Abstract
Stabilization of encoded logical qubits using quantum error correction is key
to the realization of reliable quantum computers. While qubit codes require
many physical systems to be controlled, oscillator codes offer the possibility
to perform error correction on a single physical entity. One powerful encoding
for oscillators is the grid state or GKP encoding, which allows small
displacement errors to be corrected. Here we introduce and implement a
dissipative map designed for physically realistic finite GKP codes which
performs quantum error correction of a logical qubit implemented in the motion
of a single trapped ion. The correction cycle involves two rounds, which
correct small displacements in position and momentum respectively. Each
consists of first mapping the finite GKP code stabilizer information onto an
internal electronic state ancilla qubit, and then applying coherent feedback
and ancilla repumping. We demonstrate the extension of logical coherence using
both square and hexagonal GKP codes, achieving an increase in logical lifetime
of a factor of three. The simple dissipative map used for the correction can be
viewed as a type of reservoir engineering, which pumps into the highly
non-classical GKP qubit manifold. These techniques open new possibilities for
quantum state control and sensing alongside their application to scaling
quantum computing.
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