Quantum Logic Gates in Superconducting Qubits

Successful operation of a quantum computer will require unprecedented control of quantum systems. The basic qubit operations, quantum logic gates, are described by the linear Schrodinger equation: the “analog” nature of quantum state evolution makes these logic gates fundamentally sensitive to imperfections in control and loss of energy. In contrast, conventional digital logic can correct errors due to built-in gain and non-linearity. In a quantum computer, these imperfections fortunately can be removed with error-correction protocols, which work as long as the probability for the production of errors is small enough. The performance specifications for error correction depend on details of the quantum computer architecture. Rough estimates for conventional gate-based architectures give limits below ∼ 10−4 [1], whereas more recent proposals based on surface codes may allow errors in the 10−2 range [2]. Much research in superconducting qubits has been directed towards improving the coherence of qubits and demonstrating quantum logic gates, both for single and coupled qubits. I am optimistic that quantum gates can eventually meet performance requirements needed for error correction. Here, I focus on several important issues concerning the high-level design of quantum logic gates. In particular, I will review the need to effectively turn on and off coupling interactions between qubits to produce scalable controlled-not (CNOT) gates. This is an important topic for superconducting qubits, since they typically use fixed coupling elements set by fabrication.