The ability to distribute entangled photons over long distances in both free space and through optical fiber remains challenging. Even though methods are in development to extend that distance by preparing atom-photon entangled states in remote nodes and sending them to an intermediate node for interference, entanglement of stationary qubits (whether using single atoms, quantum dots, trapped ions, or other quantum ensembles) has only reached a maximum fiber distance of 1.7 km between two physically separated nodes.
Recognizing that quantum memories—the equivalent of computer memory where instead of 1s and 0s, entangled qubits are the information stream—are a crucial element in quantum information systems, Chinese researchers have been able to extend quantum memory entanglement over standard optical fibers to 22 km using two-photon interference techniques (and even to 50 km using single-photon interference).1 The breakthrough was a collaborative effort between researchers from the University of Science and Technology of China (USTC; Hefei, China), the Jinan Institute of Quantum Technology (Jinan, China), and the Shanghai Institute of Microsystem and Information Technology (SIMIT) at the Chinese Academy of Sciences (Shanghai, China).
Using standard telecommunications components
In addition to increasing memory entanglement between nodes by a factor of 13, the researchers used difference frequency generation (DFG) in a lithium niobate waveguide to shift the near-infrared photons to the telecommunications O band (centered at 1342 nm) to enable low-loss transmission in standard optical fibers—a welcome step in the translation of this technology to real-world, scalable communications networks.
In the experimental setup, optical fibers link quantum memory nodes A and B to a measurement station in the middle (see figure). An atomic ensemble in each node is formed by a cloud of about 108 atoms trapped and cooled by lasers in a ring cavity. Using a weak “write” signal on the cloud, an extracted single photon is entangled with each of the cloud ensembles and converted (using DFG) for transport via standard optical fiber. The two extracted photons are then sent to the central measurement station for entanglement swapping. The joint measurement of the photons swaps entanglement to the two atomic ensembles.
Conversion to the telecommunications O band for the 795 nm write-out photons is accomplished through the nonlinear waveguide chip (with two integrated waveguides) and a 1950 nm pump laser. The extracted photons are interfered at the measurement module inside a beamsplitter and detected by two superconducting nanowire single-photon detectors (SNSPDs) with 50% efficiency and 100 Hz dark-count rate, heralding two entangled ensembles.
Beyond the laboratory experiment, the researchers next use two-photon interference to transmit photons from each node A and B (at USTC) along 11 km of fiber to a measurement site at Hefei Software Park. Despite the 8 dB of signal attenuation along the combined 22 km route, memory entanglement is successfully observed through a Bell state measurement. In addition, a single-photon interference method is used to entangle quantum memories in a 50 km fiber coil, resulting in a much higher probability of entanglement creation.
In summary, the quantum memory entanglement experiment at either 22 km or 50 km not only has a higher entanglement creation probability than prior experiments at only 1.7 km node separation, it also exhibits a shorter entanglement creation time (150 and 0.65 seconds, respectively, compared to 1300 seconds) with better quantum link efficiency.
The authors say that their experiment could be extended to nodes physically separated by similar distances, thus forming a functional segment of an atomic quantum network and paving the way towards establishing atomic entanglement over many nodes and over much longer distances.
REFERENCE
1. Y. Yu et al., Nature, 578, 240–245 (2020).
