Quantum reset

György Pál Gehér, Marcin Jastrzebski, Earl T. Campbell +1 more

Whether to reset qubits, or not, during quantum error correction experiments is a question of both foundational and practical importance for quantum computing. Text-book quantum error correction demands that qubits are reset after measurement. However, fast qubit reset has proven challenging to execute at high fidelity. Consequently, many cutting-edge quantum error correction experiments are opting for the no-reset approach, where physical reset is not performed. It has recently been postulated that no-reset is functionally equivalent to reset procedures, as well as being faster and easier. For memory experiments, we confirm numerically that resetting provides no benefit. On the other hand, we identify a remarkable difference during logical operations. We find that unconditionally resetting qubits can reduce the duration of fault-tolerant logical operation by up to a factor of two as the number of measurement errors that can be tolerated is doubled. We support this with numerical simulations. However, our simulations also reveal that the no-reset performance is superior if the reset duration and infidelity exceed given thresholds. For example, with the noise model we considered, we find the no-reset performance to be superior when the reset duration is greater than approximately 100 ns and the physical error probability is greater than approximately 10 −2.5 ≈ 0.003. Lastly, we introduce two novel syndrome extraction circuits that can reduce the time overhead of no-reset approaches. Our findings provide guidance on how experimentalists should design future experiments.

Tim Folger

Allen Mi, Shuwen Deng, Jakub M. Szefer

A secure reset operation could be an enabling technology that allows sharing of a quantum computer among different users, or among different quantum programs of the same user. Today, the dominant method to erase the qubit state is a full system wipe, which effectively resets all the qubits at the same time. In today's superconducting qubit machines from IBM, for example, a full system wipe takes up to 1000 μs, and it fully erases all information in the system. However, with a full system wipe there is no means for only a few qubits to be cleared and assigned to a new user or program; everything has to be erased at the same time. A secure reset operation could allow resetting only a subset of qubits, and it could be built upon existing (insecure) reset operation available from superconducting qubit machines from IBM, for example. The (insecure) reset operation is available today, which can be used to reset the state of a qubit in a time on the order of 10 μs down to 1 μs. The reset operation is thus much faster than a full system wipe. However, as this work demonstrates, it is possible to leak some information across the (insecure) reset operation as it does not perfectly reset the qubit state between two users or programs who may be sequentially scheduled on the same qubit. Further, crosstalk-like effects are observed where reset behavior of one qubit can be inferred from an adjacent qubit. This work analyzes the existing (insecure) reset operation in order to understand how a secure reset operation could be built upon it. This work then describes the design, implementation, and evaluation of the proposed secure reset operation which can reset qubits without leaking information, and retains a factor of 300 speedup over a full system~wipe.

Chuanqi Xu, Jessie Chen, Allen Mi +1 more

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.