Q & A and explanations of Sandia-Harvard quantum computing breakthrough

In an October 14, 2016, press release entitled “Diamonds Aren’t Forever: Sandia,Harvard team create first quantum computer bridge,” Sandia National Laboratories announced the success of a physics experiment involving the creation of controllable quantum emitters consisting of silicon atoms that are placed within a diamond matrix replacing one or more of the carbon atoms that originally constituted that uniform substrate.

You can read the paper upon which this press release is based, entitled “An integrated diamond nanophotonics platform for quantum optical networks,” by clicking here. 

Etopia News contacted the experimental team’s Harvard contingent and asked for more details about the mechanics and future of this exciting new technology.  Ruffin Evans, a PhD candidate in physics at Harvard, and a member of this paper’s corresponding author and Harvard physics Professor Mikhail Lukin’s research team, quickly and graciously provided an explanation of detailed aspects of this research.  He also provided a Q & A text designed for the general reader.

Here’s what Ruffin Evans told Etopia News about the research:

I'm one of the researchers at Harvard who worked on this experiment under Prof. Lukin's supervision. I have attached a brief FAQ explaining our work at the level of the general public.

Basically, the idea is the following. The diamond lattice is a very "clean" and inert material, which we then implant individual silicon atoms into. This forms a silicon-vacancy (SiV) color center which is basically an atom-like system that we can control using lasers. In other words, it is an optically addressable quantum bit. We need to do some tricks to shape the diamond around the SiV center, essentially forming tiny nanoscale optical wires that focus the light tightly around the SiV. This helps make the interaction between the SiV and the optical field strong.

This system can be used to realize an all-optical switch or even entanglement between two adjacent SiV centers. (Described in more detail in the attached txt document). These are two very important elements of quantum networks. Building an optical switch demonstrates that we can change the state of light depending of the state of the SiV center, which means that we can efficiently use SiV centers to control the flow of light on the quantum level. Entanglement is a key resource for performing complex quantum manipulations within a single quantum device. So, these two results together show that we should be able to use these techniques to build devices that can talk to each other (through light) and are also useful on their own for some basic quantum manipulations (using entanglement).

This system is very exciting because this is the first time these experiments have been performed in a single solid-state system in the optical domain. Moreover, our devices are scalable using standard semiconductor fabrication techniques, which means we could create millions of devices on a single chip. Future work may be in this direction: integrating SiV centers among multiple devices to build a complex on-chip quantum network.

Asked further about next steps in building entire quantum computing systems based on these techniques, the graduate student added:

We're already working on it, actually! Right now we're developing new techniques to link multiple SiV centers on a single device with very high efficiency. The next step is to develop technology that will increase the memory time of these devices, which basically means we have more time to engineer the interactions that are crucial for forming the network. We're hoping to accomplish those goals over the next six months. After that, so on the 6-12-month timescale, we will work on linking together multiple devices both on-chip and between multiple chips.

Here's the Q & A:

> 1. Please could you say what the most important result in your paper is, and why it is important?

We used a single color center in a diamond nanodevice to demonstrate two key building blocks for quantum networks. First, we demonstrated that a single color center can be used as an optical switch. We also used photons to entangle two color centers in a nanophotonic device. Both of these results show that we can create effective interactions between single photons and between two color centers using solid-state devices. These two types of interactions are the essential ingredients for building complex quantum states of light and matter.

> 2. Could you simply describe how you make a quantum-optical switch controlled by a single color center?

The crucial aspect of the switch is the realization of strong interactions between photons and a single color center. Specifically, we need the interaction to be strong enough that a single color center can reflect light -- a mirror made from a single quantum system. By controlling the color center, we can turn this reflectivity on and off, making a switch.

To achieve this, we chose to use silicon-vacancy color centers which have recently been shown to interact strongly with light. We placed the silicon atoms inside nanophotonic diamond devices that guide photons using the same principles as optical fibers. The cross-section of these devices is so small that a single color center can block the flow of light. In other words, the light is "focused" into a device so small that a single color center can interact with it. (We also use a set of special nanoscale mirrors -- a photonic crystal cavity -- to increase the interaction strength even more by bouncing the light back and forth over the color center thousands of times.) Once we have this strong interaction, we controlling the internal states of a color center to control the flow of light and realize a switch (see below).

> 3. How can you switch this device?

The color center can be prepared in two different electronic states by using laser fields. One of these states strongly interacts with light and reflects photons. The other state is does not interact with light and is transparent to photons. By choosing which state the color center is prepared in, we can switch the transmission either on or off.

> 4. What implications/applications do you foresee as a result of this work?

Using our techniques, we can precisely control the interactions between photons and color centers. These interactions can be used to create complex quantum states involving entanglement between multiple photons and color centers -- a key resource for quantum information processing. Our results also show that we can route and store the quantum information in photons using color centers inside nanophotonic devices.

One of the biggest advantages of our system is that it is possible to create millions of devices on a single chip using standard semiconductor fabrication techniques. (In our experiment, we create several thousand devices -- industrial techniques could do better.) So, these systems could be used to create complicated on-chip networks or even optical quantum processors with many devices all communicating using photons.

These devices also have potential applications in classical information processing, because they could be used to switch, control, and store light without first converting it into an electrical signal. Because all long-distance communication is done with optical fields in fiber optic cables, being able to control the propagation of light directly is advantageous.

> 5. What are the next stages in your investigations?

We are working on an experiment where two color centers in a single device interact directly by exchanging a photon. In other words, a photon created by one color center will be "caught" by the second color center. This would enable efficient, direct generation of interesting (entangled) quantum states between multiple color centers that would be a major step forward in the quest towards on-chip quantum networks.

One potential drawback of our system is the short memory time. In order to switch the system, recall that we "store" a photon in the color center. This changes the state of the color center and switches the incident light field. In the current system, the photon stored in the color center is lost after tens of nanoseconds. However, we are developing methods to store a photon in a color center for a long period (up to one second) such that it can be used as a long-lived memory.