Quantum Computing Hardware
What is the main purpose of frequency multiplexing in the readout of superconducting qubits, where several readout resonators with different frequencies are connected to a single transmission line?
- ATo read one qubit at several frequencies at once and eliminate measurement errors
- BTo eliminate crosstalk between qubits by giving all the resonators the same frequency
- CTo reduce the number of cables and amplifiers running into the refrigerator, limiting the heat load and the size of the system
Show answer and explanation
Answer: C. To reduce the number of cables and amplifiers running into the refrigerator, limiting the heat load and the size of the system
Providing a separate readout coaxial cable and cryogenic amplifier for every qubit makes the number of cables, the heat load and the size of the system a problem as the number of qubits grows. With frequency multiplexing, several to about ten resonators at staggered frequencies, for example in the 6–7 GHz range, are connected to one transmission line, and a microwave signal containing all their frequencies is sent to read them out together. One line and one amplifier can then be shared by several qubits. If the frequencies are not spaced far enough apart the signals mix, so there is a trade-off between the available bandwidth and the number of resonators.
- A. The aim is to assign one resonator to each qubit and read several qubits together.
- B. The resonator frequencies are not made equal but staggered so that they can be told apart.
- C. Correct. One line and one amplifier can be shared by several qubits.