First long-distance transmission of entangled ion qubits

Qubits go mobile: Ions trapped in quantum traps are often used as qubits in quantum computers. However, an entanglement of these ionic quantum bits over longer distances has not yet been done. Now physicists in Innsbruck have succeeded. For the first time, they efficiently transmitted the quantum information from ion qubits over a distance of more than 500 meters, entangling the qubits with one another. This opens up new possibilities for quantum networks.

Thanks to the phenomenon of entanglement, quantum information can be transmitted instantaneously and “uncrackably”. Because every change in the state of an entangled particle also changes its partner – immediately and regardless of the distance between the two. So far, this has already been achieved with quantum information from photons and with entanglement transferred from atoms to photons – in fiber optic sea cables , over the air and even via satellite .

How to entangle distant ion qubits?

However, efficient long-distance transmission has yet to be achieved for one important form of qubit: qubits made from trapped ions. In addition to superconducting transmon qubits , such ions held in traps made of laser beams are the second common form of processing units in quantum computers. However, the transfer of quantum information from these ion traps to transport photons has so far been inefficient and has therefore only had a short range.

“Until now, trapped ions were only entangled over a few meters in the same laboratory. This was also realized with joint control systems and with photons, which due to their wavelength are not suitable for traveling longer distances,” explains co-author Ben Lanyon from the University of Innsbruck. Now, for the first time, he and his colleagues have succeeded in entangling two ion qubits over a distance of around 500 meters and across several buildings.

Connected over 500 meters of fiber optics

In the experiment, the two quantum systems were set up in two laboratories several hundred meters apart. Each ionic qubit was then excited with a laser that emits two wavelengths simultaneously. This caused the qubit to emit a photon whose polarization depended on the wavelength the ion had absorbed – which was random. The ion qubit was thus entangled with the photon via polarization. This could now serve as a “quantum messenger” and carrier particle.

The physicists then sent the entangled photons from one laboratory building to the other via a fiber optic cable. “To do this, we sent the photons entangled with the ions over a 500-meter-long light guide and superimposed them,” explains co-author Tracy Northup from the University of Innsbruck. At the target, the photon is directed to a beam splitter, where it interacts with the photon of the local ion qubit. This transfers the entanglement of the first qubit to the second qubit.

New possibilities for quantum networks

As the researchers report, this quantum transfer achieved an efficiency of up to 88 percent – significantly more than in previous experiments of this type. Their experiment also proves for the first time that such a quantum transfer between ionic qubits also works when the qubits are manipulated via separate control systems – this is important for practical application in quantum networks.

“Our results show that trapped ions are a promising platform for the realization of future large-scale networks of quantum computers, quantum sensors and atomic clocks,” says Northup. (Physical Review Letters, 2023; doi:10.1103/PhysRevLett.130.050803 )

Source: University of Innsbruck