Skip to main content

Scalable Rare Earth Ion Quantum Computing Nodes

Periodic Reporting for period 1 - SQUARE (Scalable Rare Earth Ion Quantum Computing Nodes)

Reporting period: 2018-10-01 to 2020-03-31

Quantum technologies rely on materials that offer the central resource of quantum coherence, that allow one to control this resource, and that provide suitable interactions to create entanglement. Rare-earth ions (REI) doped into solids have outstanding potential in this context and could serve as a scalable, multi-functional quantum material. REI provide a unique physical system enabling a quantum register with a large number of qubits, strong dipolar interactions between the qubits allowing fast quantum gates, and coupling to optical photons – including telecom wavelengths – opening the door to connect quantum processors in a quantum network. The SQUARE project aims at establishing individually addressable rare-earth ions as a fundamental building block of a quantum computer with a photonic interface, and to overcome the basic roadblocks on the way towards scalable quantum hardware. The goal is to realize the fundamental elements of a multifunctional quantum processor node, where multiple qubits can be used for quantum storage, quantum gates, and for coherent spin-photon quantum state mapping. Novel schemes and protocols targeting a scalable architecture will be developed.
Technology development is the second focus of SQUARE to enable scalable REI quantum computing: highly tunable and coherent lasers, tailored cryocooler concepts, quantum-grade materials, tunable microcavities, and cryogenic nanopositioners will be developed. These elements will be directly useful for a variety of applications, ranging from other quantum technologies such as quantum communication and quantum sensing to more general applications such as microscopy, cryogenics, and advanced spectroscopy. Our results will contribute to the development of future quantum technologies, and strengthen the European high-tech industry, such as the two companies involved in the project.
Scientifically, we will chart novel grounds and address the central challenges to find a scalable approach for quantum computing and quantum networking. This will help to expand the leading role of European research in the field and contribute to advance quantum science.
Within the first funding period, the consortium has worked towards the detection and control of single rare-earth ions as qubits, developed several device elements required for scalable quantum nodes, devised theoretical frameworks to describe optically interconnected quantum nodes, and assessed achievable single-qubit gate fidelities.
Specifically, we were able to demonstrate the detection and quantum control of a single Cerium spin and used it to sense nearby Silicon nuclear spins as possible quantum memories. Further, we demonstrated dynamic tuning of strong Purcell enhancement of Erbium ions, which is a crucial ingredient for efficient single ion readout, optimized quantum gates, and interconnection of quantum nodes. Another important breakthrough was the development of a theoretical framework to realistically describe traveling quantum pulses, which is key to understand the features and limitations of quantum networks and the interconnection of quantum nodes. Also, realistic modeling to assess the limits of single-qubit gate fidelities was performed. Another focus was on the technology development of scalable device elements: A cryogenic nanopositioning platform and a vibration isolation platform for operating open-access microcavities were realized at the prototype level. Fiber cavities were advanced towards TRL8 as an enabling tool for optical quantum technologies, and commercial distribution was prepared. Growth techniques for REI-doped thin films were developed to provide a direct route for the most promising material geometry for photonic integration, and first encouraging optical properties were achieved. A scheme for a scalable laser source that can coherently address up to 100 qubits was devised in full detail, and a concept for an all-European closed-cycle cryocooler tailored for the operation of REI-based quantum nodes and other optically addressable quantum materials was worked out.
The project has achieved several important intermediate goals, and the results described above go beyond the current state of the art: E.g. sensing of a single nuclear spin via a single Cerium ion for the first time was an important step towards the possibility of harnessing external nuclear spins as quantum memories for rare-earth ions. This is a promising direction for high-density qubit implementation and will be investigated during the remaining project phase. The novel theoretical formalism for the description of traveling quantum pulses fills a gap for the realistic description of elementary processes in quantum networks and can be generalized to a broad range of questions in various fields of quantum science. Dynamical tuning of strong Purcell enhancement represents a novel capability to optically address also large numbers of qubits separated in frequency space, and could thus become a central element for scalable quantum nodes. The various devices developed within SQUARE include so far commercially unavailable technology that satisfies the demands across several fields.

With these intermediate results, we have taken first steps to evidence the potential of rare-earth quantum computing nodes and the prospects for a scalable technological implementation. Until the end of the project, we aim to demonstrate the basic elements for quantum computing with optical interconnectivity: single and two-qubit gates, an efficient spin-photon interface, scalable device elements, and protocols for scalable multi-qubit nodes. Based on this we will propose a technologically relevant roadmap for rare-earth quantum computing, directing future technological efforts. Our work has a direct connection to companies in all stages of development, from large- and medium enterprises to a currently forming start-up, and thus contributes to strengthen the near future high-tech industry sector in the EU.

On a long-term scale, REI technology is promising e.g. for 3rd generation quantum repeaters, which require error correction capability and thus 10 – 100 qubits with quantum logic at each node connected to telecom photons. It is also a promising approach to connect and interface superconducting quantum computers. Realization of REI quantum computing with a large qubit number would enable technologically relevant computations with disruptive impact.
Schematic of a tunable fiber cavity coupling to rare earth ions in a thin film