[BUG] Allow natural extension of QubitUnitary to 1x1 matrices

[dime10]

Expected behavior

QubitUnitary currently does not accept a 1x1 matrix on 0 wires, which would be equivalent to a GlobalPhase gate. Since PennyLane actually does allow 0 wire circuits and a 1x1 unitary matrix is a valid quantum operator (with an existing alias in PennyLane, see GlobalPhase), it would be nice to resolve this inconsistency.

Actual behavior

Error is raised:

ValueError: QubitUnitary: wrong number of wires. At least one wire has to be given.

Additional information

No response

Source code

No response

Tracebacks

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System information

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Existing GitHub issues

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