Optimal control of spin ensemble quantum memories

In recent years, the scientific perspective of information technology has gone through a dramatic makeover, with the emergence of quantum information theory and its foray into exciting new possibilities in physics. An important task for the development of future quantum computation and communication protocols is the seamless transfer of information between different nodes of a hybrid quantum network. Such protocols require a coherent and high- fidelity transfer of quantum information encoded in the modes of an electromagnetic wave to the states of an atomic system that can serve as storage or memory devices. The proposed research aims at developing theoretical control models to design robust and cost-effective quantum memories based on a spin-ensemble in a quantum cavity. The research will be carried out by the applicant, Himadri Shekhar Dhar, with Prof. Stefan Rotter, at the Vienna University of Technology. Hypothesis: In recent years, experiments have reported potential quantum memory operations by demonstrating the transfer of information from optical and microwave fields to solid-state spin ensembles for storage and subsequent retrieval. However, the preparation and control methods employed in these experiments make the operation practically and economically unviable for large-scale implementation. The proposed research seeks to obtain an optimal control mechanism that can overcome the physical constraints in spin ensemble quantum memory operations in a more resourceful and practically implementable manner, and offer genuine advantages over existing protocols. Methods and Innovation: To circumvent the expensive preparation techniques involved in current experimental demonstration of spin ensemble-quantum cavity based quantum memories, the project aims at developing an innovative theoretical optimal control model based on techniques applied in atomic and ion traps, cavity-QED, and NMR spectroscopy. The proposed theoretical model shall be implemented in proof-of-concept experiments, developed with envisioned collaborators, and shall also be tailored for operations using hybrid quantum systems and control mechanisms. A successful outcome of the research objectives shall provide experimentalists with the appropriate control mechanisms to design and develop resource-effective and efficient quantum memories, which will immensely benefit the research community working on quantum information and technology. Importance: The importance of the proposed research lies in the demand for innovative approaches to harness and apply quantum technology in a resourceful manner. In this respect, efficient quantum memory units are extremely valuable for developing scalable networks for quantum computation and communication. The proposed research allows us to develop quantum control techniques that are crucial for extracting optimal performance, under physical constraints, in a spin-ensemble quantum memory. Moreover, optimal control models obtained under the project shall also be applicable in the development of hybrid quantum systems to efficiently design other important quantum computation protocols.

The main outcome of the research project, funded by the Austrian Science Funds (FWF) Lise-Meitner programme, was the development of a novel theoretical framework to study light matter interactions in hybrid quantum systems. The introduced method now provides us with the tools to investigate and control the quantum dynamics of an ensemble of spins inside a quantum cavity, where the interaction between nonclassical light and matter can be fully harnessed. Such regimes were hitherto not completely accessible to researchers, and thus paves the way for interesting new physics in spin-ensemble-cavity systems, including design of efficient single photon sources for quantum computation and optimally controlled quantum memories. For instance, we have demonstrated how periodic pulses of nonclassical light, acting like good single photon sources, can be generated by sending a short coherent pulse through a spin ensemble inside a quantum cavity. Our study on this subject was recently published in Physical Review Letters. The periodic photon pulses obtained in our work can be readily harnessed in contemporary experiments in superconducting circuits, quantum dots, and other setups, and provides alternatives to mitigate some of the difficulties in obtaining nonclassical light. We note that such hybrid systems are also optimal for implementing protection against decoherence and designing quantum storage protocols. Further research on mesoscopic spin ensembles is being pursued in our group and more interesting results are expected in the near future. Our theoretical approach is based on the key insight that tensor network methods, which are widely applied in the study of solid-state physics and quantum chemistry, can also be adapted to efficiently simulate open hybrid quantum systems. So far, most known theoretical methods depend on semiclassical solutions or highly restrictive quantum regimes to study these systems. Our work therefore bridges a gap in the theoretical understanding of spin- ensemble-cavity interactions in the mesoscopic regime, and opens up new directions to investigate complex parameter regimes that have remained out of reach so far. An aspect we find particularly appealing is that our work is likely to serve as a shared theoretical platform between the disjoint communities working on numerical quantum many-body physics, on the one hand, and on hybrid quantum systems, on the other, thus triggering substantial interest and research possibilities in this area. We believe the main outcome from our research project under the Lise-Meitner programme also has the potential to impact critical studies on hybrid quantum systems, especially in designing robust quantum protocols, such as single photon sources and quantum memories, which are critical components of future quantum information and communication technology.