Ions and Photons – Two Quantum Quests

In their quest for the perfect quantum computer, scientists are taking diverse routes – each showing unique promises of its own 

By now, word has spread far and wide that quantum computers could have the potential to accomplish complex calculations at mindboggling seeds. Computations that might take millions of years to be executed by the most advanced supercomputers today can be handled by quantum computers within minutes. Scientists agree that when put to practical use, full-scale quantum computing would definitely transform every industry that require immense data crunching.

However, that would only be possible when we can develop full-scale quantum computers with multi-million qubits. As of now, such machines– mostly based on superconducting circuits –are still in the works. They work great at labs, but are not yet ready for market scale. With super conducting technology, quantum computers of that scale would require gigantic cooling set-up, and even then might not be as predictable as conventional computing. Moreover, there is a limit to cooling capabilities, making it difficult to cool large devices. That, in turn, prevents upscaling to sizes large enough to hold millions of qubits.

 Trapped Ions

This is where the trapped ion technology has proved itself. A development team has announced designing a microchip-based quantum computer based on an architecture where computation is done by shuttling trapped atomic ions. The invention has displayed admirable performance outcomes and has the potential for scaling up according to requirements. In a recent article published in Nature, the team has reported the construction and operation of a prototype microchip-based, trapped-ion quantum computer that incorporates a promising architecture based on ion shuttling.

Using trapped atomic ions for computation is not really unique. This technology has already proved to be an effective hardware platform for quantum information processing. It has been used to design quantum gates – the basic building blocks of a quantum computer – with the minimum error level on any hardware platform. However, no one has yet claimed to have effectively built a scaled-up quantum machine with immense qubit count. This is where the current developers score.

In the reported architecture, ions hover above the surface of the microchip and are moved along tracks by adjusting voltages applied to electrodes located on the surface of the chip. Being similar to charge-coupled device (CCD) array microchips, this new invention is also being dubbed the quantum-CCD architecture. This method totally bypasses the need for supercooling, thus facilitating practical quantum computers on a large scale. It is by far the most advanced implementation of the trapped-ion technology.

The paper is available at:https://www.nature.com/articles/s41586-021-03318-4

 Photons

Another technology that is making waves in the quantum fraternity is the photonic quantum computing chip. Developed by the Toronto-based start-up Xanadu, the X8 photonic quantum computing chip says their quantum computer is Python programmable, can execute multiple algorithms, and is potentially highly scalable, apart from being cloud-accessible, too!

Quantum computers based on photons scores over electron-based systems, as they can operate at room temperature. The Xanadu innovation boasts of the added advantages of scalability and speed as well. It also allows integration with existing fibre optic-based telecommunications infrastructure – paving a future highway of quantum networks and quantum Internet. The prototype measuring 4 mm by 10 mm, the silicon nitride X8 chip is actually an 8-qubit quantum computer that is compatible with conventional semiconductor industry fabrication techniques, and hence can be readily scaled up to hundreds of qubits.

Xanadu’s X8 photonic quantum computing chip
Image courtesy: Xanadu

Infrared laser pulses fired into the chip are coupled together with microscopic resonators to generate so-called “squeezed states” consisting of superpositions of multiple photons. The light next flows to a series of beam splitters and phase shifters that perform the desired computation. The photons then flow out the chip to superconducting detectors that count the photon numbers to extract the answer to the quantum computation.

Speaking to the media, Zachary Vernon, head of hardware at Xanadu, was definitely upbeat. “With these results, alongside the growing intensity of progress from academic groups and other photonic quantum computing companies, it’s becoming clear that photonics is not an underdog, but in fact one of the leading contenders.”

The X8 chip is available over the Cloud. Remote users with no prior knowledge of the hardware works can still program the device using Strawberry Fields, Xanadu’s Python library for simulating and executing programs on photonic quantum hardware, and PennyLane, the company’s Python library for quantum machine learning, quantum computing and quantum chemistry.

Obviously, it is just a prototype still and is yet far from perfection. The greatest challenge in scaling up involves reducing the lost photons that keep whizzing inside the circuitry. Another limitation isthe use of superconducting photon detectors– which part would still require supercooling as superconductivity is involved. The rest of the system is otherwise contained in a standard server rack. The company is now focussing on error correction strategies that would turn the photon-based quantum machines more tolerant of noise and defects – enabling practical applications.