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A stabilized logical qubit encoded in grid states of a superconducting cavity
Eickbusch, Alec - Yale University
Presentation on Thursday, Oct. 3, 2019, noon
Location: MIT Room 6C-442
A useful quantum computer must be able to run algorithms in the presence of noise. All
proposed quantum computing architectures are designed based on the reasonable assumption that
this noise is local: it does not act in a coordinated way on multiple sub-parts of the system. In
these architectures, protection against local noise is achieved by encoding quantum information
in non-local variables. In their seminal 2001 paper, Gottesman, Kitaev and Preskill (GKP)
demonstrated that such general protection is also achievable by encoding a qubit in the Hilbert
space of a single oscillator. They identified that, at short time, all commonly observed noise
processes only lead to small displacements in phase-space. The proposed GKP code states take
discrete values periodically spaced in both position and momentum, forming grids in phase-
space. The unique feature of this logical qubit is that any pair of orthogonal basis states have
distant support in phase-space: the code is resistant to small random shifts, and noise-induced
errors are detectable and correctable.
Our experiment implements grid state encoding and stabilization by modular quadrature
measurements of a superconducting microwave cavity. We designed and realized an original
feedback protocol that incorporates such measurements to permanently stabilize grid states with
a constrained number of photons. We showed the deterministic initialization of the logical grid-
state qubit in an arbitrary state, and the readout of its Bloch vector components. Finally, we
observed that our stabilization significantly enhances the coherence time of such encoded qubits.
These results were achieved with both square and hexagonal grid states.