Scientists' increasing mastery of quantum mechanics is heralding a new age
of innovation. Technologies that harness the power of nature's most minute
scale show enormous potential across the scientific spectrum, from computers
exponentially more powerful than today's leading systems, sensors capable of
detecting elusive dark matter, and a virtually unhackable quantum internet.
Researchers at the Department of Energy's Oak Ridge National Laboratory,
Freedom Photonics and Purdue University have made strides toward a fully
quantum internet by designing and demonstrating the first ever Bell state
analyzer for frequency bin coding.
Their findings were published in Optica.
Before information can be sent over a quantum network, it must first be
encoded into a quantum state. This information is contained in qubits, or
the quantum version of classical computing "bits" used to store information,
that become entangled, meaning they reside in a state in which they cannot
be described independently of one another.
Entanglement between two qubits is considered maximized when the qubits are
said to be in "Bell states."
Measuring these Bell states is critical to performing many of the protocols
necessary to perform quantum communication and distribute entanglement
across a quantum network. And while these measurements have been done for
many years, the team's method represents the first Bell state analyzer
developed specifically for frequency bin coding, a quantum communications
method that harnesses single photons residing in two different frequencies
simultaneously.
"Measuring these Bell states is fundamental to quantum communications," said
ORNL research scientist, Wigner Fellow and team member Joseph Lukens. "To
achieve things such as teleportation and entanglement swapping, you need a
Bell state analyzer."
Teleportation is the act of sending information from one party to another
across a significant physical distance, and entanglement swapping refers to
the ability to entangle previously unentangled qubit pairs.
"Imagine you have two quantum computers that are connected through a
fiber-optic network," Lukens said. "Because of their spatial separation,
they can't interact with each other on their own.
"However, suppose they can each be entangled with a single photon locally.
By sending these two photons down optical fiber and then performing a Bell
state measurement on them where they meet, the end result will be that the
two distant quantum computers are now entangled—even though they never
interacted. This so-called entanglement swapping is a critical capability
for building complex quantum networks."
While there are four total Bell states, the analyzer can only distinguish
between two at any given time. But that's fine, as measuring the other two
states would require adding immense complexity that is so far unnecessary.
The analyzer was designed with simulations and has demonstrated 98%
fidelity; the remaining two percent error rate is the result of unavoidable
noise from the random preparation of the test photons, and not the analyzer
itself, said Lukens. This incredible accuracy enables the fundamental
communication protocols necessary for frequency bins, a previous focus of
Lukens' research.
In the fall of 2020, Lukens and colleagues at Purdue first showed how single
frequency-bin qubits can be fully controlled as needed to transfer
information over a quantum network.
Using a technology developed at ORNL known as a quantum frequency processor,
the researchers demonstrated widely applicable quantum gates, or the logical
operations necessary for performing quantum communications protocols. In
these protocols, researchers need to be able to manipulate photons in a
user-defined way, often in response to measurements performed on particles
elsewhere in the network.
Whereas the traditional operations used in classical computers and
communications technologies, such as AND/OR, operate on digital zeros and
ones individually, quantum gates operate on simultaneous superpositions of
zeros and ones, keeping the quantum information protected as it passes
through, a phenomenon required to realize true quantum networking.
While frequency encoding and entanglement appear in many systems and are
naturally compatible with fiber optics, using these phenomena to perform
data manipulation and processing operations has traditionally proven
difficult.
With the Bell state analyzer completed, Lukens and colleagues are looking to
expand to a complete entanglement swapping experiment, which would be the
first of its kind in frequency encoding. This work is planned as part of
ORNL's Quantum-Accelerated Internet Testbed project, recently awarded by
DOE.
Reference:
Navin B. Lingaraju et al, Bell state analyzer for spectrally distinct
photons, Optica (2022).
DOI: 10.1364/OPTICA.443302
Tags:
Physics