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Optics Express

LED as laboratory test source for astronomical intensity interferometry

Andreas Zmija, Peter Deiml, Dmitry V. Malyshev, A. Zink, G. Anton, Thilo Michel, S. Funk

Published Feb 11, 2020• 8 citations
Publisher

Abstract

The challenge of astronomical intensity interferometry is to detect the small photon-bunching signals of distant sources with a broad optical bandwidth. We have built a Hanbury Brown-Twiss-like laboratory intensity interferometer with a focus on a relatively broad bandwidth ( 1 nm FWHM optical filter) and high photon rates (up to 10 MHz) per channel compared to typical (non-astronomical) intensity interferometry applications. As a light source we use a green LED to simulate starlight. The LED has proven to be a compact high-power source of stochastic light with a special advantage of a small emission area, which favours spatial coherence. Using single-photon correlations, we detect a bunching signal in the second-order correlation function with a coherence time of < 1 ps and an amplitude of < 4 ⋅ 10 − 4 and describe signal and background quantitatively for a 40 hours measurement. In this paper we show our setup, present the correlation measurements and compare them to theoretical expectations.

DOI
10.1364/oe.28.005248

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The challenge of astronomical intensity interferometry is to detect the small photon-bunching signals of distant sources with a broad optical bandwidth. We have built a Hanbury Brown-Twiss-like laboratory intensity interferometer with a focus on a relatively broad bandwidth ( 1 nm FWHM optical filter) and high photon rates (up to 10 MHz) per channel compared to typical (non-astronomical) intensity interferometry applications. As a light source we use a green LED to simulate starlight. The LED has proven to be a compact high-power source of stochastic light with a special advantage of a small emission area, which favours spatial coherence. Using single-photon correlations, we detect a bunching signal in the second-order correlation function with a coherence time of < 1 ps and an amplitude of < 4 ⋅ 10 − 4 and describe signal and background quantitatively for a 40 hours measurement. In this paper we show our setup, present the correlation measurements and compare them to theoretical expectations.
The development of quantum-enhanced technologies requires single-photon sources, as well as methods for their characterization and verification. Here, we describe a methodology for measuring the correlation function of a single-photon source using an experimental setup that comprises a laser-excited fluorescence microscope equipped with a Hanbury Brown–Twiss intensity interferometer as one of the detection systems. Measurements of the response function of the device and the reference samples are performed. The second-order autocorrelation function of the exciton state of GaAs quantum dots in AlGaAs nanowires is obtained and reveals a single-photon emission.
Thermal light such as blackbody radiation including starlight has been known to exhibit photon bunching behaviour. This is the characteristic property of thermal photons to propagate closer together than as described by random Poissonian timing statistics. Although first theorised in the 1960s, attempts to directly probe the temporal coherence of starlight has remained challenging due to the very short timescales required. This work aims to address that by using narrowband spectral filtering to increase the timescale sufficiently for a direct measurement. We present calibration tests using laboratory light sources towards an observatory measurement.
Photons emitted by incoherent sources exhibit temporal bunching, a fundamental manifestation of Bose-Einstein statistics first observed by Hanbury Brown and Twiss (HBT). While intensity correlation measurements originated in astronomy and are now central across quantum technologies, biophysics, and spectroscopy, observing temporal bunching in broadband thermal light remains limited by detector temporal resolution. Conventional approaches - such as streak cameras or optical upconversion - introduce severe trade-offs in efficiency, bandwidth, or setup versatility. Here, we propose and demonstrate a third route: pre-detection temporal magnification of the optical field prior to measuring its intensity autocorrelation function in a conventional HBT setup. Using an electro-optic time lens with a temporal magnification of 660, we resolve the temporal intensity autocorrelation of broadband thermal light generated via non-degenerate spontaneous parametric down-conversion with a bandwidth of 250 GHz. This enables direct observation of thermal photon bunching on sub-3.5 picosecond timescales without narrow spectral filtering, establishing pre-detection temporal imaging as a general platform for broadband statistical optics, with applications spanning ultrafast quantum optics, biophysical imaging, and potentially stellar intensity interferometry.
Photon anti-bunching, which is characterized by the second-order temporal coherence function g(2)(0) < g(2)(τ), has previously been measured using single-photon counting methods. We experimentally demonstrate a homodyne detection technique for measuring the second-order coherence g(2)(τ) of quantum states of light at sideband frequencies. Our scheme is based on the original Hanbury-Brown-Twiss (HBT) intensity interferometer, but instead replaces all use of photon counters with homodyne detection in each arm (see Fig. 1). The second-order coherence, g(2)(τ), is obtained from a set of quadrature amplitude measurements.
Abstract The Hanbury Brown and Twiss (HBT) interferometer was proposed to observe intensity correlations of starlight to measure a star’s angular diameter. As the intensity of light that reaches the detector from a star is very weak, one cannot usually get a workable signal-to-noise ratio. We propose an improved HBT interferometric scheme incorporating optical parametric amplifiers (OPA) into the system to amplify the correlation signal. Remarkably, for weak star light, the signal-to-noise ratio (SNR) in the new HBT interferometric scheme is much better than that of conventional HBT interferometer. Our work is valuable in measuring a star whose intensity at the detector is low and maybe also applicable in remote sensing and long-distance quantum imaging where the light passed through the object is weak after a long distance transmission.
Hanbury-Brown and Twiss (HBT) effect is the foundation for stellar intensity interferometry. However, it is a phase insensitive two-photon interference effect. Here we extend the HBT interferometer by mixing intensity-matched reference fields with the input fields before intensity correlation measurement. With the freely available coherent state serving as the reference field, we experimentally demonstrate the phase sensitive two-photon interference effect when the input fields are thermal fields in either continuous wave or non-stationary pulsed wave and measure the complete complex second-order coherence function of the input fields without bringing them together from separate locations. Moreover, we discuss how to improve the signal level by using the more realistic continuous wave broadband anti-bunched light fields as the reference field. Our investigations pave the way for developing new technology of remote sensing and interferometric imaging with applications in long baseline high-resolution astronomy.