Generation, Manipulation, and Detection of Complex States of Microwave Radiation

Abstract

Superconducting circuits represent a platform actively researched for its potential use in quantum information processing. The large couplings achieved between artificial atoms build out of lossless Josephson junctions and microwave photons are one of the most prominent features, directly implementing in a circuit the toy models of quantum electrodynamics (QED). The strong light-matter interaction enables the preparation of microwave radiation fields in non-classical states, such as states in the single photon manifold with anti-bunched statistics, that can serve as carriers of quantum bits (qubits) of information in the prospect of building a network of quantum computing nodes. The radiation emitted from the superconducting devices can be fully characterized tomographically by linear amplification and homodyne detection, yielding on average access to the complete density matrix. However, due to the low energy of microwave fields, the single-shot detection of individual itinerant photons remained elusive. In this thesis, I report on a controlled phase gate between a single itinerant photon and a transmon qubit, which enables a clicking single-shot detector for the photon. We have characterized the detector’s operation with a quantum source deterministically emitting superposition of the vacuum and single photon Fock states, and demonstrated that it performs a quantum non-demolition measurement in the Z-basis. We have then extended the capabilities of our setup by implementing a parity detector, that measures whether the number of photons reflected off of it is even or odd, without providing information on the exact number. This parity detector has allowed us to access directly the Wigner function of itinerant states of radiation. The strong projective measurement on a parity eigenspace alters the properties of the measured field, which we have demonstrated explicitly by heralding Schrödinger cat states, hallmarks of quantum superposition, from classical coherent states. Finally, I emphasize the capabilities of itinerant fields to act as photonic qubits by describing a novel versatile source of entangled states of microwave radiation modes. We have realized a unique superconducting device capable of generating a variety of states within the matrix product state family, with on the order of ten entangled modes. Their characterization, via full quantum tomography up to four modes and process maps for larger states, is extensively discussed and expands on the capabilities of the community. In conclusion, our results demonstrate the generation, manipulation and detection of complex states of microwave photonic qubits, essential steps to perform distributed quantum information processing with the superconducting circuit architecture. In particular, the demonstrated technology puts a universal quantum gate set for itinerant microwave photons, and super-sensitive photon measurements such as Heisenberg-limited phase metrology, within immediate reach.