A Historical Note on Macro-Atom Theory and the First Proposal of Superconducting Quantum Computing

This note documents early theoretical work on voltage-biased Josephson junctions in the historical context of superconducting quantum computing. The work was published in two papers in 1994 and 1996. Motivated by efforts to understand the anticipated band structures associated with Bloch oscillations in small Josephson junctions, the theoretical analysis instead showed that, in the weak-dissipation regime, a voltage-biased Josephson junction could behave as an atom-like quantum system with discrete energy levels. Based on a Hamiltonian formulation, the energy spectra and wave functions of these macroscopic quantum systems (behaving as macro-atoms) were derived. Relaxation times and spectral linewidths were also analyzed using theoretical tools developed in laser and NMR spectroscopy. Although experimental verification of the theory was still lacking at that time, the two papers explicitly noted the possibility of engineering these macro-atoms as atomic and molecular switches, possibly for quantum computing.