Towards quantum-enhanced long-baseline optical/near-IR interferometry

Microarcsecond resolutions afforded by an optical-NIR array with kilometer-baselines would enable breakthrough science. However significant technology barriers exist in transporting weakly coherent photon states over these distances: primarily photon loss and phase errors. Quantum telescopy, using entangled states to link spatially separated apertures, offers a possible solution to the loss of photons. We report on an initiative launched by NSF NOIRLab in collaboration with the Center for Quantum Networks and Arizona Quantum Initiative at the University of Arizona, Tucson, to explore these concepts further. A brief description of the quantum concepts and a possible technology roadmap towards a quantum-enhanced very long baseline optical-NIR interferometric array is presented. An on-sky demonstration of measuring spatial coherence of photons with apertures linked through the simplest Gottesman protocol over short baselines and with limited phase fluctuations is envisaged as the first step.

💡 Research Summary

The paper addresses the fundamental limitations of classical optical/near‑infrared (NIR) long‑baseline interferometry and proposes quantum‑enhanced approaches to achieve micro‑arcsecond angular resolution. Classical interferometers are constrained by photon loss in optical fibers (e.g., 0.22 dB km⁻¹ at 1550 nm) and by the difficulty of compensating optical path differences (OPD) over baselines longer than a few kilometers. Because thermal stellar emission yields extremely low photon occupation numbers, even the most efficient fibers transmit only a few percent of the signal over tens of kilometers, making kilometer‑scale baselines impractical with direct detection.

The authors introduce “quantum telescopy,” which replaces the physical transmission of astronomical photons with the distribution of entangled photon pairs. Two quantum protocols are examined in detail. The Gottesman protocol distributes an entangled pair to each telescope, performs local interference measurements, and post‑selects events where the laboratory photon and the astronomical photon arrive at the same site. This effectively teleports the astronomical photon’s phase information, eliminating the bulk of transmission loss. However, the protocol demands a continuous supply of entangled pairs at rates exceeding 10 GHz, far beyond current SPDC sources (≈100 kHz), and loses half of the astronomical photons due to the post‑selection step.

To overcome these bottlenecks, the Khabiboulline protocol incorporates quantum memories at each aperture. Incoming photons imprint their arrival time (time‑bin) onto memory qubits, and a parity‑check using shared entanglement identifies the occupied bin without revealing which telescope detected the photon. This binary encoding reduces the required entangled‑pair generation rate to match the astronomical photon arrival rate (≈10 GHz bandwidth for a 10‑m² collector observing a 10th‑mag star) and eliminates the 50 % loss of the Gottesman scheme. The protocol also reduces the number of required memory qubits to log₂(M + 1), where M is the number of time bins, making the scheme more scalable.

Technical challenges identified include: (1) developing high‑rate, high‑fidelity entangled‑photon sources; (2) building quantum memories with coherence times of milliseconds and the ability to store and process up to 20‑30 qubits per site; (3) achieving sub‑100 ps timing precision for fringe tracking, which may rely on next‑generation distributed atomic‑clock networks; and (4) integrating classical fringe‑tracking and adaptive‑optics techniques with quantum protocols for rapid phase error correction.

The paper outlines a phased roadmap: an initial on‑sky demonstration of the Gottesman protocol over short baselines using existing tabletop SPDC sources and fiber links; subsequent development of quantum‑memory‑based Khabiboulline stations; and finally scaling to baselines of several kilometers to tens of kilometers, targeting micro‑arcsecond resolution for transformative science such as exoplanet direct imaging, stellar surface mapping, and probing the immediate environment of supermassive black holes. The authors conclude that while significant engineering advances are required, the quantum‑enhanced interferometer concept offers a viable path to surpass the resolution limits of classical optical interferometry.