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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Enhancing Nonlinear Interferometers for Imaging with Undetected Photons: Seeding and High-Gain

Emma J. Pearce, Jefferson Flórez, Nathan R. Gemmell, Mariame Abire Karzazi, Yue Ma, Gabriele Bressanini, Ronan A. Battle, R. Murray, Myungshik S. Kim, Chris C. Phillips, Rupert F. Oulton, Alex S. Clark

Published Jun 26, 2023• 0 citations
Publisher

Abstract

Infrared (IR) imaging and spectroscopy is invaluable to many disciplines for its ability to probe molecular responses, from material analysis to diagnostic medicine. However, these applications are often limited by inefficient, noisy detectors. Non-degenerate nonlinear interferometers (NLIs) offer an alternative route through a technique known as imaging with undetected photons [1]. For an NLI producing visible-IR photon pairs, a change in the IR due to an object can be observed as a change to the interference of the visible photons. The IR does not need to be detected, bypassing the need for IR detectors completely.

DOI
10.1109/cleo/europe-eqec57999.2023.10232666

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Infrared (IR) imaging and spectroscopy is invaluable to many disciplines for its ability to probe molecular responses, from material analysis to diagnostic medicine. However, these applications are often limited by inefficient, noisy detectors. Non-degenerate nonlinear interferometers (NLIs) offer an alternative route through a technique known as imaging with undetected photons [1]. For an NLI producing visible-IR photon pairs, a change in the IR due to an object can be observed as a change to the interference of the visible photons. The IR does not need to be detected, bypassing the need for IR detectors completely.
Infrared (IR) imaging sees a wide range of applications due to its ability to probe molecular vibrations but is limited by detector technology available in this part of the spectrum. Imaging with undetected photons [1] offers an alternative. Using a nonlinear interferometer producing visible-IR photon pairs, transmission and phase information of an object in the IR can be observed by monitoring the interference of visible photons. No IR detection or laser is required. Recently there has been a move towards increasing the technology readiness levels of this approach, considering size, stability, and speed [2–4].
We analyse a nonlinear interferometer, also known as an SU(1,1) interferometer, in the presence of internal losses and inefficient detectors. To overcome these limitations, we consider the effect of seeding one of the interferometer input modes with either a number state or a coherent state. We derive analytical expressions for the interference visibility, contrast, phase sensitivity, and signal-to-noise ratio, and show a significant enhancement in all these quantities as a function of the seeding photon number. For example, we predict that, even in the presence of substantial losses and highly inefficient detectors, we can achieve the same quantum-limited phase sensitivity of an unseeded nonlinear interferometer by seeding with a few tens of photons. Furthermore, we observe no difference between a number or a coherent seeding state when the interferometer operates in the low-gain regime, which enables seeding with an attenuated laser. Our results expand the nonlinear interferometry capabilities in the field of quantum imaging, metrology, and spectroscopy under realistic experimental conditions.
In recent years, nonlinear quantum interferometry has been widely explored for applications like spectroscopy, optical coherence tomography, and imaging [1]. Quantum infrared spectroscopy (QIRS) uses quantum interference of generation processes of frequency-entangled visible-IR photon pairs to perform infrared spectroscopy using visible light sources and silicon-based detectors [2]. This technique utilizes a nonlinear quantum interferometer configuration where the generated IR photons pass through the sample, and the correlated visible photons are detected. The change in the detected visible photon flux with and without the sample helps extract the complex transmittance of the sample in the IR region.
Sensing with undetected photons enables the measurement of absorption and phase shifts at wavelengths different from those detected. Here, we experimentally map the balance and loss parameter space in a non-degenerate nonlinear interferometer, showing the recovery of sensitivity despite internal losses at the detection wavelength. We further explore an interaction-free operation mode with a detector-to-sample incident optical power ratio of >200. This allows changes in attowatt levels of power at 3.4 μμμm wavelength to be detected at 1550 nm, immune to the level of thermal black-body background. This reveals an ultra-sensitive infrared imaging methodology capable of probing samples effectively `in the dark'.
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].