Australasian Science Magazine

Credit: Anterovium/Adobe

By Luke Helt & Michael Steel

Single photons have weird yet useful behaviours, with applications ranging from secure communications to quantum computing. While current silicon photon sources often produce additional “noise photons” that interfere with these emerging technologies, new research has discovered a method to quieten this quantum chaos.

Not all light is the same. For example, a light bulb emits countless numbers of light particles, or photons, in all directions at any given moment, making it useful for lighting a room. In contrast, laser light emits controllable numbers of photons in controllable directions, enabling it to weld metal, read DVDs or perform eye surgery.

While not yet as common, the use of individual photons separated in space and time could find applications in secure communications and incredibly quick database searches. This is because single photons, being fundamental quantum particles, have features that set them apart from other kinds of light.

For one thing, a single photon is indivisible. If information is encoded in individual photons and sent from one party to another, an eavesdropper cannot simply intercept a small portion of the light to learn what is being transmitted without the other parties knowing. This “all or nothing” constraint enables unbreakable internet security.

Furthermore, single photons can exist in a quantum superposition. This allows a single photon to be two or more things simultaneously until it is detected; it could be both red and blue in colour, or even in different locations. Being in more than one state at the same time can allow single photons to examine multiple entries in a database simultaneously.

How Does One Create a Single Photon?

A standard method to create a single photon involves a class of substances known as non-linear optical media. While most materials simply reflect and refract light, non-linear optical materials can also directly change the colour of the light that passes through them. Common non-linear optical media include silicon, gallium arsenide and various types of glass.

Non-linear optical media are used in telecommunications to connect different parts of a fibre optic network that are transmitting different colours of laser light. For example, a device made of third-order non-linear optical media can apply green laser light to red laser light to convert it to blue laser light.

Take away the red laser light, though, and the colour conversion process becomes something weird and quantum: there becomes a very small chance that the green laser light will create a pair of photons, one red and one blue, in the same device (Fig. 1a). Similarly, there is also a very small chance that incident red laser light and blue laser light could be combined in the same device to create a pair of green photons (Fig. 1b). The key thing is that the combined energy of two incident photons is equal to the combined energy of the two photons created.

Although the aim is to create single photons, non-linear quantum processes are rare and random. Therefore, the generation of a pair of photons is quite useful. With a pair of photons, one can detect one of them to learn about the existence of its partner.

Since the photon detected announces the presence of the other, this kind of device is known as a heralded single photon source. Heralded single photon sources have been fabricated from many second-order non-linear optical materials, including lithium niobate and barium borate.

However, we believe that large-scale quantum optical technologies will only be made possible with the scalability and integration capabilities provided by third-order non-linear media, such as silicon or silicon nitride. Therefore, our most recent work has addressed an issue that is particular to heralded single photon sources made of third-order non-linear materials that attempt to produce photon pairs of the same colour.

It’s Not Quite That Easy

The problem is that, again using the example of red and blue lasers to create a pair of green photons (Fig. 1b), the red laser will also create one green photon and one deep red photon, while the blue laser will create one green photon and one purple photon (Fig. 1c). That is, there are pairs of noise photons that will make it difficult to know which green photons herald the presence of each other and which will herald the presence of a red or blue photon. Any such confusion will reduce the effectiveness of the applications mentioned above.

In particular, these noise photons will lead to situations in which someone may think they are sending a photon along a communications channel when in fact they are not. They may also cause someone to accidentally send more than one photon when it was believed that only one was being sent. Issues related to having fewer or greater numbers of photons than anticipated can also impact quantum superposition, and thus quantum-enhanced database searches.

Our Solution

We have managed to suppress these unwanted noise pairs of photons by introducing a Bragg grating into a given heralded single photon source. One way to think about a Bragg grating is as a series of alternating layers of material. As light is transmitted or reflected at each layer, it will interfere constructively or destructively depending on its colour. Given the correct thicknesses and refractive indices, these layers can create a highly reflective mirror for a specific colour of light. Bragg gratings can also be designed to reflect more than one colour of light.

But what if one tries to create a pair of photons in a Bragg grating made of a third-order non-linear optical material, with the grating designed to reflect one of the colours of photons that make up the pair? In a paper recently published in Physical Review Letters (https://goo.gl/o4uwnN), we have demonstrated that the presence of the Bragg grating can leave nowhere for a particular colour of photon to be created. And, because photons are created in pairs in these devices, this can mean that the pair creation is forbidden.

With careful engineering we can therefore design a Bragg grating that can suppress the creation of noise photon pairs without compromising the creation of desirable photon pairs. This is possible because, unlike with a colour filter, the suppression of a photon of one colour can occur with a Bragg grating designed to reflect a different colour than that of a noise photon; namely, the colour of its photon-pair partner.

We view this result, along with the calculation tools we developed, as a new tool for heralded single photon source engineering, allowing for undesirable processes to be suppressed. Along the way, our work revealed that if photons cannot be produced in pairs in a third-order non-linear material, then they will not be produced at all.

We expect these ideas will be implemented in quantum optics laboratories in the near future, helping to bring integrated chip-scale sources of single photons closer to the real world.