🔬 atomic physics

A metastable state in $\mathrm{Yb}^+$ with lifetime greater than 10 minutes

This paper reports the direct measurement of a 17.8-minute collision-free lifetime for the metastable ${}^3[11/2]_{9/2}^o$ state in $\mathrm{Yb}^+$ ions, along with the identification of a 701 nm repump transition and the characterization of collisional quenching, establishing the state's viability for atomic clockwork and quantum information processing.

Original authors: Hassan Farhat, William Liu, Wesley C. Campbell

Published 2026-09-10

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CC BY 4.0

Original authors: Hassan Farhat, William Liu, Wesley C. Campbell

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the quiet world of atomic physics, scientists work with the smallest possible building blocks of matter: single atoms stripped of an electron, leaving them as positively charged ions. These ions can be caught and held in place not by physical walls, but by invisible cages made of electric and magnetic fields. Once trapped, they can be cooled to temperatures near absolute zero, where they stop jiggling and become perfectly still. This stillness allows researchers to probe the atom's internal structure with incredible precision. One of the most important goals in this field is to find states of the atom that last a very long time without changing. These are called metastable states. Think of them as a ball resting in a deep, narrow valley; it takes a long time for the ball to roll out, meaning the atom stays in that specific condition for minutes or even hours. Such long-lived states are the foundation for the most accurate clocks in the world and are essential for the future of quantum computing, where information must be stored without fading away.

A team of researchers at the University of California, Los Angeles, has now discovered a new, exceptionally long-lasting state in the ytterbium ion, a heavy metal atom often used in these experiments. By trapping single ions and carefully watching them, they found that an excited state of the ytterbium ion can survive for nearly eighteen minutes without collapsing back to its normal state. This is a significant achievement because most excited atomic states vanish in a fraction of a second. The researchers measured this time directly, observing how long the ion stayed in this special state before a random collision with a stray gas molecule knocked it out. Even in their vacuum chamber, where the air is thinner than the atmosphere on Mars, these collisions happen occasionally. By measuring how the lifetime changed as they adjusted the pressure, they calculated that in a perfect vacuum, free from any collisions, the ion would remain in this state for 17.8 minutes. This duration is long enough to be useful for building advanced atomic clocks or for storing quantum information, which requires the delicate state of an atom to be preserved for extended periods.

To find this state, the scientists used a clever method involving a small crystal made of just three ions. They arranged these ions in a line and then used electric fields to create a situation where the middle ion could sit in one of two stable positions, slightly above or below the center line. This setup acts like a sensitive pressure gauge. When a stray gas molecule hits the crystal, it gives the middle ion a tiny nudge, causing it to jump, or "hop," between the two positions. By counting how often these hops occurred, the team could determine exactly how many gas molecules were present in the chamber at any given moment. They then populated the ions with the special long-lived state and watched to see how long the ion stayed there before a collision forced it out. The longer the ion stayed, the fewer the collisions, and the more accurate their measurement of the state's natural lifetime became.

The researchers also solved a practical problem that often plagues experiments with long-lived states: how to get the atom back to normal once the experiment is over. If an atom gets stuck in a long-lived state, it stops glowing, making it invisible to the detectors used to read the results. The team identified a specific laser color, a deep red light at 701 nanometers, that could act as a "rescue" beam. This laser gently nudges the ion from the long-lived state into a different, higher-energy state that naturally falls back down to the normal cooling cycle. This allows the researchers to quickly reset the experiment and read the results without losing the atom. They measured the exact frequency of this rescue laser for different versions of the ytterbium atom, confirming that this pathway works reliably.

The implications of this work extend beyond just measuring time. The specific state the team studied has a complex internal structure that could be used to protect information from errors. In the world of quantum computing, information is fragile and easily corrupted by the environment. However, certain atomic structures can be designed to be naturally resistant to these errors. The ytterbium ion's new state offers a large, robust platform for such error protection, potentially allowing for more powerful and reliable quantum computers. The team confirmed that this state does not have hidden, fast pathways to decay that would ruin its usefulness, making it a prime candidate for future technologies. By proving that this state can last for nearly eighteen minutes and by showing how to control it with lasers, the researchers have opened a new door for using ytterbium ions in the next generation of precision instruments and quantum machines.

Technical Summary: Metastable State in Yb+ with Lifetime Greater Than 10 Minutes

Problem and Context
Trapped ytterbium ions (Yb+) serve as versatile platforms for optical frequency standards and quantum information processing. While the $2F_{7/2}$ state in odd isotopes ( $^{171}$ Yb+, $^{173}$ Yb+) hosts metastable qubits, recent theoretical and experimental efforts have focused on expanding the angular momentum manifold ( $J$ ) to implement advanced quantum error correction (QEC) codes. These codes often require large- $J$ states ( $J > 3$ to $J > 10$ ) to increase code distance and suppress logical errors. However, many high- $J$ states in Yb+ possess magnetic dipole (M1) decay pathways that limit their utility for long-term storage. The $3[11/2]^o_{9/2}$ state (at 30,224 cm $^{-1}$ ) was identified as a prime candidate for an additional qubit manifold because it lacks M1 decay pathways to other $J$ - $K$ coupled states, theoretically offering a long radiative lifetime suitable for quantum memory or clockwork. Prior to this work, direct experimental verification of its lifetime and the development of practical control mechanisms (such as repumping) were required to validate its utility.

Methodology
The authors performed laser spectroscopy and lifetime measurements on trapped Yb+ ions ( $^{171}$ Yb+, $^{172}$ Yb+, and $^{174}$ Yb+) in a radio frequency Paul trap with a 4.00 G magnetic field.

State Preparation: Ions were cycled between the ground state ( $2S_{1/2}$ ) and the metastable $2F_{7/2}$ state via the $2S_{1/2} \leftrightarrow 2D_{5/2}$ electric quadrupole (E2) transition. Population was then transferred to the target $3[11/2]^o_{9/2}$ state using a resonant laser on the $2F_{7/2} \leftrightarrow 3[11/2]^o_{9/2}$ E2 transition.
Lifetime Measurement: The collision-free lifetime was determined by observing "quantum jumps" where ions returned from the shelved $3[11/2]^o_{9/2}$ state to the cooling cycle. To isolate the radiative lifetime from collisional quenching, the authors utilized a bi-stable Coulomb-crystal manometer. A three-ion crystal was configured in a "zig-zag" kinked state, where the central ion hops between two stable positions ("zig" and "zag") upon collision with background gas. By measuring the hopping rate ( $\Gamma_h$ ) as a proxy for local pressure and correlating it with the observed dwell time in the metastable state, the authors extrapolated the collision-free lifetime ( $\tau_0$ ) to the limit of $\Gamma_h \to 0$ .
Repump and Spectroscopy: To address the long lifetime of the metastable state, the authors investigated a repump pathway. They identified and drove the $3[11/2]^o_{9/2} \leftrightarrow 3[7/2]^o_{5/2}$ transition at 701 nm. The excited $3[7/2]^o_{5/2}$ state decays via electric dipole (E1) channels to the $2D_{3/2}$ and $2D_{5/2}$ states, which are already efficiently repumped in the standard cooling cycle. Hyperfine structure was also mapped for $^{171}$ Yb+ to determine hyperfine $A$ coefficients.

Key Results

Radiative Lifetime: The collision-free radiative lifetime of the $3[11/2]^o_{9/2}$ state was measured to be 17.8(6) minutes. This value exceeds the 10-minute threshold required for many quantum information applications and is consistent with recent theoretical calculations (though the theoretical value cited is 40.93 minutes, the experimental lower bound is robust).
Transition Frequencies: The authors measured the $2F_{7/2} \leftrightarrow 3[11/2]^o_{9/2}$ transition frequencies for $^{172}$ Yb+ (263.985 96(2) THz) and $^{174}$ Yb+ (263.983 92(2) THz). The $3[11/2]^o_{9/2} \leftrightarrow 3[7/2]^o_{5/2}$ repump transition frequencies were measured as 427.898 78(2) THz ( $^{172}$ Yb+) and 427.898 86(2) THz ( $^{174}$ Yb+).
Hyperfine Coefficients: The hyperfine $A$ coefficient for the $3[11/2]^o_{9/2}$ state in $^{171}$ Yb+ was determined to be 389(5) MHz. For the $3[7/2]^o_{5/2}$ state, the coefficient was measured as −310(10) MHz.
Collisional Quenching: Using the bi-stable crystal manometer, the authors observed that collisional quenching of the $3[11/2]^o_{9/2}$ manifold by background gas is more efficient than for the $2F_{7/2}$ state, yet the lifetime at the apparatus's base pressure still exceeds 10 minutes.

Significance and Claims
The paper establishes the $3[11/2]^o_{9/2}$ state as a viable, long-lived qubit manifold for trapped Yb+ ions. The demonstrated lifetime of nearly 18 minutes is sufficient for use in atomic clockwork and quantum information processing. The successful identification and benchmarking of the 701 nm repump transition provide a mechanism for rapid population recovery, which is essential for quantum nondemolition (QND) measurements of sub-structure within the metastable manifold and for efficient readout in clock or qubit applications.

The authors claim that this work enables the operation of a state-of-the-art hyperfine qubit in $^{171}$ Yb+ with a relative ease of implementation, given the accessible radio-frequency hyperfine splitting (1.94(2) GHz) and the long lifetime. Furthermore, the $3[11/2]^o_{9/2}$ manifold augments the existing "omg" scheme by providing an optically isolated qubit manifold during active operation. In even isotopes, this manifold offers a platform for spin-cat encoding, where computational states are defined on the stretch states ( $m_J = \pm 9/2$ ), potentially allowing for the suppression of logical errors that scales exponentially with angular momentum. The work validates the $3[11/2]^o_{9/2}$ state as a critical resource for expanding the capabilities of trapped-ion systems in precision measurement and quantum error correction.

Technical Summary: Metastable State in Yb+ with Lifetime Greater Than 10 Minutes

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