Quantum networks promise to be an option for creating an ultra-secure future. Quantum key distribution (QKD) is an integral part of quantum communication that is utilized for security when exchanging information.
To date, researchers have constantly been devising methods to realize a scalable and efficient quantum key distribution network.
The latest experiment, by the researchers at Nanjing University, focused on achieving optimal time-bin Bell state measurement and enhancing the key rate in a measurement-device-independent quantum key distribution (MDI QKD) network.
Niobium nitride (NbN) nanowire is employed to detect single photons in superconducting single-photon detectors. Image used courtesy of Pernice et al
Integrated quantum photonic devices include integrated single-photon detectors such as superconducting nanowire single-photon detectors (SNSPDs).
What are SNSPDs? And, how might these detectors be helpful to achieve optimal time-bin Bell state measurement for time-bin qubits in fiber-based quantum communication?
Quantum Photonic Integrated Devices
Past experiments on integrated quantum photonic technology have achieved notable milestones in advancing robust and efficient quantum photonic systems.
The combination of wafer-scale fabrication technologies and single photons of light sets up an integrated quantum photonic (IQP) technology. The IQP technology includes integrated quantum photonic platforms, integrated single-photon sources, and integrated single-photon detectors.
The IQP technology finds applications in quantum computing and communication. It enables a scalable phase-stable quantum circuitry with robust functionalities such as single-photon detection, quantum state manipulations, and entangled state generation.
Schematic of a star-like MDI QKD network. Image used courtesy of Zheng et al
IQP platforms made up of silicon nitride (Si3Ni4) often serve as optical waveguides to route photons emitted by integrated single-photon sources such as a parametric photon-pair source. However, integrated single-photon detectors, like the superconducting nanowire single-photon detectors and avalanche photodiodes, can provide the means to absorb the photon and get the quantum information.
With the IQP technology, it seems promising that researchers would be able to solve challenges that are above the limits of classical or traditional technologies.
Common Photon Detectors in Quantum Networks
A suitable photon detector in a quantum cryptography system that researchers have always employed is the superconducting nanowire single-photon detector first fabricated in 2001.
This superconducting detector is made with niobium nitride (NbN), works at cryogenic temperatures, and provides:
- A high quantum efficiency
- Low jitter
- Good timing resolution
- Low dark count rates in quantum key distribution networks
Similar to the superconducting nanowire single-photon detector as a single-photon detector, the avalanche photodiode has a low performance and lesser wavelength sensitivity in contrast with the superconducting nanowire single-photon detector.
More Solutions on Quantum Network Problems
In their latest experiment, the researchers at Nanjing University in China exploited IQP technology. In doing so, they combined a chip based on silicon photonics and a superconducting nanowire single-photon detector to achieve optimal time-bin Bell state measurement to enhance secure key rate in quantum communication.
Generally, low dead time of single-photon detection is hard to achieve in traditional superconducting nanowire single-photon detectors due to the shorter nanowire length. This challenge, in turn, leads to lower detection efficiency.
An MDI QKD that network employs a central node (Charlie) to perform a Bell state measurement. Image [modified] used courtesy of Zheng et al
The researchers solved this challenge by utilizing the evanescent coupling between the optical waveguides and superconducting nanowire. This result obtained a high on-chip detection efficiency and reduced the dead time of single-photon detection by more than an order of magnitude.
Researchers marked a big milestone with the obtained results (high on-chip detection efficiency and low dead time of single-photon detection). For the first time, the researchers were able to realize optimal Bell-state measurement of time-bin encoded qubits in fiber-based quantum communication.
The researchers, however, remarked that a more advanced waveguide-integrated superconducting nanowire single-photon detector promises an improvement in detection efficiency.
Interested in other photonics research? Find out more in the articles down below.
Integrated Laser-on-Silicon Photonics Gets a Boost from DARPA
Solving Quantum Challenges with Photonic Quantum Chips
Purdue’s Magnetic-free Optical Isolator Aims to Push Photonic ICs Forward
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