When atoms lose their sense of colour
A newly identified colour of light is “invisible” to excited atoms, but still traps atoms in their ground state
A research team led by Monika Aidelsburger at the Max Planck Institute of Quantum Optics (MPQ) and the Ludwig-Maximilian University Munich (LMU) has identified a colour of light at which atoms become selectively “colourblind”: At this wavelength, the light has no effect on excited-state atoms, but strongly confines atoms in the ground state. The results, published in PRX Quantum, provide a powerful new tool for analogue quantum simulation and novel computing architectures.
Laser light is the most important resource for controlling neutral atoms. By choosing the right colour of laser light, physicists can trap atoms, cool them to extremely low temperatures, or shift their energy levels with high precision. These techniques underpin modern quantum technologies, from atomic clocks to quantum simulators and quantum computers.
However, independently controlling atoms in different internal energy states has remained a major experimental challenge, particularly for highly excited states. Researchers at MPQ and LMU Munich have now discovered a specific wavelength of light that affects atoms differently depending on their energy: atoms in an excited state remain “blind” to it, while ground-state atoms are strongly trapped.
This phenomenon is known as a tune-out wavelength. At such a wavelength, the light-induced energy shift of a particular atomic state vanishes, even at high light intensity. While tune-out wavelengths have been measured for atoms in their ground state, accurately determining them for excited-state atoms has so far remained elusive.
High-fidelity control of excited-state atoms
Excited atoms are notoriously difficult to work with. They typically have a short lifetime and are highly sensitive to their environment. Collisions between excited-state atoms and photon scattering from the trapping light can rapidly eject atoms from the trap, severely limiting measurement precision. These effects have so far prevented an accurate determination of tune-out wavelengths for excited atomic states.
“To find this wavelength, we needed to measure tiny light-induced perturbations on fragile atoms,” explains Tim Höhn, the lead author of the study. “Any extra heating, collisions, or photon scattering immediately reduces the achievable precision.”
An ultra-cold clock-magic crystal
The researchers succeeded in identifying the tune-out wavelength by combining several experimental techniques. They worked with ultracold atoms trapped in a two-dimensional optical lattice forming a crystal-like structure. At the so-called “magic wavelength”, the trapping potential is exactly the same for both the ground and excited atomic states. This allows them to cool the atoms using an “optical clock line” – an ultranarrow transition whose exceptional stability forms the basis of atomic clocks.
This setting improves the lifetime of the excited-state atoms in two important ways. First, the lattice isolates atoms from each other, strongly suppressing detrimental collisions. Second, the ultralow temperatures allow the researchers to reduce the amount of trapping light, minimising unwanted photon scattering and extending excited-state lifetimes from just a few hundred milliseconds to over five seconds.
Together, these advances enabled the team to probe the atomic response to additional laser light with unprecedented sensitivity and to accurately determine the tune-out wavelength of the excited state.
At this newly identified wavelength, the excited atoms experience no light-induced energy shift at all – effectively becoming invisible to the laser – while ground-state atoms remain attracted to regions of high light intensity.
Quantum control of colourblind atoms
This form of state-dependent “colourblindness” gives researchers a powerful new tool. By choosing the appropriate wavelength, they can now selectively manipulate atoms depending on their internal state, opening new possibilities for designing tailored quantum systems.
In particular, the technique enables the simulation of complex quantum many-body systems, in which particles in different internal states experience different potential-energy landscapes. It is especially promising for analogue quantum simulation of strongly correlated systems. The method also offers new routes toward the fast and scalable assembly of dense atom arrays, a key ingredient for quantum simulation and quantum computation.
More generally, the ability to control excited states with such precision paves the way for high-fidelity local addressing in quantum algorithms and for advanced architectures of neutral-atom quantum computers.
By making excited atoms selectively insensitive to light, the researchers have added a new colour to the quantum toolbox — one that lets atoms “see” light only when it’s wanted.
