Microwave signals comprising correlated photons could be used to code information for quantum computing and may offer an alternative use of optical systems to build quantum computers. Researchers at Aalto University chilled a microwave resonator to nearly absolute zero temperature — the point at which any thermal motion freezes — to correspond to a state of perfect darkness. In this quantum vacuum state no photon is present, but there exist fluctuations that can bring photons in and out of existence for a very short time. Artistic depiction of the generation of three correlated photons from quantum vacuum. Courtesy of Antti Paraoanu. When the researchers converted these fluctuations into real photons, with different frequencies, they found that the photons were correlated with each other. “This all hints at the possibility of using the different frequencies for quantum computing. The photons at different frequencies will play a similar role to the registers in classical computers, and logical gate operations can be performed between them,” said professor Sorin Paraoanu. In the vacuum state, fluctuations occurring at different frequencies were uncorrelated. However, the researchers found that if a parameter in the Lagrangian of the field was modulated by an external pump, vacuum fluctuations stimulated spontaneous down-conversion processes, creating squeezing between modes symmetric with respect to half of the frequency of the pump. They showed that it was possible to generate coherence between photons in separate frequency modes through double parametric pumping of a superconducting microwave cavity. The coherence correlations were tunable and were established by a quantum fluctuation that stimulated the simultaneous creation of two photon pairs. Experimental results indicated that the origin of this vacuum-induced coherence was the absence of which-way information in the frequency space. “With our experimental setup we managed to create complex correlations of microwave signals in a controlled way,” said doctoral student Pasi Lähteenmäki. From this research, a novel approach to quantum computing may emerge — the ability to engineer the quantum vacuum to create novel devices and protocols for quantum technologies. “Today the basic architecture of future quantum computers is being developed very intensively around the world,” said professor Pertti Hakonen. “By utilizing the multifrequency microwave signals, an alternative approach can be pursued which realizes the logical gates by sequences of quantum measurements. Moreover, if we use the photons created in our resonator, the physical quantum bits or qubits become needless.” The research was published in Nature Communications (doi:10.1038/ncomms12548).