  In recent years, quantum computing has promised a revolution in computing
performance, based on massive parallelism enabled by many entangled qubits.
Josephson junction integrated circuits have emerged as the key technology to
implement such a universal digital quantum computer. Indeed, prior experiments
have demonstrated simple Josephson qubit configurations with quantized energy
levels and long coherence times, which are a necessary prerequisite for a
practical quantum computer. However, these quantized states do not directly
prove the presence of entanglement or macroscopic superposition, which are
essential for the superior speed of such a digital quantum computer. On the
contrary, an alternative realistic foundation for quantum mechanics has
recently been proposed, with coherent transitions between quantized states, but
without entanglement. A new experiment is proposed that may test whether
superconducting quantum circuits can exhibit quantized states without
macroscopic entanglement or superposition. Specifically, a flux qubit (a
bi-stable SQUID) may be configured with a resonant input line for excitation
and a single-flux quantum output line for simultaneous direct measurement of
quantized energy and flux states, which are incompatible measurements in
standard quantum theory. Such an observation could undermine the assumptions of
superposition and entanglement, bringing into question the foundation and the
ultimate performance of a universal digital quantum computer.
