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While the underlying technology promises extremely good scalability, single qubit
performance still is a major challenge to all groups in the field.
The decision to pursue the Josephson qubit approach was based on several
factors that promised to naturally address some of the DiVincenzo criteria.
2.1.1 Long Coherence Time
The qubit is formed by quantum states in a superconductor, a material owing
its name to the fact that it exhibits no resistance to electrical currents. Early
experiments with superconducting magnets have lead to estimated lifetimes of
established electrical currents inside such magnets in the hundred thousand year
range [Gallop, 1990].
Since energy dissipation is one of the major sources of
qubit decoherence, the apparent absence of resistance as a loss mechanism in
superconducting circuits lead to the hope that these systems could support qubit
states coherently for a very long time.
2.1.2 Scalability
The quantum states in Josephson qubits consist of currents and voltages inside
electrical circuits built from mostly standard circuit elements like wires, inductors,
capacitors, transformers, etc. As such, the majority of the circuit’s behavior can be
readily analyzed using standard circuit theory. This greatly simplifies the design
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