Thrust I: Room temperature quantum repeater

Vision: This thrust of the project will be dedicated to engineer all the available quantum hardware into unifying experiments showing the proof of principle capabilities needed to build a quantum repeater prototype. The SBU team (PI Figueroa plus two graduate students) will demonstrate the heralding of quantum memories, the storage of entanglement using several quantum memories and finally entanglement swapping experiments using four quantum memories.

Vision for Thrust I – QLAN1 Quantum Repeater Prototype: We envision finalizing a four-memory quantum repeater outperforming direct propagation at a distance of \sim 11km when operated at a wavelength of 795nm. This milestone requires a network with two rubidium tuned entangled sources, four quantum memories, and one Bell-state measurement station. All these quantum devices will be deployed in five different physical locations on the Stony Brook campus (the two QIT laboratories in the Physics Building, the ECC fiber exchange building, the Qunnect CEWIT lab and a dedicated room in the basement of the Health Science Tower).

Improvements in quantum hardware. We currently have four operational single-photon-level qubit memories. These memories will be the foundation of a first table-top quantum repeater prototype. In order to attain the performance thresholds required for quantum repeater operation, we have identified key areas of further development: a) millisecond storage times, b) heralded operation and c) fast-duty-cycle operation.

Heralded quantum memories at room temperature. Heralding a single-photon-generated spin wave increases the chances of performing entanglement swapping despite having non-unitary storage efficiencies. We propose to herald our quantum memories by combining the DV qubit detection capabilities of our current setups together with the CV homodyne state tomography of a heralding field. We envision to use the change in the quadrature and phase of a heralding field caused by the presence of a stored photon as a heralding event. The time tag of this measurement can be used to post-select the successful retrieval of the stored photon. We plan to develop the tools to analyze the CV quadrature changes in a real-time, in a shot-by-shot fashion. Not only is this scheme novel, but its application for a quantum repeater will address the remaining outstanding challenge of improving the effective entanglement generation-rate after the memories. It will also show the capabilities of the network to simultaneously transmit and detect DV and CV states.

Storage of entanglement in two memories. Using the available quantum memories and entanglement sources, we will explore the interconnection of many devices. In doing so, we will create elementary quantum networks aiming to tailor the functionality required for quantum repeater operation. The SB-improved entanglement source will be integrated into anexperiment where each photon of the entangled pair is stored in an independent quantum memory. Preservation of the entanglement will be tested after synchronizing the retrieval of the photons from the memories and verifying their non-classical correlations.

Storage of entanglement in remote locations. A parallel experiment will oversee the realization of entanglement distribution by storing entanglement in two memories located in remote locations over several kilometers apart. We have already tested a medium-distance network using several fiber links between the Stony Brook QIT laboratories and the Qunnect laboratory in the CEWIT building, approximated 11 kilometers apart. A network of optical fibers has already been placed between the aforementioned buildings and the basement of the Health Science Tower, distributing laser light and qubits. The development of such memory assisted fiber-based network will allow us to test the operational principles of entanglement storage when the entanglement has undergone significant losses in the fiber and to measure the relation between signal to background and fidelity after fiber propagation.

Deliverable of Thrust I: A working prototype of a quantum repeater at 795 nm.: Two entangled sources are located in the SBU QIT laboratory and the Engineering ECC building. Two distant portable quantum memories are located in the CEWIT and Health Science Tower buildings. A Bell state measurement station equipped with two room temperature memories is located in the SBU QIT II laboratory.

Remote control and monitoring of deployed devices. The quantum network components need to be modular and easy to operate for a long-term operational test-beds to be viable. In particular, they need to be remotely controllable and monitored. We have already pioneered this idea by controlling several remote room-temperature quantum memories directly from a web interface. The web interface allows us to monitor the device status and to control the quantum signal synchronization. As we move towards the development of quantum repeaters and more extensive quantum networks, the number of devices will increase, and the monitoring and control complexity will grow rapidly. We will upgrade the electronic control systems and software subsystems associated with the network operation, as well as designing ”plug-&-play” quantum memories. This will eventually include a move towards state-of-the-art FPGAs for fast-feedback systems and tuning, in particular for system stabilization (i.e., polarization in long fibers) and automation (i.e., noise removal at the single-photon level) as well as network status monitoring.

Quantum repeater node implementation with four memories and entanglement swapping at 795nm. Using the portable entangled source developed in BNL, the entangled source already located in SBU and the medium distance fiber networks described above, our next target will be to perform entanglement swapping experiments mediated by quantum memories. In this case, the distances to the measurement setups containing the memories and a Hong-Ou-Mandel interference experiment will not necessarily be equal, resulting in relative time delays. To show the capabilities of a fully combined setup, we will perform storage of retrieval of entangled photons propagating in four independent fiber links. The achieved coherence time of the memories will allow us to synchronize the qubits regardless of the time delay. We will perform several first-ever experiments with a quantum network of four quantum memories in a quantum repeater node configuration. Two EPR pairs will be stored in two pairs of quantum memories. Successful storage and retrieval of the two entangled states will allow preliminary experiments on entanglement swapping. Temporal shaping of the memory-retrieved photons will assure high visibility of the HOM interference and thus the Bell State measurement. Furthermore, heralding mechanisms applied to the memories will optimize the rates of successful Bell-state measurements, thus increasing the achievable entanglement swapping rates.

Distant node time synchronization. A variety of time synchronization methods will be employed. A local Stratum-1 NTP server combined with a pulsed synchronized trigger will enable high-precision synchronous time-stamping at remote locations within the same QLAN. In this case, a precise time-base distribution can be established through the fiber network using the White Rabbit (WR) network protocol, a Bridged Local Area Network that uses Ethernet to interconnect switches/nodes and the Precision Time Protocol (PTP) to synchronize them to sub-nanosecond accuracy at distances of up to 15km between elements. From the WR’s PPS/10MHz outputs, a high-frequency synchronized trigger can be derived for comparative time-of-arrival photon measurements regardless of the QLAN topology. We already employed the WR system for the entanglement characterization described above to synchronize two detection stations with good results.

For more spatially extensive configurations, such as the proposed QWANs, a different approach is necessary to extend synchronization. Similarly to cell-phone/wireless networks, GPS receivers at each are node locked onto GPS satellites, locking their internal oscillator clock to the network time-base, independently of location. This will provide a relatively accurate window of +/-10ns for photon arrival coincidences.

Success metrics: This experiment will be the first implementation of such a sophisticated quantum repeater type network and will form the test bed for further developments to achieve longer distance operation. Our target is to measure memory-assisted entanglement rates of several kHz, equivalent to the state-of-the-art systems without quantum memories.