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. The team's results, which are published
in PRX Quantum, mark the culmination of years of related research.
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—particles of light—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—a feat not previously accomplished on a network—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, a Wigner Fellow and research scientist at ORNL as well as the
team's electrical engineering expert. "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—the sources of noise in the network—and further
improve the QLAN's quality of service.
Reference:
Muneer Alshowkan et al, Reconfigurable Quantum Local Area Network Over
Deployed Fiber. PRX Quantum (2021).
DOI: 10.1103/PRXQuantum.2.040304
Tags:
Physics