A protocol for entangling microwave and optical photons has been demonstrated by researchers in Austria. This has the potential to help to overcome one of the central issues in the formation of a quantum internet by allowing microwave frequency circuits to exchange quantum information through optical fibres.
The central vision underpinning a quantum internet – first articulated back in 2008 by Jeff Kimble of Caltech in the US – is that networked quantum processors could exchange quantum information, much as classical computers exchange classical information via the Internet. Transferring quantum information is far more difficult, however, because background noise can destroy quantum superpositions in a process called decoherence.
Many of the most powerful quantum computers in existence, such as IBM’s Osprey, use superconducting qubits. These work at microwave frequencies, which makes them extremely vulnerable to disruption by background thermal radiation – and explains why they need to be kept at cryogenic temperatures. It also makes transferring information between superconducting qubits extremely difficult. “[One way] is to build ultracold links,” explains Johannes Fink of the Institute for Science and Technology Austria in Klosterneuburg. “The record was just published in Nature [by Andreas Wallraff’s group at ETH Zurich in Switzerland and colleagues]: 30 m at 10–50 mK – that has some challenges for scaling up.” In contrast, he says, “fibre optics works really well for communication – we use it all the time when we surf the Internet”.
A scheme whereby quantum information could be transferred between microwave qubits by sending photons down optical fibres would therefore be extremely valuable. The most direct approach is quantum transduction, in which, by the interaction with a third photon, a microwave photon is up-converted to an optical photon that can be sent along fibres.
Unfortunately, practical implementations of this process also introduce both loss and noise: “You send ten photons and maybe only one of them gets converted…and maybe your device adds some extra photons because it was hot or for some other reason,” says Fink’s PhD student Rishabh Sahu, who is joint first author on a paper describing this latest research. “Both of these bring the fidelity of transduction down.”
An alternative way to transfer quantum information is called quantum teleportation and was first demonstrated experimentally in 1997 by Anton Zeilinger’s group at the University of Innsbruck – for which Zeilinger shared the 2022 Nobel Prize for Physics. When a qubit interacts with one photon in an entangled pair, its own quantum state gets entangled with the second photon.
A quantum network could be produced under ambient conditions if this second photon could travel down a low-loss optical fibre to interact with an identically prepared transmission photon from a second network node through a so-called Bell state measurement. This would perform an “entanglement swap” between the remote superconducting qubits.
Entangled photon pairs are generated by a process called spontaneous parametric down-conversion, whereby one photon splits into a two. However, nobody had previously managed to generate an entangled pair of photons whose energy differed by a factor of more than 10,000. This difference encompasses a photon at an optical telecoms wavelength of about 1550 nm; and another at a microwave wavelength of about 3 cm.
Fink’s group pumped a lithium niobate optical resonator that was part of a microwave resonator with a high-power laser at telecom wavelengths. The vast majority of the laser light simply came back out of the resonator unchanged and was filtered out. However, approximately one photon per pulse split into two entangled photons – one microwave and the other at a wavelength just slightly longer than the pump photons.
Entangled light source is fully on-chip
“We verified this entanglement by measuring the covariances of the two electromagnetic field fluctuations. We found microwave-optical correlations that are stronger than classically allowed, which signifies that the two fields are in an entangled state.” says Liu Qiu, a postdoctoral researcher and joint first author on the paper describing the work. The researchers now hope to extend this entanglement to qubits and room temperature fibres, implement quantum teleportation and entangle qubits in separate dilution refrigerators.
Alexandre Blais of the Université de Sherbrooke in Canada collaborated on Wallraff’s Nature paper and he is impressed with Fink and colleague’s work, “Normally optics and microwaves don’t talk to each other. Optics is really high energy and tends to ruin the quantum coherence properties of your microwave circuits. Now [the researchers] have standing photons: if I want to transfer that information into another fridge I need to transfer that information into a flying photon in an optical fibre, and there will be loss there. And that photon then has to travel down that fibre, enter the second fridge and do some magic…We should not think that this makes everything easy now – it’s just the beginning, but that doesn’t take away from the quality of the experiment.”
The research is described in Science.