Oct 15, 1998
Josephson Junction Qubits and the Quantum Measurement Process
Dr. Gerd Schon, ITP & Univ Karlsruhe
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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.
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cond-mat/9808067, cond-mat/9801125, cond-mat/9706016
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