May 2019
Abstracts of the QSIT Lunch Seminar, Thursday, May 9, 2019
Entanglement Stabilization using Parity Detection and Real-Time Feedback in Superconducting
Christian Kraglund Andersen – Quantum Device Lab (Wallraff group), ETH Zurich
In recent years, quantum computing has seen a surge of progress both theoretically and experimentally. However, the long-term success of quantum computers relies on the ability to perform fault-tolerant quantum computations using quantum error correction. In this approach, errors are detected through the repeated measurement of multi-qubit parity operators and corrected using feedback operations conditioned on the measurement outcomes. In our work [1], we experimentally demonstrate the use of an ancillary qubit to repeatedly measure the ZZ and XX parity operators of two data qubits and to thereby project their joint state into the respective parity subspaces. By applying feedback operations conditioned on the outcomes of individual parity measurements, we demonstrate the real-time stabilization of a Bell state with a fidelity of F≈74% in up to 12 cycles of the feedback loop. The ability to stabilize parity over multiple feedback rounds with no reduction in fidelity provides strong evidence for the feasibility of executing stabilizer codes on timescales much longer than the intrinsic coherence times of the constituent qubits.
[1] C.K. Andersen, et al., arXiv:1902.06946 (2019)
Quantum error correction in the presence of a continuous symmetry
Philippe Faist – Quantum Information Theory (Renner group), ETH Zurich
The ability of a code to protect quantum information against errors is severly limited if the code conforms to a continuous symmetry. We derive a quantitative limit to this accuracy, and we show that this limitation vanishes in the regime of large physical subsystems. Our results have a wide scope of applications. First, we provide an approximate version of the Eastin-Knill theorem, by quantifying the accuracy limit of a code that would admit a universal set of transversal gates. Second, we obtain a characterization of the accuracy of approximate quantum error-correcting codes that appear as energy eigenstates in many-body systems. I will furthermore discuss implications of our results for global symmetries in quantum gravity as well as for limitations in using quantum error correction to improve sensitivity for quantum metrology in the presence of noise.
Joint work with Sepehr Nezami, Victor V Albert, Grant Salton, Fernando Pastawski, Patrick Hayden and John Preskill, arXiv:1902.07714.
See also related work by Woods and Alhambra, arXiv:1902.07725.