Nature Podcast：潘建伟谈量子互联网

来源：Nature自然科研

又到了每周一次的 Nature Podcast 时间了！欢迎收听本周由Benjamin Thompson和 Shamini Bundell 带来的一周科学故事，本期播客片段讨论两个相距50公里的量子存储器的纠缠。欢迎前往iTunes或你喜欢的其他播客平台下载完整版，随时随地收听一周科研新鲜事。

Interviewer: Benjamin Thompson

The internet is great, right? It connects people and lets us send data around the world in a flash. It even allows us to visit amazing websites like nature.com/podcast. But the internet doesn’t just include computers. You’ve got fridges, printers, modems – you name it. Each of these are called nodes, but this is the classic internet. Now, researchers around the world are looking even further. They’re trying to build the quantum internet by connecting quantum nodes together. It’s hoped that a quantum internet would offer new computational opportunities that aren’t possible using a regular network. Here’s Tracy Northup, a quantum physicist from the University of Innsbruck in Austria.

Interviewee: Tracy Northup

One aspect is the idea that small-scale quantum computers might not be powerful enough by themselves to do meaningful computations but that by linking them together we can gain this new computational power. And also, there have been proposals to link together things like telescopes or atomic clocks, and then if we can harness quantum mechanics for this link, we can gain new sensitivity for these kinds of measurements.

Interviewer: Benjamin Thompson

Connecting distant nodes relies on a quantum effect called entanglement. Once two nodes are entangled, the state of one node affects the other, and so information can be shared between the two. One of the ways that two nodes can be entangled involves sending photons down optical fibres, but this method does have its drawbacks, explains Jian-Wei Pan from the University of Science and Technology of China.

Interviewee: Jian-Wei Pan

The quantum signal has its drawbacks of the photon loss caused by unavoidable absorption of the fibre, so then the photo signal will be weak, so then the distance of quantum communication is very much limited. So, we have to find a way how to send the photon signal over larger distances.

Interviewer: Benjamin Thompson

Because photons produced by nodes can be readily absorbed by the optical fibres that carry them, it’s hard to get entanglement over longer distances – something that will be needed for the quantum internet to become a reality. So far, the longest entanglement between two nodes using this method is 1.3 kilometres, but that might be about to change. This week, Jian-Wei and his colleagues have published a paper in Nature demonstrating entanglement of two nodes over dozens of kilometres, by combining a number of techniques. Now, these two nodes were actually in the same lab, but they were connected by fibre-optic cables to an intermediate station across town. At the heart of these nodes was a cloud of laser-cooled atoms. To entangle the nodes, each cloud was coaxed to produce a photon, which were fired along the fibre-optic cables. If these photons met in the intermediate station at precisely the right time, the nodes would become entangled together. This might sound simple, but there is a catch if you want to entangle nodes over longer distances using optical fibres. Owing to the wonders of quantum mechanics, photons don’t only exist as particles – they are also waves. And the wavelength of a photon influences how much it is absorbed by the fibres its travelling down. In this case, Jian-Wei’s cold atom clouds produced photons at an unhelpful wavelength.

Interviewee: Jian-Wei Pan

Normally, our cold atomic clouds only emit photons with a wavelength of about 800 nanometres. Then if you send a special photon to the fibre, then the photon loss is huge.

Interviewer: Benjamin Thompson

At 800 nanometres, many of the photons would be absorbed by the fibres, making entanglement difficult. One of the ways that Jian-Wei and his colleagues used to get around this involved converting the wavelengths of the photons produced by the nodes.

Interviewee: Jian-Wei Pan

We managed to convert our frequency from 795 nanometres to 1,342 nanometres. That’s one of the key technologies we have been pursuing for the last ten years. 

Interviewer: Benjamin Thompson

This change in frequency put the photons firmly inside a range used for telecommunications around the world. Photons at this frequency are much less likely to be absorbed by the fibres, making it a much more efficient way of establishing entanglement. Overall, Jian-Wei and colleagues demonstrated they were able to get entanglement between two nodes to occur at over 50 kilometres, and he says they’re looking to go even further. Tracy Northup, who you heard from earlier and who wasn’t part of the research, was impressed with the work.

Interviewee: Tracy Northup

I am impressed. I think there’s a handful of technological advancements, and combining those in a single experiment is really challenging, so they’ve just done a lot of things to try to scale up the system and it’s challenging to do those all at once.

Interviewer: Benjamin Thompson

There is, of course still a lot of work to do, before a system like this could become part of a larger network. For example, Tracy explained that entangled states are fragile and can easily be lost, and right now, in Jian-Wei’s system, they are lost faster than new ones can be generated, but she thinks that this is something that can be overcome. While some of the principals of quantum mechanics have been used in a limited form to encrypt data along regular communication networks for a while, a network of multiple interconnected quantum nodes around the globe remains a way off just yet. This work appears to be another step in the right direction, but it could be a long road before we reach a quantum internet and its promise of radically different applications in science and computing. 

Interviewee: Tracy Northup

I think that for all of these different applications, we’ll just start to see kind of the first test beds emerging, and then I think we’ll start to see, within the next ten years, that we can link together quantum computers and do computations over these distributed networks or distributed sensors. That’s something I’m certainly looking forward to.

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