The Josephson Parametric Amplifier
A Josephson parametric amplifier boosts faint qubit-readout signals with noise near the quantum limit, sitting at the coldest stage of the cryostat.
Amplifying without adding noise
Reading out a superconducting qubit means detecting an extremely weak microwave signal, often at the single-photon level. Any amplifier adds some noise, and standard amplifiers add far too much to resolve such signals in a single measurement. A Josephson parametric amplifier, or JPA, adds noise close to the fundamental minimum allowed by quantum mechanics, making high-fidelity single-shot readout possible.
How it works
The amplifier is a resonant circuit whose inductance comes from Josephson junctions and is therefore nonlinear. A strong pump tone modulates this nonlinear inductance, and energy from the pump is transferred to the weak signal, amplifying it. Because the gain comes from a nearly lossless reactive element rather than from a dissipative transistor, the added noise can approach the half-photon quantum limit set by the uncertainty principle.
Its place in the readout chain
The JPA sits at the mixing chamber, right after the qubit, as the first stage of the cold-electronics stack. By adding almost no noise while providing gain, it lets the following HEMT amplifier at 4 kelvin, which is noisier, operate on an already-boosted signal without degrading it much. The overall noise of the chain is dominated by whatever comes first, so a quiet first stage sets the performance of the whole readout.
Limitations
Parametric amplifiers have limited bandwidth and a limited dynamic range: too strong a signal saturates them. They also require careful pump-tone management and can be sensitive to magnetic fields. Traveling-wave parametric amplifiers were developed to widen the bandwidth, letting many frequency-multiplexed qubits be read through one amplifier, which is important for the scaling of readout.
- Adds noise near the quantum limit of half a photon
- Gain from nonlinear Josephson inductance driven by a pump
- First stage of the readout chain at the mixing chamber
- Limited bandwidth and dynamic range; TWPAs widen bandwidth