When the predecessor of the Internet today delivered its first message in 1969, clunky and functional classic computers already existed. For decades.. Today, physicists are designing a whole new early thread on the Internet to move and manipulate a radically different type of information: qubits, or “qubits.” And this time, we are not waiting for the corresponding computer to exist first.
The two teams have demonstrated an ensemble of technologies that are essential for building the backbone of such networks, a device known as a quantum repeater. For the first time, researchers have succeeded in using light particles to combine two crystals tens of meters apart into a single quantum mechanical system and verify the connection in a simple way.Experiments foresee a future in which institutions on Earth can take advantage of a strange type of connection called Entanglement..
“This is certainly a new step in quantum repeater applications,” said Julian Laurat, a physicist at the University of Sorbonne in France, who was not involved in the study.
Store light in matter
One of the pillars of quantum information technology is the qubit, a system (like particles) that exists in a combination of two states called “superposition”. The rich behavior of cubits compared to classic bits (which can only exist as 0s or 1s) allows for new calculation modes so that 6-sided dies are suitable for games different from double-sided coins.
In a recent experiment, a team from the University of Science and Technology of China (USTC) and the Institute of Photon Science (ICFO) in Spain created a cubic using photons or photoparticles. In past experiments, photon information was often stored in laser-controlled gas clouds, but USTC and ICFO researchers have advanced a new type of “solid” quantum hard drive. It was. -Earth metal. In previous experiments, ions replaced the gas and the glass held them in place.
“Our doped crystals can be thought of as almost frozen clouds,” said Samuele Grandi, an ICFO physicist who worked on one of the experiments.
When a photon enters a crystal, it collides with an ion (researchers have carefully prepared it to react to the incoming particles) and transfers that energy to the ion. At that moment, the crystal holds the qubit of the photon and functions as a quantum memory, which is a storage device for quantum information.
The second pillar of quantum communication is the ether link called entanglement. This link allows two particles or groups of particles to function as one system, even if they are separated by a large distance. This phenomenon is at the heart of the quantum Internet, where fiber optic cables and radio waves yoke quantum devices in the same way they connect classical computers. Quantum networks can be expanded to the extent that quantum memory can be involved, and cannot be expanded any further.
The problem is, unlike the bits on the hard drive Quantum mechanics Prohibits copying and relaying qubits in quantum memory (a property that theoretically helps prevent hacking of quantum messages). To overcome this obstacle, researchers envision daisy-chaining quantum memory with repeaters. For example, one day you can entangle Boston’s memory with New York’s repeater’s memory, and New York’s repeater with Washington, DC’s memory, in order to entangle the memory between Boston and Washington, DC.
Grundy and his collaborators have taken a notable step towards such a device. Their device starts with two laser-like devices, one on each side. Both can generate intertwined photon pairs. Even this first step is a challenge, and each device has only a one-thousandth chance of doing so.
But with persistence, one device will eventually fire twin photons. One photon goes directly into the corresponding quantum memory (doped glass) and the other photon goes through the fiber optic cable. Between the two devices (and their memory), this photon hits the beam splitter. This is a material that allows photons to pass for half the time.
Quantum magic happens here. When Grundy and his collaborators see the photons popping out of the beam splitter, they don’t know if they came from the right side or the left side. Therefore, we do not know if the partner photon is in the memory on the right or in the memory on the left. Quantum mechanics has serious consequences for this uncertainty. The stored photons can be in the right or left memory, so the right and left overlays, both present and nonexistent, so that the two crystals are intertwined. Must exist together.
“The fact that you don’t know which direction you came from [from]”This creates a tangle between the memories that currently hold one photon,” Grundy said.
Successful, the group’s device stored one photon between two intertwined memories in adjacent laboratories 10 meters (33 feet) apart. This result is often described mathematically in quantum textbooks, but is rarely experienced in the real world.
“This was daunting for me,” Grundy told Live Science. “You know it works, but then you’ll see it, and this is really counterintuitive.”
The important thing is that the team can easily see the surreal connection. The photons coming out of the beam splitter mean that the memories are intertwined. Researchers call this particle a foretelling photon because it “foretells” entanglement. Other physicists have previously entangled various types of quantum memory, but the ICFO and USTC experiments were the first to entangle crystalline memory with this well-defined entangled signal.
ICFO devices also used the same wavelengths of light used in fiber optic cables, demonstrating that their memory can attempt multiple entanglements at the same time. This is a step towards a quantum network that carries different messages at the same time. In contrast, the USTC Group has realized a form of entanglement between two photons. This has shortened the life of the connection, but it helps immediately.The team described their work in two the study Published in the journal Nature on June 2nd.
These results “make significant advances in the components of the future quantum repeater chain,” Ronald Hanson, a quantum communications researcher at the Delft University of Technology in the Netherlands, told Live Science in an email. “For areas working on solid-state ensemble-based memory, these drive cutting-edge technology significantly.”
long way to go
The ICFO experiment was the culmination of a decade of work led by physicist Hugues de Riedmatten, who developed the steps, materials, and devices needed to create foretold links. Grundy and his ICFO colleague Dario Lago-Rivera also made extreme efforts to separate the rudimentary repeater components from the turmoil of the world. For example, if a cable that is several meters long is stretched by tens of nanometers due to vibrations from a building or blowing hot air, the disturbance will ruin the experiment.
Despite progress, practical quantum repeaters that can reliably entangle memories between cities (much fewer continents) remain years ahead. ICFO memory can store qubits in just 25 microseconds. This is enough time to entangle with another memory within 3 miles (5 km). Grumpy systems are also unreliable, with a 25% chance of trying to write a photon to memory.
Nevertheless, researchers have different ideas on how to improve the setup. Backed by the successful combination of so many quantum elements, they believe they are on the road to entanglement and expansion of quantum communications from adjacent laboratories to adjacent cities.
“This was the starting point for demonstrating the principles,” said Grandi. I just want to see if everything works.
Originally published in Live Science.
Physicists link “quantum memory” in the early stages towards the quantum internet
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