Two Quantum Computers Shatter A Distance Record, With A Possible Connection Over 2,000 Km Away

Connecting two quantum computers together is like trying to keep the flame of a candle lit on the hood of a car going at full speed on the highway: very complicated. Unlike our classical computer networks, where information circulates in the form of bits, quantum computing requires extremely fragile qubits. As the distance increases, the qubits lose their quantum state (decoherence), taking with them any possibility of reliable communication. Previously, even when two quantum computers were separated by a few kilometers, it was very difficult to keep them connected

This is one of the biggest constraints of quantum computing, which we absolutely must circumvent if we really wish to one day build the quantum internet, the future network which will connect all quantum computers together. The distance factor is, for the moment, a technological ceiling, but American researchers have found a way to open a small opening there. They demonstrated that by greatly extending the duration of quantum coherence, two computers could theoretically stay connected and exchange information over more than 2,000 kilometers of optical fiber. Remarkable work, which was the subject of a publication on November 6 in the journal Nature Communications.

From a few kilometers to more than 2,000: a giant leap for quantum

To understand the magnitude of this record, it is necessary to return to one of the fundamental principles of quantum computing: quantum coherence. When we seek to connect two quantum computers, we entangle atoms (or qubits): this results in the creation of a deep quantum link between them, such that the state of one instantly depends on the other. This is what we call thequantum entanglement. The problem being that this entangled state only survives a very short time before losing its coherence (decoherence). However, the longer this coherence time, the greater the connection distance between the computers.

Previously, this coherence lasted only 0.1 milliseconds, far too short a time to allow entanglement (and therefore information) to travel long distances in an optical fiber. In this new study, Tian Zhong’s team managed to extend this quantum memory up to more than 10 milliseconds, with a maximum measured at 24 milliseconds.

A small gain of time which is enough, in theory, to allow the entanglement to propagate over more than 2,000 kilometers, or even up to 4,000 kilometers in ideal conditions. That’s the distance that separates Chicago from Colombia!

How did they manage to multiply coherence time so effectively? By rethinking the manufacture of crystals serving as support for the entangled atoms, which constitute the memory quantum networks. Until now, all other teams working on quantum memory devices based on rare earth atoms used crystals made by fusion at very high temperatures, then cut afterwards. This proven method, called Czochralskigives imperfect crystals, filled with microscopic defects which disrupt quantum states and their transmission.

A new manufacturing technique

Tian Zhong’s team preferred to assemble the crystal layer by layer at the atomic level using a technique called molecular beam epitaxy (MBE), rather than starting from an already formed block of material, like 3D printing, as the team explains. This method allowed them to more precisely control the position of the atoms within the crystal and to prevent it from having too many defects. It is therefore exceptionally purer and provides entangled atoms with a healthier environment to transmit information.

Even if we cannot expect that one quantum computer located in New York and the other in Chicago be connected in the coming monthsit is certain that Tian Zhong and his collaborators are on the right path. Their next goal: to test this technology on a small scale in their laboratoryby connecting qubits distributed across several cryogenic refrigerators using 1,000 kilometers of coiled cable. This phase will allow them to simulate long distances without requiring external infrastructure and to validate the viability of the technique MBE. Without this demonstration, no increase in scale would be scientifically justifiableand this is one of the conditions sine qua non so that one day we can hope to build the first foundations of the quantum internet.