Absorption imaging of quantum gases near surfaces using incoherent light

This paper presents a modular absorption imaging technique for ultracold atomic gases near surfaces that utilizes a rotating diffuser to reduce transverse spatial coherence, thereby eliminating interference artifacts while preserving the necessary spectral bandwidth for reliable imaging in micron-scale proximity to complex structures.

The Gist

The "Flickering Lightbulb" Solution for Seeing Quantum Clouds

Imagine you are trying to take a clear photograph of a delicate, invisible cloud of atoms floating just millimeters away from a shiny, complex piece of metal. This is a common task for physicists studying quantum gases. However, there's a major problem: the light you use to take the picture is too perfect.

The Problem: The "Laser" Glitch

Standard lasers are like a choir of a million singers all hitting the exact same note at the exact same time. This makes the light incredibly "coherent" (organized). While this is great for many things, when this organized light hits a surface near your atoms, it bounces back and interferes with itself.

Think of it like dropping two stones into a calm pond. The ripples from the stones cross each other, creating a messy pattern of high and low waves. In your photo, this creates:

• Standing Waves: Like a frozen ripple pattern that looks like stripes across your image.
• Speckles: Like static on an old TV screen.
• Diffraction: Blurry edges caused by the light bending around the metal structures.

These artifacts make it impossible to see the actual shape of the atom cloud. It's like trying to read a book while someone is shining a strobe light and a laser pointer directly into your eyes. You can't tell if the weird patterns are the text or just the light messing with your vision.

The Solution: The "Rotating Frosted Glass"

The researchers at the University of Sussex came up with a clever, low-tech solution to fix this. They realized they didn't need to stop the light from being organized; they just needed to scramble it while it was traveling.

They built a simple add-on module that works like this:

1. The Diffuser: They shine the laser beam through a piece of tracing paper (a diffuser). This turns the smooth, organized laser beam into a chaotic, "fuzzy" mess of light, similar to how a frosted bathroom window scatters light from a bulb.
2. The Spinner: Here is the magic trick. They mount this piece of paper on a spinning drone motor. As the paper spins, the "fuzzy" pattern of light changes incredibly fast—thousands of times per second.
3. The Camera's Eye: The camera takes a photo that lasts for a tiny fraction of a second (microseconds). Because the light pattern is spinning so fast, the camera doesn't see the individual fuzzy spots. Instead, it sees the average of all those spots, which results in a perfectly smooth, even sheet of light.

The Analogy: Imagine trying to paint a wall with a bucket of water that has a hole in the bottom, creating a chaotic spray. If you stand still, you get a messy splatter. But if you spin the bucket around your head very fast, the water spray blends together, and you end up painting the wall with a smooth, even coat. That's what the spinning diffuser does to the laser light.

Why This Matters

By using this "spinning frosted glass" technique, the team achieved three major breakthroughs:

1. Seeing the Invisible: They could finally take clear photos of atoms sitting right next to complex surfaces (like the metal chips used to trap them). Before, the "noise" from the light made the atoms look like they were in the wrong place or had weird shapes. Now, the images are clean.
2. Measuring Distances: Because the images are clear, they can measure exactly how close the atoms are to the surface (even within a few microns). This is crucial for calibrating their experiments.
3. The "Truth Detector": The researchers discovered that sometimes, what looks like a new physical discovery (like a weird bump in the atom cloud) is actually just an optical illusion caused by the laser's organization. By switching between "organized" (coherent) light and "scrambled" (incoherent) light, they can tell the difference.
   • If the weird pattern disappears when they use the spinning diffuser, it was just a glitch.
   • If the pattern stays, it's a real physical feature of the atoms.

The Bottom Line

This paper introduces a simple, modular tool that turns a "too-perfect" laser into a "just-right" light source. It's like taking a high-speed, strobe-light camera and replacing it with a steady, soft lamp. This allows scientists to finally see quantum gases clearly near surfaces, opening the door to better sensors, quantum computers, and a deeper understanding of how atoms interact with the world around them.

Technical Summary

1. Problem Statement

Absorption imaging is the standard technique for characterizing ultracold neutral atomic gases. However, when imaging atoms in close proximity to surfaces (e.g., atom chips or nano-structured substrates), coherence-induced artifacts severely degrade image quality.

• The Issue: Standard imaging uses highly coherent laser light (long coherence length) to match narrow atomic linewidths. Near surfaces, this leads to:
  • Standing waves formed by reflections.
  • Edge diffraction from mounting structures.
  • Speckle patterns caused by optical contaminants.
• Consequences: These interference effects create high-contrast fringes and intensity modulations that are often mistaken for physical features of the atomic cloud.
• Limitations of Current Solutions:
  • Post-processing algorithms often rely on assumptions about atomic distributions (e.g., Gaussian) and fail where illumination vanishes.
  • Standard background subtraction (taking shadow, reference, and dark images) fails against time-dependent phase modulations (e.g., air turbulence) or high-contrast static fringes that imprint onto the optical density.
  • In some regions near complex surfaces, adequate illumination is impossible to achieve with coherent light.

2. Methodology

The authors propose a method to suppress transverse spatial coherence while preserving the narrow spectral bandwidth required for resonant absorption imaging.

• Core Technique: A modular extension to standard imaging setups using a rotating diffuser.
  • Mechanism: A laser beam is incident on a diffuser (tracing paper) mounted on a rotating motor. The rotation induces temporal variation in the speckle pattern.
  • Condition for Incoherence: The imaging exposure time ($t_{exp}$) must be significantly longer than the correlation time ($t_c$) of the speckle pattern. In this experiment, $t_c \approx 1\,\mu\text{s}$, while exposure times were $>12\,\mu\text{s}$, effectively averaging out the speckle to produce homogeneous illumination.
• Tunability: The degree of spatial coherence is tunable without modifying imaging optics. By adding a focusing lens before the diffuser, the beam diameter on the diffuser is varied:
  • Large beam: High number of incoherent modes $\rightarrow$ Incoherent light.
  • Small beam (focused): Fewer modes $\rightarrow$ Partially coherent to fully coherent light.
• Experimental Setup:
  • Light Source: 780 nm laser (resonant with Rubidium).
  • Diffuser: Tracing paper disk rotating at $\approx 90$ Hz.
  • Optics: A condenser lens collects scattered light to maximize transmission into the vacuum chamber.
  • Geometry: Atoms are trapped near a quartz substrate with a nano-structured sample. Imaging is performed from below at grazing incidence to capture both the real atomic cloud and its mirror image.

3. Key Contributions

1. Modular Implementation: Demonstrated that incoherent imaging can be added to existing setups as a plug-and-play module without altering the core imaging optics or camera alignment.
2. Coherence Control: Established a method to continuously tune the spatial coherence of the imaging light (from fully coherent to fully incoherent) simply by adjusting the beam focus on the diffuser.
3. Artifact Identification: Introduced a diagnostic protocol where comparing images taken with coherent, partially coherent, and incoherent light allows researchers to distinguish genuine physical features from coherence-induced artifacts.

4. Key Results

• Imaging Near Complex Surfaces:
  • Coherent Light: Produced complex interference patterns and standing waves that obscured the atomic cloud and its mirror image, preventing reliable measurement of atom-surface distances ($d_{as}$) in the sub-100 $\mu$m range.
  • Incoherent Light: Provided uniform illumination extending well beyond the limits of coherent imaging. This enabled the reliable extraction of $d_{as}$ and robust calibration of trapping currents, even at distances as small as 5.8 $\mu$m from the surface.
• Elimination of Speckle Artifacts:
  • In Time-of-Flight (TOF) expansion experiments, coherent light produced speckle-like background patterns that persisted even after averaging multiple shots (because the speckle pattern was static relative to the shot-to-shot timing).
  • Incoherent imaging yielded smooth, Gaussian density profiles after averaging, confirming the removal of speckle-induced noise.
• Diagnostic of Optical Aberrations:
  • The authors observed "complementary" density profiles at different TOF times when using coherent light with an aberrated imaging system.
  • Switching to incoherent light suppressed these artifacts, revealing that the anomaly was an interplay between optical aberrations and spatial coherence, rather than a physical property of the gas.
• Trade-offs:
  • Incoherent imaging slightly reduced image contrast and resolution compared to coherent light (due to the nature of incoherent transfer functions). However, this was deemed acceptable given the elimination of severe artifacts, and density scales could be calibrated against coherent measurements.

5. Significance

• Reliability in Proximity Experiments: This technique enables the study of atom-surface interactions, local magnetic fields, and current flow irregularities in regimes (micron-scale proximity) previously inaccessible due to imaging artifacts.
• Diagnostic Power: It provides a critical tool for validating experimental data. Researchers can now verify if observed features in quantum gas density distributions are physical or merely imaging artifacts caused by coherence.
• Broad Applicability: The method is simple, robust, and applicable to various fields including cavity quantum electrodynamics, quantum control, optical sensing, and cold atoms near fibers. It opens pathways for improved precision in quantum technologies where imaging near surfaces is essential.