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Feedback-stabilised quantum states in a mixed-species ion system

September 25, 2018 - 10:00am
Vlad Negnevitsky
ETH, Zurich

Multi-qubit parity measurements are critical for
feedback-based quantum error correction, as well as a powerful
building block in quantum information processing. We repeatedly and
nondestructively measure the parity of two beryllium ions using a
calcium ancilla, and apply low-latency conditional corrections to
deterministically stabilize particular parity subspaces as well as
Bell states. The ancilla is also used to sympathetically recool the
three-ion crystal, which allows us to re-initialize the system without
collapsing the entangled beryllium state. The phases of qubit gates
are calculated and set in real-time based on the Stark shifts caused
by the sequence of gates applied so far, which varies due to the
conditional feedback.
The stabilized subspaces and Bell states decay an order of magnitude
more slowly than in the unstabilized case. The primary decay mechanism
in the stabilized cases arises from population leakage out of the
beryllium hyperfine qubit subspace. The Bell states retain coherence
for more than 50 rounds of measurement and feedback.

The experiment demonstrates multiple basic elements of a quantum
hardware system and its attendant classical control, suited to
larger-scale stabilization and feedback experiments including quantum
error correction and other quantum feedback algorithms.

Host- Joseph Britton

PSC 2136
College Park, MD 20742