Superconducting Qubits: Wiring up Quantum Entanglement

Superconducting microwave-frequency circuits afford a powerful laboratory for testing the fundamental tenets of quantum mechanics while advancing the frontier of computing and sensing hardware that harnesses quantum entanglement for performance gains over classical technology.  Quantum bits can be thought of high-Q oscillators whose ringdown time sets the number of logical gates that can be performed per unit time, and must be increased to a level to permit error correction. Coherence is lost to dissipation in the environment arising from both the finite impedance of circuity needed to control and measure the qubits, and to imperfections in the embedding circuit—both microscopic and macroscopic. From a design perspective, long-lived quantum circuits leverage oscillators designed to weakly couple to lossy modes, passive control circuitry that has a frequency response tailored to reduce coupling to a 50 Ohm bath for write pulses while enhancing coupling at the frequency of readout pulses for maximal SNR, active circuits that amplify signals without wasting information, and pulse sequences designed to induced entanglement between qubits on demand. In this talk, we will give an overview of how these principles of microwave design are implemented in contemporary superconducting qubits.