Controlling and protecting quantum information in superconducting oscillators
Modern quantum experiments allow the precise manipulation and measure- ment of many-body quantum states, pushing quantum mechanics from a test- able theory to a utilizable technology. The central promise of these systems is to process quantum information for exponential advantages in computing, sensing, and communication. An interesting way to achieve such a processor is to store and manipulate quantum information in the continuous-variable (bosonic) phase space of light. Since photons in free space do not talk to each other, such an approach necessarily requires the introduction of nonlinearity through strong light-matter couplings. However, since all matter is lossy, this inevitably introduces a trade-off between the speed of control and the inherit- ed decoherence of the light. This thesis achieves controllable bosonic systems by trapping microwave light in the standing waves of a superconducting oscillator, and making it interact with Josephson junction-based nonlinearities. I first demonstrate novel ways to exchange single photons between two oscillators through carefully construct- ed driven nonlinearities, achieving several orders of magnitude higher fidelity than previously possible. I then introduce ways to utilize such a high-fidelity coupler to dynamically couple light and matter, in a way that breaks the trade- off between universal control and inherited decoherence. Finally, I theoretically show how such universal control can autonomously protect any appropriate error-correction code in the bosonic mode, against a full Lindbladian error channel. Together, this thesis provides a promising path toward fault-tolerant bosonic quantum processors.
Thesis Advisor: Robert Schoelkopf Committee: Michel Devoret, Shruti Puri, Steve Girvin, Archana Kamal