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Springer eBooks

Bayesian Quantum Measurement of a Single-Cooper-Pair Qubit

Alexander N. Korotkov

Published Jan 1, 2002• 0 citations
Publisher
DOI
10.1007/978-1-4615-0737-6_2

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We study the quantum charge noise and measurement properties of the double Cooper-pair resonance point in a superconducting single-electron transistor (SSET) coupled to a Josephson charge qubit. Using a density-matrix approach for the coupled system, we obtain a full description of the measurement backaction; for weak coupling, this is used to extract the quantum charge noise. Unlike the case of a nonsuperconducting SET, the backaction here can induce population inversion in the qubit. We find that the Cooper-pair resonance process allows for a much better measurement than a similar nonsuperconducting SET, and can approach the quantum limit of efficiency.
The aluminum single Cooper-pair box (SCB) qubit has demonstrated potential as a candidate for quantum computing. Measurements of coherent manipulation of the relevant SCB states have been demonstrated with various charge and quantum capacitance readout methods. We performed studies of quasiparticle dynamics in an SCB that yielded poisoning probabilities and rates of quasiparticle tunneling processes. Attempts to prevent poisoning with a differential SCB isolated from ground and other quasiparticle sources revealed the possibility for quasiparticle generation on superconducting islands of diminutive size. We have gone on to conduct more advanced measurements of coupled qubits as an important step towards the realization of multi-qubit systems and other precursors that are pivotal in the evolution of reliable quantum computing.
A superconductor is a natural source of spin-entangled spatially separated electron pairs. Although the first Cooper-pair splitter devices have been realized recently, an experimental confirmation of the spin state and the entanglement of the emitted electron pairs has been lacking up to now. In this paper, a method is proposed to confirm the spin-singlet character of individual split Cooper pairs. Two quantum dots (QDs), each of them holding one spin-prepared electron, serve as the detector of the spin state of a single Cooper pair that is forced to split when it tunnels out from the superconductor to the QDs. The number of charges on the QDs, measured at the end of the procedure, carries information on the spin state of the extracted Cooper pair. The method relies on the experimentally established toolkit of QD-based spin qubits: resonant spin manipulation, Pauli blockade, and charge measurement.