  The adoption of decentralized, tamper-proof ledger systems is paving the way
for new applications and opportunities in different contexts. While most
research aims to improve their scalability, privacy, and governance issues,
interoperability has received less attention. Executing transactions across
various blockchains is notably instrumental in unlocking the potential of novel
applications, particularly in the financial sector, where their potential would
otherwise be significantly diminished. Therefore, interoperable ledgers are
crucial to ensure the expansion and further adoption of such a technology in
various contexts.
  In this paper, we present a protocol that uses a combination of trusted
execution environment (TEE) and blockchains to enable interoperability over
independent semi-centralized CBDC ledgers, guaranteeing the atomicity of
inter-bank transfers. Our interoperability protocol uses a custom adaptation of
atomic swap protocol and is executed by any pair of CBDC instances to realize a
one-way transfer. It ensures features such as atomicity, verifiability,
correctness, censorship resistance, and privacy while offering high scalability
in terms of the number of CBDC instances. Our approach enables to possible
deployment scenarios that can be combined: (1) CBDC instances represent central
banks of multiple countries, and (2) CBDC instances represent the set of retail
banks and a paramount central bank of a single country. We provide a detailed
description of our protocol as well as an extensive analysis of its benefits,
features, and security.
  In this WIP paper, we made a proof-of-concept implementation and made a
partial evaluation, while the more extensive evaluation will be made in our
future work.
