Using a Dynamical Bifurcation to Readout a Quantum Bit Irfan Siddiqi Department of Applied Physics Yale University As advances in nanotechnology drive the miniaturization of electronics, quantum effects which once appeared only in thought experiments are now being tested for real applications. To harness many of the potential gains in speed and sensitivity of solid-state quantum devices, the fundamental principles of superposition and entanglement must be applicable to macroscopic variables. I will show how a dynamical bifurcation, as implemented using a superconducting Josephson tunnel junction, can be used to create a quantum limited amplifier for transferring information to classical electronics. In particular, I will present measurements of the Josephson bifurcation amplifier (JBA)[1] as a high-fidelity readout for superconducting qubits. The bifurcation amplifier principle provides both the technical hardware and the conceptual framework for testing the foundations of macroscopic quantum mechanics at a deeper level. [1] I. Siddiqi, R. Vijay, F. Pierre, C.M. Wilson, M. Metcalfe, C. Rigetti, L. Frunzio, and M.H. Devoret, Phys. Rev. Lett. 93, 207002 (2004).