Scheme of the experimental setup. In each node, located in buildings 400 m apart, there is a single 87Rb atom is charged in an optical dipole trap. Both atoms are excited synchronously to the 5 . state2p3/2|F′=0, mf’= 0⟩ to generate entanglement of atoms and photons in the subsequent spontaneous decay. The single photons emitted at a wavelength of 780 nm are collected using high numerical aperture objectives and coupled to single-mode fibers leading to the QFC devices. There they are converted to telecom wavelength ( = 1.517 nm) by difference frequency generation in a periodically poled lithium niobate (PPLN) waveguide located in a Sagnac interferometer-type arrangement, such a configuration fully maintains the polarization quantum state of the photon. The converted photons are conducted via fiber optic links up to 16.5 km long to a middle station, where the entanglement is converted to the atoms by a BSM. After successfully generating atom-atom entanglement, the atoms are independently analyzed by a readout pulse whose polarization, set by a half-wave plate (HWP) and quarter-wave plate (QWP), defines the measurement setting. PC, polarization controllers. Credit: Nature(2022). DOI: 10.1038/s41586-022-04764-4
A network in which data transmission is perfectly protected against hacking? If physicists have their way, this will one day become a reality using the quantum mechanical phenomenon known as entanglement. For entangled particles, the rule is: if you measure the state of one of the particles, you automatically know the state of the other. It doesn’t matter how far apart the entangled particles are. This is an ideal state of affairs for transmitting information over long distances in a way that makes eavesdropping impossible.
A team led by physicists Prof. Harald Weinfurter from LMU and Prof. Christoph Becher from Saarland University now has two quantum memories over a 33 kilometer long fiber optic connection. This is the longest distance to date anyone has ever managed to entangle over a telecom fiber.
The quantum mechanical entanglement is mediated through photons emitted from the two quantum memories. A decisive step was the shifting of the wavelength of the emitted light particles by the researchers to a value that is used for conventional telecommunications. “By doing this, we were able to significantly reduce photon loss and create entangled quantum memories even over long distances of fiber optic cable,” Weinfurter says.
In general, quantum networks consist of nodes of individual quantum memories, such as atoms, ions or defects in crystal lattices. These nodes can receive, store and transmit quantum states. Mediation between the nodes can be done with light particles that are exchanged either via the air or in a targeted manner via a fiber optic connection. For their experiment, the researchers use a system consisting of two optically confined rubidium atoms in two labs on the LMU campus. The two locations are connected via a 700-metre long fiber optic cable, which runs under Geschwister Schollplein in front of the main university building. By adding extra fibers on spools, connections up to 33 kilometers in length can be realized.
A laser pulse excites the atoms, after which they spontaneously fall back to their ground state, each emitting a photon. Due to the conservation of angular momentum, the atom’s spin is entangled with the polarization of its emitted photon† These light particles can then be used to create a quantum mechanical coupling of the two atoms. To do this, the scientists sent them through the fiber optical cable to a receiving station, where a joint measurement of the photons indicates an entanglement of the quantum memories.
Credit: Jan Greune
However, most quantum memories emit light with wavelengths in the visible or near infrared range. “In fiber optics, these photons only make it a few kilometers before they are lost,” explains Christoph Becher. Therefore, the physicist from Saarbrücken and his team optimized the wavelength of the photons for their journey in the cable. Using two quantum frequency converters, they increased the original wavelength from 780 nanometers to a wavelength of 1,517 nanometers.
“This is close to the so-called telecom wavelength of about 1,550 nanometers,” says Becher. The telecom band is the frequency range in which the transmission of light in fiber optics has the lowest losses. Becher’s team achieved the conversion with an unprecedented efficiency of 57%. At the same time, they managed to maintain a high degree of quality of the information stored in the photons, which is a prerequisite for quantum coupling.
“The significance of our experiment is that we are actually entangled with two stationary particles, i.e. atoms that function as quantum memories,” said Tim van Leent, lead author of the paper published in Nature † “This is much more difficult than photon entanglement, but it offers a lot more application possibilities.”
The researchers believe that the system they have developed can be used to build large-scale quantum networks and to implement secure quantum communication protocols. “The experiment is an important step towards the quantum internet based on existing fiber optic infrastructure,” says Harald Weinfurter.
Tim van Leent et al, Single atom entanglement over 33 km telecom fiber, Nature(2022). DOI: 10.1038/s41586-022-04764-4
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Quote: Researchers achieve record entanglement of quantum memories (2022, July 7) retrieved July 7, 2022 from https://phys.org/news/2022-07-entanglement-quantum-memories.html
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