Abstract
We demonstrate a photonic circuit with integrated long-lived quantum memories. Precharacterized quantum nodes—diamond microwaveguides containing single, stable, negatively charged nitrogen-vacancy centers—are deterministically integrated into low-loss silicon nitride waveguides. These quantum nodes efficiently couple into the single-mode waveguides with collected into the waveguide, have narrow single-scan linewidths below 400 MHz, and exhibit long electron spin coherence times up to . Our system facilitates the assembly of multiple quantum nodes with preselected properties into a photonic integrated circuit with near unity yield, paving the way towards the scalable fabrication of quantum information processors.
- Received 23 August 2014
DOI:https://doi.org/10.1103/PhysRevX.5.031009
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Published by the American Physical Society
Popular Summary
Compact, phase-stable quantum networks made of individual quantum memory nodes connected via photons may enable quantum computing schemes, or serve as nodes in a long-distance, fault-tolerant quantum communication network. Solid-state quantum memories are stable, easily manipulated, and can be integrated into on-chip devices. In particular, the negatively charged nitrogen vacancy center in diamond is a leading choice as a quantum memory since it can store a quantum state for up to a second in its electronic spin. Moreover, the spin state can be optically initialized, manipulated, and read out, which allows for direct photonic integration and interconnection between nodes. However, nitrogen vacancy centers, which are solid-state defects in general, can suffer from nanoscopic differences in their crystal environment that cause deviations from their ideal properties. These deviations lead to a low yield of high-quality defects. Finding multiple defects that have the same fluorescence transition and thus emit indistinguishable photons (a necessity for quantum effects to arise) is therefore challenging. It is very difficult to fabricate a full, monolithic network in which every node has a quantum memory at the correct position and the necessary spin and spectral properties. Here, we address this problem of scalability by introducing a method for the preselection and subsequent integration of nodes into a prefabricated on-chip photonic network.
We create large arrays of diamond waveguides that contain single nitrogen vacancy quantum memories. We prescreen the waveguides and select the ones with the desired properties. Only these waveguides are then placed into a photonic circuit fabricated in silicon nitride. In this way, we ensure that every node in our quantum network contains a high-quality quantum memory that is well coupled to the photonic network. Our simulations and experiments demonstrate efficient coupling from the nitrogen vacancy memory into the photonic circuit. Using 532-nm laser excitation, we demonstrate that an individual nitrogen vacancy center is well coupled optically to the photonic circuit with more than one million photons coupled into one direction of the single-mode waveguide per second. We also use microwave radiation to control and store the nitrogen vacancy center electron state for more than 120 microseconds, showing that our integrated quantum nodes can indeed act as quantum memories.
This demonstration marks the first reported integration of long-lived quantum memories into a photonic circuit, representing another step toward building scalable quantum networks.