Has Macroscopic Superposition in Superconducting Qubits Really Been Demonstrated

Daniel N. Rogovin

Timothy P. Spiller, Terrence D. Clark, Robert J. Prance +2 more

We discuss the quantum mechanical superposition of macroscopically different states in the light of experiments performed on superconducting circuits, and conclude that compelling evidence for this effect exists.

Philip R. Johnson, William T. Parsons, Frederick W. Strauch +4 more

Jan Ivar Korsbakken, Frank K. Wilhelm, K Birgitta Whaley

Flux qubits, small superconducting loops interrupted by Josephson junctions, are successful realizations of quantum coherence for macroscopic variables. Superconductivity in these loops is carried by $\sim 10^6$ -- $10^{10}$ electrons, which has been interpreted as suggesting that coherent superpositions of such current states are macroscopic superpositions analogous to Schrödinger's cat. We provide a full microscopic analysis of such qubits, from which the macroscopic quantum description can be derived. This reveals that the number of microscopic constituents participating in superposition states for experimentally accessible flux qubits is surprisingly but not trivially small. The combination of this relatively small size with large differences between macroscopic observables in the two branches is seen to result from the Fermi statistics of the electrons and the large disparity between the values of superfluid and Fermi velocity in these systems.

Kosuke Kakuyanagi, George C. Knee, Mao-Chuang Yeh +6 more

Alexandre M. Zagoskin, Alexandre Blais

From a physicist's standpoint, the most interesting part of quantum computing research may well be the possibility to probe the boundary between the quantum and the classical worlds. The more macroscopic are the structures involved, the better. So far, the most "macroscopic" qubit prototypes that have been studied in the laboratory are certain kinds of superconducting qubits. To get a feeling for how macroscopic these systems can be, the states of flux qubits which are brought in a quantum superposition corresponds to currents composed of as much as 10^5-10^6 electrons flowing in opposite directions in a superconducting loop. Non-superconducting qubits realized so far are all essentially microscopic.