I'm trying to make sense of the 'memory qubits' e.g. 'spatial modes' in the paper. Do you know if they are they a sort of...quantum flip flop? Is this also a h

I'm trying to make sense of the 'memory qubits' e.g. 'spatial modes' in the paper. Do you know if they are they a sort of...quantum flip flop? Is this also a hypothetical structure, or have they been built and shown to be able to reliably store and retrieve quantum state in such spatial modes? 430 million is a much, much larger number than their headline 13436 qubits...