Boron arsenide semiconductor sets record in quantum vibrations

Cubic boron arsenide has set a record for how long quantum vibrations persist in a semiconductor — and the reason why is a small lecture in the physics of heat and noise.

A team at Rice University, working with collaborators at the University of Houston and Texas Tech University, reported March 20 in Physical Review Letters that optical phonons in boron-11-enriched cubic boron arsenide completed nearly 1,000 vibration cycles before decaying, compared to fewer than 100 cycles typical of other semiconductors. The quality factor — a measure of how sharply the material resonates — exceeded 3.7 × 10³ below 100 kelvin. The paper, led by first author Tong Lin, a Rice doctoral alumna, and corresponding author Hanyu Zhu, the William Marsh Rice Chair and associate professor of materials science and nanoengineering at Rice University, identifies why this material behaves so differently.

In most semiconductors, the dominant friction on vibrating atoms comes from three-phonon scattering — an optical phonon interacting with two acoustic phonons and dissipating energy as heat. In boron arsenide, that process is physically blocked. An optical phonon in this material carries more energy than any possible combination of two outgoing acoustic phonons, so the standard three-phonon scattering channel simply cannot occur. The much less probable four-phonon scattering takes over instead. "The friction against one optical phonon by two acoustic phonons does not occur," Zhu said in the Rice news release. That is the mechanism, stated plainly.

The finding also clarifies what destroys coherence when everything else is working. The team determined that residual boron-10 isotopes — the minority boron isotope present even in enriched samples — are the main culprit of coherence loss at the quantum ground state. Removing that isotope impurity entirely could extend phonon lifetime by another factor of 10, according to the paper. Structural defects, by contrast, had no measurable effect on optical phonon coherence, a result the researchers called both surprising and fortunate.

The practical stakes are not abstract. Phonons are the quantum of lattice vibration: acoustic phonons conduct heat, and optical phonons govern infrared thermal radiation. Both processes are central to how semiconductors manage energy at the nanoscale — and to how quantum circuits shed heat without corrupting qubit states. Longer-lived phonon coherence does not automatically translate to a better qubit, but it does mean this material offers a unusually clean mechanical degree of freedom, a property that could eventually matter for phonon-based quantum memories or for managing thermal load in dense classical electronics.

The team discriminated three separate decoherence mechanisms by measuring how the damping rate changed with temperature and fitting the contribution of each. They used high-resolution Raman spectroscopy and Fourier transform infrared spectroscopy — standard tools, but applied here with enough resolution to separate channels that usually blend. The research was funded by the Welch Foundation, the Air Force Office of Scientific Research, the Department of Energy, the National Science Foundation, and Qorvo Inc.

The result raises a question the paper does not answer: whether this coherence advantage survives at temperatures closer to where quantum hardware actually operates. Below 100 kelvin is cold, and the regime where four-phonon scattering dominates may behave differently as temperature rises. The boron-10 isotope problem also does not disappear at higher temperatures. But the underlying physics — a material whose dominant scattering channel is intrinsically suppressed — is a genuine structural feature, not a parameter that can be tuned away. Whether it translates to a useful quantum or classical device depends on solving the rest of the stack.

The record is real. The mechanism is interesting. The path from here to a lab advantage is still long enough to require a receipt.