A team from the U.S. Department of Energy’s Oak Ridge National Labora­tory, Stanford University and Purdue University developed and demons­trated a novel, fully functional quantum local area network, or QLAN, to enable real-time adjustments to infor­mation shared with geographi­cally isolated systems at ORNL using entangled photons passing through optical fiber. This network exempli­fies how experts might routinely connect quantum computers and sensors at a practical scale, thereby realizing the full potential of these next-generation techno­logies on the path toward the highly anti­cipated quantum internet. 

Local area networks that connect classical computing devices are nothing new, and QLANs have been successfully tested in tabletop studies. Quantum key distri­bution has been the most common example of quantum communications in the field thus far, but this procedure is limited because it only establishes security, not entanglement, between sites. “We’re trying to lay a foundation upon which we can build a quantum internet by under­standing critical functions, such as entangle­ment distribution bandwidth,” said Nicholas Peters, the Quantum Information Science section head at ORNL. “Our goal is to develop the funda­mental tools and building blocks we need to demons­trate quantum networking appli­cations so that they can be deployed in real networks to realize quantum advantages.”

When two photons are paired together, or entangled, they exhibit quantum corre­lations that are stronger than those possible with any classical method, regardless of the physical distance between them. These interactions enable counter­intuitive quantum communi­cations protocols that can only be achieved using quantum resources. One such protocol, remote state preparation, harnesses entanglement and classical communi­cations to encode information by measuring one half of an entangled photon pair and effectively converting the other half to the preferred quantum state. Peters led the first general experi­mental realiza­tion of remote state preparation in 2005 while earning his doctorate in physics. The team applied this technique across all the paired links in the QLAN and demons­trated the scala­bility of entanglement-based quantum communi­cations.

This approach allowed the team to link together three remote nodes, known as “Alice,” “Bob” and “Charlie” – names commonly used for fictional characters who can communicate through quantum trans­missions – located in three different research labora­tories in three separate buildings on ORNL’s campus. From the laboratory containing Alice and the photon source, the photons distributed entanglement to Bob and Charlie through ORNL’s existing fiber-optic infra­structure. Quantum networks are incompatible with amplifiers and other classical signal boosting resources, which interfere with the quantum correlations shared by entangled photons. With this potential drawback in mind, the team incor­porated flexible grid bandwidth provi­sioning, which uses wavelength-selective switches to allocate and reallocate quantum resources to network users without dis­connecting the QLAN. This technique provides a type of built-in fault tolerance through which network operators can respond to an unanticipated event, such as a broken fiber, by rerouting traffic to other areas without disrupting the network’s speed or compro­mising security protocols.

“Because the demand in a network might change over time or with different confi­gurations, you don’t want to have a system with fixed wavelength channels that always assigns particular users the same portions,” said Joseph Lukens, research scientist at ORNL. “Instead, you want the flexi­bility to provide more or less bandwidth to users on the network according to their needs.” Compared with their typical classical counterparts, quantum networks need the timing of each node’s activity to be much more closely syn­chronized. To meet this requirement, the researchers relied on GPS, the same versatile and cost-effective techno­logy that uses satellite data to provide everyday navigation services. Using a GPS antenna located in Bob’s laboratory, the team shared the signal with each node to ensure that the GPS-based clocks were synchro­nized within a few nanoseconds and that they would not drift apart during the experiment.

Having obtained precise timestamps for the arrival of entangled photons captured by photon detectors, the team sent these measurements from the QLAN to a classical network, where they compiled high-quality data from all three labora­tories. “This part of the project became a challenging classical networking experiment with very tight tolerances,” Lukens said. “Timing on a classical network rarely requires that level of precision or that much attention to detail regarding the coding and synchroni­zation between the different labora­tories.”

Without the GPS signal, the QLAN demons­tration would have generated lower quality data and lowered fidelity, a mathematical metric tied to quantum network performance that measures the distance between quantum states. The team anticipates that small upgrades to the QLAN, including adding more nodes and nesting wavelength-selective switches together, would form quantum versions of inter­connected networks – the literal defi­nition of the internet. “The internet is a large network made up of many smaller networks,” said Muneer Alshowkan, a postdoctoral research associate at ORNL who brought valuable computer science expertise to the project. “The next big step toward the development of a quantum internet is to connect the QLAN to other quantum networks.”

Addi­tionally, the team’s findings could be applied to improve other detection techniques, such as those used to seek evidence of elusive dark matter, the invisible substance thought to be the universe’s pre­dominant source of matter. “Imagine building networks of quantum sensors with the ability to see funda­mental high-energy physics effects,” Peters said. “By developing this technology, we aim to lower the sensi­tivity needed to measure those phenomena to assist in the ongoing search for dark matter and other efforts to better understand the universe.” The researchers are already planning their next experiment, which will focus on imple­menting even more advanced timing synchroni­zation methods to reduce the number of acci­dentals and further improve the QLAN’s quality of service. (Source: ORNL)

Reference: M. Alshowkan et al.: Reconfigurable Quantum Local Area Network Over Deployed Fiber, PRX Quantum 2, 040304 (2021); DOI: 10.1103/PRXQuantum.2.040304

Link: Quantum Information Science Group, Oak Ridge National Laboratory, Oak Ridge, USA