When we look at matter on the scale of atoms and electrons, we see counterintuitive behaviour. Quantum particles, for example, can exist in two states at once, a phenomenon called superposition. Famously, Erwin Schrödinger used the image of a cat that is both dead and alive to illustrate this dissonance between what quantum theory tells us and what we observe in our everyday experience. But even for quantum physicists, the notion of getting information from a computer without turning it on seems odd. “Everyone thinks it's weird. It is unintuitive,” says Onur Hosten, a graduate student at the University of Illinois at Urbana-Champaign.

Hosten's work demonstrating counterfactual computation (CFC) — the process by which a quantum computer's solution to a computation may be learnt without actually running the computer — appears on page 949 of this issue. Strange as it is, CFC is not a new concept. “It was shown to be theoretically possible in 1998, but thought to be no better than random guessing,” explains Hosten.

When Hosten started working on the project in 2004, at the suggestion of his supervisor, Paul Kwiat, few people were investigating the phenomenon. “It was thought odd and of no practical utility,” says Hosten. But what began as an exercise in theory led to interesting and potentially useful results. He demonstrated that CFC was physically possible. “I then came up with the idea of beating the random-guessing limit,” he adds. Hosten showed that CFC could be used to infer the outcome of a computation with a higher probability of getting the right answer than random guessing. He then spent the following year thinking about possible applications for his finding. “We found that with CFC you can avoid some errors that are happening in the computer,” he explains, a discovery that could lead to methods for making quantum computing more robust and feasible.

The key to Hosten's success was finding a way to superpose the states ‘running’ and ‘not running’ in a quantum computer. He used an optical approach: a single photon could travel through the computer (representing the computer running) or an empty path (computer not running). Because in quantum physics the photon, like Schrödinger's cat, can simultaneously exist in different places or states, this allowed Hosten to superpose the computer's on and off states and infer an answer to a computation without running the computer. “Although you could get the result without running the computer, you still need the computer to be there, and you need to give it the same time to function as if it were running. Otherwise you cannot superpose the two histories to get the result,” says Hosten. Sounds complicated? It is. “I understand the process in terms of the mathematics and physics involved, but if I get into very deep philosophical questions, I will fail,” Hosten adds.

While others in the field ponder his study, Hosten has moved to the next challenge, quantum non-demolition measurements, or the ability to detect a photon's presence without destroying it in the process. Once again, the process has an established theory, but no one has put it to the test yet. “That's what I do,” says Hosten.