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A leap towards practical photonic quantum computing

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New method can be used to measure large-scale quantum correlation of single photons, which until now would have required thousands of single photon counters

Researchers have demonstrated a way to map and measure large-scale photonic quantum correlation with single-photon sensitivity. The ability to measure thousands of instances of quantum correlation is critical for making photon-based quantum computing practical.

Published earlier this year in Optica, a multi-institutional group of researchers reports the new measurement technique, which is called correlation on spatially-mapped photon-level image (COSPLI). The team also developed a way to detect signals from single photons and their correlations in tens of millions of images.

This illustration represents a new approach for mapping and measuring photonic correlation with direct single-photon imaging. Credit: Xian-Min Jin, Shanghai Jiao Tong University

Research team leader Xian-Min Jin, from Shanghai Jiao Tong University, China, said: ‘COSPLI has the potential to become a versatile solution for performing quantum particle measurements in large-scale photonic quantum computers. This unique approach would also be useful for quantum simulation, quantum communication, quantum sensing and single-photon biomedical imaging.’

Interacting photons

Quantum computing technology promises to be significantly faster than traditional computing, which reads and writes data encoded as bits that are either a zero or one. Instead of bits, quantum computing uses qubits that can be in two states at the same time and will interact, or correlate, with each other. These qubits, which can be an electron or photon, allow many processes to be performed simultaneously.

One important challenge in the development of quantum computers is finding a way to measure and manipulate the thousands of qubits needed to process extremely large datasets. For photon-based methods, the number of qubits can be increased without using more photons by increasing the number of modes encoded in photonic degrees of freedom – such as polarisation, frequency, time and location – measured for each photon. This allows each photon to exhibit more than two modes, or states, simultaneously. The researchers previously used this approach to fabricate the world’s largest photonic quantum chips, which could possess a state space equivalent to thousands of qubits.

However, incorporating the new photonic quantum chips into a quantum computer requires measuring all the modes and their photonic correlations at a single-photon level. Until now, the only way to accomplish this would be to use one single-photon detector for each mode exhibited by each photon. This would require thousands of single-photon detectors and cost around 12 million dollars for a single computer.

‘It is economically unfeasible and technically challenging to address thousands of modes simultaneously with single-photon detectors,’ said Jin. ‘This problem represents a decisive bottleneck to realising a large-scale photonic quantum computer.’

Single-photon sensitivity

Although commercially available CCD cameras are sensitive to single photons and much cheaper than single-photon detectors, the signals from individual photons are often obscured by large amounts of noise. After two years of work, the researchers developed methods for suppressing the noise so that single photons could be detected with each pixel of a CCD camera.

The other challenge was to determine a single photon’s polarisation, frequency, time and location, each of which requires a different measurement technique. With COSPLI, the photonic correlations from other modes are all mapped on to the spatial mode, which allows correlations of all the modes to be measured with the CCD camera.

To demonstrate COSPLI, the researchers used their approach to measure the joint spectra of correlated photons in 10 million image frames. The reconstructed spectra agreed well with theoretical calculations, thus demonstrating the reliability of the measurement and mapping method, as well as the single-photon detection. The researchers are now working to improve the imaging speed of the system from tens to millions of frames per second.

‘We know it is very hard to build a practical quantum computer, and it isn’t clear yet which implementation will be the best,’ said Jin. ‘This work adds confidence that a quantum computer based on photons may be a practical route forward.’

Generating high-quality single photons for quantum computing

In separate research that could advance practical quantum computing, a team from the Massachusetts Institute of Technology (MIT) has designed a way to generate, at room temperature, more single photons for carrying quantum information.

MIT researchers designed a single-photon emitter that generates, at room temperature, high-quality photons that could be useful for practical quantum computers. Credit: MIT

Quantum emitters generate photons that can be detected one at a time. Consumer quantum computers and devices could potentially leverage certain properties of those photons as quantum bits (‘qubits’) to execute computations. 

A key challenge, however, is producing single photons with identical quantum properties — known as ‘indistinguishable’ photons. To improve the indistinguishability, emitters funnel light through an optical cavity where the photons bounce back and forth, a process that helps match their properties to the cavity. Generally, the longer photons stay in the cavity, the more they match.

But there’s also a trade-off. In large cavities, quantum emitters generate photons spontaneously, resulting in only a small fraction of photons staying in the cavity, making the process inefficient. Smaller cavities extract higher percentages of photons, but the photons are lower quality, or ‘distinguishable.’

In a paper published in May in Physical Review Letters, the researchers split one cavity into two, each with a designated task. A smaller cavity handles the efficient extraction of photons, while an attached large cavity stores them a bit longer to boost indistinguishability.

Compared to a single cavity, the researchers’ coupled cavity generated photons with around 95 per cent indistinguishability, compared to 80 per cent indistinguishability, with around three times higher efficiency.

‘In short, two is better than one,’ said first author Hyeongrak Choi, a graduate student in the MIT Research Laboratory of Electronics (RLE). ‘What we found is that in this architecture, we can separate the roles of the two cavities. The first cavity merely focuses on collecting photons for high efficiency, while the second focuses on indistinguishability in a single channel. One cavity playing both roles can’t meet both metrics, but two cavities achieves both simultaneously.’

The relatively new quantum emitters, known as ‘single-photon emitters,’ are created by defects in otherwise pure materials, such as diamonds, doped carbon nanotubes, or quantum dots. Light produced from these ‘artificial atoms’ is captured by a tiny optical cavity in photonic crystal – a nanostructure acting as a mirror. Some photons escape, but others bounce around the cavity, which forces the photons to have the same quantum properties – mainly, various frequency properties. When they’re measured to match, they exit the cavity through a waveguide.

But single-photon emitters also experience large amounts of environmental noise, such as lattice vibrations or electric charge fluctuation, which produce different wavelength or phase. Photons with different properties cannot be ‘interfered’, such that their waves overlap, resulting in interference patterns. That interference pattern is what a quantum computer observes and measures to do computational tasks.

Photon indistinguishability is a measure of photons’ potential to interfere. In that way, it’s a valuable metric to simulate their usage for practical quantum computing. ‘Even before photon interference, with indistinguishability, we can specify the ability for the photons to interfere,’ Choi explained. ‘If we know that ability, we can calculate what’s going to happen if they are using it for quantum technologies.’

In the researchers’ system, a small cavity sits attached to an emitter, which in their studies was an optical defect in a diamond, called a ‘silicon-vacancy centre’ – a silicon atom replacing two carbon atoms in a diamond lattice. Light produced by the defect is collected into the first cavity. Because of its light-focusing structure, photons are extracted with very high rates. Then, the nano-cavity channels the photons into a second, larger cavity. There, the photons bounce back and forth for a certain time. When they reach a high indistinguishability, the photons exit through a partial mirror formed by holes connecting the cavity to a waveguide.

Importantly, Choi noted, neither cavity has to meet rigorous design requirements for efficiency or indistinguishability as traditional cavities, called the ‘quality factor (Q-factor)’. The higher the Q-factor, the lower energy loss in optical cavities. But cavities with high Q-factors are challenging to make.

In the study, the researchers’ coupled cavity produced higher quality photons than any possible single-cavity system. Even when its Q factor was roughly one-hundredth the quality of the single-cavity system, they could achieve the same indistinguishability with three times higher efficiency.

The cavities can be tuned to optimise for efficiency versus indistinguishability – and to consider any constraints on the Q factor – depending on the application. That’s important, Choi added, because today’s emitters that operate at room temperature can vary greatly in quality and properties.

Next, the researchers are testing the ultimate theoretical limit of multiple cavities. One more cavity would still handle the initial extraction efficiently, but then would be linked to multiple cavities for photons of various sizes to achieve some optimal indistinguishability. But there will most likely be a limit, Choi said: ‘With two cavities, there is just one connection, so it can be efficient. But if there are multiple cavities, the multiple connections could make it inefficient. We’re now studying the fundamental limit for cavities for use in quantum computing.’

 

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