Chris Lee of Ars Technica has spent much of his adult life playing with lasers, so he’s a big fan of photon-based quantum computers. Even as various forms of physical hardware such as superconducting wires and embedded ions made progress, it was possible to watch it flow over an optical quantum computer assembled by a Canadian startup called Xanadu. But in the year since Xanadu described its hardware, companies using that other technology continued to make progress by reducing errors, exploring new technologies and increasing the number of qubits.
But the advantage of optical quantum computing hasn’t gone away, and now Xanadu is back with a reminder that it hasn’t gone away either. Thanks to some tweaks to the design it described a year ago, Xanadu can now sometimes perform operations over 200 qubits. And it has been shown that simulating the behavior of just one of those operations on a supercomputer would take 9,000 years, while the optical quantum computer can do them in just a few tens of milliseconds.
This is a completely contrived benchmark: Just like Google’s quantum computer did, the quantum computer is just itself as the supercomputer tries to simulate it. The news here is more about the scale potential of Xanadu’s hardware.
stay in the light
The benefits of optically based quantum computing are significant. Nearly all modern communications depend on optical hardware at some point, and improvements in that technology have the potential to be applied directly to quantum computing hardware. Some of the manipulations we might need can be done with hardware that’s scaled down enough that we can etch it onto a silicon chip. And all hardware can be kept at room temperature, avoiding some of the challenges of getting signals into or out of equipment that is near absolute zero.
Xanadu seems convinced that these advantages are big enough to build a business around them. The hardware Lee described last year relies on a single chip to put photons into a specific quantum state and then force photon pairs to interact in a way that entangles them. These interactions form the basis of qubit manipulations that can be used to perform calculations. The photons can then be sorted according to their state, with the number of photons in each state answering the calculation.
There are challenges to scale up this technology. Since the photons can only interact in pairs, adding another photon means you need to have enough hardware features for the necessary interactions. That means scaling the processor to a higher number of qubits means scaling all of this hardware on the chip. It’s not a problem now, but it could easily be one as things scale from hundreds to thousands.
Choose your own adventure
That scaling-up is probably why Xanadu’s new system, called Borealis, involves a significant overhaul of its architecture. His earlier machine used a set of identical photons all entering the chip in parallel and traveling through it simultaneously. In Borealis, the photons sequentially enter the system and follow a path that is somewhat like a “choose your own adventure” game.
The first piece of hardware the photons hit is a programmable beam splitter, which can perform two functions. If two photons arrive at the same time, they can interfere with each other and become entangled. And depending on its condition, the beam splitter can deflect photons off the main path and into a loop of optical fibers. Traveling around that loop adds a delay to the photon’s travel, allowing it to exit the fiber the moment a new photon arrives at the beam splitter, allowing it to become entangled with a later photon.
Once past the first beam splitter, the photons enter a second, with a longer loop of optical fiber introducing a longer delay for all the photons sent down it. And then on to a third with an even longer loop. The optional delays allow photons to become entangled with other photons that arrive at the hardware long after. As Xanadu puts it, each of the three beam splitters in Borealis is like adding an extra dimension to the entanglement matrix, going from no entanglement to three dimensions of potential entanglement.
Once through, the photons are sorted according to their properties and sent to a series of detectors. The detectors keep track of how many photons arrive and when, which will provide an answer to any calculations they make. As configured, it could process over 200 individual photons as part of a calculation.
