Magnetic atoms with a large electric dipole moment

This study experimentally demonstrates that dysprosium atoms can be prepared in a long-lived metastable state with a large induced electric dipole moment of over 1 Debye and a reduced transition dipole moment of 7.65 Debye, enabling efficient single-photon excitation from the ground state through a three-state Stark interaction.

The Gist

Imagine you have a tiny, invisible magnet. In the world of atoms, some are naturally magnetic, like a compass needle that always points north. The Dysprosium (Dy) atom is the "champion" of these magnetic atoms; it has a super-strong magnetic personality.

But here's the catch: while it's great at magnetism, it's terrible at electricity. It doesn't naturally have an "electric personality" (an electric dipole moment). Usually, to get an atom to act like a tiny electric magnet, you have to look at complex molecules, not single atoms.

The Big Discovery
This paper is about a team of scientists who found a way to wake up a "sleeping giant" inside the Dysprosium atom. They managed to give this single atom a massive electric personality, making it behave like a tiny, super-charged magnet for electricity.

Here is how they did it, explained through a few simple analogies:

1. The Twin Towers (The Doublet)

Inside the atom, there are two specific energy levels (think of them as two floors in a very tall building) that are almost identical in height but have opposite "handedness" (parity).

• Floor A (The Basement): This is a very stable, long-lived floor. If you put an atom here, it can stay for about 30 seconds. In the world of atoms, that's an eternity!
• Floor B (The Attic): This floor is just a tiny step above Floor A, but it's a bit more unstable.

Normally, these two floors are separated by a tiny gap. The scientists used microwaves (like a very precise radio signal) to measure the exact distance between these two floors. They found that for all five stable versions of Dysprosium, the gap is incredibly consistent, down to the level of a few thousandths of a hair's width.

2. The Electric Shove (Inducing the Dipole)

Here is the magic trick: When you apply a strong electric field (a "push" from an electric force) to these two floors, they start to talk to each other.

• The Analogy: Imagine two people standing on a seesaw. If you push down on one side, the other goes up. But in this quantum world, when you push them with an electric field, they don't just move; they mix. They become a hybrid of both floors.
• The Result: This mixing creates a huge electric dipole moment. The scientists measured this to be about 7.65 Debye. To put that in perspective, that's huge for a single atom! It's comparable to the electric personality of complex molecules like water. They successfully induced an electric dipole moment of more than 1 Debye, which was the goal.

3. The Three-Step Staircase (The Shortcut)

You might ask, "How do you get the atom up to these special floors in the first place?"
Usually, you can't jump directly from the ground (the atom's resting state) to these high floors because the rules of quantum mechanics say "No Entry."

However, the scientists found a clever workaround using a third floor (a third energy state) that sits a bit higher up.

• The Analogy: Imagine you want to get to a roof you can't reach directly. You can't jump from the ground to the roof. But, there is a balcony (the third state) that you can jump to. Once you are on the balcony, the electric field acts like a slide that connects the balcony to your target roof.
• By using a laser to get the atom to the balcony, and then applying a strong electric field, the atom can slide down into the special "Twin Tower" state. This allows them to prepare the atoms exactly how they want them.

Why Does This Matter?

Why do we care about giving a single atom a giant electric personality?

1. The Best of Both Worlds: Most atoms are either magnetic OR electric. This new state of Dysprosium is both. It has a huge magnetic moment (it's a strong magnet) AND a huge electric moment (it's a strong electric dipole).
2. New Physics Playground: Scientists can now use two different knobs to control these atoms: a magnetic field knob and an electric field knob. This allows them to create new, exotic states of matter that have never been seen before.
3. Quantum Computing: These atoms could be used as building blocks for quantum computers. Because they can be controlled so precisely and interact with each other over long distances (like magnets pulling on each other from far away), they are perfect for creating "entangled" pairs, which is the secret sauce of quantum computing.

In Summary
The scientists took a naturally magnetic atom, found a secret, long-lived "attic" inside it, and used a strong electric field to make that attic behave like a super-charged electric magnet. They figured out how to get atoms into that attic using a clever three-step staircase. This opens the door to building a new kind of "quantum gas" made of atoms that are both magnetic and electric, potentially revolutionizing how we build future computers and understand the universe.

Technical Summary

1. Problem Statement

The field of quantum simulation and quantum computing has seen significant advances using systems with large dipole moments. While magnetic dipole moments are inherent to many atoms (like Dysprosium, Dy) due to electron spin, large electric dipole moments (EDMs) are typically restricted to heteronuclear molecules or Rydberg atoms.

• The Challenge: Atoms generally possess symmetric structures that preclude large permanent EDMs. While opposite-parity doublets in atoms can be induced by external electric fields, most known atomic doublets suffer from short radiative lifetimes or small induced dipole moments, limiting their utility in dipolar physics.
• The Gap: There is a need for a system that combines a large intrinsic magnetic dipole moment (for magnetic control) with a tunable, large electric dipole moment (for long-range, anisotropic interactions), all within a single atomic species to avoid the complexity of ultracold molecules.

2. Methodology

The authors employed an atomic beam experiment combined with optically detected microwave spectroscopy and high-field Stark spectroscopy to characterize a specific opposite-parity doublet in the Dysprosium (Dy) atom.

• Experimental Setup:
  • Source: A pulsed supersonic beam of Dy atoms was generated via ablation of a rotating Dy rod using a 1064 nm laser, seeded in Helium or Argon gas.
  • Interaction Region: The beam passed through a chamber where it could be subjected to:
    • Weak electric fields ($\le 150$ V/cm) and magnetic fields ($\le 10$ G) for microwave spectroscopy.
    • Strong electric fields (up to $150$ kV/cm) for optical Stark spectroscopy.
  • Detection: Atoms were ionized via Resonance-Enhanced Multi-Photon Ionization (REMPI) schemes and detected using a time-of-flight mass spectrometer, allowing for isotope-resolved spectroscopy.
• Target System: The study focused on a specific opposite-parity doublet located approximately 17,513 cm$^{-1}$ above the ground state, consisting of a lower $J=10$ state ($|a\rangle$) and an upper $J=9$ state ($|b\rangle$).
• Spectroscopic Techniques:
  • Microwave Spectroscopy: Used to measure the energy splitting ($\Delta E$) between $|a\rangle$ and $|b\rangle$ and observe the Stark shift in weak fields.
  • High-Field Optical Spectroscopy: Used to excite atoms directly from the ground state ($|g\rangle$) to the doublet states ($|a\rangle, |b\rangle$) in strong electric fields, utilizing a three-level Stark interaction involving a third state ($|c\rangle$ at 17,727 cm$^{-1}$).

3. Key Contributions

• Experimental Characterization: The first precise experimental determination of the energy levels and transition properties of the long-lived opposite-parity doublet in Dy, which had been theoretically predicted but not fully characterized.
• Measurement of Isotope Shifts: Precise determination of the doublet spacing for all five stable bosonic isotopes of Dy ($^{156}$Dy, $^{158}$Dy, $^{160}$Dy, $^{162}$Dy, $^{164}$Dy) with kHz-level accuracy.
• Extraction of Dipole Moments: Derivation of the reduced transition dipole moment ($\mu_{TDM}$) between the doublet states using both low-field quadratic Stark shifts and high-field linear Stark shifts.
• Demonstration of Direct Preparation: Proof that the three-state Stark interaction (involving $|g\rangle$, $|b\rangle$, and $|c\rangle$) allows for the direct single-photon preparation of atoms in the metastable doublet from the ground state.

4. Key Results

• Energy Levels and Splitting:
  • The doublet is located at $\approx 17,513$ cm$^{-1}$.
  • The energy splitting ($\Delta E = E_b - E_a$) for $^{162}$Dy was measured as 33,711.892(3) MHz (approx. 1.12 cm$^{-1}$).
  • Isotope shifts were measured for all five stable bosonic isotopes, showing excellent agreement with theoretical field shift predictions.
• Radiative Lifetimes:
  • The lower state $|a\rangle$ ($J=10$) has a predicted lifetime of $\tau_a \approx 28$ seconds (experimentally confirmed to be long-lived, $\sim$half a minute).
  • The upper state $|b\rangle$ ($J=9$) has a predicted lifetime of $\tau_b \approx 34$ $\mu$s.
• Transition Dipole Moment:
  • From the quadratic Stark shift in weak fields ($<150$ V/cm): $\mu_{TDM} = 7.8 \pm 0.2$ D.
  • From the linear Stark shift in high fields ($90-150$ kV/cm) using a three-level model: $\mu_{TDM} = 7.65 \pm 0.05$ D.
  • This value is comparable to large permanent dipole moments found in polar molecules.
• Induced Electric Dipole Moment (EDM):
  • In high electric fields, the doublet states mix, allowing for the induction of large EDMs.
  • For $M \le 4$, the induced EDM exceeds 1 Debye.
  • At $E = 1$ kV/cm, the induced EDM for the $M=0$ level is $\approx 0.044$ D, with a lifetime of $\approx 100$ ms.
• Three-State Interaction:
  • The doublet interacts with a third state $|c\rangle$ ($J=9$, $17,727$ cm$^{-1}$).
  • This interaction enables the mixing of $|c\rangle$ into $|b\rangle$, making the transition from the ground state $|g\rangle$ to the doublet weakly allowed, facilitating direct optical preparation.

5. Significance

• First Doubly Polar Quantum Gas: This work provides the experimental pathway to creating the first quantum gas of atoms that possesses both large magnetic and large electric dipole moments. This enables the study of doubly dipolar interactions, where both magnetic and electric forces can be tuned independently using static fields.
• Quantum Simulation: The system offers a unique platform for simulating new many-body phases of matter and exploring complex spin models that are difficult to realize with molecules due to their chemical complexity and short lifetimes.
• Fundamental Physics: The long-lived, highly polarizable states are suitable for precision tests of fundamental physics, such as searching for time-variation of fundamental constants or parity violation, similar to previous Dy experiments but with significantly enhanced control and interaction strength.
• Scalability: Unlike molecules, which require complex cooling and state preparation, this atomic system leverages existing laser cooling and trapping techniques for Dy, potentially offering a more robust route to doubly polar quantum gases.

In summary, Seifert et al. have successfully validated the existence of a "textbook exception" in atomic physics: a metastable atomic state in Dysprosium that can be induced to have a large electric dipole moment while retaining its massive magnetic moment, opening new frontiers in quantum matter research.