Quantum-computing company Xanadu and the University of Alberta are collaborating to design better cancer drugs of an unusual kind: ones activated by light.
Announced August 14, 2026, the work with chemistry professor Alex Brown will develop quantum algorithms to speed the design of photosensitizers — the molecules at the heart of photodynamic therapy.
How photodynamic therapy works
A photosensitizer is administered and accumulates in tumour tissue, then light of a specific wavelength is directed at the site. The molecule absorbs that light, enters an excited state, and transfers the energy to nearby oxygen — generating reactive species that kill surrounding cells.
The appeal is spatial control. The drug is inert until illuminated, so toxicity is confined to where light is delivered rather than distributed throughout the body. It is used clinically in certain skin, oesophageal and bladder cancers, and in some non-oncology conditions.
The limits are equally clear: light penetrates tissue poorly, so deep tumours are inaccessible, and existing photosensitizers have imperfect selectivity and absorption characteristics.
Why designing these molecules is unusually hard
“Photosensitisers are challenging systems because their performance depends on excited-state processes,” Brown said — and that phrase identifies exactly why computational chemistry struggles here.
Most drug design concerns molecules in their ground state: how a compound is shaped, how it binds a target, how stable it is. Those calculations are demanding and tractable, and modern computational chemistry handles them reasonably well.
A photosensitizer must be characterised in excited states — what happens after absorbing a photon. That involves electrons redistributed into higher-energy configurations, transitions between states of different spin, and energy transfer to oxygen. Each step depends on quantum mechanical properties that approximate poorly.
Classical simulations are slow, costly, or miss critical quantum interactions, and the approximations that make ground-state calculations feasible are precisely the ones that break down in excited states.
Why quantum computing suits this specifically
Chemistry is frequently cited as the most promising near-term application of quantum computing, and the reason is structural rather than promotional.
Simulating a molecule means representing the quantum states of its electrons. On a classical computer, the resources required grow exponentially with the number of electrons, which is why exact calculations are limited to very small systems and everything larger relies on approximation.
A quantum computer represents quantum states natively, using quantum systems to model quantum systems. For problems where the difficulty is the quantum mechanics, that is an inherent advantage rather than merely a faster classical machine.
Excited-state chemistry is among the clearest examples, because it is where classical approximations fail most badly.
What the collaboration will build
The partners will develop a quantum-computing framework to simulate light-matter interactions and pinpoint which molecules best trigger cancer-cell death at the right wavelengths.
The wavelength requirement is a real design constraint rather than a detail. Light penetration through tissue depends strongly on wavelength — red and near-infrared travel considerably further than blue, which is absorbed within a fraction of a millimetre.
A photosensitizer absorbing only blue light is therefore restricted to surface lesions regardless of how efficiently it generates reactive species. Designing for absorption in the red or near-infrared window while retaining efficient energy transfer is the central trade-off, and it is exactly the kind of multi-property optimisation that computational design is meant to accelerate.
The honest position on quantum computing
Current quantum hardware remains limited. Devices available today have modest qubit counts, substantial error rates, and cannot yet run chemistry calculations at scales beyond what classical computers manage.
Work like this is therefore largely algorithm development — building the methods that would run on hardware once it matures, and testing them on small systems or classical simulations of quantum computers. That is legitimate and necessary preparatory work, and it is not the same as solving the chemistry problem now.
Why it is still worth doing
For Xanadu, the project extends its quantum drug-design workflow into a concrete, high-value problem, which serves a purpose beyond the chemistry.
Quantum computing has an application problem: the hardware advances steadily while convincing demonstrations of practical advantage remain scarce. Identifying specific problems where the classical approach genuinely fails — rather than merely being slower — is how the field establishes where the advantage will eventually appear.
The light-delivery problem quantum chemistry cannot fix
Better photosensitizers would address one half of photodynamic therapy’s limitation. The other half is physical and no amount of molecular design resolves it.
Light attenuates rapidly in tissue. Even at the most penetrating wavelengths, useful intensity extends only a centimetre or two, which is why the therapy is used on skin lesions and on surfaces reachable by endoscope — the oesophagus, the bladder, the airways — rather than on solid organs.
Workarounds exist and are cumbersome. Optical fibres can be inserted into tumours to deliver light internally, and implantable light sources have been explored. Both convert a non-invasive treatment into an invasive one, which removes much of the original appeal.
So improved photosensitizers would most plausibly expand the therapy within its existing anatomical territory — better selectivity, fewer side effects, treatment of lesions currently too diffuse to target — rather than opening it to the deep tumours where the greatest need lies. That is a real but bounded prize, and worth keeping in view when the technology is described in general terms.
Excited-state photochemistry is a defensible choice for that, and photodynamic therapy attaches it to a clinical need. It is early-stage research rather than a product, and a sign of quantum methods inching into real drug discovery. R&D news.