Sufficient condition for universal quantum computation using bosonic circuits

We present a new method for quantifying the resourcefulness of continuous-variable states in the context of promoting otherwise simulatable circuits to universality. The simulatable, albeit non-Gaussian, circuits that we consider are composed of Gottesman-Kitaev-Preskill states, Gaussian operations, and homodyne measurements. We first introduce a general framework for mapping a continuous-variable state into a qubit state. We then express existing maps in this framework, including the modular subsystem decomposition and stabilizer subsystem decomposition. Combining these results with existing results in discrete-variable quantum computation provides a sufficient condition for achieving universal quantum computation. These results also allow us to demonstrate that for states symmetric in the position representation, the modular subsystem decomposition can be interpreted in terms of resourceless (simulatable) operations - i.e., included in the class of Gaussian circuits with input stabilizer Gottesman-Kitaev-Preskill states. Therefore, the modular subsystem decomposition is an operationally relevant mapping to analyze the logical content of realistic Gottesman-Kitaev-Preskill states, among other states.Comment: 30 pages, 13 figure