1. SQUID Based Quantum Bits James McNulty

2. What’s a SQUID? Superconducting Quantum Interference Device

3. How A SQUID Works Flux Quanta ▫ Current density zero! (Meissner Effect) ▫ Unit of flux: Josephson Junction ▫ Tunneling : ▫ Phase difference: ▫ Current Produced with no Voltage! ▫ Due to phase difference Superconducting ring

4. How A SQUID Works Two Josephson Junctions ▫ Phase difference around loop ▫ Supercurrent  Maximum Value  Creates interference pattern ▫ Very sensitive to changes In flux

5. Flux Qubits A superconducting ring with a Josephson Junction ▫ External source of flux Behaves as particle in double potential well Each well represents supercurrents in opposite directions: CW, CCW At Superposition state of symmetric anti U Φ SC Ring SQUID C.H. van der Wal, et al. Quantum superposition of macroscopic persistent-current states. Science 290(5492):773-777, 2000.

6. Flux Qubits Irradiate JJ with microwaves ▫ 1-10 GHz to transition between states Change qubit from to and vice versa when ▫ Higher states not accessible Φ ext = Φ 0 /2 Clarke, J., Wilhelm, F.K. Superconducting Quantum Bits, Nature 453,1031-1042(2008).

7. Flux Qubits SQUID detects the flux produced in qubit Averages current (flux) from qubit over multiple trials As Flux moves away from state switching requires more energy F(GHz) C.H. van der Wal, et al. Quantum superposition of macroscopic persistent-current states. Science 290(5492):773-777, 2000.