Why climb mountains when you can tunnel through them? Harnessing quantum tunneling
holds great promise to speed up solutions for a broad range of optimization problems. I will
present experiments on the disordered Ising ferromagnet, L(Ho,Y)F4, that quantitatively
compare quantum and classical annealing protocols, and demonstrate quantum speedup for
reasons that can be understood at a microscopic level. This approach follows from Richard
Feynman’s concept of a quantum computer and underlies the power of D-Wave machines. In
the dilute limit, where there is no long-range ferromagnetic order, the Ho dipoles form clusters
of several hundred spins that bind together and can be excited resonantly. By analogy to laser
excitation of atoms, we use a pump-probe magnetic technique to drive the system out of the
linear regime, and study both the nature of the excitations and the coupling of the excitations
to the spin bath. By applying a magnetic field transverse to the Ising axis, we are able to tune
the dynamics of the quantum degrees of freedom such that localized clusters (the “qubits”)
essentially decouple from their environment.
Thomas Rosenbaum is the Sonja and William Davidow Presidential Chair, a Professor of Physics, and the President of the California Institute of Technology.
Physics Colloquium: Tom Rosenbaum, CalTech
Meeting ID: 962 9210 8492