreed with conventional FTIR within statistical uncertainty.
See the passage in the paper
The QFTIR results agree with the conventional FTIR measurements within the statistical uncertainty over most of the spectral range.
Authors’ interpretation The authors argue that GaSe-based photon-pair generation makes quantum infrared sensing across the molecular fingerprint region possible.
See the passage in the paper
Together with previously reported results, this work demonstrates that QIRS can cover the entire molecular fingerprint region.

Limitations

The demonstrated measurements required long acquisition times, which the authors identify as the system’s practical limitation.
See the passage in the paper
At present, however, the practical limitation of the system is the long acquisition time of the quantum interferograms (Sec. 3).
A substantial fraction of collected signal photons did not contribute to interference because their conjugate idler photons were outside the idler arm’s acceptance.
See the passage in the paper
The 3-mm iris, which matches the detected signal photons to the acceptance of the idler arm, discards a large fraction of the signal photons collected with the full aperture in Sec. 2, because their conjugate idler photons, emitted at 10–24 times larger angles, are not returned to the crystal.
The phase-matching calculation beyond the range specified for the refractive-index equations relies on extrapolation.
See the passage in the paper
The refractive indices of the GaSe crystal were determined using the Sellmeier equations provided in Ref.[54], which are specified for the 0.65–18 µm range; the calculation at idler wavelengths beyond 18 µm is therefore an extrapolation.

Why it matters

The authors present the work as filling a gap in infrared photonic quantum sensing by extending QIRS into the long-wavelength region up to 20 µm. They argue that the GaSe source may also support other quantum-sensing modalities, including imaging and optical coherence tomography.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: quantum infrared spectroscopy gallium selenide spontaneous parametric down-conversion molecular fingerprint region Fourier-transform spectroscopy

Read further

Comprehensive Summary
Layman's Summary
Blog article
Original abstract

The paper investigates whether quantum infrared spectroscopy (QIRS) can be extended beyond approximately 10 micrometers into the 10–20-micrometer range. This long-wavelength region of the molecular fingerprint spectrum contains vibrational features useful for identifying materials and studying molecular structure. Conventional infrared measurements in this range, however, can be hindered by thermal background and detector noise. Rather than detecting infrared photons directly, QIRS probes a sample with infrared photons and detects their correlated visible or near-infrared partners. Earlier QIRS systems had not reached this wavelength range because suitable photon-pair sources were unavailable. To address this limitation, the authors developed a tunable source based on spontaneous parametric down-conversion in a 100-micrometer-thick gallium selenide (GaSe) crystal. A 785-nanometer continuous-wave laser pumps the crystal, generating near-infrared signal photons paired with far-infrared idler photons. Adjusting the pump’s incidence angle tunes the phase-matched wavelengths. The team measured the signal spectra and compared them with phase-matching calculations. At incidence angles of 42, 51, and 61 degrees, the signal peaks ranged from approximately 822 to 864 nanometers, corresponding to idler wavelengths from 17.4 to 8.6 micrometers. After accounting for detection efficiency, the measured rates agreed with the theoretical pair-generation rates to within 25 percent. The estimated generation rate was about one million signal photons per second within the collection cone, and the source operated in the low-gain regime. For spectroscopy, the researchers built a Michelson-type nonlinear interferometer. Infrared idler photons travel through the sample path and return to the crystal, while their near-infrared partners are detected by a silicon avalanche photodiode. Interference between pair-generation processes on the forward and return passes produces a signal that varies with path length. Fourier-transforming this interferogram yields the infrared spectrum without directly detecting the idler photons. The authors first observed interference across idler wavelengths extending to 20 micrometers. This demonstrated that photon pairs were generated at these long wavelengths and remained coherent, despite the lack of suitable photon-counting detectors in this range. They then measured the transmittance of a 30-micrometer polystyrene film from 8.6 to 20 micrometers, using three pump angles and a scan-based Fourier-transform method. The resulting spectra had a resolution of 4 inverse centimeters and reproduced characteristic polystyrene absorption features near 1070, 1030, 900, 760, 700, and 540 inverse centimeters. The bands near 760 and 700 inverse centimeters are associated with out-of-plane C–H deformation in the phenyl ring, while the band near 540 inverse centimeters is sensitive to local chain conformation. Across most of the measured range, the QIRS results agreed with those from a conventional Fourier-transform infrared spectrometer. However, acquiring the QIRS data took about 42 hours, compared with roughly one minute for the conventional measurements. The authors attribute the long acquisition time largely to the small fraction of generated photon pairs that contribute to interference, because the idler collection optics accept only a limited range of emission angles. They suggest that larger-aperture or shorter-focal-length optics, along with a moderately thicker crystal, could improve performance. A thicker crystal, however, would narrow the spectral bandwidth and could reduce interference visibility. The study demonstrates GaSe-based QIRS across a broad wavelength range of 8.6–20 micrometers, including a region previously inaccessible to this technique. Although the work establishes a route to long-wavelength quantum spectroscopy, its acquisition speed is not yet competitive with conventional infrared spectroscopy.

Quantum infrared spectroscopy reaches a new wavelength frontier

Some of the most useful clues to a material’s molecular structure lie beyond the reach of familiar infrared measurements. In particular, the 10–20-micrometer region of the molecular fingerprint spectrum contains vibrational features that can help identify substances and reveal how their molecules are arranged. A new study, “Ultra-broadband quantum infrared spectroscopy extending to 20 μm,” by Yu Mukai, Ryo Okamoto, and Shigeki Takeuchi, demonstrates a way to investigate this long-wavelength region using quantum correlations rather than direct infrared-photon detection.

The work extends quantum infrared spectroscopy (QIRS) to wavelengths as long as 20 micrometers. It also shows that the technique can recover characteristic absorption features in a polystyrene film. The results open a route to quantum-enhanced measurements in a spectral region that had previously been out of reach for QIRS, although the present method is much slower than conventional infrared spectroscopy.

Why look beyond 10 micrometers?

Infrared spectroscopy identifies molecular vibrations by measuring which wavelengths a sample absorbs. The resulting absorption pattern acts like a molecular fingerprint. Long-wavelength features can provide useful information about a material’s chemical structure and molecular environment.

Measuring this region directly can be difficult. Thermal background and detector noise may obscure weak signals, and suitable photon-counting detectors are not available across the entire 10–20-micrometer range. QIRS takes a different approach: infrared photons interact with the sample, but the measurement detects correlated visible or near-infrared photons that are generated alongside them. The detected partner carries information about what happened to the infrared photon.

Until now, extending QIRS into the long-wavelength fingerprint region had been limited by the lack of suitable sources of correlated photon pairs. Mukai and colleagues addressed that challenge by developing a tunable source based on a thin gallium selenide (GaSe) crystal.

A tunable source of infrared photon pairs

The researchers directed a continuous-wave laser with a wavelength of 785 nanometers into a 100-micrometer-thick GaSe crystal. Through a process called spontaneous parametric down-conversion, the crystal produced pairs of photons: a near-infrared signal photon and a far-infrared idler photon. The idler is the photon that probes the sample, while the signal is the one measured by the detector.

By changing the angle at which the pump laser entered the crystal, the team adjusted which wavelengths were produced efficiently. Measurements at three incidence angles—42, 51, and 61 degrees—gave signal peaks between about 822 and 864 nanometers. These corresponded to idler wavelengths spanning 17.4 to 8.6 micrometers. The measured signal rates agreed with phase-matching calculations to within 25 percent after accounting for detection efficiency.

The estimated generation rate was about one million signal photons per second within the collection cone. The source operated in the low-gain regime, meaning that photon-pair creation remained relatively sparse rather than building into a high-intensity output.

Reading infrared spectra with a visible-light detector

To perform spectroscopy, the researchers built a Michelson-type nonlinear interferometer. Infrared idler photons traveled through the sample path and returned to the GaSe crystal. Their near-infrared partners were detected by a silicon avalanche photodiode.

Photon-pair generation on the outward and return passes through the crystal interfered with one another. As the path length changed, the detected signal varied, producing an interferogram. The team then Fourier-transformed this signal to obtain the infrared spectrum. In this way, the sample’s response was read out through the near-infrared partners rather than by directly detecting the long-wavelength photons.

First, the researchers observed interference across idler wavelengths extending to 20 micrometers. This was an important milestone: it showed both that photon pairs could be generated at these wavelengths and that their coherence was preserved, despite the lack of suitable photon-counting detectors for the idler light.

Testing the method on polystyrene

The team next measured the transmittance of a 30-micrometer polystyrene film across 8.6–20 micrometers. They used three pump angles and a scan-based Fourier-transform method, achieving a spectral resolution of 4 inverse centimeters.

The resulting spectra reproduced recognizable polystyrene absorption features near 1070, 1030, 900, 760, 700, and 540 inverse centimeters. The bands near 760 and 700 inverse centimeters are linked to out-of-plane C–H deformation in the phenyl ring. The feature near 540 inverse centimeters is sensitive to local chain conformation. Across most of the measured range, the QIRS results agreed with those from a conventional Fourier-transform infrared spectrometer.

Promise—and a practical challenge

The main limitation was speed. Collecting the QIRS data took about 42 hours, compared with roughly one minute for the conventional measurement. The researchers attribute much of this difference to the small fraction of generated photon pairs that contribute to the interference signal. The idler collection optics accept only a limited range of emission angles, restricting how many useful pairs reach the measurement.

Possible improvements include using larger-aperture or shorter-focal-length optics to collect more of the emitted light. A moderately thicker crystal could also help, though it brings a trade-off: increasing crystal thickness narrows the spectral bandwidth and may reduce interference visibility.

Overall, the study establishes GaSe-based QIRS across a broad 8.6–20-micrometer range, including wavelengths previously inaccessible to the technique. It demonstrates that quantum correlations can support molecular spectroscopy well into the long-wavelength fingerprint region. Before QIRS can compete with conventional infrared instruments in routine measurements, however, its collection efficiency and acquisition speed will need substantial improvement.

Created on 10 Nov. 2026

Was this synthesis useful?

Score: 0

Ask questions about this paper to our AI assistant

How to chat with this paper? Illustrated guide

Compare with the PDF

You can also chat with multiple papers at once here.

AI assistant instructions?

Tools for this paper

Explain it differently 1 free rewrite left today

Save to a collection Group papers in collections to compare them, chat with them and export them. Continue with Google Create a free account Log in

Export BibTeX · RIS (Zotero, Mendeley, EndNote) · Obsidian (Markdown) · Notion (Markdown)

Advanced tools Premium

For researchers who follow a field closely: citation alerts, journal-club slides, version comparison and Zotero export. Discover Premium

Zotero

Citation alerts Citation alerts guide

Who works on it? The people and institutions who publish on its topic

Weekly letter Presented in "Quantum physics this week" of 9 November 2026. Quantum physics this week: Protecting fragile states, exposing fragile advantages

Journal club Premium+ Slides to present it Propose for the journal club

Versions What changed between versions

Talk about it in public Premium+ Press kit: release, questions, hype check Press kit guide

Disclaimer: The AI-based summarization tool and virtual assistant provided on this website may not always provide accurate and complete summaries or responses. We encourage you to carefully review and evaluate the generated content to ensure its quality and relevance to your needs.

Ultra-broadband quantum infrared spectroscopy extending to 20 {\textmu}m

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

Quantum infrared spectroscopy reaches a new wavelength frontier

Why look beyond 10 micrometers?

A tunable source of infrared photon pairs

Reading infrared spectra with a visible-light detector

Testing the method on polystyrene

Promise—and a practical challenge

Ask questions about this paper to our AI assistant

Tools for this paper

Advanced tools Premium

Papers around this one

Closest papers

Prior work it builds on

Go further

Cite this paper (BibTeX)