This paper published today in Phys. Rev. B Rapid Communications. The idea here is to lock pairs of qubits together (think taking pairs of physical qubits and locking them together to form a logical qubit) and see if the composite object still behaves quantum mechanically. It does!

This paper describes a particular pair in a single unit cell. There are a total of 16 possible pairs. We did the same experiments on all 16 pairs and you get the same behavior.

The main high-level message is that coupling qubits through CJJ couplers maintains the quantum mechanical nature of the composite (qubit+coupler+qubit) system.

Here is the title & abstract:

**Cotunneling in pairs of coupled flux qubits**

We report measurements of macroscopic resonant tunneling between the two lowest energy states of a pair of magnetically coupled rf-superconducting quantum interference device flux qubits. This technique provides both a direct means of measuring the energy gap of the two-qubit system and a method for probing of the environment coupled to the pair of qubits. Measurements of the tunneling rate as a function of qubit flux bias show a Gaussian line shape that is well matched to theoretical predictions. Moreover, the peak widths indicate that each qubit is coupled to a local environment whose fluctuations are uncorrelated with that of the other qubit.

Phys. Rev. B 82, 060512(R) (2010) [4 pages]

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