There comes a moment in every physicist's life when they think the unthinkable: I wish I were an engineer. I suspect this thought crossed the minds of the 14-odd physicists involved in creating a key demonstration of the scalability of quantum computing using light.

At the moment, if you had to bet on the technology most likely to win the quantum computing race, most people would put their chips on a spread of superconducting rings. But I’d put the house and kids on light. Why? Because lasers make everything better. More seriously, quantum computing architectures based on superconducting devices have made remarkable progress in the last five to ten years. By contrast, progress on the light front has been ominously slow. But it should be easier to work with light-based qubits if we can ever get them off the ground.

Why I love photons

Photons, as far as I’m concerned, still make the best quantum bits (qubits). This is because photons mostly pass through the world unhindered. A photon, in a super-special quantum state, can go from air to an optical fiber to air, through a silicon chip, back into air, and into a fiber again, all without destroying its quantum state. About all you need to ensure is that your photon detector is in the dark so that only the qubit photons hit it.

Superconducting qubits are made up of electrons, which are sensitive to everything. It takes real experimental skill and good engineering to ensure that superconducting qubits maintain their quantum state.

You should be wondering why, if light is so good, light-based quantum computers lag so far behind. It is for exactly the same reason: photons don’t notice each other, but electrons do. Imagine you want to switch the state of one qubit based on the state of another qubit. For electrons, that’s simple because they have an electric and magnetic field through which they can manipulate each other. Photons, however, just pass right through each other without noticing. The simple way to implement quantum operations is actually very, very hard using photons.

Instead, you have to use multiple combinations of linear operators. What is the difference? Put it like this: in a nonlinear operation, two photons might collide to create a single photon, or a single photon might split to create two photons. But, in a linear operation, the number of photons is preserved, and only their paths are modified.

To perform a two-qubit operation you need a minimum of one gate and three qubits—two that are being operated on, and one controlling the operation. To perform the same operation using only linear operations (as required for photons) requires four qubits and four gates. The complexity grows horribly quickly.

This is where the shift from physics to engineering is required. To implement a single gate is complex but doable. But, what about implementing all possible gates for, say, two qubits? That requires the design of a custom integrated optical chip. This is where the engineers come in.

The chip that the researchers produced is quite remarkable. It takes in a single laser light source and, from there, generates pairs of photonic qubits. The qubits then pass through a single gate that consists of a maze of interferometers (the linear operation used to construct the gates). Each waveguide has a small heating element attached that allows the researchers to control the exact distance the photons travel between and in each interferometer. This control determines the path that photons take through the maze. Or more specifically, the control, combined with the quantum state of the photon, determines the path through the maze.

The researchers demonstrated this by implementing 98 different two-qubit gates on the same hardware. And, along with each, they performed a full set of measurements (about 1,000 measurements per gate). The gates are about as reliable as any others you will find in the quantum computing world, which is to say that operations complete successfully around 93 percent of the time. For comparison, ion-based quantum computers are at 95 to 99 percent and superconducting quantum computers are around 90 to 95 percent.

To show that the chip was capable of more than just a single operation, the researchers showed that you can run an optimization algorithm on it.

Two-bit heaven

This is still just two qubits, which is nothing compared to superconducting quantum computers, which are now in the 20-qubit range. So, why am I excited? The point is that this paper shows that many of the big problems have been overcome. The researchers showed that you can design, fabricate, and control a chip with the precision required for programmable quantum computing. There is not much to stop the design of the gate from being scaled up to more elaborate circuits that could run bigger programs (if the chip had enough qubits and they could be detected).

Therein lie a couple of potential stumbling points. The photon detectors were not on the chip. Instead the light was piped out to external detectors. For two qubits (just two external detectors), that is feasible. For 100 qubits, that is probably not going to work. Maybe two chips—one for computation and one for detection—are going to have to be glued together.

Likewise, the light source is going to be difficult. The researchers' production process for entangled photons is a random process. Each laser pulse might or might not generate entangled photons. In the case of this chip, which has just two computational qubits, there are four independent locations where these photons are generated. For a computation to take place, two locations have to produce a pair of photons independently. This takes place for about a quarter of the laser pulses.

To increase the number of entangled photons, you need to generate them simultaneously at multiple sites. The chances of that happening are not great; a 100-qubit computer would never be expected to work. There are, I should say, many other ways to produce the required photons. And many of these are deterministic: you push a button and get a photon. But, the big question is if they can be integrated into the chip technology developed here.

In any case, I’m pretty excited by developments on the computation side. And very wary of the road ahead for generating the required photons and detecting the result of the computation.

Nature Photonics, 2018, DOI: 10.1038/s41566-018-0236-y. (About DOIs)