Scientists have demonstrated quantum wave mixing (QWM) on a single superconducting artificial atom. In their exploration of the different regimes of QWM, they showed that superposed and coherent states of light were mapped into a quantized spectrum of narrow peaks, and they found that the number of peaks was determined by the number of interacting photons. Researchers further found that the artificial atom visualized photon-state statistics, distinguishing coherent, one- and two-photon superposed states with the finite number of peaks in the quantum regime. Physicists uncovered the unusual wave mixing effects on a single artificial atom in the gigahertz frequency range. Courtesy of MIPT Press Office. According to the research team from the Moscow Institute of Physics and Technology (MIPT) and Royal Holloway, University of London, quantum wave mixing has never before been observed in a system of this type. Results could provide insight into nonlinear quantum effects in microwave optics with artificial atoms and could be useful for building new types of on-chip quantum electronics. The graph above shows the results of the experiment: Wave frequencies are plotted on the horizontal axis against radiation intensity on the vertical axis. The two highest peaks correspond to source radiation frequencies. The peaks labeled in pink, orange and purple are attributed to the phenomenon of quantum wave mixing on a single atom. The top graph illustrates the experiment using an artificial atom with two energy levels, while the bottom graph corresponds to the case with three energy levels. Courtesy of MIPT Press Office. “Our paper reports the findings of an experiment demonstrating unusual wave mixing effects on a single artificial atom in the gigahertz frequency range. We examined a qubit strongly coupled to the electromagnetic field in the transmission line and observed the mixing of the photonic quantum state prepared in the qubit with that of the coherent light in the transmission line,” said researcher Aleksei Dmitriev. The researchers believe that this observation could lead to a way to visualize the quantum state statistics of source photons. Signals incoming via an on-chip superconducting stripe are shown in blue. The artificial atom is depicted as a square in the lower left part of the chip. It is coupled to the superconducting stripe and to the ground contact. Courtesy of MIPT Press Office. Beyond its capacity to uncover quantum optical effects, an artificial atom — considered a staple of quantum optics experiments — can double as a qubit, the basic element of a quantum computer that enables computations using units of information that, unlike conventional units, can be in two states (one and zero) at the same time. The research was published in Nature Communications (doi:10.1038/s41467-017-01471-x).