  The high expenses imposed by current quantum cloud providers, coupled with
the escalating need for quantum resources, may incentivize the emergence of
cheaper cloud-based quantum services from potentially untrusted providers.
Deploying or hosting quantum models, such as Quantum Neural Networks (QNNs), on
these untrusted platforms introduces a myriad of security concerns, with the
most critical one being model theft. This vulnerability stems from the cloud
provider's full access to these circuits during training and/or inference. In
this work, we introduce STIQ, a novel ensemble-based strategy designed to
safeguard QNNs against such cloud-based adversaries. Our method innovatively
trains two distinct QNNs concurrently, hosting them on same or different
platforms, in a manner that each network yields obfuscated outputs rendering
the individual QNNs ineffective for adversaries operating within cloud
environments. However, when these outputs are combined locally (using an
aggregate function), they reveal the correct result. Through extensive
experiments across various QNNs and datasets, our technique has proven to
effectively masks the accuracy and losses of the individually hosted models by
upto 76\%, albeit at the expense of $\leq 2\times$ increase in the total
computational overhead. This trade-off, however, is a small price to pay for
the enhanced security and integrity of QNNs in a cloud-based environment prone
to untrusted adversaries. We also demonstrated STIQ's practical application by
evaluating it on real 127-qubit IBM\_Sherbrooke hardware, showing that STIQ
achieves up to 60\% obfuscation, with combined performance comparable to an
unobfuscated model.
