Quantum Computation with Schrödinger cat states (QuCoS)

After decades of experimental and theoretical research, it is now possible to control and manipulate quantum systems with high precision and use them for quantum information applications. Such a quantum computer would allow researchers to tackle problems that are intractable or very difficult to solve on a classical computer. Despite the huge progress it is still a long way towards a full fledge quantum computer which consists of several thousands of quantum bits. One of the main problems is the realization of a scalable system where quantum-error-correction protocols can be implemented i.e. errors in the qubits can be detected and corrected on the fly. The goal of this QUANTERA project CuQoS is to realize a quantum system based on superconducting resonators and circuits that allows continuous error correction paving the way towards a quantum computer. The key point of the project is the encoding of quantum information in so-called Schrödinger cat states that are stored in high coherence Microwave resonators. The realization of dedicated superconducting circuits will allow the detection of errors, which combined with fast feedback, allows the correction of the same. Furthermore, we will investigate the realization of quantum gate operations on this encoded qubit to realize a quantum computer. CuQoS is a collaboration of several theoretical and experimental research groups in the EU, which are specialists in superconducting circuits. To realize this project several novel superconducting circuits have to be realized and novel experimental methods have to be developed. The results of this consortium consisting of 6 research groups led by G. Kirchmair (University of Innsbruck) B. Huard (ENS Lyon), Zaki Leghtas und Mazyar Mirrahimi (ARMINES Paris), Ioan Pop (Karlsruhe Institute for Technology), Yonatan Cohen (Quantum Machines Israel) und L. Buimaga-Iarinca (National Institute for Research Romania) will have far reaching consequences for future Quantum technologies and the realization of a quantum computer.

The FWF funded project QuCos aims to demonstrate the same level of quantum error correction provided by a few hundreds of qubits (with properties beyond the state of the art) in today's mainstream approach of the so-called surface code architecture. Our alternative approach is known as cat codes, because it employs multiple interconnected high coherence cavity modes with non-linear dissipation, to encode a qubit in superpositions of Schrödinger cat states. Our project combines realizing the quantum processor architecture as well as the control system and the protocols that drive it, building towards a full-stack error-corrected quantum computer. The partners in our collaboration form a strong synergetic group that has the full range of expertise needed to design and realize these systems, and to obtain these challenging goals. Furthermore, all partners of our project, including both industry and academia, have worked together and published works in the fields of quantum computing and quantum information processing. We aim to implement error protected qubits, fault tolerant operations, and demonstrate the scalability of this approach by realizing a repetition code. Our project will enable quantum experiments towards the ambitious and well-defined goal of constructing a logical qubit, on which we can perform gates, and most importantly, quantum error-correctio (QEC). All algorithms with theoretically proven quantum speedup require QEC, therefore, with this project we are realizing an essential building block of a European error corrected quantum processor. In this poject jointly with international collaborators we could demonstrate the increase of the life-time of such an encoded state by factor of 1 million and demonstrated the implementation of novel gate operations on these encoded qubits. Furthermore, we investigate the non-linearity of a Josephson junction, demonstrated a novel architecture to control a high coherence resonator and showed that classical thermal states can be brought into a quantum superposition.