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Entangled ions set long-distance record

12 May 2023 Ali Lezeik 
Photo of the ion trap and optical cavity used in the experiment
One of the nodes: An ion trap between the two mirrors forming the optical cavity. (Courtesy: Northup lab)

Using light and optical fibres to send information from point A to B is today a standard practice, but what if we could skip the “sending and carrying” steps entirely and simply read information instantaneously? Thanks to quantum entanglement, this idea is no longer a work of fiction, but a subject of ongoing research. By entangling two quantum particles such as ions, scientists can put them into a fragile joint state where measuring one particle gives information about the other in ways that that would be impossible classically.

Researchers from the University of Innsbruck, Austria, have now performed this tricky entanglement process on two calcium ions trapped in optical cavities 230 m apart – equivalent to around two football pitches – and connected via a 520 m long optical fibre. This separation is a record for trapped ions and sets a milestone in quantum communication and computation systems based on these quantum particles.

Towards a quantum network

Quantum networks are the backbone of quantum communication systems. Among their attractions is that they could link the world with unprecedented computing power and security while enhancing precision sensing and time measurement for applications ranging from metrology to navigation. Such quantum networks would consist of quantum computers – the nodes – connected through the exchange of photons. This exchange can be done in free space, similarly to how light travels through space from the Sun to our eyes. Alternatively, the photons can be sent through optical fibres similar to those used to transmit data for Internet, television and phone services.

Quantum computers based on trapped ions offer a promising platform for quantum networks and quantum communication for two reasons. One is that their quantum states are relatively easy to control. The other is that these states are robust against external perturbations that can disrupt the information carried between and at the nodes.

Trapped calcium ions

In the latest work, research teams led by Tracy Northup and Ben Lanyon at Innsbruck trapped calcium ions in Paul traps – an electric field configuration that produces a force on the ion, confining it in the centre of the trap. Calcium ions are appealing because they have a simple electronic structure and are robust against noise. “They are compatible with technology needed for quantum networks; and they are also easily trapped and cooled, therefore suited for scalable quantum networks,” explains Maria Galli, a PhD student at Innsbruck who was involved in the work, which is described in Physical Review Letters.

The researchers began by placing a single trapped ion inside each of two separate optical cavities. These cavities are spaces between pairs of mirrors that allow precise control and tuning of the frequency of light that bounces between them (see image above). This tight control is crucial for linking, or entangling, the information of the ion to that of the photon.

After entangling the ion-photon system at each of the two cavities – the nodes of the network – the researchers performed a measurement to characterize the entangled system. While the measurement destroys the entanglement, the researchers had to repeat this process multiple times to optimize this step. The photons, each entangled with one of the calcium ions, are then transmitted through the optical fibre that connects the two nodes, which are located in separate buildings.

Exchanging information

While the researchers could have transferred the photons in free space, doing so would have risked disrupting the ion-photon entanglement due to several noise sources. Optical fibres, in contrast, are low loss, and they also shield the photons and preserves their polarization, allowing longer separation between the nodes. However, they are not ideal. “We did observe some drifts in the polarization. For this reason, every 20 minutes we would characterize the polarization rotation of the fibre and correct for it.” says Galli.

The two photons exchange the information of their respective ion-photon systems through a process known as a photon Bell-state measurement (PBSM). In this state-selective detection technique, the photons’ wavefunctions are overlapped, creating an interference pattern that can be measured with four photodetectors.

By reading the measured signals on the photodetectors, the researchers can tell whether the information carried by the photons – their polarization state – is identical or not. Matching pairs of outcomes (either horizontal or vertical polarization states) consequently herald the generation of entanglement between the remote ions.

Trade-offs for successful entanglement

The researchers had to balance several factors to generate entanglement between the ions. One is the time window in which they do the final joint measurement of the photons. The longer this time window is, the more chance the researchers have of detecting photons – but the trade-off is that the ions are less entangled. This is because they aim to catch photons that arrive at the same time, and allowing a longer time window could lead them to detect photons that actually arrived at different times.

The researchers therefore needed to carefully check how much entanglement they managed to achieve for a given time window. Over a time window of 1 microsecond, they repeated the experiment more than 13 million times, producing 555 detection events. They then measured the state of the ions at each node independently to check the correlation, which was 88%. “Our final measurement step is in fact to measure the state of both ions to verify that the expected state correlation is there,” Galli says. “This confirms that we have succeeded in creating entanglement between the two ions.”

From a sprint to a marathon

Two football pitches may seem like a large distance over which to create a precarious quantum entangled state, but the Innsbruck team has bigger plans. By making changes such as increasing the wavelength of photons used to transmit information between the ions, the researchers hope to cover a much greater distance of 50km – longer than a marathon.

While other research groups have previously demonstrated entanglement over even longer distances using neutral atoms, ion-based platforms have certain advantages. Galli notes that the fidelities of quantum gates performed with trapped ions are better than those of quantum gates performed on atoms, mainly because interactions between ions are stronger and more stable than interactions between atoms and the coherence time of ions is much longer.

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