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Record entanglement of quantum memories + Quantum flute manipulates photons

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Record entanglement of quantum memories is an important step to the quantum internet
https://www.lmu.de/en/newsroom/news-over...ories.html

RELEASE: Researchers from LMU and Saarland University have entangled two quantum memories over a 33-kilometer-long fiber optic connection -- a record and an important step toward the quantum internet.

A network in which data transmission is perfectly secure against hacking? If physicists have their way, this will one day become a reality with the help of the quantum mechanical phenomenon known as entanglement. For entangled particles, the rule is: If you measure the state of one of the particles, then you automatically know the state of the other. It makes no difference how far away the entangled particles are from each other. This is an ideal state of affairs for transmitting information over long distances in a way that renders eavesdropping impossible.

A team led by physicists Prof. Harald Weinfurter from LMU and Prof. Christoph Becher from Saarland University have now coupled two atomic quantum memories over a 33-kilometer-long fiber optic connection. This is the longest distance so far that anyone has ever managed entanglement via a telecom fiber. The quantum mechanical entanglement is mediated via photons emitted by the two quantum memories. A decisive step was the researchers' shifting of the wavelength of the emitted light particles to a value that is used for conventional telecommunications. "By doing this, we were able to significantly reduce the loss of photons and create entangled quantum memories even over long distances of fiber optic cable," says Weinfurter.

Generally speaking, quantum networks consist of nodes of individual quantum memories -- such as atoms, ions, or defects in crystal lattices. These nodes are able to receive, store, and transmit quantum states. Mediation between the nodes can be accomplished using light particles that are exchanged either over the air or in a targeted manner via fiber optic connection. For their experiment, the researchers use a system comprised of two optically trapped rubidium atoms in two laboratories on the LMU campus. The two locations are connected via a 700-meter-long fiber optic cable, which runs underneath Geschwister Scholl Square in front of the main building of the university. By adding extra fibers on coils, connections of up to 33 kilometers in length can be achieved.

A laser pulse excites the atoms, after which they spontaneously fall back into their ground state, each thereby emitting a photon. Due to the conservation of angular momentum, the spin of the atom 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 optic cable to a receiver station, where a joint measurement of the photons indicates an entanglement of the quantum memories.

However, most quantum memories emit light with wavelengths in the visible or near-infrared range. "In fiber optics, these photons make it just a few kilometers before they are lost," explains Christoph Becher. For this reason, 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 around 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 accomplished the conversion with an unprecedented efficiency of 57 percent. At the same time, they managed to preserve the quality of the information stored in the photons to a high degree, which is a condition of quantum coupling.

"The significance of our experiment is that we actually entangle two stationary particles -- that is to say, atoms that function as quantum memories," says Tim van Leent, lead author of the paper. "This is much more difficult than entangling photons, but it opens up many more application possibilities." The researchers think that the system they developed could be used to construct large-scale quantum networks and for the implementation of secure quantum communication protocols. "The experiment is an important step on the path to the quantum internet based on existing fiber optic infrastructure," says Harald Weinfurter.

PAPER: http://dx.doi.org/10.1038/s41586-022-04764-4


Scientists invent 'quantum flute' that can make particles of light move together
https://news.uchicago.edu/story/uchicago...e-together

RELEASE: University of Chicago physicists have invented a "quantum flute" that, like the Pied Piper, can coerce particles of light to move together in a way that's never been seen before.

Described in two studies published in Physical Review Letters and Nature Physics, the breakthrough could point the way towards realizing quantum memories or new forms of error correction in quantum computers, and observing quantum phenomena that cannot be seen in nature.

Assoc. Prof. David Schuster's lab works on quantum bits -- the quantum equivalent of a computer bit -- which tap the strange properties of particles at the atomic and sub-atomic level to do things that are otherwise impossible. In this experiment, they were working with particles of light, known as photons, in the microwave spectrum.

The system they devised consists of a long cavity made in a single block of metal, designed to trap photons at microwave frequencies. The cavity is made by drilling offset holes -- like holes in a flute.

"Just like in the musical instrument," Schuster said, "you can send one or several wavelengths of photons across the whole thing, and each wavelength creates a 'note' that can be used to encode quantum information." The researchers can then control the interactions of the "notes" using a master quantum bit, a superconducting electrical circuit.

But their oddest discovery was the way the photons behaved together.

In nature, photons hardly ever interact -- they simply pass through each other. With painstaking preparation, scientists can sometimes prompt two photons to react to each other's presence.

"Here we do something even weirder," Schuster said. "At first the photons don't interact at all, but when the total energy in the system reaches a tipping point, all of a sudden, they're all talking to each other."

To have so many photons "talking" to one another in a lab experiment is extremely strange, akin to seeing a cat walking on hind legs.

"Normally, most particle interactions are one-on-one -- two particles bouncing or attracting each other," Schuster said. "If you add a third, they're usually still interacting sequentially with one or the other. But this system has them all interacting at the same time."

Their experiments only tested up to five "notes" at a time, but the scientists could eventually imagine running hundreds or thousands of notes through a single qubit to control them. With an operation as complex as a quantum computer, engineers want to simplify everywhere they can, Schuster said: "If you wanted to build a quantum computer with 1,000 bits and you could control all of them through a single bit, that would be incredibly valuable."

The researchers are also excited about the behavior itself. No one has observed anything like these interactions in nature, so the researchers also hope the discovery can be useful for simulating complex physical phenomena that can't even be seen here on Earth, including perhaps even some of the physics of black holes.

Beyond that, the experiments are just fun.

"Normally quantum interactions take place over length and time scales too small or fast to see . In our system, we can measure single photons in any of our notes, and watch the effect of the interaction as it happens. It's really quite neat to 'see' a quantum interaction with your eye," said UChicago postdoctoral researcher Srivatsan Chakram, the co-first author on the paper, now an assistant professor at Rutgers University.
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