June 2016

Abstracts of the QSIT Lunch Seminar, Thursday, June 2, 2016

Nonlocal Polarization Feedback in a Fractional Quantum Hall Ferromagnet

Szymon Hennel, Nanophysics Grroup, ETH Zurich

In a quantum Hall ferromagnet, the spin polarization of the two-dimensional electron system can be dynamically transferred to nuclear spins in its vicinity through the hyperfine interaction. The resulting nuclear field typically acts back locally, modifying the local electronic Zeeman energy. Here we report a nonlocal effect arising from the interplay between nuclear polarization and the spatial structure of electronic domains in a ν=2/3 fractional quantum Hall state. In our experiments, we use a quantum point contact to locally control and probe the domain structure of different spin configurations emerging at the spin phase transition. Feedback between nuclear and electronic degrees of freedom gives rise to memristive behavior, where electronic transport through the quantum point contact depends on the history of current flow. We propose a model for this effect which suggests a novel route to studying edge states in fractional quantum Hall systems and may account for so-far unexplained oscillatory electronic-transport features observed in previous studies.

Optimized Circuits for Transmon Dispersive Readout

Theo Walter, Quantum Device Lab, ETH Zurich

High single-shot fidelity is quintessential for achieving a universal fault-tolerant quantum computer in order to measure qubit state errors as they occur and possibly apply feedback. Current schemes for fault-tolerant computing implement error correcting algorithms on arrays of physical qubits to create a single logical qubit. As these algorithms and others, such as feed-forward and Bell tests, rely on qubits with finite coherence and dephasing times, measurement speed and absolute fidelity are two important factors to maximize the fidelity of a larger algorithm. Dispersive readout using transmon qubits has demonstrated to be both fast and accurate. In our work we attempt to bring this technology to its current limits, focusing on maximizing the fidelity and speed of the state measurement. In our experiment we achieve the highest reported fidelity to date of 98.6(3)% for this system, with a qubit lifetime of 7.6  μs. Further, this was obtained in 72 ns, a measurement twice as fast as previously reported. This is accomplished by optimizing the circuit parameters. We constrained the optimization to not affect general qubit functionality and the outlook of this work suggests that 99.6% fidelity can be achieved in 50 ns with our parameters and it is implementable in the multiplexed scaling architecture for transmon superconducting circuits.

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