Quantum Networks

In the near future, advanced quantum communication and networking technologies driven by quantum information processing will revolutionize traditional communication methods. Quantum information technology generally deals with the storage, transmission and processing of information using quantum-mechanical systems. Quantum communication also allows the transmission of sensitive information, such as cryptography keys, in a way that protects its confidentiality. Any attempt by an eavesdropper to listen-in on the exchange will be detected by the recipient. Where transmission over longer distance is required, measures must be taken to counteract the unavoidable losses of the transmitted signal. Traditionally a signal repeater receives, amplifies, and then forwards the signal. However, for quantum systems each measurement results in a change to the photons, with the result that the same fundamental principle which protects quantum communication from listening in also prevents the traditional amplification of the signal. Instead, the role of amplifiers are fulfilled by quantum repeaters which work with entangled photon pairs. Without the latter, quantum communication connections are limited to distances of at most _ 200km, hindering the possibility of a true quantum network.

Quantum Key Distribution


As soon as the first quantum computers become available, today’s protected information will no longer be secure without using quantum methods for encryption, such as quantum key distribution (QKD), currently considered to be the most powerful data encryption scheme ever developed. In QKD two parties use single photons that are randomly polarized to states representing ones and zeroes to transmit a series of random sequences that are used as cryptographic keys. This sequence of numbers becomes a quantum key to lock (or unlock) encrypted messages. Because the transmitted photons cannot be intercepted without being destroyed, the act of interception tips off the message receiver. This is a consequence of the uncertainty principle, around which quantum mechanics as well as quantum key distribution algorithms are based. Researchers have only been able to transmit quantum keys through short-distance optical fibers. While these distances have been useful to create small-scale QKD networks connecting closely-spaced bank branches or localized government offices, these systems fail at greater distances due to photons being absorbed by normal optical fiber. Practically, the rate at which information can be communicated securely decays almost exponentially with the distance over which the communication occurs. However, this loss can be mitigated by the use of quantum memory assisted schemes.

Introduction to room temperature quantum repeaters

Novel ideas have been proposed to combine individual quantum systems to serve as building blocks of a quantum communication network. The technological cornerstone enabling such realization is the quantum repeater, as it provides a pathway to overcome the non-cloning theorem restrictions. Despite numerous proposals regarding quantum repeater architectures an experiment connecting several quantum devices in a quantum repeater configuration remains an extraordinary challenge. Most of the experimental progress has been targeted to realize the DLCZ proposal, which is based upon a low repetition rate probabilistic scheme to generate entanglement. Recent studies have demonstrated that fast room temperature quantum memories can be used as an alternative strategy to circumvent this restriction. Further advancement along this line will have an enormous impact on the field towards building a quantum repeater, as warm vapor alleviates the need for laser trapping and cooling in vacuum or cooling to cryogenic temperatures. This will lead to inexpensive and commercialization-friendly designs that substantially reduce the cost of many-device quantum networks.

Technological building blocks of a quantum repeater node.

Our goal is to create a prototype of a quantum repeater node that uses high-duty-cycle room-temperature quantum memories in combination with high-rate polarization entanglement sources.