Simple chip design generates spectrally pure single photons
The group of CQT Fellow Zhu Di designed the chip for applications in quantum computing and networking
The CQT team’s lithium niobate chip creates photons with desirable properties for quantum technologies.
Thanks to companies pursuing photonic quantum computing, there is buzz about the idea of building quantum computers that calculate with light. Singapore researchers have designed a new chip good at generating the identical photons needed to start a calculation. Their work is published on 23 June in Physical Review Letters.
CQT Fellow Zhu Di and his collaborators made the photon source using the material thin-film lithium niobate, which is a promising platform for integrated quantum photonics. On a chip smaller than a fingernail, they control the modes of light to create photon pairs with two desirable properties: they have the same polarisation but are only weakly correlated in frequency.
The first property simplifies the chip design, while the second means that a single photon can be extracted with high ‘spectral purity’ – a technical term that refers to the photon being prepared in the same, well-defined quantum state. Photons identical in this way interfere reliably with one another, which is essential for successful computation. Such single photon sources also have applications in quantum networking.
One from two
One common approach to create single photons is to use sources of entangled photon pairs. In a process called spontaneous parametric down-conversion (SPDC), one photon entering a nonlinear material, like thin-film lithium niobate, can split into two lower-energy photons. One photon from each pair, the idler, is detected to herald the arrival of the other, the signal photon, which is routed for quantum information processing tasks.
The problem is that SPDC naturally makes the two photons strongly correlated in frequencies. After the measurement of the idler, the signal photons can be left in a mix of different frequency states – in other words, they have low spectral purity. Photonic computations are done by interfering the photons, and this interference is made less reliable by poor spectral purity.
Wang Xiaojie, CQT Affiliate and first author of the new paper, explains: “If the photons do not interfere as intended, it leads to incorrect measurement outcomes and increased operation errors.”
From their new chip, Di, Xiaojie and their co-authors report generating photons with purities of about 94%, competitive with other single-photon sources.
Shapes of light
The group suppress the frequency correlations by taking advantage of the different spatial shapes – called ‘modes’ – that light can take inside a waveguide. The chip is etched with a waveguide that directs the incoming laser light to where a second waveguide runs in parallel. Entangled photons appear in these dual channels because of the way the light and material interact.
The team’s innovation was to design the waveguides to control the modes of light. In each pair, one photon is generated in the fundamental mode, having a simple shape, and the other in a higher-order mode with a more complex shape. These modes affect the photon’s speed of travel in the waveguide, helping to weaken the correlation between their frequencies. After generation, an on-chip mode converter changes the higher-order mode back into the fundamental mode. The two photons then exit through the separate channels.
“By carefully tuning how fast each photon travels, we can reduce the usual one-to-one link between the signal and idler frequencies,” says Xiaojie. “This makes the two photons much less spectrally correlated.”
To improve purity, the researchers also implemented a known technique called ‘Gaussian-apodised poling’. Lithium niobate is a ferroelectric material, which means that it has an electric dipole orientation. Poling involves creating an up-down-pattern in the electric dipole along the waveguide.
Scaling up
For next steps, the researchers want to improve their device fabrication to integrate more single photon sources on one chip. “Beyond that, we want to be able to produce many chips on a wafer,” says Di. “Eventually for a utility-scale quantum computer, we are talking about millions of such sources.”
The researchers are based at the Department of Materials Science and Engineering at the National University of Singapore (NUS) and the Agency for Science, Technology and Research (A*STAR). They made the chip in fabrication facilities at NUS.
