Anomalous heating of ions near electrodes

Short answer: Ions held near a metal electrode heat up much faster than thermal noise in the metal would predict. A 2006 Physical Review Letters paper reported that cooling the electrodes from 300 to 150 kelvin cut the heating rate by an order of magnitude, a 2015 Physical Review A paper measured heating at trap frequencies of about 0.6 to 1.5 MHz and temperatures of about 4 to 295 K, and a 2018 Physical Review Letters paper measured a distance exponent of minus 3.79 in a planar trap. The mechanism is still not fully understood.

The effect

The 2018 abstract says cold ions near conductive surfaces experience heating of their oscillatory motion, typically orders of magnitude greater than expected from thermal electric field fluctuations of electrons in the conductors. This is known as anomalous heating and is not fully understood. The 2006 abstract says it matters for decoherence in trapped-ion quantum information, and perhaps in other charge-based systems.

2006: moveable electrodes

The authors measure anomalous heating of an ion in the lowest quantum levels of a novel radio-frequency trap with moveable electrodes. They measure how heating scales with electrode proximity, and when the electrodes are cooled from 300 to 150 K, the heating rate drops by an order of magnitude. They say this provides direct evidence that anomalous heating comes from microscopic noisy potentials on the electrodes that are thermally driven.

2015: frequency, temperature and baking

The Physical Review A paper (Bruzewicz, Sage and Chiaverini) presents heating rate measurements of a trapped ion at several trap frequencies, about 0.6 to 1.5 MHz, and temperatures between about 4 and 295 K. They also test the effect of adsorbed surface contaminants with boiling points below about 105 degrees Celsius, by measuring heating before and after locally baking the trap chip under ultrahigh vacuum. They compare the rates with those calculated from available electric-field noise models and say they can tightly constrain a subset of the models by their expected scaling with frequency and temperature. Discrepancies between measurement and prediction, they say, point to the need for refined models.

2018: distance dependence

The Physical Review Letters paper (Boldin, Kraft and Wunderlich) addresses an open question: how heating depends on the ion-electrode separation. They measure it directly in an ion trap of simple planar geometry. They find heating rates from images of a single ytterbium-172 ion's resonance fluorescence after a variable heating time, deducing temperature from the average oscillation amplitude. Assuming a power law for heating versus separation, they measure an exponent of minus 3.79 (uncertainty 0.12).

Limits

The papers use different traps and measurement methods, so their numbers are not interchangeable. None identifies the microscopic source of the noise. The 2006 abstract supports a thermal, surface-noise explanation, while the 2015 paper says existing models do not fully match.

FAQ

What is anomalous heating?

Motional heating of trapped ions far above thermal estimates for the electrodes.

Does cooling the trap help?

Yes. Cooling electrodes from 300 to 150 K reduced heating tenfold in 2006.

How does heating depend on distance?

A power-law exponent of minus 3.79 in the 2018 planar trap.

Why does it matter?

It causes decoherence for ion qubits, the 2006 authors say.

Is the cause known?

Not fully. The 2018 abstract says it remains not fully understood.