Entangled qubit states sent through a communication cable

February 26, 2021 // By Rich Pell
Entangled qubit states sent through a communication cable
Researchers at the University of Chicago say they have sent entangled qubit states through a communication cable linking one quantum network node to a second node for the first time.

This represents a breakthrough for quantum computing, say the researchers, as it lays the groundwork for future quantum communication networks.

"Developing methods that allow us to transfer entangled states," says Prof. Andrew Cleland, who led the research, "will be essential to scaling quantum computing,"

The researchers also amplified an entangled state via the same cable first by using the cable to entangle two qubits in each of two nodes, then entangling these qubits further with other qubits in the nodes. To send the entangled states through the communication cable – a one-meter-long superconducting cable – the researchers created an experimental set-up with three superconducting qubits in each of two nodes.

They connected one qubit in each node to the cable and then sent quantum states, in the form of microwave photons, through the cable with minimal loss of information. The fragile nature of quantum states, say the researchers, makes this process quite challenging.

The researchers were able to develop a system in which the whole transfer process – node to cable to node – takes only a few tens of nanoseconds, allowing them to send entangled quantum states with very little information loss. The system also allowed them to "amplify" the entanglement of qubits.

To do so, the researchers used one qubit in each node and entangled them together by essentially sending a half-photon through the cable. They then extended this entanglement to the other qubits in each node. When they were finished, all six qubits in two nodes were entangled in a single globally entangled state.

In the future, say the researchers, quantum computers will likely be built out of modules where families of entangled qubits conduct a computation. These computers could ultimately be built from many such networked modules, similar to how supercomputers today conduct parallel computing on many central processing units connected to one another.

The ability to remotely entangle qubits in different modules, or nodes, say the researchers, is a significant advance to enabling such modular approaches.

"These modules will need to send complex quantum states to each other, and this is a big step toward that," says Cleland.

A quantum communication network, say the researchers, could also potentially take advantage of this advance. The researchers asy they hope to next extend their system to three nodes to build three-way entanglement.

For more, see "Deterministic multi-qubit entanglement in a quantum network."

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