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SPIE Optical and Infrared Interferometry and Imaging

Increasing baselines and precision of optical interferometers using two-photon interference effects

Michael Keach, Steve Bellavia, Zhi Chen, Jesse Crawford, Denis Dolzhenko, Eden V. Figueroa, Aaron Mueninghoff, ANDREI NOMEROTSKI, Jonathan Schiff, Rom Simovitch, Anže Slosar, Paul Stankus, Stephen Vintskevich

Published Jul 15, 2022• 2 citations
Publisher

Abstract

Observations using interferometers provide sensitivity to features of images on angular scales much smaller than any single telescope, on the order of Δθ ∼ λ/b where b is the interferometric baseline. Present-day optical interferometers are essentially classical, interfering single photons with themselves. However, there is a new wave of interest in interferometry using multiple photons, whose mechanisms are inherently quantum mechanical, which offer the prospects of increased baselines and finer resolutions among other advantages. We will discuss recent ideas and results for quantum-assisted interferometry using the resource of entangled pairs, and specifically a two-photon amplitude technique aimed at improved precision in astrometry.

DOI
10.1117/12.2632122

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Observations using interferometers provide sensitivity to features of images on angular scales much smaller than any single telescope, on the order of Δθ ∼ λ/b where b is the interferometric baseline. Present-day optical interferometers are essentially classical, interfering single photons with themselves. However, there is a new wave of interest in interferometry using multiple photons, whose mechanisms are inherently quantum mechanical, which offer the prospects of increased baselines and finer resolutions among other advantages. We will discuss recent ideas and results for quantum-assisted interferometry using the resource of entangled pairs, and specifically a two-photon amplitude technique aimed at improved precision in astrometry.
Improved quantum sensing of photons from astronomical objects could provide high resolution observations in the optical benefiting numerous fields, including general relativity, dark matter studies, and cosmology. It has been recently proposed that stations in optical interferometers would not require a phase-stable optical link if instead sources of quantum-mechanically entangled pairs could be provided to them, potentially enabling hitherto prohibitively long baselines. A new refinement of this idea is developed, in which two photons from different sources are interfered at two separate and decoupled stations, requiring only a slow classical information link between them. We rigorously calculate the observables and contrast this new interferometric technique with the Hanbury Brown \& Twiss intensity interferometry. We argue this technique could allow robust high-precision measurements of the relative astrometry of the two sources. A basic calculation suggests that angular precision on the order of 101010 microarcseconds in the relative opening angle could be achieved in a single night's observation of two bright stars.
Classical optical interferometry requires maintaining live, phase-stable links between telescope stations. This requirement greatly adds to the cost of extending to long baseline separations and limits on baselines will in turn limit the achievable angular resolution. Here we describe a novel type of two-photon interferometer for astrometry, which uses photons from two separate sky sources and does not require an optical link between stations. Such techniques may make large increases in interferometric baselines practical, even by orders of magnitude, with a corresponding improvement in astrometric precision benefiting numerous fields in astrophysics. We tested a benchtop analogue version of the two-source interferometer and unambiguously observe correlated behavior in detections of photon pairs from two thermal light sources, in agreement with theoretical predictions. This work opens new possibilities in future astronomical measurements.
It has been recently suggested that optical interferometers may not require a phase-stable optical link between the stations if instead sources of quantum-mechanically entangled pairs could be provided to them, enabling extra- long baselines and benefiting numerous topics in astrophysics and cosmology. We developed a new variation of this idea, proposing that two photons from different sources could be interfered at two decoupled stations, requiring only a slow classical information link between them. We show that this approach could allow high- precision measurements of the relative astrometry of the two sources, with a basic calculation giving angular precision of 10 µas in a few hours’ observation of two bright stars. We also give requirements on the instrument for these observations, in particular on its temporal and spectral resolution. Finally, we discuss possible technologies for the instrument implementation and first proof-of-principle experiments.
The idea of exploiting quantum features of light to perform more precise astrophysical measurements is a long-desired goal of both the optics and astrophysics communities, but only recently have theoretical proposals started to be experimentally implemented. One important goal is to increase the baseline distance between telescopes in interferometry, and thus improve the angular resolution of the celestial object. Some of the earlier proposals rely on the use of quantum repeaters to connect the two baseline stations. However, another proposal employs other quantum optical approaches to circumvent the need for quantum repeaters, while still providing a quantum advantage. I will discuss the on-going project for building quantum-enhanced telescopes for precision astrometry, i.e., measurements of position and velocity of celestial objects. I will present a proof-of-principle experimental demonstration of a two-photon interferometer for such goal. Moreover, I will present a single-photon-sensitive spectrometer, based on a linear array of 512 single-photon avalanche diode detectors, with 0.04 nm spectral and 40 ps temporal resolutions. This device enables us to implement a spectral binning technique, essentially allowing us to perform multiple measurements together, because each frequency can be treated as an independent measurement. Such device could also be used in other applications. Lastly, I will report on our frequency-resolved Hanbury Brown-Twiss experiments. Our work represents an important step towards quantum telescopes.
Quantum technologies may revolutionize optical interferometry through new means to distribute and preserve electric-field amplitude and phase correlations between widely separated telescopes. After reviewing existing techniques, I survey proposed quantum-sensing and quantum-networking applications and highlight recent laboratory demonstrations of key building blocks. By easing field-transport demands for baselines beyond $\sim$1 km, quantum-enhanced interferometry could enable sub-milliarcsecond imaging and microarcsecond-class differential astrometry. That said, "going quantum" is not a magic shortcut to sensitivity because many of the most compelling science cases remain photon- and turbulence-limited. I therefore emphasize the practical constraints -- loss, bandwidth, coherence time, synchronization, and wavelength limits of quantum interfaces -- that must also be overcome to make these capabilities useful for astronomy, whether deployed on the ground or from space.