A recent Physics Today article and other scientific reviews highlight the importance of rare earth ion ensembles in the development of robust quantum memory for light. These solid-state systems store and recall the quantum states of light with high fidelity and have been shown to be competitive with or better than alternative approaches. Analysis and demonstrations by international groups have shown that rare-earth systems offer the potential to simultaneously achieve multimode quantum storage with 90% efficiency over timescales of seconds with GHz access bandwidths. Quantum memories that satisfy all these properties simultaneously are critical in the race to develop quantum networks, long-distance quantum communication and quantum cryptography, and linear optics quantum computing.
This project focuses directly on the demonstration of efficient solid-state quantum memories through development of enabling triply ionized Thulium (Tm3+)-doped solid-state LiNbO3 optical waveguides in collaboration with the experimental quantum communication group of Prof. Wolfgang Tittel at the Institute for Quantum Information Science, University of Calgary, a leader in many of the quantum memory demonstrations. The broad goal of our work is to develop a solid-state material satisfying all requirements for quantum teleportation of quantum states between light and matter. In recent demonstrations of a high-fidelity solid-state quantum memory for entangled photons by Tittel, et al. (Nature, 2011), the 2% total system recall efficiency was limited by an unexplained excess decoherence in the Tm3+-doped LiNbO3 waveguide storage material. From our past decoherence studies of bulk Tm3+-doped LiNbO3 single crystals, it is known that excess decoherence that can be encountered in waveguides is not a fundamental limit of the material system.
In this project, we are investigating and resolving the differences between the waveguide storage material used in the Nature demonstration by Calgary and the superior properties of our bulk crystals through coordinated experiments carried out in Montana and Calgary. Experimental study of optical decoherence combined with theoretical modeling of the material physics is used to reveal mechanisms for excess decoherence in the waveguide material to guide improvement in performance through routes including new LiNbO3 materials, alternate waveguide fabrication processes, material processing, and manipulation of properties through controlled external perturbations such as applied fields and temperature.
This project's results have immediate applications in designing solid-state materials for quantum information processing, optical signal processing, laser frequency references, and laser materials. Quantum memories that enable long distance quantum key distribution for secure communication are of strategic importance given the need to transmit confidential information in a way that keeps it secure even 50 years from now. There is close collaboration with local groups in these areas at MSU, AdvR Inc., Scientific Materials Corp., and S2 Corp. -- a company whose technology was enabled by our scientific research.
The techniques employed provide ideal educational programs for students at all stages, from undergraduates to post-doctoral researchers. This work offers students opportunities to gain proficiency with concepts and methods for research and development of optical materials in academic or industrial environments and is further enhanced by exchanges between MSU and Calgary. In particular, it provides a much-needed pool of skilled graduates for over twenty local Montana optical industries with whom close ties are maintained.
The rapidly developing field of quantum information science (QIS) employs the unique properties of quantum systems (systems governed by quantum physics) to process, store, and transmit data in ways that are impossible to achieve with previous classical information systems. Practical applications of QIS, including quantum computers, secure long distance communications, quantum networks for computers, and non-classical metrology (measurements), urgently need a "quantum memory" that can store and retrieve quantum states of matter and light on demand. In particular, quantum memories enable long distance distribution of the "quantum keys" that are of strategic importance for transmitting confidential information in a way that remains secure far into the future. Of all the optically addressed quantum memories being investigated, rare-earth ions in transparent crystals at cryogenic temperatures stand out as one of the most promising that can store and recall the quantum states of light with high fidelity, efficiency, and signal bandwidth. The broad goal of this project was to enable transformative advances in solid-state quantum memories by uniquely integrating the knowledge of fundamental physics of materials and applied engineering of these optical materials developed by researchers at Montana State University with the expertise in practical quantum optics and quantum cryptography of researchers at the University of Calgary. By combining related efforts spanning quantum information science, basic materials chemistry, spectroscopy, crystal growth, atomic physics, nonlinear optics, and integrated photonics, the properties of the existing quantum memory material thulium-doped lithium niobate were dramatically enhanced, reaching critical milestones required for practical development of devices. Specifically, we were able to reduce the rate at which information is lost in this storage material by nearly two orders of magnitude, reaching new regimes of performance with this system and surpassing our initial goals of controlling how information stored in quantum states of matter inevitably degrades over time by interactions with the environment, a process known as decoherence. Simultaneously, we were also able to increase the lifetime of quantum states in this system by nearly three orders of magnitude, considerably simplifying programming of the memory (material preparation and initialization process), a key development for practical devices. Of even greater significance, we introduced a new material, thulium-doped yttrium gallium garnet, for quantum memory applications that offers even greater improvements in performance. The capabilities of that new system are still being explored in ongoing work, and initial results indicate that it enables fast (high-bandwidth) quantum communication over long distances of hundreds of kilometers. The fundamental scientific and technical advances in the exceptional, high-quality, rare-earth-activated materials pursued in this project have direct applications in a broad range of other rare-earth-enabled optical/photonic technologies including classical optical computing, optical processing of RADAR signals, ultra-wide-bandwidth (UWB) spectrum analysis of microwave signals, slow light and meta-materials, and improvements to atomic clocks through provision of ultra-stable Laser Local Oscillators (LLO’s). The materials development impacts materials science in general by offering new insights into crystal imperfections, material fabrication processes, laser system performance, and the light-matter interaction itself; these impacts arise from the extreme sensitivity of our quantum measurements to the nano-scale perfection of materials,. As a result, our targeted experimental measurements offer broader new insights into material physics, and the improved fabrication methods that were achieved in this project accelerate the development and commercialization of both the targeted applications as well as entirely new ones that exploit the unique capabilities of these advanced materials. In fact, the new thulium-doped yttrium gallium garnet material developed in this project is already being investigated by industry for electronic signal analysis applications with high time-bandwidth performance. The extensive participation of students, junior scientists, and visiting researchers was an essential component of this effort that also benefits the larger science, technology, engineering, and mathematics (STEM) community. In addition to senior personnel, other researchers directly contributed to the work at Montana State University, including four undergraduate students, one graduate student, one postdoctoral researcher, one junior scientist, and six visiting scientists. Furthermore, junior researchers at the University of Calgary also directly participated in the effort under this project, including two postdoctoral researchers and three graduate students. Portions of this project were carried out in collaboration with industrial partners, including AdvR Inc, S2 Corp., and Scientific Materials Corp., facilitating technology transfer and exposure of students to both academic and industrial environments. Extensive interactions between all the researchers involved in this project were an essential component of the project’s success.
