This image shows Joseph Lukens performing experiments. (Image Credit: Jason Richards/ORNL, U.S. Dept. of Energy)
Thanks to quantum technology advancements, we're witnessing innovations masterminded by scientists, from computers more powerful than existing systems to sensors detecting dark matter and an unhackable quantum internet. Researchers from the Department of Energy's Oak Ridge National Laboratory and Purdue University developed a frequency bin coding Bell state analyzer, progressing toward a fully operational quantum internet.
Data must first be encoded into a quantum state before it transmits over a quantum network. This data is held in qubits, which remain in a state where they cannot be described independently of one another. Maximized entanglement between two qubits occurs whenever they are in a bell state.
These bell states need to be measured so that protocols can provide quantum communication and transmit entanglement across a quantum network. Although teams have performed measurements for many years, the researchers' technique is the first-ever Bell state analyzer designed for frequency bin coding.
"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."
"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 [an] 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."
The analyzer can measure two Bell states, which is sufficient. Measuring the other two states would require more complex implementations on the analyzer. The team designed the analyzer, which has shown 98% fidelity, using simulations. Meanwhile, the other 2% isn't sourced from the analyzer. Instead, it comes from unavoidable noise due to random test photon preparations. The high accuracy rate enables the required communication protocols for frequency bins.
The team is now exploring ways to perform complete entanglement swapping experiments with the analyzer, which could be the first in frequency encoding.
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