testInG quaNtum physIcs by countIng micrOwave photoNs – IGNITION

Despite almost a century of academic investigations and technological developments, quantum physics still firmly holds to its counter-intuitive postulates. Surprisingly amid this wealth of experimental support for the theory, it was recently proposed that quantum mechanics could simply fail at predicting the interference pattern between three or more alternatives. Quantum mechanics could then well be the limit behavior of a more general theory in some regime, such as classical mechanics is the limit behavior of quantum mechanics for macroscopic objects. Until now this fascinating prospect of a violation of Born's rule has been barely experimentally investigated and it is argued that severe loopholes may have canceled out deviations to quantum predictions in these experiments that use propagating photons or nuclear magnetic resonance.

With the IGNITION project, we want to benefit from the high level of control of superconducting circuits to measure interference at third order, thus trying to observe deviations to quantum predictions. By developing new devices, control and measurement protocols, we will both evade the loopholes of previous attempts and reduce the uncertainty in the measurement hence performing the most discriminating test to date of these deviations. Several technological developments will be realized over the course of this project including microwave engineering and fabrication process improvement, quantum control optimization, integrated directional quantum limited amplifiers, phase detectors and engineered microwave dissipation.

Beyond the main goal of providing a new meaningful challenge for quantum mechanics, the above technological developments are timely and relevant to quantum information processing and computing with superconducting circuits. In this direction, we propose three ancillary objectives that directly use the tools we will create. First, we will elaborate a new scheme for photocounting based on resonance fluorescence of a qubit. Second, we will implement a new quantum error correction protocol using Schrödinger cat states of microwave light. Using the many levels of a single oscillator instead of a qubit register indeed seems a very promising alternative to other quantum computing architectures currently under investigation. Finally, we will develop and test a detector of the optical phase and investigate the associated measurement back-action.