Generation of long-distance matter-matter entanglement
Our partnership with BNL has recently demonstrated a milestone experiment, the longest (~158 km) quantum communication between quantum light-matter interfaces using telecom qubits. We have observed Hong-Ou-Mandel (HOM) interference between indistinguishable telecom photons produced in two independent room temperature quantum memories, separated by a distance of 158 km. We obtained an interference visibility after long-distance propagation of 49% for single-photon level experimental input. For this elementary quantum network prototype to evolve into a large-scale memory-assisted quantum-repeater-based network, further enhancements are necessary, particularly regarding quantum hardware improvements and the addition of highly non-linear atomic systems to provide the aforementioned on-demand capabilities to the entanglement generation and distribution.
Overview of part of the testbed on Long Island, New York. It consists of 4 commercially available fibers connecting the Quantum Internet Laboratory (QIT) in the Physics Building in SBU to the Quantum Information Science and Technology Laboratory (QIST) in the Instrumentation Building in BNL. Two quantum memories (Alice and Bob) are located in the SBU QIT laboratory and are connected independently to the network. The interference setup and telecom compatible single photon nano-wire detectors (Charlie station) are located in the QIST laboratory in BNL. The other two fibers are used to transport classical timing triggers and sequencing information.
The BNL-SBU matter-matter entanglement generation quantum network is controlled by purpose-built classical data and control planes. Each node includes a timing switch that supports the White Rabbit protocol. This system has been used to achieve ns-level timing synchronization between the two campuses. Additionally, we use a hardware controller for quantum device triggering and network orchestration, and a software-defined node controller. Two fiber strands are used for carrying all necessary classical signals for data, timing, and orchestration. These fibers are connected to DWDM systems at each end that multiplex/demultiplex several C-band channels. The DWDM systems include transponders, preamplifiers, booster amplifiers, and dispersion compensation modules to enable reliable transmission of signals over the 70km fiber span. Additionally, Filter Wavelength Division Multiplexers (FWDMs) are used at each end to inject O-band signals into the fibers along with C-band signals. The classical side of the network has one standard switch at each location used to interconnect all quantum node control devices within a local classical network connecting both campuses. Additional attenuated laser signals in the O-band range are used to perform polarization stabilization on both quantum channels via a feedback loop that matches the polarization axes at the QIS creation and measurement stations.
The current hybrid classical/quantum BNL-SBU testbed, a first example of a classically controlled quantum network. The BNL and SBU campuses are connected with 4 fiber strands, 2 are used for quantum signals and 2 for multiplexed classical data, timing, and control signals using DWDM and FWDM systems. At SBU, the Alice and Bob stations host qubit generators, while the Charlie station at BNL hosts single photon superconducting nanowire detectors and measurement devices, such as a BSM device. All nodes include White Rabbit timing switches, servers hosting node controlling software, and control electronics for driving the quantum devices.
In this QN, the operations are orchestrated using classical control. We have built quantum nodes encompassing one or more quantum devices. Each quantum node is a conglomerate of a set of other devices with specific functionality and distinct roles in the overall operation. A subset of these devices is controlled via electronic devices, such as signal, delay, and pulse generators that are equipped with network interfaces and thus can be configured, managed, and monitored over the classical network. They are precision-driven by sequences of triggering signals generated by a device trigger controller, a device that directs the operation of the node (also called an orchestrator). In turn, the trigger sequences drive optical hardware, such as electro-optical and acousto-optical modulators, as well as various lasers, that operate the quantum devices. The orchestrator device is also connected to the classical network with a dual interface, standard data connection for management, and a White Rabbit timing connection that can provide sub-nanosecond precision. At the core of the intra-node network of the quantum node is a classical data switch to which all devices with a network interface are connected to. Finally, a server or embedded system hosts the software-defined node controller that supervises the operation of the node.
Schematic of the internal network of a quantum node containing a configurable quantum memory. The intranode network includes a software-defined node controller, classical switch, a White Rabbit timing switch, an orchestrator (device trigger controller) and a QNIC. The controlling electronics are connected to the classical network for configuration, management, and monitoring and are driven by the orchestration which is synchronized with the rest of the network through the timing switch. The QNIC electronics interface with the electronics of the photon source and the orchestrator, while the two outputs of the photon source are connected to the optical combiner part of the QNIC where quantum signals are combined with classical optical analog signals that need to co-propagate with the quantum signals in the same band (O-band). The (classical) NIC part of the QNIC is also connected to the classical switch for control communications. The node has 4 optical connections to the world, one for data, one for timing, and two for quantum signals. The signals can be further multiplexed and routed to other nodes using optical switches.