  Interaction blockade occurs when strong interactions in a confined few-body
system prevent a particle from occupying an otherwise accessible quantum state.
Blockade phenomena reveal the underlying granular nature of quantum systems and
allow the detection and manipulation of the constituent particles, whether they
are electrons, spins, atoms, or photons. The diverse applications range from
single-electron transistors based on electronic Coulomb blockade to quantum
logic gates in Rydberg atoms. We have observed a new kind of interaction
blockade in transferring ultracold atoms between orbitals in an optical
lattice. In this system, atoms on the same lattice site undergo coherent
collisions described by a contact interaction whose strength depends strongly
on the orbital wavefunctions of the atoms. We induce coherent orbital
excitations by modulating the lattice depth and observe a staircase-type
excitation behavior as we cross the interaction-split resonances by tuning the
modulation frequency. As an application of orbital excitation blockade (OEB),
we demonstrate a novel algorithmic route for cooling quantum gases. Our
realization of algorithmic cooling utilizes a sequence of reversible OEB-based
quantum operations that isolate the entropy in one part of the system, followed
by an irreversible step that removes the entropy from the gas. This work opens
the door to cooling quantum gases down to ultralow entropies, with implications
for developing a microscopic understanding of strongly correlated electron
systems that can be simulated in optical lattices. In addition, the close
analogy between OEB and dipole blockade in Rydberg atoms provides a roadmap for
the implementation of two-qubit gates in a quantum computing architecture with
natural scalability.
