Hyperfine and Zeeman Optical Pumping and Transverse Laser Cooling of a Thermal Atomic Beam of Dysprosium Using a Single 421 nm Laser

This paper demonstrates the simultaneous transverse laser cooling and Zeeman-hyperfine optical pumping of a thermal dysprosium atomic beam to a specific polarized ground state using a single 421 nm laser, preparing the system for fundamental physics experiments such as parity violation measurements.

The Gist

Imagine you have a chaotic crowd of people (atoms) running down a hallway at different speeds and facing random directions. Your goal is to get everyone to stop running, stand perfectly still, and face the exact same direction so you can take a perfect group photo. This is essentially what the scientists in this paper did, but instead of people, they were working with Dysprosium atoms, and instead of a hallway, they used a beam of light.

Here is a breakdown of how they did it, using simple analogies:

1. The Problem: A Chaotic Crowd

The researchers started with a beam of Dysprosium atoms shooting out of a hot oven.

• The Speed Issue: The atoms were moving sideways (transversely) at about 20 meters per second. It's like trying to photograph a sprinting runner while they are also swaying left and right.
• The Direction Issue: The atoms were spinning and facing every which way. Some were looking left, some right, some up, some down.
• The Complexity: Dysprosium is a "complicated" atom. It has many internal "rooms" (energy levels) it can hide in. To study it properly, you need to get every single atom into one specific room and facing one specific way.

2. The Solution: The "Magic" Laser and the "Tuning Fork"

To fix the chaos, the team used a single laser beam (blue-violet light at 421 nm) and a special device called an Electro-Optic Modulator (EOM).

• The Laser as a "Stop Sign" and "Turn Signal":
  The laser acts like a traffic cop. When the atoms hit the laser, they get "kicked" in the opposite direction of their motion. This slows them down (cooling). At the same time, the laser pushes the atoms to spin in a specific direction (polarization).

  • Analogy: Imagine a wind tunnel blowing against a runner. The wind slows the runner down (cooling) and forces them to lean forward into the wind (polarization).

• The EOM as a "Choir of Tuning Forks":
  Because Dysprosium atoms are so complex, a single laser tone isn't enough to catch them all. Some atoms are in "Room A," some in "Room B," etc.
  The researchers used the EOM to take their single laser and split it into five different frequencies (like hitting five different tuning forks at once).

  • Analogy: Imagine you are trying to get a group of people to line up, but they are wearing different colored hats. If you only shout "Red Hats, line up!", the Blue Hats ignore you. The EOM allows the laser to shout "Red Hats, Blue Hats, Green Hats..." all at the same time, ensuring every atom hears a command it understands and moves to the correct spot.

3. The Process: "Optical Pumping" and "Cooling"

The team combined two techniques:

• Optical Pumping (The Sorting Hat):
  They used the laser to force the atoms to climb a ladder of energy levels until they reached the very top rung (a specific state called $F=10.5, m_F=10.5$). Once they reached the top, they couldn't go any higher, so they stayed there.

  • Result: Almost all the atoms were forced into this one specific "VIP room."

• Laser Cooling (The Brake Pedal):
  While sorting them, they also used a standing wave of light (like a mirror reflecting the laser back on itself) to act as a brake. This reduced the atoms' sideways wobble.

  • Result: The atoms slowed down from a chaotic sprint to a gentle stroll.

4. The Results: A Perfect Lineup

When they checked the results, they saw two major improvements:

1. Brighter Signal: The signal from the atoms became 5.9 times brighter. This proved that almost all the atoms were successfully herded into that one specific "VIP room." Before, they were scattered in many rooms; now, they were all in one.
2. Sharper Focus: The "blur" in their measurement disappeared. The atoms were moving much slower and more uniformly. The width of their signal dropped from a blurry 57 MHz to a sharp 2.3 MHz. This meant the atoms were cooled down to the theoretical limit of how cold they could get with this method.

5. A Happy Accident

While working on the main target (an isotope called $^{163}$Dy), they accidentally did the same thing to a different isotope ($^{161}$Dy). The "choir of tuning forks" (the EOM) happened to hit the right notes for this second group too, organizing them even though they didn't plan to.

Why Does This Matter?

The paper states that this organized, cold, and perfectly aligned beam of atoms is now ready for a very specific job: searching for "Parity Violation."

• The Goal: Parity violation is a fundamental physics concept where nature treats "left" and "right" differently. Dysprosium is a special atom that might show this effect clearly.
• The Benefit: By getting 100 times more atoms into the perfect state (compared to previous methods), the researchers believe they can finally detect this tiny effect if it exists.

In summary: The scientists built a high-tech "herding machine" using a single laser and a frequency-splitting device to catch a chaotic swarm of atoms, slow them down, and force them all to face the same way. This creates a super-clean beam of atoms ready to help solve a deep mystery in physics.

Technical Summary

Technical Summary: Hyperfine and Zeeman Optical Pumping and Transverse Laser Cooling of a Thermal Atomic Beam of Dysprosium

Problem and Motivation
Dysprosium (Dy) is a premier system for fundamental physics experiments, including searches for temporal variations of the fine-structure constant, tests of local Lorentz invariance, and searches for parity non-conservation (PNC). These experiments often rely on the "accidental" degeneracy of excited states with opposite parity, which can be tuned to exact degeneracy using low magnetic fields. However, the sensitivity of these measurements is limited by the signal-to-noise ratio, which depends on the number of atoms prepared in specific internal states and their velocity distribution.

Controlling the internal states of Dy is challenging due to its complex hyperfine structure, particularly in isotopes like $^{163}\text{Dy}$ and $^{161}\text{Dy}$ which possess nuclear spin. Efficiently optically pumping atoms to a single Zeeman sublevel ($m_F$) and a specific hyperfine level ($F$) typically requires multiple repumper laser beams or complex modulation setups. Furthermore, thermal atomic beams have large transverse velocity spreads, leading to significant Doppler broadening that reduces interaction efficiency with narrow-linewidth lasers. This work addresses the need for a method to simultaneously achieve efficient hyperfine and Zeeman optical pumping and transverse laser cooling of a thermal Dy atomic beam using a simplified optical setup.

Methodology
The experiment utilizes a thermal atomic beam of Dysprosium generated by heating a sample to $\approx 1500^\circ\text{C}$. The atoms interact with a single 421 nm laser beam (corresponding to the $4f^{10}6s^2 (J=8) \to 4f^{10}6s6p (J'=9)$ transition) and a weaker 599 nm probe laser.

1. Optical Pumping and Modulation: To address the hyperfine structure of $^{163}\text{Dy}$, a custom-built broadband electro-optic modulator (EOM) is employed. The pump laser is locked to the $F=10.5 \to F'=11.5$ hyperfine component. The EOM generates five frequency sidebands (detuned by 57, 235, 503, 830, and 1190 MHz) to cover all necessary ground-state hyperfine transitions, enabling the transfer of population to the $F=10.5$ ground state.
2. Polarization Control: The pump laser is circularly polarized ($\sigma^+$) to drive Zeeman pumping toward the $m_F = 10.5$ sublevel.
3. Transverse Laser Cooling: A standing wave laser beam, orthogonal to the atomic beam, is used to reduce transverse velocities via the optical molasses technique.
4. Detection: A 599 nm probe laser ($J=8 \to J'=7$) is scanned across resonance to monitor the population distribution and velocity spread. The fluorescence is detected by a photomultiplier tube (PMT). The probe polarization is switched between $\sigma^+$ and $\sigma^-$ to distinguish between Zeeman pumping efficiency (dark state detection) and cooling (population amplification).

Key Contributions

• Single-Laser Multi-Functionality: The paper demonstrates that a single 421 nm laser beam, combined with a custom multi-tone EOM, can simultaneously perform hyperfine optical pumping, Zeeman optical pumping, and transverse laser cooling. This eliminates the need for multiple repumper lasers or complex AOM/EOM arrays typically required for such tasks.
• Accidental Pumping: The technique inadvertently achieves hyperfine and Zeeman pumping for the $^{161}\text{Dy}$ isotope due to the resonance of the EOM sidebands with its specific hyperfine transitions, despite the system being tuned for $^{163}\text{Dy}$.
• State Preparation Efficiency: The method successfully prepares the atomic beam in the $|F=10.5, m_F=10.5\rangle$ ground state, a critical requirement for high-sensitivity PNC experiments.

Experimental Results

• Zeeman Pumping: Using a $\sigma^+$ pump and $\sigma^-$ probe, the amplitude of the $^{164}\text{Dy}$ spectral line increased by a factor of 1.7(2), indicating successful population transfer to the $m_J=8$ dark state.
• Hyperfine and Zeeman Pumping ($^{163}\text{Dy}$): When the EOM was active, the amplitude of the $F=10.5 \to F'=9.5$ probe transition increased by a factor of 5.9(7). This aligns closely with the theoretical expectation of 5.4, confirming that nearly all atoms in the beam were pumped to the $|F=10.5, m_F=10.5\rangle$ state.
• Laser Cooling: The full width at half maximum (FWHM) of the probe spectrum for $^{163}\text{Dy}$ was reduced from 56.9(6) MHz (uncooled) to 2.3(2) MHz (cooled). The extracted Doppler width ($\sigma_g \approx 0.61$ MHz) is consistent with the theoretical Doppler cooling limit for this transition, indicating the atomic beam has been cooled to the Doppler limit.
• Accidental Pumping ($^{161}\text{Dy}$): An amplification factor of 3.0(7) was observed for the $^{161}\text{Dy}$ $F=10.5 \to F'=9.5$ transition. However, no significant laser cooling was observed for this isotope, as the closest EOM sideband was detuned by $\approx 700$ MHz, preventing efficient cycling for cooling.

Significance
The authors state that this work represents an important step toward improving the signal-to-noise ratio in ongoing searches for parity non-conservation (PNC) in Dysprosium. By achieving a preparation efficiency that increases the number of atoms in the target $|F=10.5, m_F=10.5\rangle$ state by a factor of $\approx 10^2$ compared to previous unoptimized methods, the technique offers a substantial boost in sensitivity. This enhanced sensitivity is crucial for detecting the long-sought PNC signals in Dysprosium, potentially allowing the experiment to probe the expected range of the weak-mixing amplitude more effectively. The paper concludes that this multi-tone EOM technique is a versatile tool that could be adapted for other systems, such as trapped atoms, molecules, and ions, to achieve high-efficiency optical pumping.