This paper studies what happens when a moving quantum wavepacket hits a region that removes particles. The authors build a one-dimensional “synthetic” lattice from internal states of a rubidium Bose–Einstein condensate. By tuning how strongly sites in one half of the chain lose atoms, they observe three behaviors: nearly lossless ballistic passage, strong absorption, and strong reflection caused by the quantum Zeno effect.

The experimental lattice uses the hyperfine ground states of 87Rb as the sites of a chain. The two subchains are the F=2 manifold (non-dissipative, left side) and the F=1 manifold (dissipative, right side). Radio-frequency fields set the hopping inside each subchain with an average tunneling ¯J ≈ 0.98 ΩRF (ΩRF/(2π)=1.968(4) kHz, so ¯J≈2π×1.929(4) kHz). A microwave field controls the tunneling across the interface (J0). Loss on the right side is created by optically coupling F=1 to an excited state (5P3/2, F′=0) that decays with lifetime 26.3 ns; atoms that decay pick up momentum and leave the measured population. Experiments start with about NT≈10^5 atoms prepared on the leftmost site |j=−4⟩ and track site populations over hundreds of microseconds using Stern–Gerlach separation and time-of-flight imaging.

At small loss rate γ the packet just travels through both halves and returns nearly unchanged. As γ grows to about the size of the tunneling rate (γ/¯J ≈ 1) absorption becomes large. If γ is made much larger than ¯J, absorption drops and the wavepacket is mostly reflected back into the non-dissipative side. The authors explain this as an impedance-matching condition: maximal absorption happens when tunneling into the lossy region and the loss rate are balanced. Extremely large loss acts like an imaginary potential step that reflects the packet, an effect related to the quantum Zeno effect. The paper quantifies these observations: the single-pass absorption proxy peaks near γmax/¯J = 1.34(5), and maximum absorption also occurs for an interface tunneling J0,max/¯J = 1.32(7).