Securing NISQ Quantum Computer Reset Operations Against Higher-Energy State Attacks

Enabling the sharing of quantum computers among different users requires a secure reset operation that can reset the state of a qubit to ground state |0 > and prevent leakage of the state to a post-reset circuit. This work highlights that the existing reset operations available in superconducting qubit NISQ quantum computers are not fully secure. In particular, this work demonstrates for the first time a new type of higher-energy state attack. Although NISQ quantum computers are typically abstracted as working with only energy states |0 > and |1 >, this work shows that it is possible for unprivileged users to set the qubit state to |2 > or |3 >. By breaking the abstraction of a two-level system, the new higher-energy state attack can be deployed to affect the operation of circuits or for covert communication between circuits. This work shows that common reset protocols are ineffective in resetting a qubit from a higher-energy state. To provide a defense, this work proposes a new Cascading Secure Reset (CSR) operation. CSR, without hardware modifications, is able to efficiently and reliably reset higher-energy states back to |0 >. CSR achieves a reduction in |3 >-initialized state leakage channel capacity by between 1 and 2 orders of magnitude, and does so with a 25x speedup compared with the default decoherence reset.