Parametric amplifiers for high fidelity, QND qubit measurement

Quantum information processing requires high-fidelity, quantum non-demolition (QND) readout of quantum bits. In superconducting circuits, qubits are coupled to short-lived 'readout' oscillators and measured via coherent pulses which gain qubit information as they pass through the readout mode. A central challenge is that these pulses must be very short compared to qubit lifetimes (to achieve high fidelity) and contain relatively few photons (to remain QND due to poorly understood measurement-induced qubit transitions). Given these limitations, the amplifier which processes these pulses is of vital importance. We desire these amplifiers to have high saturation power, large bandwidth, achieve the quantum limit on added noise, and be directional so that we can connect them directly to quantum circuits. In this talk I will discuss our efforts to master all these requirements in cavity-based, parametrically driven amplifiers. I will show a simple single-mode, non-directional amplifier which is nearly-quantum limited with good bandwidth and a very high saturation power (-90 dBm), and demonstrate high fidelity qubit readout with this device. I will also describe our efforts to build so-called 'embedded amplifiers' which can act directionally and be directly integrated into quantum processors without the use of bulky ferrite-based microwave circulators. Finally, I will discuss how close we are to the point where further amplifier improvements cease to benefit us and further readout fidelity increase must come from the qubit's increased tolerance for strong, short readout pulses.