Dr. Gerd Schon, ITP & Univ Karlsruhe, Josephson Junction Qubits and the Quantum Measurement Process

Schedule Oct 15, 1998 Josephson Junction Qubits and the Quantum Measurement Process Dr. Gerd Schon, ITP & Univ Karlsruhe Several physical realizations of quantum bits have been proposed, including trapped ions, NMR systems and spins in nano-structures, quantum optical systems, and nano-electronic devices. The latter appear most suitable for large-scale integration and potential applications. We suggest to use low-capacitance Josephson junctions, exploiting the coherence of tunneling in the superconducting state combined with the possibility to control individual charges by Coulomb blockade effects. These systems constitute constitute quantum bits (qubits), with logical states differing by one Cooper-pair charge. Single- and two-bit operations can be performed by applying a sequence of gate voltages. The phase coherence time is sufficiently long to allow a series of these steps. We propose a design, with controlled Josephson couplings, which is close to ideal. In addition to the manipulation of qubits the resulting quantum state has to be read out. This can be accomplished by coupling a single-electron transistor capacitively to the qubit. To describe this quantum measurement process we study the time evolution of the density matrix of the coupled system. Only when a transport voltage is turned on, the transistor destroys the phase coherence of the qubit; in this case within a short time. The measurement is accomplished after a longer time scale, when the signal resolves the different quantum states. We present a suitable set of system parameters, which can be realized by present-day technology. The crucial steps on the way to quantum computation, such as entanglement of qubits' states or demonstrations of the laws of quantum mechanics in controlled fabricated solid state devices, should be within reach of experiments.
cond-mat/9808067, cond-mat/9801125, cond-mat/9706016

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