Engineering multi-photon dissipation in superconducting circuits for quantum error correction – ENDURANCE

Technologies exploiting quantum properties such as entanglement and superposition, could revolutionize our computational abilities. In the last two decades, impressive theoretical and experimental progress have brought us closer than ever to building these technologies. However, some outstanding challenges still stand in the way of a scalable quantum computer. One of them is to engineer a logical quantum bit (qubit): a system which encodes a qubit of information, and on which we can efficiently perform gates and quantum error correction (QEC). QEC consists in prolonging the coherence of a system by detecting and correcting when errors occur due to an undesired interaction with the environment. This requires redundantly encoding information in a large Hilbert space, which is usually chosen to be the state space of a multi-qubit register. However, engineering and controlling large numbers of qubits remains a difficult challenge.

The goal of this project is to tackle this problem from a completely new direction. We plan on constructing a logical qubit based on the subtle interplay between coherent and dissipative multi-photon processes. Although dissipation usually destroys quantum coherence, some specifically engineered dissipative multi-photon processes can actually protect a system against decoherence. The key idea is that a non-linearly driven-dissipative system can develop, not a single ground state, but rather a ground state manifold, in which we can encode information. This manifold is composed of all quantum superpositions of many steady states, and hence can serve as a quantum memory, which is robust against some decoherence channels.

In a proposal [Mirrahimi et. al. NJP (2014)], we have shown that driven-dissipative QEC could be realized using a few oscillators and qubits, thus avoiding the complication of large arrays of qubits [Fowler et. al. PRA (2012)]. The multi-photon processes we aim to demonstrate rely on non-linear couplings which are larger than the spurious couplings to the environment. Josephson superconducting circuits are an ideal system to combine strong non-linearities and low loss. We have recently demonstrated that it is possible to obtain a two-photon dissipation rate of at least the same order as the natural dissipation rate of a superconducting cavity [Leghtas et. al. Science (2015)]. For the purpose of QEC, we need to go further and design a four-photon dissipation rate which overcomes the natural decay of the oscillator. This challenging task will be addressed by:

First: we will design a superconducting circuit incorporating Josephson junctions, which exhibits a four-photon dissipation rate which is larger than other parasitic processes.

Second: we will build the experimental apparatus to characterize this device. We will need to quantify the four-photon dissipation rate and compare it other undesired terms.

Third: once the four-photon loss rate is sufficiently larger than other processes, we will parametrically apply squeezing operators to our mode, which will act as a fault-tolerant logical X rotation.

Fourth, we will combine this new technology with fast parity tracking [Sun et. al. Nature (2014)] to correct for single photon loss.
These ambitious tasks will require solving conceptual and technical challenges. The candidate has published convincing preliminary results related to this proposal, which puts us in a good position to succeed. We are convinced that this proposal opens new avenues within the fields of circuit quantum electrodynamics and quantum information (QI).