Schröder, Tim

Photonic graph state creation with diamond spin defects – exploring fascinating entangled objects as resource for quantum information

Tim Schröder - Humboldt-Universität & Ferdinand-Braun-Institut, Berlin, Germany

Measurement-based quantum computing (MBQC) is a model of quantum computation that relies on quantum measurements rather than unitary operations to implement algorithms [1,2]. In MBQC, a highly entangled state, a so-called resource graph state, is prepared initially. Algorithms are then executed by performing a sequence of adaptive single-qubit measurements on this state. Each measurement outcome determines the basis for subsequent measurements, allowing complex computations to unfold without the need for direct gate operations. This approach is particularly interesting because it separates the preparation of entanglement from the computation process, offering potential advantages in fault-tolerant quantum computing and making it compatible with various physical implementations of quantum processors.
In this presentation I will introduce the fundamental concepts of MBQC through ongoing research our group is conducting, focusing on the physical implementation with photonic graph states [3,4]. There are two main challenges that the scientific community as well as commercial companies a presently facing towards the implementation of MBQC. The first one is the creation of the required resource state consisting of photons that are entangled with each other in at least two dimensions. For the creation of such graph states [5], we are exploring the suitability of atom-like optically active spin defect centers in diamond nanostructures [6]. To this end, we theoretically investigate the achievable resource state size [7] and the required gate operations for graph state creation [8,9]. Experimentally, we demonstrate optical and microwave qubit control gates [10]. To provide the required high photon creation efficiencies, we have developed an efficient spin-photon interface with up to 99% creation-to-fiber coupling efficiency [11,12]. The second challenge is the implementation of adaptive single-qubit measurements on the photons of the entangled state [2] which requires photonic integrated circuits with feed-forward operations. Towards this goal we have made progress in using AlGaN as photonic platform [13] and are developing feed-forwarding methods.
By addressing both of these challenges, our group explores photonic graph states as fascinating entangled objects and the required physical systems to turn them into resources for quantum information.

References
[1] R. Raussendorf and H. J. Briegel, "A One-Way Quantum Computer," Phys. Rev. Lett. 86, 5188 (2001).
[2] H. J. Briegel et al., "Measurement-based quantum computation," Nature Physics 5, 19–26 (2009).
[3] I. Schwartz et al., "Deterministic generation of a cluster state of entangled photons," Science 354, 434 (2016).
[4] P. Thomas et al., "Efficient generation of entangled multiphoton graph states from a single atom," Nature 608, 7924 (2022).
[5] J. Borregaard et al., "One-Way Quantum Repeater Based on Near-Deterministic Photon-Emitter Interfaces," PRX 10, (2020).
[6] T. Schröder et al., "Quantum nanophotonics in diamond [Invited]," Journal of the Optical Society of America B 33, B65 (2016).
[7] G. Pieplow et al., "Deterministic Creation of Large Photonic Multipartite Entangled States with Group-IV Color Centers in Diamond," arXiv:2312.03952 (2023).
[8] L. Orphal-Kobin et al., "Coherent microwave, optical, and mechanical quantum control of spin qubits in diamond," Advanced Quantum Technologies, 2300432 (2024).
[9] G. Pieplow et al., "Efficient Microwave Spin Control of Negatively Charged Group-IV Color Centers in Diamond," Phys. Rev. B 109, 115409 (2024).
[10] C. G. Torun et al., "SUPER and subpicosecond coherent control of an optical qubit in a tin-vacancy color center in diamond," arXiv:2312.05246 (2023).
[11] J. M. Bopp et al., "‘Sawfish’ Photonic Crystal Cavity for Near-Unity Emitter-to-Fiber Interfacing in Quantum Network Applications," Advanced Optical Materials, 2301286 (2023).
[12] T. Pregnolato et al., "Fabrication of Sawfish photonic crystal cavities in bulk diamond," APL Photonics 9, 036105 (2024).
[13] S. Gündogdu et al., "AlGaN/AlN heterostructures: an emerging platform for integrated photonics," npj Nanophotonics 2, 1 (2025).