A GHz-Gated Dual-Anode InGaAs/InP SPAD Front-End for OAM Cat-Qubit Syndrome Extraction

Abstract

The OAM cat-qubit encoding developed in a companion note [1] defines an operationalprotocol for extracting the channel’s mode-hopping rate γ from the linear growth of OAMpopulation variance, Var(l) = γz, measured via repeated single-photon counting at the outputports of a telecom-band OAM mode sorter. The quality of this regression — and hence of everydownstream erasure/bit-flip probability computed from it — depends directly on the cleanlinessof the underlying click statistics: afterpulsing and gate-transient artifacts in the single-photondetector bias the extracted γ in ways the model of the companion note does not account for.This note identifies and combines two published single-photon detection results as a candidatefront-end for this measurement: a dual-anode InGaAs/InP SPAD (DA-SPAD) using a two-portsubtraction circuit to cancel gate-transient artifacts [2], and asynchronous GHz sinusoidal gatingenabling free-running-like operation at count rates up to 500 Mcount/s [3]. We outline why thiscombination is a plausible match for the companion note’s measurement requirements, notethe accepted timing-jitter trade-off (∼168 ps) is not a concern for a spatial/mode-populationregression, and record this as an open hardware question — we do not have access to this classof detector or its RF control electronics, and welcome correspondence from groups working withcomparable hardware.