(Phys.org)—Due to quantum effects, it's possible to build a quantum computer that computes without running—or as the scientists explain, "the result of a computation may be learned without actually running the computer." So far, however, the efficiency of this process, which is called counterfactual computation (CFC), has had an upper limit of 50%, limiting its practical applications.
Now in a new paper, scientists have experimentally demonstrated a slightly different version called a "generalized CFC" that has an efficiency of 85% with the potential to reach 100%. This improvement opens the doors to realizing a much greater variety of applications, such as low-light medical X-rays and the imaging of delicate biological cells and proteins—in certain cases, using only a single photon.
The researchers, led by Prof. Jiangfeng Du at the University of Science and Technology of China and Prof. Liang Jiang at Yale University in the US, have published a paper on the high-efficiency counterfactual computing method in a recent issue of Physical Review Letters.
"The main keys to achieving high-efficiency CFC include the utilization of exotic quantum features (quantum superposition, quantum measurement, and the quantum Zeno effect), as well as the use of a generalized CFC protocol," Du told Phys.org.
How counterfactual computing works
By "not running," the scientists mean that the computer—which can operate in either an "on" subspace or an "off" subspace—stays in its "off" subspace for the entire computation. Physically maintaining the computer in the "off" subspace, in this scheme, involves controlling the spin properties of a diamond system, which acts as a quantum switch. Some of the spins must be kept in a superposition state, in which they occupy two states at the same time.
To control the spin superposition, the physicists took advantage of the quantum Zeno effect, in which frequent measurements on a system can "freeze" the system in its current state. By applying a sequence of pulses to the system, the scientists could keep the system in its "off" subspace, and so keep it from running.
"The procedure comprises a quantum switch and a quantum register," Jiang explained. "For each repetition, we prepare the quantum switch into a quantum superposition state, including two coherent parts ('on' and 'off'). Then the 'algorithm,' a NOT gate on the quantum register in our case, is performed in the 'on' subspace. Although it seems the computer has run in this step, a consequent projective measurement will remove all the changes in the 'on' subspace, since the probability of the whole system collapsing into the 'off' subspace during the measurement is very large (approaches 100% as the number of repetitions tends to infinity utilizing the quantum Zeno effect)."
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