Curator's Take
This groundbreaking research tackles a critical security challenge as quantum computing moves toward shared cloud infrastructure, introducing the novel concept of "covert quantum computing" where users can perform calculations without adversaries detecting their activity even when sharing the same quantum processor. The work cleverly adapts techniques from quantum game theory and reveals that current quantum computers may be more vulnerable to side-channel attacks than previously thought, with the researchers discovering unexpected long-range coupling effects that extend beyond the theoretically predicted boundary qubits. This has immediate practical implications for quantum cloud providers like IBM and IQM, as it exposes potential privacy vulnerabilities in multi-tenant quantum systems and provides a mathematical framework for understanding how much computational activity can remain hidden from co-located adversaries. As quantum computing transitions from isolated laboratory experiments to shared commercial platforms, this type of privacy-preserving research becomes essential for building trust in quantum cloud services.
— Mark Eatherly
Summary
As quantum computers become available through multi-tenant cloud platforms, ensuring privacy against adversaries sharing the same quantum processing unit becomes critical. We introduce and explore \emph{covert quantum computing}, a new concept that ensures an adversary with access to all other quantum computational units (QCUs) of a quantum computer cannot detect computation on the subset that they cannot access. Analogous to covert communication, we employ information theory. However, since here the adversary controls the systems used for detection, we require a richer framework for covertness analysis that accounts for the use of quantum memories and adaptive operations. Thus, we adopt the \emph{quantum-strategy} framework used in quantum game theory and memory channel discrimination. Current quantum computers use planar graph circuit layouts and typically assume nearest-neighbor crosstalk. We derive discrete isoperimetric inequalities to show that, for an $n$-qubit circuit under this model, only $\mathcal{O}(\sqrt{n})$ border qubits provide detection information to the adversary. We then explore this scaling law on IQM's 54-qubit \emph{Emerald} processor and IBM's 156-qubit \emph{ibm\_fez} machine employing the Heron 2 architecture. We implement Ramsey experiments on qubits not used in computation, and detect nearest-neighbor crosstalk, as expected. However, we also observe long-range coupling effects beyond the border qubits, revealing a side channel that the adversary can exploit. We hypothesize that this long-range crosstalk is induced by leakage from the drive and control lines. Beyond weakening covertness, it exposes co-tenants to both adversarial and unintended crosstalk and degrades circuits that span spatially distributed qubits, motivating further work on spatial isolation and crosstalk characterization.