The Traveling-Wave Parametric Amplifier
A traveling-wave parametric amplifier gives broadband, near-quantum-limited gain by spreading amplification along a long nonlinear transmission line.
Beyond the Resonant Amplifier
A Josephson parametric amplifier concentrates amplification in a resonator, which gives excellent noise performance but only over a narrow band and with limited power handling. Reading many qubits at once needs an amplifier that is both broadband and able to handle a comb of tones. The traveling-wave parametric amplifier, or TWPA, meets this need by removing the resonator entirely.
How It Works
A TWPA is a long transmission line whose inductance is nonlinear, built either from a chain of Josephson junctions or from a superconducting film with kinetic inductance. Signal and pump propagate together down the line, and the nonlinearity mixes them continuously so the signal grows exponentially with distance. Because there is no resonator, the gain spans several gigahertz of bandwidth.
Phase Matching
Efficient amplification requires that the signal, pump, and generated idler stay in phase as they travel, a condition called phase matching. Dispersion in the line otherwise causes them to drift out of step and the gain to saturate. TWPAs engineer the dispersion, often with periodic loading of the line, to keep the pump phase-matched to the signal over the desired band.
- Broadband gain covers many multiplexed readout channels at once.
- Higher saturation power tolerates a comb of simultaneous tones.
- Noise performance can approach the quantum limit across the band.
Trade-offs and Role
TWPAs are longer and more complex to fabricate than resonant amplifiers, may add slightly more noise, and require careful pump filtering to keep the strong pump tone out of the qubits. Even so, their bandwidth and dynamic range make them the enabling amplifier for large-scale multiplexed readout, sitting at the front of the output chain and feeding the warmer semiconductor amplifiers above.
Together with frequency multiplexing, the TWPA is a key reason many qubits can be read through a single output line without sacrificing fidelity.