Can a lossy spot perfectly swallow interacting matter waves?
A single lossy site in a lattice of ultracold atoms can completely absorb incoming matter waves from both sides, and the atoms' own interactions help lock the system into this state.
Source
Coherent perfect absorption of nonlinear matter waves
Study at a glance
- Design
- Other — Laboratory experiment on a Bose-Einstein condensate in a 1D optical lattice with one site made lossy by an electron beam, compared with a tight-binding Gross-Pitaevskii model and simulations.
- N
- No participant-style N; each lattice site holds roughly 700 atoms and simulations used about 200 sites.
- Population
- Atomic Bose-Einstein condensate in an optical lattice
- Outcome
- Atom number in the dissipative site over time, phase profile of currents, dissipation threshold for CPA
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What they did
The team loaded a Bose-Einstein condensate into a one-dimensional optical lattice and used an electron beam to remove atoms from one central site, making it an absorber. They solved a nonlinear lattice model to find when incoming waves from both sides would be perfectly absorbed, checked the stability of those states, and compared simulations with measured atom numbers for weak and strong dissipation.
What they found
The model shows perfect absorption survives interactions, but only the slow incoming currents are stable; the fast ones are dynamically unstable. In experiments at moderate loss, the atom number in the lossy site stayed constant and equal to its neighbours, the signature of coherent perfect absorption. Above a critical loss the site emptied, but the breakdown happened near J/ħ, lower than the predicted 4J/ħ.
The limits
What it doesn't show
Strict perfect absorption requires an infinite lattice fed from infinity, so the experiment only shows a quasi-steady version in a finite system. The measured breakdown threshold disagrees with the simple model by about a factor of four, which the authors attribute to transverse instabilities and loading conditions not captured by the tight-binding picture. The matter-wave laser suggested by time reversal was not demonstrated.
Key terms
- Coherent perfect absorption (CPA)
- Complete absorption of waves arriving from both sides of an absorber, achieved by interference that cancels all reflected and transmitted waves.
- Bose-Einstein condensate
- A gas of bosonic atoms cooled so that many occupy the same quantum state and behave as one coherent matter wave.
- Optical lattice
- A periodic potential made by interfering laser beams that traps atoms in a row of wells.
- Time reversal of lasing
- CPA is the time-reversed counterpart of a laser: swapping loss for gain turns perfect absorption into coherent emission.
- Dynamical instability
- Small perturbations of a state grow in time, so the state cannot persist in practice.
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Quiz yourself
What made the central lattice site absorptive?
Common questions
Why is nonlinearity usually a problem for CPA?
Linear CPA relies on simple interference and a transfer matrix; with interactions, waves mix and there is no general recipe, and plane waves can be unstable.
Why did nonlinearity actually help here?
Interactions plus dissipation act like an attractor that pulls the condensate toward the CPA state, so it appeared over a range of loss strengths without fine tuning.
Why did the experiment break down at lower loss than predicted?
Real lattice sites have transverse extent that allows extra instabilities, and refilling of the lossy site by tunnelling must be fast enough to keep up with the loss.
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