## WHY THIS MATTERS IN BRIEF

## The more information a photon can carry the sooner we can create quantum communications systems that are immune from hackers and eavesdropping.

Single photons are ideally suited for sending information in digital form because they can be used to encode 0 and 1’s. As a result it is easy to imagine that this is all the data that a single photon can hold. But surprisingly that’s not the case. In theory, there is no limit to the amount of information a single photon can encode. And that raises an interesting question. How much information can physicists pack into a single photon in practice? And what does current technology allow?

Today we get an answer thanks to the work of Tristan Tentrup and pals at the University of Twente in the Netherlands. They have packed more than 10 bits into a single photon for the first time.

Their method is straightforward, in theory. The approach is to associate a single photon with a unique member of an alphabet. When the alphabet contains lots of members, the photon carries lots of information.

It’s not hard to see why. When an alphabet contains only two members, such as binary code, each member encodes one bit of information. This is the amount of information needed to describe each symbol in the alphabet.

But when the alphabet is bigger, it takes more information to uniquely describe each member. So each member can encode that amount of data.

The actual amount of information is given by the log to base 2 of the number of members. For example, in an alphabet of 10 symbols, such as each decimal number, each symbol encodes about 3.3 bits. In an alphabet of 26 symbols, such as the English alphabet, each symbol encodes 4.7 bits. And so on.

Tentrup and team have managed to achieve their goal by creating an alphabet with 9,072 symbols. In that case, each symbol encodes more than 13 bits of information.

Creating this alphabet is simple. The team did it by defining a 112 x 81 grid of pixels – that’s 9,072 of them. Each pixel represents a different symbol of the alphabet. To encode a photon with one of these symbols, all they have to do is point the photon toward that part of the grid. So when a specific pixel registers the arrival of a photon, it registers that symbol.

The “Grid”

The tricky part is doing this accurately with single photons. One way to steer photons is with a tilting mirror that simply reflects them in a specific, controllable direction. But they used a more flexible device called a spatial light modulator which modifies a photon’s wavefront as it reflects it. This uses diffraction effects to steer the photon toward its target.

Detecting single photons is also a potential banana skin, since any stray light can overwhelm the signal but again the team have a handy trick for preventing this. Instead of creating single photons, they create them in pairs and encode just one of them with information using this steering mechanism. They look out for the other as a warning that the first is about to arrive at the pixel.

This allows them to switch on the pixel at the very instant the first photon arrives. And this dramatically reduces the chances of a stray photon swamping the signal. Nevertheless, noise still has an impact and the photons end up carrying slightly less information than the theoretical maximum.

The results are nonetheless impressive.

“We demonstrate high-dimensional encoding of single photons reaching 10.5 bit per photon,” says Tentrup. That significantly improves on the previous record of just seven bits per photon and immediately suggests ways to encode even more by increasing the size of the grid and the breakthrough has immediate applications. Physicists already use information encoded in single photons for applications such as the distribution of keys in quantum cryptography, known as Quantum Key Distribution (QKD).

This information is currently encoded in single photons using the binary code of 0s and 1s but the new technique immediately allows each photon to carry an order of magnitude more.

“A very promising direction for this work would then be the implementation of a large-spatial-alphabet encoding for quantum key distribution,” says Tentrup.

And when that happens it will take us another step closer to realising the promise of secure communications that can’d be hacked or eavesdropped on.