Single atom surprisingly chill after laser treatment

CQT researchers (clockwise from front) Chow Chang Hoong, Vindhiya Prakash, Christian Kurtsiefer and Ng Boon Long found another way to cool single neutral atoms.

Colder atoms are better for building quantum computers and quantum simulators. That should drive interest in a discovery by CQT researchers: they find an established cooling method is unexpectedly effective for neutral atoms in an optical trap. The method both makes the atom colder and is easier to implement than conventional techniques.

In a paper published on 13 May 2024 in Physical Review Research, CQT researchers Chow Chang Hoong, Ng Boon Long, Vindhiya Prakash and Christian Kurtsiefer detail how they demonstrate electromagnetically induced transparency (EIT) cooling for an optically trapped single neutral Rubidium atom. The paper is highlighted as an Editor’s Suggestion.

The team successfully cooled the atom to a temperature of 5.7 µK, 2.5 times lower than achieved with conventional techniques. The method also used just two lasers, where others may need six or more before.

The method might be extended to neutral atom arrays that have hundreds of single atoms held in grids by optical tweezers. Neutral atom arrays are a promising platform for quantum information processing tasks, pursued in companies as well as at university labs.

Introducing EIT cooling

EIT is a phenomenon where an atom becomes ‘transparent’, allowing a laser that is resonant to its energy level to pass right through. This can happen when a second laser probes the atom. The two lasers are known as the coupling laser and probe laser.

EIT cooling has been applied before in other platforms such as trapped ions and quantum gas microscopy, but not for optical traps. “This cooling technique has not been explored in our trap profile yet,” says newly minted PhD Chow Chang Hoong, who covered this work in his thesis on control techniques for cold atoms. “I think people find it interesting that we can draw a connection.”

A cold atom is a still atom. Absolute zero is the theoretical minimum temperature at which everything is absolutely stationary, 0K at -273.15C. This state is unreachable but researchers can nudge an atom closer by addressing its motional energy levels.

An atom in a trap has energy levels corresponding to the atom’s oscillation in the trap. Depending on the trap’s frequency, the atom could oscillate faster or slower.

The team uses a kind of optical trap common in atomic physics, known as a far-off resonant optical dipole trap. It has small trap frequencies in the tens of kHz. This causes the spacing of the atom’s motional energy levels to be extremely small, making it difficult for researchers to distinguish the different levels for cooling.

The researchers observed what is called the ‘Fano spectrum’, a characteristic of EIT, to help them distinguish the motional energy levels.

A surprising detour

To observe the Fano spectrum, the researchers change the frequency of the probe laser and repeatedly record the atomic fluorescence — the light ‘scattered’ by the atom as it spontaneously decays after absorbing the light from the first coupling laser. In particular, the researchers record the scattering of light in the backward direction.

The Fano spectrum has an asymmetric shape featuring a dip and then a sharp peak. The dip is where the atom has become transparent. This is also known as the ‘dark state’ of the atom. For atoms in their dark state, the researchers can use the narrow Fano spectrum to distinguish the different motional energy levels. This lets them calculate the frequencies and intensities of the lasers that could address those levels.

Next, the researchers can make use of this knowledge for cooling atoms not in the dark state. When a trapped atom is ‘hot’ or oscillating in the trap, it is decoupled from its dark state. Then, the researchers can use the lasers to address its motional energy levels. The oscillating atom sees slightly different frequencies from the lasers due to the Doppler shift effect, like how you can hear the pitch change from a passing ambulance siren. This causes the oscillating atom to scatter light and lose energy, cooling down. If the atom is not oscillating and cool, it ends up predominantly staying in the dark state.

Chang Hoong says, “Our group has been interested in studying properties of light that is emitted from a system so I would say this is a surprising detour.”