Co-planar waveguide flux qubits

1. A qubit device comprising:

an elongated thin film uninterrupted by Josephson junctions;

a superconducting quantum interference device (SQUID) in direct electrical contact with a proximal end of the elongated thin film, wherein the SQUID comprises less than three Josephson junctions; and

a ground plane that is co-planar with the elongated thin film and is in direct electrical contact with a distal end of the elongated thin film;

wherein the elongated thin film, the SQUID and the ground plane comprise a superconductor material,

wherein the elongated thin film comprises a first side and a second opposite side and wherein each of the first side and the second side is separated from the ground plane by a respective gap.

2. The qubit device of claim 1 wherein a width of each respective gap is constant along a length of the elongated thin film.

3. The qubit device of claim 1 , wherein each of the elongated thin film, the SQUID, and the ground plane comprises aluminum or niobium.

4. The qubit device of claim 1 , wherein the SQUID comprises a superconducting loop interrupted by the less than three Josephson junctions.

5. The qubit device of claim 4 , wherein the SQUID comprises two Josephson junctions interrupting the superconducting loop.

6. The qubit device of claim 1 , further comprising a substrate, wherein the elongated thin film, the ground plane and the SQUID are on a surface of the substrate.

7. The qubit device of claim 6 , wherein the substrate is sapphire.

8. The qubit device of claim 1 , wherein the elongated thin film comprises a first section extending away from the SQUID along a first direction and a second section extending away from the SQUID along a second direction different from the first direction.

9. The qubit device of claim 1 , wherein the elongated thin film is a single-branched co-planar waveguide.

10. The qubit device of claim 1 , wherein the elongated thin film is a double-branched co-planar waveguide.

11. A qubit control system comprising:

a qubit comprising an elongated co-planar waveguide and a superconducting quantum interference device (SQUID), wherein a first end of the elongated co-planar waveguide is in direct electrical contact with the SQUID and a second end of the elongated co-planar waveguide is in direct electrical contact with a ground plane;

a first bias control device arranged to electromagnetically couple to the elongated co- planar waveguide during operation of the qubit control system; and

a second bias control device arranged adjacent to the SQUID to electromagnetically couple to the SQUID,

wherein the elongated co-planar waveguide comprises an elongated thin film uninterrupted by Josephson junctions, wherein each of a first side of the elongated thin film and a second side of the elongated thin film is separated from the ground plane by a respective gap, wherein the first side of the elongated thin film is opposite to that of the second side of the elongated thin film.

12. An apparatus comprising:

a plurality of qubits, each qubit of the plurality of qubits comprising a corresponding elongated co-planar waveguide and a corresponding superconducting quantum interference device (SQUID), wherein, for each qubit, a first end of the corresponding elongated co-planar waveguide of the qubit is in direct electrical contact with the corresponding SQUID of the qubit, and a second end of the corresponding elongated co-planar waveguide of the qubit is in direct electrical contact with a ground plane,

wherein each qubit is operatively coupled to at least one other qubit of the plurality of qubits, and

wherein the corresponding co-planar waveguide of each qubit crosses over the corresponding co-planar waveguide of at least one other qubit of the plurality of qubits at a corresponding waveguide crossing location.

13. The apparatus of claim 12 , comprising, at each waveguide crossing location, a corresponding coupling element arranged to electromagnetically couple the crossing co-planar waveguides at the waveguide crossing location.