Photonic Chips for Fault-Tolerance Quantum Computing

Article By : Maurizio Di Paolo Emilio

The collaboration between Xanadu and Imec involves fabricating low-loss silicon nitride circuits that can correct qubit errors and increase capacity.

Xanadu and Imec have partnered to develop photonic chips for fault-tolerant quantum computing. Their common goal is to create a machine based on quantum theory capable of executing any algorithm, detecting and correcting any error that may affect the calculation, thus accommodating a large number of qubits.

Xanadu and Imec said they aim to harness photonics to build million-qubit quantum computers that are immediately available to customers. Their collaboration involves fabricating low-loss silicon nitride circuits that can correct qubit errors and increase capacity.

In an interview with EE Times Europe, Amin Abbasi, business development manager at Imec, and Zachary Vernon, head of hardware at Xanadu, have highlighted the prospects of quantum computers for future applications. “There are some applications that may not require full fault-tolerance to deliver some value. Examples exist in machine learning and optimization. Xanadu has already made some chips available to users via its cloud platform that can have an impact in these areas. However, the biggest value likely lies in chemistry applications which cannot be addressed without a device having millions of physical qubits and implementing error correction,” said Vernon.

He added, “Demonstrating robust error correction and achieving fault-tolerance is the biggest hurdle that quantum computing must overcome before it can address the most valuable applications. Good materials fabrication and high-quality lithography and chip packaging are critical.”


Quantum computing potentially offers an exponential acceleration over classical computing, for specific tasks. A central and outstanding challenge to make quantum computing practical is the achievement of fault tolerance, which means that calculations of any length or size can be performed in the presence of noise, provided this does not exceed a certain threshold.

At its most basic level, a quantum computer is a machine that generates qubits in certain states, transforms them using quantum gates, and then measures them. Quantum gates that act on states make use of the entanglement, which fuses qubits so that a description of each qubit’s state is not possible.

In addition to this gate model, a quantum computer can be seen as a cluster state and it can be represented as beads on a wire. As Xanadu points out in one of its blogs, the beads here are qubits, and the wire represents the entanglement between them. The goal here is to measure the qubits. The measurement is not just a reading of the computation; it is the actual computation. A different measurement setup will result in a different gate applied to the qubit.

For the computer to be universal, the cluster state must be at least two-dimensional; for it to be fault-tolerant, it must be three-dimensional. In standard computers, we can protect each bit of information from errors through repetition or redundancy. In quantum computers, redundancy is forbidden. Thus, 3D clusters are used, and their size and arrangement allow topological quantum discrete variable (DV) error correction.

Because they have built-in but challenging error-correcting capabilities, GKP states are ideal qubits. The Gottesman-Kitaev-Preskill (GKP) code is a potential approach to fault-tolerant quantum computation, as it encodes logical qubits into grid states of harmonic oscillators. The quality of the grid states, on the other hand, must be exceptionally high for the code to be fault-tolerant.

Silicon nitride allows creating of compressed states, which can be used to synthesize qubits instead of single photons. The compressed states are deterministically created and can be utilized to distill error-tolerant qubits (GKP states).

“The leading approaches are based on trapped ions, superconducting circuits, and photonics. Photonics is much easier to scale up, because chips can be networked using optical fiber. However, the number of individual components needed per qubit is also larger. Luckily, chip integration and volume manufacturing offered by semiconductor processes help with this,” said Vernon.

Xanadu’s photonic architecture essentially consists of four blocks: the state preparation factory, the multiplexer, the calculation module and the photonic quantum processing unit (QPU). The multiplexer performs many generations of states in parallel to increase the probability of producing a GKP state. The QPU performs the necessary measurements to implement any quantum algorithm and correct errors.

Figure 1: GKP States (Source: Xanadu)

The collaboration

The photonic chip transports light instead of electrons. Xanadu’s approach offers several advantages such as scalability up to one million qubits through the optical network, room temperature computing, and the natural ability to leverage manufacturing R&D centers such as Imec, a semiconductor R&D center for advanced technologies on advanced 200mm and 300mm lines, as well as volume production on their 200mm line.

“To deliver on its full potential, quantum computing must incorporate error correction,” said Vernon. “Doing so means scaling up existing prototypes for qubit chips to thousands of identical modules. Achieving this scale up while maintaining high performance – low losses and good uniformity – and the ability to interconnect these chips is a big challenge and something that the photonic approach is very well suited to address.”

“Imec’s role is to provide extremely low loss and wafer-scale integrated platform to its customers, both on industrial and academic segments,” said Abassi. “Quantum computing based on integrated photonic requires low loss, highly scalable and high performance circuits. At Imec, we are working on these requirements to deliver the state-of-the-art photonic wafers to our customers.”

The modular architecture of photonic chips simplifies the design, fabrication, and integration of quantum and other optoelectronic solutions. Cryogenic requirements are reduced and the homodyne detectors set the device clock speed in the QPU, which operates efficiently and fast. The forthcoming architectures’ modularity, speed and room-temperature operation will enable photonics to contribute to the rapid construction of quantum computers.

This article was originally published on EE Times Europe.

Maurizio Di Paolo Emilio holds a Ph.D. in Physics and is a telecommunication engineer and journalist. He has worked on various international projects in the field of gravitational wave research. He collaborates with research institutions to design data acquisition and control systems for space applications. He is the author of several books published by Springer, as well as numerous scientific and technical publications on electronics design.


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