University of Illinois researchers are in it passed to determine the result of an algorithm without ever running the algorithm. This was achieved by combining quantum calculations with a quantum question and using an effect called ‘counterfactual computation’. A prerequisite for this effect is that a quantum computer is available that is programmed to perform a calculation as soon as it is turned on. If there is a possibility that the computer is already performing the calculation, when in reality the computer has not performed the calculation, it is possible that the result of a calculation becomes known without starting the calculation. The scientists developed an optical quantum computer to demonstrate this effect.
Quantum interrogation, also known as ‘measuring without interaction’, is a technique that makes use of the property that, for example, photons sometimes behave like a wave and sometimes like a particle. This dualistic property is used to seek out a particular region in a space, without actually entering that region. Using a clever arrangement of three interferometers, the team managed to Grover’s quantum search algorithm search a four-element database. ‘By placing the photon in a superposition of the search algorithm rotating and non-rotating, we got information about the answer even when the photon was not executing the search algorithm,’ says Onur Hosten, lead author of the article on the study that appears in Nature. We also theoretically demonstrated how to get the answer without ever starting the algorithm, using the ‘chained Zeno’ effect‘.
By making clever use of interference properties and dividing rays, the researchers can place each photon in a superposition of taking two paths. While a photon can occupy multiple positions at once, it can only appear in one place at a time. The appearance then determines the path and that, in a very strange way, may mean that the search algorithm does not need to be started. Kwiat, head of the research team, says the possibility that the algorithm could start somehow prevents the algorithm from starting. “This is the heart of quantum question schemes, and quantum mechanics doesn’t get much more mysterious to me than this.” Although the researchers’ optical quantum computer cannot be scaled up, using such questioning techniques could reduce the number of errors in future quantum computers. “All efforts to reduce the error rate of quantum computers make it more likely that a large-scale quantum computer can eventually be made,” said Kwiat.