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

Abstract: 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.

• The paper presents quantum entanglement and memory methods that overcome photon loss and phase errors in long-baseline interferometry.
• The study compares the Gottesman and Khabiboulline protocols, highlighting their trade-offs in entanglement rates and signal usability.
• Initial laboratory demonstrations and on-sky applications indicate that these quantum enhancements could achieve microarcsecond angular resolutions in astronomical imaging.

Towards Quantum-Enhanced Long-Baseline Optical/Near-IR Interferometry

This paper explores the utilization of quantum technology to enhance optical and near-infrared (NIR) interferometry, particularly for achieving resolutions at the microarcsecond level. Traditional long-baseline interferometry has enabled significant astronomical observations, yet it is constrained by current technological barriers, such as photon loss and phase errors. The advancement of quantum-enhanced techniques presents a promising avenue for overcoming these limitations, allowing for much longer baselines and, consequently, higher resolution measurements.

Challenges of Classical Interferometry

The current state-of-the-art in classical optical interferometry primarily relies on combining optical beams over distances comparable to available telescope separations, which typically cap at a few hundred meters. The physical limitations include photon losses and the need to compensate for differential optical path differences over large scales, often rendering longer baselines impractical with existing technology. Furthermore, free-space propagation and mechanical compensation on such scales are technically demanding, especially at shorter wavelengths where transmission losses are substantial.

Quantum Augmentation through Entanglement

The proposition to integrate quantum mechanics into interferometry is predicated on using quantum entanglement to link apertures across vast distances without the traditional physical transmission of light. The Gottesman and Khabiboulline protocols are central to the authors' arguments, each offering a method of utilizing entangled states to facilitate phase measurement and exceed the current baseline constraints.

• Gottesman Protocol: This protocol involves linking entangled photon states between two separate apertures, facilitating measurement by teleportation of photon states without the need for physical transmission. The downside includes a significant loss of usable signal, and the requirement for high entanglement rates poses considerable technical challenges.
• Khabiboulline Protocol: This approach introduces the use of quantum memories to buffer photonic data, decoupling the necessity of live entanglement generation. Quantum memories store arrival times and photon states, allowing interference measurements and visibility determination without excessive entanglement rates. This approach potentially alleviates the demands of the Gottesman Protocol by relying on binary encoding to reduce the number of entangled states needed.

Practical Implications and Future Developments

The integration of quantum-enhanced methods necessitates advancements in several key areas:

1. Entanglement Distribution: The high-frequency generation of entangled states must become more feasible, requiring significant innovations beyond current 100 kHz limits.
2. Quantum Memory: Enhancing the life span and fidelity of quantum memory cells is crucial to achieving stable interference measurements.
3. Phase Tracking and Resolution: Accurate timing and synchronization, possibly down to hundreds of picoseconds, are essential for effective interference measurement and compensation of optical path differences across long baselines.

Laboratory Demonstrations: Initial experiments have demonstrated principles related to these protocols, yet the robustness and practicality of applying these on larger, astronomical scales remain to be proven. Notably, current entanglement generation rates and memory element lifetimes must improve significantly to support realistic astronomical applications.

On-Sky Demonstrations and Community Integration

The authors highlight a phased approach for practical demonstration, beginning with modest applications using existing telescopes and evolving toward incorporation at established astronomical arrays like the CHARA Array. These initiatives have cultivated growing interest across the quantum and astronomical communities, underscoring the potential for quantum-enhanced interferometry to achieve microscale angular resolutions and provide unprecedented insights into stellar and planetary phenomena, as well as galactic structures.

Summary and Challenges

Overall, quantum-enhanced interferometry offers a promising path toward overcoming the current limitations of classical optical interferometry. However, noteworthy challenges persist, particularly concerning quantum state generation, synchronization, and data integrity over extended baselines. Collaborative efforts between astronomers and quantum physicists will be pivotal in addressing these challenges and pushing the boundaries of astronomical observation.

Ultimately, further development in quantum technologies could revolutionize the field, providing tools to achieve ultra-high-resolution imaging that enables groundbreaking scientific discoveries.