CC Lab

In our lab, we focus on controlling the interaction between ultracold atoms, which is the origin of almost all many-body phenomena and a powerful tool in both fundamental researches and quantum simulations.

In most experiments, this interaction is tuned by a magnetic Feshbach resonance. This method is simple and elegant, but it is limited. A coil has a large inductance, so the field cannot be switched faster than a few kilohertz. A coil is also centimetre-sized, so the interaction cannot be patterned at the scale of a single lattice site. And the crossing is given by atomic structure, so for atom pairs without a suitable crossing there is nothing to tune.

This platform is to replace the magnetic field by light: we use an intensity-modulated, far-detuned laser to shake the collisional energy levels, and the interaction becomes programmable in time, in space, in frequency, and in channel. Based on this method, we have finished two works as follow:

1. Modulation-induced Feshbach resonance

This platform is to realize a Feshbach resonance that lives in the frequency domain rather than in the magnetic-field domain. The periodic shaking connects the free-scattering state to shallow molecular states through Floquet sidebands, and at particular modulation frequencies we observe a strong resonant atom loss (Phys. Rev. Lett. 135, 203201). The resonance condition keeps exactly the same analytic form as the magnetic one, with the magnetic field replaced by integer multiples of the modulation frequency and the coupling weighted by Bessel functions J_m(A/ω). The essential difference is that no level crossing is needed in advance: a molecular state that stays almost parallel to the scattering state, and never crosses it, can still be brought into resonance by a small shaking. In this way we can directly scan the energy spectrum of shallow molecular states, including the molecular states embedded in the continuum, without synthesizing any molecule. And because the knob is now a laser, the interaction can be switched fast, patterned in space, and delivered at several frequencies at once.

2. Matter-wave induced transparency

This experiment is to apply the interference between matter-wave scattering pathways in order to suppress collisional loss. We couple the free-atom pair to two Feshbach molecular states, 4g(4) and 6s, and use the modulation to bring the second one into a two-photon-like resonance, so that the three states form a Λ-type system. Under the dark-state condition, the two pathways interfere destructively and generate a dark dressed state with very small overlap on the lossy molecular component; a narrow transparency window therefore opens at the very centre of a broad and strongly dissipative resonance, where the loss would be maximal in the two-level case (arXiv 2607.03820). The window can be moved in the magnetic field–modulation frequency plane, its linewidth is controlled by the modulation-induced coupling, and the participating pathways can be selected by multifrequency Floquet modulation. This is the matter-wave analogue of electromagnetically induced transparency, except that the coupling comes from collisions themselves, so the transparency is the transparency of the interaction: the dissipation is suppressed while the resonant interaction remains tunable.

Future

In the future, we will work on extending this control from small modulation regions (A<<ω) to large modulation regions (A>>ω), the more complex spectroscopy with coherent destruction of tunneling (CDT) and higher-order resonances would be observed. By combining the optical lattice and radio frequency, we will focus on the non-Markov dissipation dynamics based on degenerate ultracold cesium gas system. Besides, we will also use the steep dispersion of the transparency window for preparing ultracold molecules and non-linear atom-molecule dynamics.