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When you shine a torch into a dusty room, not all the photons reach their destination. Simon Greig (xrrr)

‘Louder’ light could power a brighter quantum future

All of the light we see around us comes in chunks of energy known as photons. As well as making up light, photons can be used to carry and process information and their quantum properties make possible new tasks in secure communication, ultra-fast quantum computing and more.

To develop these new technologies and the science behind them, we need to learn to manipulate single-photon quantum states in new and powerful ways.

With collaborators at the University of Queensland and the University of Science and Technology of China, my research group at Griffith University has done just that.

We’ve experimentally demonstrated the noiseless amplification of information encoded in a single photon that has been subjected to loss. Our results were published in Nature Physics on Sunday.

Loss? Noise? Amplification?

Amplifiers are ubiquitous in modern technology. They’re used to enlarge singers’ voices at concerts and to boost long-distance data transmission for the internet.

Modern amplifiers pretty much just work, and signals come through crystal clear nearly all of the time. Any imperfections in an amplifier’s output signal are mostly due to technical noise – stuff in the amplifier not working as well as it could.

But through precise engineering design and lots of hard work, this technical noise can be virtually eliminated. That leaves just quantum noise – uncertainty in the signal due to its fundamentally quantum origin.

This quantum noise is usually so much smaller than the signal being amplified that it can be completely ignored. Unless the signal is a quantum system itself!

Quantum communications

In recent years, scientists have been working towards the development of quantum technologies – devices that will use the fundamental properties of the quantum world to outperform the best current (or “classical”) technologies.

Among the many approaches being explored is the idea of encoding information into the quantum states – the quantum description of properties such as polarisation – of photons.

Information transmission and processing with photons has the potential to guarantee secure communications, and the ability to calculate solutions to certain problems that can’t be practically solved by an ordinary computer – now or ever.

Let’s consider the communication task as an example.

In order to be secure, each quantum bit (or qubit) of information is typically sent in a single photon. Heisenberg’s Uncertainty Principle means if an eavesdropper tries to measure the information, she disturbs the photon – the quantum version of “you can’t have your cake and eat it too.”

The point is that the intended recipient can detect the intrusion, or proceed with certainty if none exists.

But, crucially, the range of quantum communication is currently limited by the loss of the photons.

Losing light

If you shine a flashlight around a dark house, you’ll notice a few things. If you haven’t vacuumed for a while, you might be able to see where the light beam goes by the light that is scattered off dust particles.

And if you shine the flashlight out through a window to see who’s dog is barking, some of the light bounces back off the window. In both cases, not all of the light reaches its intended destination.

Optical communication signals travelling through the atmosphere, or an optical fibre network, also suffer these kinds of loss.

For single photons, loss like this is devastating, because there really isn’t much light to lose! Losing single photons drastically limits the range of quantum-secured communications and other quantum science tasks that require light to travel significant distances.

Lost or not?

The funny thing about quantum mechanics is that when a single photon encounters loss, we don’t know whether it’s lost or not until we try to detect it.

It’s just like the famous Schrodinger’s cat thought experiment in which a hypothetical cat in a box is, because of quantum physics, both alive and dead at the same time.

In fact, quantum physics suggests that it’s the act of looking – opening the lid – that forces a “decision” about the cat’s fate.

Now, if you were a cat-lover confronted with the box containing Schrodinger’s cat, you may wish to try to make it more probable the cat is alive before you open the box. That would mean you’d need to do something to the quantum state of the cat without looking.

And this is exactly what our photon amplifier does. It amplifies the likelihood of the photon being present, without learning any of the information carried in the photon’s quantum state.

The secret to this operation is the idea of quantum teleportation.

Into the realms of science fiction

The word “teleportation” brings to mind our favourite science fiction shows, where some kind of matter (often people) are moved over long distances without traversing the space in between. In the quantum version, it’s not matter that’s moved, but information.

The idea in our qubit amplifier scheme is to teleport the information carried by the lossy photon on to another light beam where it’s much more likely a photon is present.

In our experiment, which was based on an idea by University of Geneva researchers, we took a signal where photons were present just 4% of the times we expected them (the remainder being lost), and amplified it until photons were present 20% of the times expected.

In each case, the quality of the information was preserved to a high degree. In principle, it’s possible to amplify to close to 100% photon presence, but this actually requires improvements in the quality of sources of extra photons used to power the amplifier – a problem on which Australian and international researchers continue to make steady progress.

Things can only get better

The laws of quantum physics say that you can’t accurately copy an unknown quantum state, a fact known as the “no cloning theorem”. Because of this kind of idea, it turns out that the qubit amplifier can’t work every time – sometimes it tries to teleport the information but fails.

But when it succeeds it sends a separate success signal. Thus we know when we can use its output and when we can’t.

It turns out that this is enough to make future versions (with higher amplification) very useful in various quantum communication tasks, as well as for quantum-enhanced measurements, and in the scientific exploration of other strange and useful properties of the quantum world.

Perhaps in the future, qubit amplifiers will be as common in quantum technologies as amplifiers are in classical devices today.

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