April 15, 2024

Researchers take a different approach with measurement-based quantum computing

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Atsushi Sakaguchi and his team are exploring the possibility of using light to produce quantum computers based on measurements instead of gates. Credit: RIKEN

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Atsushi Sakaguchi and his team are exploring the possibility of using light to produce quantum computers based on measurements instead of gates. Credit: RIKEN

The race to develop quantum computers has greatly intensified in recent years. Next-generation systems can now run simple algorithms using dozens of qubits (or quantum bits), which are the building blocks of quantum computers.

Much of this success has been achieved in so-called gate-based quantum computers. These computers use physical components, primarily superconducting circuits, to house and control qubits. This approach is quite similar to that of classic computers based on conventional devices. Therefore, the two computing architectures are relatively compatible and could be used together. Additionally, future quantum computers could be made by taking advantage of technologies used to make conventional computers.

But the Optical Quantum Computing research team at the RIKEN Center for Quantum Computing has taken a very different approach. Instead of optimizing gate-based quantum computers, Atsushi Sakaguchi, Jun-ichi Yoshikawa, and team leader Akira Furusawa have been developing measurement-based quantum computing.

Measurement-based computing

Measurement-based quantum computers process information in a complex quantum state known as a cluster state, which consists of three (or more) qubits linked together by a non-classical phenomenon called entanglement. Entanglement occurs when the properties of two or more quantum particles remain linked, even when they are separated by large distances.

Measurement-based quantum computers work by performing a measurement on the first qubit in the cluster state. The result of this measurement determines what measurement to make on the second entangled qubit, a process called feedforward. This then determines how to measure the third. In this way, any quantum gate or circuit can be implemented by appropriately choosing the measurement series.

Measurement-based schemes are very efficient when used in optical quantum computers, as it is easy to entangle a large number of quantum states in an optical system. This makes a measurement-based quantum computer potentially more scalable than a gate-based quantum computer. For the latter, qubits must be manufactured and tuned precisely for uniformity and physically connected to each other. These problems are solved automatically by using a measurement-based optical quantum computer.

Importantly, measurement-based quantum computing offers programmability in optical systems. “We can change the performance simply by changing the measurement,” says Sakaguchi. “This is much easier than changing hardware, as gate-based systems require in optical systems.”

But progress is essential. “Feedforward is a control methodology in which we feed measurement results to a different part of the system as a form of control,” explains Sakaguchi. “In measurement-based quantum computing, feedforward is used to compensate for the inherent randomness of quantum measurements. Without feedforward operations, measurement-based quantum computing becomes probabilistic, while practical quantum computing will have to be deterministic. “.

The Quantum Optical Computing research team and their coworkers (from the University of Tokyo, Palacký University in the Czech Republic, the Australian National University and the University of New South Wales, Australia) have now demonstrated a more advance advance: non-linear. Advance. Nonlinear feedforward is required to implement the full range of potential gates in optics-based quantum computers. The findings are published in the journal. Nature Communications.

“We have experimentally demonstrated nonlinear quadrature measurement using a new nonlinear feed technology,” explains Sakaguchi. “This type of measurement had previously been a barrier to performing universal quantum operations in quantum computing based on optical measurements.”


Gate-based quantum computers are becoming more common. But the Optical Quantum Computing research team at the RIKEN Center for Quantum Computing has been developing measurement-based quantum computing, with digital circuits for electrical-optical control (pictured). Measurement-based systems are potentially more scalable than gate-based quantum computing. Credit: RIKEN

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Gate-based quantum computers are becoming more common. But the Optical Quantum Computing research team at the RIKEN Center for Quantum Computing has been developing measurement-based quantum computing, with digital circuits for electrical-optical control (pictured). Measurement-based systems are potentially more scalable than gate-based quantum computing. Credit: RIKEN

Optical computers

Optical quantum computers use qubits made of packets of light waves. At other institutions, some members of the current RIKEN team had previously built the large optical cluster states needed for measurement-based quantum computing. Linear feedforward has also been achieved to construct simple gate operations, but more advanced gates require nonlinear feedforward.

In 2016, a theory for the practical implementation of nonlinear quadrature measurement was proposed. But this approach presented two major practical difficulties: generating a special auxiliary state (which the team achieved in 2021) and performing a non-linear forward operation.

The team overcame this latest challenge with complex optics, special electro-optic materials, and ultrafast electronics. To do this, they used digital memories, in which the desired nonlinear functions were previously calculated and recorded in the memory. “After the measurement, we transform the optical signal into an electrical one,” explains Sakaguchi. “In the linear feed, we simply amplify or attenuate that signal, but we needed to do much more complex processing for the non-linear feed.”

The key advantages of this non-linear feed technique are its speed and flexibility. The process must be fast enough that the output can be synchronized with the optical quantum state.

“Now that we have shown that we can perform nonlinear feedforward, we want to apply it to quantum computing based on real measurements and quantum error correction using our previously developed system,” says Sakaguchi. “And we hope to be able to increase the speed of our nonlinear feed for high-speed optical quantum computing.”

“But the key message is that, although approaches based on superconducting circuits may be more popular, optical systems are a promising candidate for quantum computer hardware,” he adds.

More information:
Atsushi Sakaguchi et al, Nonlinear Feedforward Enabling Quantum Computing, Nature Communications (2023). DOI: 10.1038/s41467-023-39195-w

Magazine information:
Nature Communications

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