Quantum computers based on photons may possess key advantages over those based on electrons. To benefit from those advantages, quantum computing startup Xanadu has, for the first time, made a photonic quantum computer publicly available over the cloud.
Whereas classical computers switch transistors either on or off to symbolize data as ones and zeroes, quantum computers use quantum bits or "qubits" that, because of the surreal nature of quantum physics, can be in a state known as superposition where they can act as both 1 and 0. This essentially lets each qubit perform two calculations at once.
If two qubits are quantum-mechanically linked, or entangled, they can help perform 2^2 or four calculations simultaneously; three qubits, 2^3 or eight calculations; and so on. In principle, a quantum computer with 300 qubits could perform more calculations in an instant than there are atoms in the visible universe.
A number of companies, such as IBM, Rigetti, Amazon and Microsoft have made quantum computers publicly available over the cloud. These all rely on qubits based either on superconducting circuits or trapped ions. One drawback with these approaches is that they both demand temperatures colder than those found in deep space, because thermal vibrations can disrupt the qubits. The expensive, bulky systems required to hold qubits at such frigid temperatures can also make it an extraordinary challenge to scale these platforms up to high numbers of qubits.
In contrast, quantum computers that rely on qubits based on photons can, in principle, operate at room temperature. They can also readily integrate into existing fiber optic–based telecommunications infrastructure, potentially helping connect quantum computers together into powerful networks and even a quantum Internet. With the addition of so-called “time multiplexing” architectures, photonic quantum computing can in principle scale up to millions of qubits.
On 2 September,Toronto-based Xanadu announced the release of the world's first publicly available photonic quantum computing platform. Applicants can access 8, 12, and soon 24 qubit machines over the cloud.
According to Christian Weedbrook, Xanadu’s founder and CEO, the company can roughly double the number of qubits in its cloud systems every six months. In the coming months, Xanadu will release a blueprint for photonic quantum computing that is essentially a primer on “how to scale to millions of qubits in a fault-tolerant manner,” says Weedbrook.
The classic approach to photonic quantum computing, linear optical quantum computing, relies on qubits each based on a single photon. This strategy manipulates photons with mirrors, beam splitters, and phase shifters. Single photon detectors are then used to help read the results of what these devices have done. The problem with this approach is that single photons are difficult to experiment with, generally limiting this strategy to a handful of photons, Weedbrook says.
In contrast, Xanadu's strategy, known as continuous variable quantum computing, does not employ single-photon generators. Instead, the company relies on so-called “squeezed states” consisting of superpositions of multiple photons.
Squeezed states take advantage of a key tenet of quantum physics: Heisenberg's uncertainty principle, which states that one cannot measure a feature of a particle, such as its position, with certainty without measuring another feature of that particle, like its momentum, with less certainty. Squeezed states take advantage of this tradeoff to “squeeze” or reduce the uncertainty in the measurements of a given variable while increasing the uncertainty in the measurement of another variable the researchers can ignore. This improved certainty can in principle help Xanadu entangle large numbers of photons.
Xanadu's X8 photonic quantum processing unit, showing the inputs (connected to 'squeezed light' sources) and optical gates.Image: Xanadu
Sequences of laser pulses fired into Xanadu's microchips couple with microscopic resonators to generate squeezed states. The light next flows to a network of beam splitters and phase shifters, which perform the desired computation. The photons then zip out of the chips to superconducting detectors that count the photon numbers to extract the answer to the computation.
“Prior to Xanadu, nobody had put in the effort to fully automate such a complex photonic system before, which is necessary to make it available to users writing high-level code, rather than just for scientists in a lab,” says Weedbrook.
Xanadu notes that a current limitation of its systems stems from the superconducting photon counters they use. Those counters require ultra-cold temperatures less than 1 degree above absolute zero. However, the company notes that future detectors may not require superconductivity or cryogenic temperatures.
A past criticism of photonic quantum computing is that it lacked fault tolerance and error correction, Weedbrook says. “However, this is starting to change, but people may not be aware of the important advances in this area,” he says. “Photonics has made a significant amount of progress only in the last few years.” Specifically, he notes Xanadu's strategy of continuous variable quantum computing is compatible with more sophisticated strategies for error correction and fault tolerance than earlier photonic approaches were.
In addition to its quantum cloud, Xanadu, which is part of IBM's quantum computing Q network, has made a variety of open-source tools widely available on Github. These include Strawberry Fields, its cross-platform Python library for simulating and executing programs on quantum photonic hardware, and PennyLane, its software library for quantum machine learning, quantum computing, and quantum chemistry.
Xanadu's partners, which include Amazon's Quantum Solutions Lab, already helped test the company's quantum cloud before release. In the 36 hours immediately following its release, the Xanadu Quantum Cloud received 150 applicants, says Weedbrook. “The response has been overwhelmingly positive,” he notes. “We are currently prioritizing institutions with dedicated quantum researchers over individual contributors, but that will change in the short term.”
All in all, “we are laying the groundwork for our vision of the future: a global array of photonic quantum computers, networked over a quantum Internet,” says Weedbrook.
Charles Q. Choi is a science reporter who contributes regularly to IEEE Spectrum. He has written for Scientific American, The New York Times, Wired, and Science, among others.