The Quantum Doorway: How a Crowd Changes the Rules of Tunneling ✛
Quantum tunneling — the ability of particles to pass through barriers forbidden by classical physics — powers stars with thermonuclear fusion, ignites primordial nucleosynthesis, and gives us flash memory. But when not a lone particle but a whole bunch seeps through a barrier, their behavior resembles a crowd trying to squeeze through a narrow arch: interactions between them fundamentally change the picture. For a long time, it remained a mystery exactly how interparticle forces renormalize the fundamental scaling law of tunneling, linking the critical 'traversability' of the barrier to temperature.
Physicists from Peking University and the University of Geneva turned a Bose-Einstein condensate of rubidium-87 atoms into a quantum simulator, trapping it in an optical lattice of crossed laser beams (quantum optics). By changing the polarization, they reconfigured the geometry of the two-dimensional lattice from hexagonal to triangular, and by varying the intensity, they controlled the tunnel coupling between sites — analogous to the width of a doorway. To pinpoint the critical point where tunneling freezes, the researchers measured the fraction of atoms that remained with zero momentum after ballistic expansion (quantum measurement). Thus, by tracking the crowd of particles at different temperatures and interaction strengths, they mapped out a universal power law.
The results of numerical simulations using the quantum Monte Carlo method confirmed that the curve Jc ∝ T^α with a non-integer α describes a wide class of one-dimensional systems. The exponent α could be expressed through the Luttinger parameter K, which depends on the dimensionless interaction γ: α = (4K−1)/(2K). For weak interactions (K ≫ 1) we get α ≈ 2, and as interaction strengthens, the exponent slowly slides, until — in the limit of very strong coupling — tunneling becomes single-particle and α tends to one. This crossover from collective dance to a solo breakthrough has been witnessed in the lab for the first time.
The discovery promises a breakthrough in understanding high-temperature superconductivity and designing quantum computers, where controlling the tunneling of Cooper pairs or qubits is critical. Moreover, the universality of the scaling suggests that similar laws may operate in organic superconductors and even in astrophysical systems. This work lays the foundation for 'tunneling crowd engineering' — the ability to tune the passage of quantum information through complex media, taking another step toward practical quantum information devices.
The transition from collective to single-particle tunneling as the lattice depth increases resembles how a crowd of people moving through a narrow door changes behavior under strong compression: interactions make everyone 'feel' each other, but when the passage becomes absolutely tight, each one seeps through on their own.