The investigation of the quantum properties of propagating microwave fields was believed to be limited by the lack of single photon detectors at these frequencies. In a ground-breaking set of experiments we have demonstrated the detection of single photon anti-bunching and sub-poissonian photon statistics in both pulsed  and continuous  microwave frequency single photon sources (Fig. 1).
|Figure 1: Microscope image of a single photon source connected to a beam-splitter realized in an integrated micro-chip fabricated in the ETH clean-room facilities FIRST. (Image: D. Bozyigit et al., Nat. Phys. 7, 154–158 (2011))|
We have also shown that detection schemes based on linear amplification and quadrature amplitude detection enable the full reconstruction of the quantum state of propagating single photons  even in the presence of noise or limited detection efficiency. These insights have already enabled the generation and detection of entanglement between propagating photons and localized qubits, and the observation of the coalescence of indistinguishable photons at a beam-splitter, known as the Hong-Ou-Mandel effect, at microwave frequencies . These techniques have an impact on both measurement and detection technology of microwaves and for realizing small-scale quantum networks.
 Antibunching of microwave-frequency photons observed in correlation measurements using linear detectors, Nat. Phys. 7, 154–158 (2011)
 Observation of Resonant Photon Blockade at Microwave Frequencies Using Correlation Function Measurements, Phys. Rev. Lett. 106, 243601 (2011), also in arXiv:1102.0461
 Experimental State Tomography of Itinerant Single Microwave Photons, Phys. Rev. Lett. 106, 220503 (2011), also in arXiv:1011.6668
 Correlations, indistinguishability and entanglement in Hong-Ou-Mandel experiments at microwave frequencies, Nat. Phys. advance online publication, – (2013), also in arXiv:1301.4458