Ars Technica’s Chris Lee has spent a good chunk 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 such as superconducting wires and trapped ions advanced, it was possible that he was raving about an optical quantum computer being built by a Canadian startup called Xanadu. But in the year since Xanadu described its hardware, companies using this other technology have continued to make strides, lowering error rates, researching new technologies, and increasing the number of qubits.
But the optical quantum computing advantage hasn’t gone away, and now Xanadu is back, reminding us that it hasn’t gone either. Thanks to some changes to the design outlined a year ago, Xanadu is now able to perform operations with sometimes more than 200 qubits. And it has been shown that simulating the behavior of just one of these operations would take 9,000 years on a supercomputer, while its optical quantum computer can do it in just tens of milliseconds.
This is an entirely made-up benchmark: just like Google’s quantum computer, the quantum computer is just itself, while the supercomputer tries to simulate it. The news here relates more to the scaling potential of Xanadu’s hardware.
Stay in the light
The benefits of optically based quantum computing are significant. Almost all modern communications will eventually depend on optical hardware, and improvements in this technology have a chance of being directly applied to quantum computing hardware. Some of the manipulations we might need can be done with hardware miniaturized 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 in or out of devices that are near absolute zero.
Xanadu seems convinced that these advantages are so great that it makes sense to build a company around them. The hardware Lee described last year relies on a single chip to put photons into a specific quantum state and then force pairs of photons into an interaction that entangles them. These interactions form the basis for qubit manipulations that can be used to perform computations. The photons can then be sorted by their state, with the number of photons in each state providing an answer to the calculation.
There are challenges in scaling this technology. Since the photons can only interact in pairs, adding another photon means you need to include enough hardware capabilities for the necessary interactions. This means that scaling the processor to a higher qubit count involves scaling all of the hardware on the chip. It’s not a problem now, but it could easily become one as things scale from hundreds to thousands.
Choose your own adventure
This scaling is likely why Xanadu’s new system, called Borealis, involves a significant architectural overhaul. His earlier machine used a series of identical photons, all of which entered the chip in parallel and traversed it simultaneously. In Borealis, the photons enter the system sequentially, following a path that’s a bit like a choose-your-own-adventure game.
The first piece of hardware the photons encounter is a programmable beam splitter that can serve two functions. If two photons hit it at the same time, they can interfere and entangle each other. And depending on its condition, the beamsplitter can redirect photons out of the main path and into a fiber optic loop. Traveling around this loop adds a delay to the photon’s journey, allowing it to exit the fiber at the same time as a new photon arrives at the beamsplitter, allowing it to become entangled with a later photon.
Once through the first beamsplitter, the photons travel into a second, with a longer loop of fiber optics that introduces a longer delay for any photons they send down. And then on to a third with an even longer loop. The optional delays allow photons to entangle with other photons that arrived at the hardware long after them. As Xanadu presents, each of the three beamsplitters in Borealis is like adding an extra dimension to the entanglement matrix, expanding it 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, providing an answer to any calculations they perform. As configured, it could handle more than 200 individual photons as part of a calculation.
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