Processing microwave photons at the quantum limit with 3-wave mixing Josephson elements
ABSTRACT :
In the field of superconducting quantum circuits, a key need is efficient processing of microwave photons. It is critical for high-fidelity non-demolition qubit readout, as well as more complex tasks like remote entanglement of distant qubits. How do we engineer circuits best suited to perform these tasks? I will discuss two elementary circuits in which Josephson junctions are combined to realize 3-wave mixing, a convenient primitive for quantum signal processing. The first circuit is the Josephson Ring Modulator (JRM), a quadrupole element, consisting of 4 identical junctions arranged in a Wheatstone's bridge. This versatile element has been used in a variety of devices such as parametric amplifiers, frequency-converting tunable beam-splitters as well as non-reciprocal devices such as microwave circulators and directional amplifiers. A straightforward method to improve these devices would be to combine multiple JRMs to achieve increased power handling and/or information throughput. However, combining multiple quadrupole elements is a difficult task, and hence, in the second part of the talk, I will introduce a dipole element which we call the Superconducting Nonlinear Asymmetric Inductive element (SNAIL). This element, which is similar in construction to the RF-SQUID and flux qubit, has been employed in a traditional parametric amplifier as well as to realize a novel nonlinear coupling between a qubit and a microwave cavity. It promises higher dynamic range and/or bandwidth than existing parametric amplifiers based on 3-wave mixing.