Physics of Two Qubit Gates
Daniel D. Stancil, Gregory T. Byrd · 2022
Distinguishing characteristics of quantum computing include the use of the quantum mechanical phenomena of superposition and entanglement. The simplest manifestation of entanglement is a two-qubit state that cannot be expressed as a product of single-qubit states. The Bell State is a classic example. Turning on and off the interaction by tuning the qubits requires tuning control lines in addition to the RF signal lines. This additional complexity impacts the scalability of the system. In contrast, the cross-resonance scheme uses fixed-frequency qubits with fixed coupling, and controls the interaction with RF signals—simplifying qubit control. The CNOT is a fundamental two-qubit gate and is necessary for universal quantum computing. It plays a large role in the logic of quantum algorithms. The density matrix and partial trace approach can easily be extended to more than two qubits.