ECSU team realizes high-efficiency optical quantum memory
Optical quantum memories, which allow the reversible mapping of quantum states of light onto matter, are an indispensable component of long-distance quantum communications and distributed quantum networks based on quantum repeaters.
Optical waveguide-based optical quantum memory devices connected to other integrated quantum devices, such as quantum light sources, photonic circuits, and single-photon detectors, will open the way for integrating multifunctional quantum architectures. Rare-earth ion-doped optical waveguides are the best candidates for the development of on-chip quantum memory devices due to their compactness, scalability, and enhanced light-matter interactions.
Much effort has been invested in fabricating on-chip memory devices by various methods on the basis of which photonic quantum memories have been demonstrated in different systems.
To realize high-capacity quantum repeaters, on-chip quantum memories with broadband and multimode characteristics have been demonstrated. For practical integrated quantum memories, an efficient approach is the development of fiber-optic integrated quantum memory chips, which can be directly interconnected with current fiber-optic systems, thus facilitating the application of integrated devices in quantum networks. To date, such fiber-integrated quantum memory chips with multimode capacity in the telecom band have not been demonstrated, especially for broadband storage of nonclassical light-a key step toward achieving large-scale, high-rate quantum networks that are compatible with existing telecom infrastructures.
"Telecom-band-integrated multimode photonic quantum memory."
In a recent article published in Science Advances, a collaboration between Qiang Zhou's group at the University of Electronic Science and Technology, Feng Chen's group at Shandong University, and Lixing You's group at the Chinese Academy of Sciences demonstrated telecom-band-integrated multimode storage in Er3+ :LiNbO3 waveguides.
Specifically, the laser-written waveguides fabricated by FLM were directly coupled to single-mode fiber pigtails at both ends via optical collimators, ensuring compatibility with fiber-optic communication systems. An on-chip quantum memory system is demonstrated with a fiber pigtail waveguide based on the atomic frequency comb (AFC) protocol.
With a 4 GHz-wide AFC, the team experimentally achieved multimode quantum storage of 330 predicted single-photon timing modes in the 1532 nm band-a 167-fold increase in coincidence detection compared to single-mode.
Preparation and calibration of Er3+ :LiNbO3 waveguides.
Experimental setup
Characterization of the quantum memory
This quantum memory chip paves the way for integrated memory-based quantum networks compatible with fiber-optic communication infrastructures.
Despite these important results, some upgrades are needed to realize a functional device for quantum networks. In the paper, the experimental team says, "The storage times we demonstrate are limited by the AFC preparation setup. For example, our AFC-pumped laser has a linewidth of about 100 kHz and a frequency drift on the order of MHz, which results in a minimum tooth spacing of a few MHz for the preparation of AFCs, leading to a maximum storage time of a few hundred nanoseconds. Therefore, there is a need to update the laser system to prepare AFCs using a super-stabilized laser system."
"From the material side, it is necessary to extend the optical coherence time of Er3+ ions in the waveguide, which can be achieved by optimizing the doping concentration of Er3+ ions."
Finally, a promising avenue to achieve larger memory capacities is to combine multiple degrees of freedom. In addition to multiple temporal mode operation, more spectral channels can be prepared in the large inhomogeneous spread of the Er3+ :LiNbO3 waveguide using optical frequency combs. In addition, by fabricating multiple waveguides in the crystal, more spatial channels can be utilized in the on-chip quantum memory.
In summary, the above approach combines the reliability of fiber-optic integrated devices compatible with fiber-optic communication infrastructures, broadband multiplexed storage characteristics, and the promise of laser-written components.
"The next step may be to store quantum bits, create entanglement between remote storage chips, and implement feed-forward control and on-demand recall (on-demand recall)."
