Research Bits: June 4

Ultra-pure silicon

Researchers from the University of Manchester and University of Melbourne developed a technique to engineer ultra-pure silicon that could be used in the construction of high-performance qubit devices that extend quantum coherence times.

The highly purified silicon chips house and protect the qubits so they can sustain quantum coherence much longer, enabling complex calculations with greatly reduced need for error correction, explained David Jamieson, a professor at the University of Melbourne, in a release. “We believe silicon is the leading candidate for quantum computer chips that will enable the enduring coherence required for reliable quantum calculations. The problem is that while naturally occurring silicon is mostly the desirable isotope silicon-28, there’s also about 4.5 percent silicon-29. Silicon-29 has an extra neutron in each atom’s nucleus that acts like a tiny rogue magnet, destroying quantum coherence and creating computing errors.”

Using a standard ion implanter tuned to a specific configuration, the researchers directed a focused, high-speed beam of pure silicon-28 at a silicon chip so the silicon-28 gradually replaced the silicon-29 atoms in the chip, reducing silicon-29 from 4.5% to two parts per million (0.0002%).

“Now that we can produce extremely pure silicon-28, our next step will be to demonstrate that we can sustain quantum coherence for many qubits simultaneously. A reliable quantum computer with just 30 qubits would exceed the power of today’s supercomputers for some applications,” Jamieson added. [1]

Diamond qubit array on CMOS

Researchers from Massachusetts Institute of Technology, Cornell University, and MITRE Corporation demonstrated a scalable, modular hardware platform that integrates thousands of interconnected qubits onto a customized “quantum-system-on-chip” (QSoC). By tuning qubits across 11 frequency channels, the QSoC architecture allows for a protocol of “entanglement multiplexing” for large-scale quantum computing.

The researchers used qubits made from diamond color centers, solid-state systems that carry quantum information and are compatible with semiconductor fabrication processes.

“The conventional assumption in the field is that the inhomogeneity of the diamond color center is a drawback compared to identical quantum memory like ions and neutral atoms. However, we turn this challenge into an advantage by embracing the diversity of the artificial atoms: Each atom has its own spectral frequency. This allows us to communicate with individual atoms by voltage tuning them into resonance with a laser, much like tuning the dial on a tiny radio,” said Dirk Englund, professor of EECS at MIT, in a statement.

Integrating a large array of diamond color center qubits onto a CMOS chip provided the control dials. The chip can be incorporated with built-in digital logic that rapidly and automatically reconfigures the voltages, enabling the qubits to reach full connectivity.

The researchers demonstrated a 500-micron by 500-micron area transfer for an array with 1,024 diamond nanoantennas, but they could use larger diamond arrays and a larger CMOS chip to further scale up the system. They also found that with more qubits, tuning the frequencies requires less voltage for this architecture. [2]

Tiny quantum light detector

Researchers from the University of Bristol integrated a tiny quantum light detector onto a silicon electronic-photonic quantum chip.

At 80 micrometers by 220 micrometers, the small size of circuit improves its operational speed without sacrificing sensitivity. It was built with a commercially accessible foundry.

“These types of detectors are called homodyne detectors, and they pop up everywhere in applications across quantum optics,” said Jonathan Matthews, a professor and director of the Quantum Engineering Technology Labs at Bristol, in a statement. “They operate at room temperature, and you can use them for quantum communications, in incredibly sensitive sensors — like state-of-the-art gravitational wave detectors — and there are designs of quantum computers that would use these detectors.” [3]

References

[1] Acharya, R., Coke, M., Adshead, M. et al. Highly 28Si enriched silicon by localised focused ion beam implantation. Commun Mater 5, 57 (2024). https://doi.org/10.1038/s43246-024-00498-0

[2] Li, L., Santis, L.D., Harris, I.B.W. et al. Heterogeneous integration of spin–photon interfaces with a CMOS platform. Nature (2024). https://doi.org/10.1038/s41586-024-07371-7

[3] Joel F. Tasker et al., A Bi-CMOS electronic photonic integrated circuit quantum light detector. Sci. Adv. 10, eadk6890 (2024). https://doi.org/10.1126/sciadv.adk6890