Department of Physics
Condensed Matter Seminar

Qubits based on Josephson Junctions are prime candidates for solid state quantum computing as they take advantage of inherently scalable semiconductor fabrication techniques. The recent news with these devices is good: Rabi oscillations, Ramsey fringes and spin echo measurements, coherence times of about 1 μs, and full control on the Bloch sphere. And bad: Spurious resonators and materials problems. And much is ugly, but in this talk I single out state readout. Ideally, readout should be fast, non-destructive, cause minimal disturbance to the device and its neighbors, and provide the correct answer. Following work by Siddiqi et al.*, I will show how we (are endeavoring to) use a weakly coupled anharmonic LC oscillator to monitor the state of our qubit. This gives us two qubit readout possibilities; a destructive readout which causes minimal qubit heating; and a projective readout which leaves the qubit in either the ground or excited state. As a bonus, the readout oscillator circuit also couples to noise in our qubit—a source of decoherence—allowing us to perform flux noise measurements on our qubit in a minimally invasive way. * Phys. Rev. Lett. 94, 027005 (2005).