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Advanced Photonics Research

Interaction‐Free Quantum Spectroscopy

Yu Chen, Yujie Cai, Xing-Tong Li, Kun Huang, Jin‐Ming Liu, E Wu

Published May 6, 2021• 8 citations
Publisher

Abstract

Interaction‐free measurement (IFM)‐based single‐photon interference is developed as a powerful platform for revealing object information in quantum metrology, where the presence of a sample is identified without photon “touching” in an interferometer. Herein, a method of obtaining the spectroscopic properties with interaction‐free quantum spectroscopy (IFQS) based on the spectrum‐correlated photon pairs by the spontaneous parametric down‐conversion (SPDC) is proposed and implemented. Infrared signal photons around 1557 nm from the SPDC pairs are sent to a Mach–Zehnder interferometer (MZI), which contains a semitransparent sample in one of the arms. The transmission spectrum of the sample is characterized by recording the coincidence detection of the spectrum‐correlated near‐infrared heralding photons around 808 nm from the SPDC. The IFQS can not only distinguish the existence of the sample but also recognize the sample by its transmission spectrum without interaction. The approach unlocks the potential to facilitate the applications of correlation‐based quantum spectroscopy and reduce the power‐induced optical damage of fragile materials.

DOI
10.1002/adpr.202000206

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Interaction‐free measurement (IFM)‐based single‐photon interference is developed as a powerful platform for revealing object information in quantum metrology, where the presence of a sample is identified without photon “touching” in an interferometer. Herein, a method of obtaining the spectroscopic properties with interaction‐free quantum spectroscopy (IFQS) based on the spectrum‐correlated photon pairs by the spontaneous parametric down‐conversion (SPDC) is proposed and implemented. Infrared signal photons around 1557 nm from the SPDC pairs are sent to a Mach–Zehnder interferometer (MZI), which contains a semitransparent sample in one of the arms. The transmission spectrum of the sample is characterized by recording the coincidence detection of the spectrum‐correlated near‐infrared heralding photons around 808 nm from the SPDC. The IFQS can not only distinguish the existence of the sample but also recognize the sample by its transmission spectrum without interaction. The approach unlocks the potential to facilitate the applications of correlation‐based quantum spectroscopy and reduce the power‐induced optical damage of fragile materials.
Abstract The authors demonstrate a form of two‐photon‐counting interferometry by measuring the coincidence counts between single‐photon‐counting detectors at an output port of a Mach–Zehnder Interferometer (MZI) following injection of broad‐band time‐frequency‐entangled photon pairs (EPP) generated from collinear spontaneous parametric down conversion into a single input port. Spectroscopy and refractometry are performed on a sample inserted in one internal path of the MZI by scanning the other path in length, which acquires phase and amplitude information about the sample's linear response. Phase modulation and lock‐in detection are introduced to increase detection signal‐to‐noise ratio and implement a “down‐sampling” technique for scanning the interferometer delay, which reduces the sampling requirements needed to reproduce fully the temporal interference pattern. The phase‐modulation technique also allows the contributions of various quantum‐state pathways leading to the final detection outcomes to be extracted individually. Feynman diagrams frequently used in the context of molecular spectroscopy are used to describe the interferences resulting from the coherence properties of time‐frequency EPPs passing through the MZI. These results are an important step toward the implementation of a proposed method for molecular spectroscopy—quantum‐light‐enhanced 2D spectroscopy.
Infrared (IR) spectroscopy is an indispensable tool for many practical applications including material analysis and sensing. Existing IR spectroscopy techniques face challenges related to the inferior performance and the high cost of IR-grade components. Here, we develop a new method, which allows studying properties of materials in the IR range using only visible light optics and detectors. It is based on the nonlinear interference of entangled photons, generated via Spontaneous Parametric Down Conversion (SPDC). In our interferometer, the phase of the signal photon in the visible range depends on the phase of an entangled IR photon. When the IR photon is traveling through the media, its properties can be found from observations of the visible photon. We directly acquire the SPDC signal with a visible range CCD camera and use a numerical algorithm to infer the absorption coefficient and the refraction index of the sample in the IR range. Our method does not require the use of a spectrometer and a slit, thus it allows achieving higher signal-to-noise ratio than the earlier developed method.
Recent years have seen the development of quantum sensing concepts utilizing nonlinear interferometers based on correlated photon pairs generated by spontaneous parametric down-conversion (SPDC). Using SPDC far from frequency degeneracy allows a “division of labor” between the mid-infrared photon for the strongest sample interaction and the correlated near-infrared photon for low-noise detection. The small number of photons provided by SPDC and the resulting inferior signal-to-noise ratio are, however, a limiting factor preventing the wide applicability of the novel sensing concept. Here, we demonstrate a nonlinear interferometer based on pump-enhanced SPDC with strongly improved emission rates while maintaining broadband spontaneous emission. For validation of the concept, we demonstrate high-resolution mid-infrared spectroscopy with near-infrared detection, showcasing improved accuracy. Although the number of mid-infrared photons is about five orders of magnitude smaller than in classical spectrometers, the sensitivity of the quantum spectrometer becomes comparable, marking an essential step toward real-world applications.
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.
Abstract Nonlinear interferometry with entangled photons allows for characterizing a sample without detecting the photons interacting with it. This method enables highly sensitive optical sensing in the wavelength regions where efficient detectors are still under development. Recently, nonlinear interferometry has been applied to interferometric measurement techniques with broadband light sources, such as Fourier‐transform infrared spectroscopy and infrared optical coherence tomography. However, they have been demonstrated with photon pairs produced through spontaneous parametric down‐conversion (SPDC) at a low parametric gain, where the average number of photons per mode is much smaller than one. The regime of high‐gain SPDC offers several important advantages, such as the amplification of light after its interaction with the sample and a large number of photons per mode at the interferometer output. This work presents broadband spectroscopy and high‐resolution optical coherence tomography with undetected photons generated via high‐gain SPDC in an aperiodically poled lithium niobate crystal. To prove the principle, reflective Fourier‐transform near‐infrared spectroscopy with a spectral bandwidth of 17 THz and optical coherence tomography with an axial resolution of 11 µm are demonstrated.
We demonstrate a new scheme for infrared spectroscopy with visible light sources and detectors. The technique relies on the nonlinear interference of correlated photons, produced via spontaneous parametric down conversion in a nonlinear crystal. Visible and infrared photons are split into two paths and the infrared photons interact with the sample under study. The photons are reflected back to the crystal, resembling a conventional Michelson interferometer. Interference of the visible photons is observed and it is dependent on the phases of all three interacting photons: pump, visible and infrared. The transmission coefficient and the refractive index of the sample in the infrared range can be inferred from the interference pattern of visible photons. The method does not require the use of potentially expensive and inefficient infrared detectors and sources, it can be applied to a broad variety of samples, and it does not require a priori knowledge of sample properties in the visible range.