Thrust III: Increasing the operational distance of the quantum repeater prototype using telecom functionality

Vision: A successful set of experiments regarding memory-assisted entanglement swapping (Thrust I), frequency conversion and long distance entanglement preservation (Thurst II) will put us in a prime position to demonstrate a quantum repeater prototype capable of challenging direct propagation already at telecom wavelengths. The three teams will exchange quantum devices and equipped all the participating nodes with the necessary quantum hardware to attempt memory assisted entangled swapping in a network of approx. 200km in fiber distance.

Vision for Thrust III: A QWAN Quantum Repeater prototype connecting BNL and SBU}. In this envisioned configuration light from two memory-tuned high repetition rate entangled sources is distributed through the fiber connections in the ECC (SBU) and SDCC (BNL) data centers after frequency conversion to telecom. The BNL QLAN is connected to the SBU QLAN via approx. 60km long Crown Castle fiber while fiber loops at the two campuses lengthen the path to exceed 200km. We will test the distribution of entanglement over a long distance network using memory-assisted entanglement swapping at telecom wavelengths.

Connecting the SBU and BNL QLANs using telecom wavelength. We will enhance the existing quantum hardware infrastructure to equip all participant nodes. We will deploy a quantum memory in the QIST laboratory in BNL and two extra frequency converters will be added in the Stony Brook QIT II laboratory (provided by Qunnect LLC). The final configuration will have one high repetition entanglement source and two frequency converters in each the BCF (BNL) and ECC (SBU). Four quantum memories will be located in the Qunnect CEWIT laboratoy, the QIT I laboratory in SBU (together with a Bell state measurement setup) and the BNL QIST laboratory. All fiber connections will be expanded with fiber loops provided by the SBU DoIT department, ESnet and Crown Castle to form a approx. 200km fiber length network.

Dual frequency conversion/storage of light experiments. Having entangled photons tuned to telecom atomic transitions will be the basis to explore the reciprocal frequency conversion and storage of the entangled photons in the same room temperature quantum memory. The idea will be to use the the adiabatic conversion in the quantum memory controlled by a telecom laser, to map the photon to the D1 or D2 rubidium transitions. While this new field is being adiabatically created, simultaneously we will also modify the EIT control field to create a spin wave, thus storing the long distance travelled photon. Characterization of the achieved dual conversion efficiencies will be performed.

Memory-assisted entanglement swapping at telecom wavelenghts. The final stage of the network will be the implementation of quantum connections between BNL, SBU,using frequency conversion units to match telecom wavelengths. The reminder quantum light matter interfaces will be provided by Qunnect, the spin off company of the SBU QIT laboratory. Sharing entanglement between remote quantum memories will be the basis to explore the performance of the instrument in a quantum repeater node configuration. Here two Einstein-Podolsky-Rosen photon pairs traverse the long distance fiber links and reach quantum memories for storage. Photon production will be repeated until heralding of the capture of photons in all the memories is received. In SBU there will be a BSM setup based upon Hong-Ou-Mandel interference. After the heraldings are received, the memories in the middle of the configuration will be read and sent to the BSM unit. Successful detection of a Bell state after storage will herald the swapping of entanglement among the most distant quantum memories in nodes located at CEWIT and BNL. Preservation of the entanglement will be tested after synchronizing the retrieval of the photons from the memories at CEWIT and BNL and verifying their non-classical correlations.

Storage of CV squeezed light in quantum memories after fiber propagation. Having the infrastructure to transfer and store telecom wavelength photons after long distance fiber propagation will provide the backbone to test the performance of the quantum repeater infrastructure for CV quantum light. As mentioned above, it is straightforward to convert the entangled sources into squeezed light sources, and having the frequency conversion and storage capabilities will allow us to test if CV quantum interference and CV entanglement swapping operations can be performed efficiently, using the the same network infrastructure and room temperature quantum devices. Homodyne detectors, quantum state tomography, and quantum process tomography techniques can then be used to characterize the performance of the CV entanglement swapping operations.

Deliverable of Thrust III: Infrastructure for a Quantum Repeater Network across Long Island}: Combining the individual QLANs at BNL and SBU/CEWIT with a long connection to Garden City establishes the necessary infrastructure to conduct true long distance entanglement swapping experiments at telecom wavelength, a key milestone in the development of a functioning quantum repeater.

Metrics of success: In this part of the proposal the main milestone will be the deployment of all necessary quantum devices at all required locations. Achieving interconnections between all the nodes using frequency conversion and telecom entanglement will be considered a major success. Performing entanglement swapping with entangled photons or CV squeezing covering the long distances will be a major breakthrough.