Manipulating photons for microseconds tops 9,000 years on a supercomputer

Ars Technica's Chris Lee has spent a good portion of his adult life playing with lasers, so he's a big fan of photon-based quantum computing. Even as various forms of physical hardware like superconducting wires and trapped ions made progress, it was possible to find him gushing about an optical quantum computer put together 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 cutting down error rates, exploring new technologies, and upping the qubit count.

But the advantage of optical quantum computing didn't go 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 is now able to sometimes perform operations with more than 200 qubits. And it's shown that simulating the behavior of just one of those operations on a supercomputer would take 9,000 years, while its optical quantum computer can do them in just a few dozen milliseconds.

This is an entirely contrived benchmark: just as Google's quantum computer did, the quantum computer is just being itself while the supercomputer is trying to simulate it. The news here is more about the potential of Xanadu's hardware to scale.

Remain in light

The advantages of optical-based quantum computing are considerable. Nearly all modern communications depend on optical hardware at some point, and improvements in that technology have the chance to be directly applied to quantum computing hardware. Some of the manipulations we might need can be done with hardware that's miniaturized to the point where we can etch it onto a silicon chip. And all of the hardware can be kept at room temperature, avoiding some of the challenges of getting signals into or out of equipment that sits near absolute zero.

Xanadu appears to be convinced that these advantages are substantial enough that building a company around them makes sense. The hardware that Lee described last year relies on a single chip to put photons in a specific quantum state and then force photon pairs to interact in ways that entangle them. These interactions form the basis of qubit manipulations that can be used to perform calculations. The photons can then be sorted based on their state, with the number of photons in each state providing an answer to the calculation.

There are challenges to scaling this technology. Since the photons can only interact in pairs, adding another photon means you have to include enough hardware features for its necessary interactions. That means that scaling the processor to a higher qubit count involves scaling all of this hardware on the chip. It's not a problem now, but it could easily be one as things scale through the hundreds to the thousands.

Choose your own adventure

That scaling is probably why Xanadu's new system, called Borealis, involves a significant revision to the architecture. Its earlier machine used a bunch of identical photons that all entered the chip in parallel and traveled through it simultaneously. In Borealis, the photons enter the system sequentially and follow a path that's a bit like a "choose your own adventure" game.

The first bit of hardware the photons hit is a programmable beam splitter, which can serve two functions. If two photons arrive at it simultaneously, they can interfere with each other and become entangled. And depending on its state, the beam splitter can deflect photons out of the main path and into a loop of optical fiber. Traveling around that loop adds a delay to the photon's travel, allowing it to exit the fiber at the same time as a new photon is arriving at the beam splitter, allowing it to become entangled with a later photon.

Once past the first beam splitter, the photons run into a second, with a longer loop of optical fiber that introduces a longer delay to any 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 only arrived at the hardware well after they did. As Xanadu presents it, each of the three beam splitters in Borealis is like adding an additional dimension to the entanglement matrix, taking it up from no entanglement to three dimensions of potential entanglement.

Once through, the photons are sorted based on 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 it's performing. As configured, it could handle more than 200 individual photons as part of a calculation.