All Reflective Field-widened Unbalanced Interferometer for Quantum Sensing and Communication Applications

This paper presents a compact, passive, all-reflective field-widened unbalanced interferometer utilizing spherical mirror cavities to achieve high-visibility interference for time-bin encoded quantum signals in spatially multimode and turbulent free-space optical channels, offering a robust alternative to adaptive optics.

The Gist

Imagine you are trying to listen to a whisper in a very windy, chaotic room. Normally, to hear that whisper clearly, you would need a complex, expensive machine with moving parts (like adaptive optics) to cancel out the wind and focus the sound. This paper introduces a new, simpler device that acts like a "smart ear" for light, allowing it to hear whispers (quantum signals) even when the air is turbulent, without needing those complex moving parts.

Here is a breakdown of what the researchers built and why it matters, using everyday analogies:

The Problem: The "Windy Room" of Light

In quantum communication (sending information using single particles of light), we often use "time-bin" encoding. Think of this like sending a message by tapping a drum: a short tap means "1" and a long tap means "0." To read this message, you need to mix two versions of the tap together to see if they match.

However, when light travels through the air (free space), the atmosphere acts like a bumpy, wavy road. This turbulence distorts the light waves, making them arrive at the receiver out of sync. Traditionally, fixing this requires heavy, complex equipment called "adaptive optics" that constantly adjusts the path of the light, like a camera lens that refocuses itself thousands of times a second. This is bulky and hard to fit on small devices like drones or satellites.

The Solution: The "All-Mirror" Magic Box

The team built a new device called the Offner Relay Interferometer (ORI). Instead of using glass lenses (which can bend different colors of light by different amounts, causing a rainbow blur), they used only mirrors.

• The Analogy: Imagine trying to fold a long piece of rope into a small backpack. A standard rope is stiff and takes up a lot of space. The ORI is like a clever folding technique that lets you pack a very long rope (a long delay between light signals) into a tiny, compact box.
• How it works: The device uses a spherical mirror (curved like a bowl) and a flat mirror arranged in a specific "cavity." Light bounces back and forth between them multiple times. This creates a long path for the light to travel inside a very small physical space.

Why This Design is Special

The paper highlights three main "superpowers" of this design:

1. It's "Field-Widened" (The Wide-Angle Lens):
   Most delicate light-measuring devices require the light to hit them perfectly straight-on, like a laser pointer hitting a bullseye. If the light comes in at a slight angle (which happens often in the real world), the measurement fails.

   • The Analogy: Think of a standard interferometer as a narrow tunnel where you must walk perfectly straight. The ORI is like a wide, open plaza; you can walk in from many different angles, and the device still works perfectly. The researchers showed it works even when the light hits at slight angles, maintaining a very high "visibility" (clarity) of over 97%.

2. It's "Achromatic" (The Colorblind Mirror):
   Glass lenses act like prisms; they split white light into rainbows, which messes up the timing of the signal. Mirrors, however, reflect all colors the same way.

   • The Analogy: If you use a glass lens, it's like trying to run a race where everyone has to wear shoes of different weights depending on their shoe size. With the ORI's mirrors, everyone wears the same weight shoes. This means the device works perfectly with "broadband" light sources (light that contains many colors), which is crucial for many quantum systems.

3. It's Compact and Rugged:
   Because the light bounces back and forth inside a "folded" path, the device doesn't need to be long.

   • The Analogy: A standard device for this job might be as long as a dining table. The ORI folds that length up so it fits on a shelf. This makes it perfect for small platforms like drones or satellites where space and weight are at a premium.

What They Actually Proved

The researchers didn't just simulate this on a computer; they built a prototype and tested it.

• The Test: They shone a laser through the device. They tested it with a single, perfect beam of light and with "multimode" light (a messy, spread-out beam that mimics real-world conditions).
• The Result: Even with the messy, spread-out light, the device achieved a clarity (visibility) of 0.97 (out of 1.0). This is nearly perfect. In comparison, a standard device without these special features dropped to about 0.53 with the same messy light.
• Imaging Demo: They also showed that you can look through the device and see an object (a target) while simultaneously measuring the light's phase. This proves it could be used for "quantum sensing" or LiDAR (laser radar) to see objects clearly even if the light is scattered.

The Bottom Line

This paper presents a new, compact, and robust way to measure quantum light signals in the real world. By using only mirrors and a clever folding trick, they created a device that is:

• Small enough to fit on a drone or satellite.
• Sturdy enough to handle light coming from different angles without needing complex adjustments.
• Versatile enough to work with many different colors of light.

It's a step toward making quantum communication and sensing practical for everyday use in the sky and space, without the need for heavy, expensive, moving parts.

Technical Summary

Technical Summary: All Reflective Field-widened Unbalanced Interferometer for Quantum Sensing and Communication Applications

Problem Statement
Time-bin encoded signals used in free-space optical channels for quantum communication and sensing typically suffer from wavefront distortions caused by atmospheric turbulence. Conventional solutions rely on active adaptive optics, which introduce significant technical overhead and complexity, particularly for resource-constrained platforms like drones and satellites. While passive field-widened interferometers offer an alternative by correcting angle-of-incidence induced phase shifts without active correction, existing designs face limitations. Refractive field-widened interferometers, which use optically dense materials, suffer from chromatic dispersion that limits their compatibility with broadband quantum light sources. Furthermore, standard unbalanced interferometers with large physical footprints are susceptible to thermal and vibrational noise, while simple approaches are often restricted to small fields-of-view, where minor deviations from normal incidence cause significant drops in interference visibility.

Methodology
The authors propose and demonstrate a novel design called the Offner Relay Interferometer (ORI). This is a field-widened, unbalanced interferometer that relies exclusively on reflective optics to minimize chromatic dependence and dispersion. The core of the design is a cavity imaging system formed between a spherical concave mirror (CM) and a flat mirror (FM). This configuration creates a folded optical path, allowing for long relative path delays within a compact physical footprint.

Key design objectives included:

1. Compactness: Utilizing a folded path to achieve delays longer than the physical dimensions of the device.
2. Simplicity: Employing off-the-shelf spherical mirrors, which are easier to align than parabolic mirrors and more readily available than specialized refractive components.
3. Broadband Operation: Avoiding refractive materials entirely to ensure the system is effectively achromatic within the range of beam splitter coatings.

The team utilized numerical optical ray tracing simulations to verify the design's performance across various configurations (varying mirror radii and reflection counts). They subsequently built a prototype using parameters aligned with the simulations ($R_1=200$ mm, $R_2=300$ mm, 3 reflections per arm) and tested it using a 785 nm continuous wave external cavity diode laser.

Key Contributions and Results

• High Visibility with Multimode Beams: The primary achievement is the demonstration of high interference visibility (>0.97) for spatially multimode beams without the need for adaptive optics. In experimental tests, the ORI achieved a maximum multimode visibility of 0.979 ± 0.022. This significantly outperformed a standard unbalanced Michelson interferometer tested under the same conditions, which yielded a visibility of only 0.536 ± 0.023.
• Robustness to Angular Offsets: The ORI functions effectively as a field-widened interferometer. Simulations showed that even with input angular offsets of up to 0.3° horizontally and 0.58° vertically, the system maintained high visibility (0.9939). The design tolerates a maximal input angle of 1.72° before a 4% drop in visibility occurs.
• Achromatic Performance: Numerical simulations confirmed that the all-reflective design is effectively achromatic across a broad wavelength range (600 nm to 1000 nm). In contrast, simulations of a refractive field-widened interferometer using N-BK10 and N-SF66 glass showed strong chromatic dependence due to material dispersion.
• Compact Form Factor: The folded cavity configuration allows for smaller physical footprints compared to refractive field-widened Michelson interferometers with equivalent time delays. For example, a design with a 1020 ps delay required a footprint of 4572 mm² in the ORI configuration, compared to 10021 mm² for a comparable refractive Michelson design.
• Imaging Capability: The authors demonstrated the ORI's utility in imaging interferometry by observing a target scattering light through the system. The setup successfully imaged the target while simultaneously measuring phase information, showing interference fringes with a visibility of approximately 0.49 when both arms were active, compared to 0.11 when one arm was blocked.

Significance and Claims
The paper claims that the Offner Relay Interferometer represents a step toward robust, field-deployable quantum interferometers. By combining a reflective-only design with a cavity imaging system, the ORI addresses the trade-offs between chromatic dispersion, physical size, and alignment sensitivity. The authors assert that the design is particularly applicable to spatially multimode and turbulent optical channels, such as those found in satellite communications, and is well-suited for quantum systems utilizing time-bin encoded qubits.

The work highlights that the ORI offers a practical alternative to adaptive optics for free-space quantum links, enabling direct interference of multimode beams with high visibility. The authors note that while the current investigation is limited to specific configurations, the design principles allow for scalability in path delay and form factor, potentially enabling integration with additive manufacturing techniques for future field testing.