The Alice laboratory houses the quantum equipment, which includes the photon source as well as the first node of the teams network. Credit: Carlos Jones/ORNL, U.S. Dept. Energy
The U.S. Department of Energy, Stanford University, and Purdue University created and demonstrated a fully functional quantum local network (or QLAN) that allows real-time adjustments of information with ORNL's geographically isolated systems. It uses entangled photons through optical fiber.
This network shows how experts could connect quantum computers to sensors on a practical scale. In this way, they can realize the full potential of next-generation technologies that are leading to the anticipated quantum internet. These results are the result of years of research.
QLANs, which connect classic computing devices, are not new. They have been tested successfully in tabletop studies. Although quantum key distribution is the most well-known example of quantum communication, it has limited utility as it does not establish entanglement between sites.
Nicholas Peters, Quantum Information Science section chief at ORNL, stated that "we're trying to lay the foundation upon which we can create a quantum Internet by understanding critical functions such as entanglement distributor bandwidth." "Our goal is the development of the basic tools and building blocks necessary to demonstrate quantum networking applications in real networks so they can realize quantum benefits," said Nicholas Peters, head of Quantum Information Science section at ORNL.
Two photonsparticles light are entangled when they are paired together. This creates quantum correlations stronger than any other classical method regardless of their physical distance. These interactions allow for counterintuitive quantum communication protocols that are only possible with quantum resources.
Remote state preparation is one such protocol. It uses entanglement as well as classical communications to encode information. This involves measuring half of an entangled pair of photons and then converting the remaining half to the desired quantum state. Peters earned his doctorate in Physics while leading the first experimental realization of remote-state preparation. This technique was applied to all paired links of the QLAN, a feat never before achieved on a network. It also demonstrated the scaleability of entanglement based quantum communications.
This allowed the team to connect three remote nodes, named "Alice", "Bob", and "Charlie". These names are commonly used to refer to fictional characters that can communicate via quantum transmissions. They were located in three different buildings on ORNL's campus in three different research labs. The photons were distributed to Bob and Charlie by ORNL's fiber-optic infrastructure, starting from the laboratory that contained Alice.
Quantum networks can't be used with amplifiers or other signal boosting resources that interfere with quantum correlations between entangled photons. This potential problem was considered by the team. They implemented flexible grid bandwidth provisioning which uses wavelength-selective switches to reallocate and reallocate quantum resources to network customers without disconnecting QLAN. This technology provides network operators with a built-in fault tolerance that allows them to respond to unanticipated events such as a broken fiber by routing traffic to other areas, without compromising security protocols or slowing down the network's speed.
Joseph Lukens (a Wigner Fellow, research scientist at ORNL, and the team's expert in electrical engineering), stated that "the demand in a network might vary over time or with different configurations." Instead, you want to be able to offer more or less bandwidth to users according to their requirements.
Quantum networks require that the timing of each node’s activity is more closely synchronized than their classical counterparts. GPS was used by the researchers to meet this requirement. It is a versatile and affordable technology that uses satellite data for everyday navigation services. The team used a GPS antenna in Bob's lab to share the signal with each node. This ensured that the GPS-based clocks synchronized within a few seconds and did not drift during the experiment.
The team obtained exact timestamps to determine when entangled photons were captured by photon detectors. They then sent the measurements from the QLAN into a classic network where they compiled high quality data from all three labs.
Lukens stated that this part of the project was a complex classical networking experiment with tight tolerances. "Timing a classical network requires a lot of precision and attention regarding the coding and synchronization.
The QLAN demonstration without the GPS signal would have produced lower quality data and lowered reliability. This mathematical metric is tied to quantum network performance, which measures distance between quantum states.
The team believes that minor upgrades to QLAN would create quantum versions of interconnected networks, which could include nesting wavelength-selective switch pairs together and adding more nodes.
Muneer Alshowkan (a postdoctoral researcher associate at ORNL) said that the internet is a network of many smaller networks. He also brought valuable computer science expertise. The next step towards the development of a quantum Internet is to connect QLAN with other quantum networks.
The team's findings can also be used to improve other detection methods, such as those used for finding evidence of elusive darkness, an invisible substance believed to be the main source of matter in the universe.
Peters stated, "Imagine building quantum sensors that can see fundamental high-energy effects in physics." "By developing this technology we aim to lower sensitivity to measure these phenomena in order to aid in the ongoing search to find dark matter and other efforts that help us better understand the universe.
Researchers are already planning the next experiment. This will be focused on developing more advanced timing sync methods to reduce accidental noise sources in the network and further improve QLAN's service quality.
Continue exploring A quantum internet is closer than ever, thanks to this switch
Muneer Alshowkan and colleagues, Reconfigurable Quantum Local Area Network over Deployed Fiber. PRX Quantum (2021). Muneer Alshowkan et al, Reconfigurable Quantum Local Area Network Over Deployed Fiber.(2021). DOI: 10.1103/PRXQuantum.2.040304
