states have been generated experimentally by various approaches. In the optical regime, cat states of small size, which are sometimes called Schrödinger kittens, have been generated by subtracting one photon from squeezed states of light [@Ourjoumtsev2006a; @Wakui2007a]. Optical cat states of a little larger size have been generated by other methods [@Ourjoumtsev2007a; @Sychev2017a]. Note that these optical cat states are of *traveling* fields and generated *probabilistically*.

In the microwave regime, cat states of larger size have been generated experimentally [@Deleglise2008a; @Vlastakis2013a; @Leghtas2015a; @Touzard2018a]. The generation using Rydberg atoms [@Deleglise2008a] is heralded by measurement results of the atomic states, where the parity, ‘even’ or ‘odd’, of the cat state is determined randomly according to the measurement results. *On-demand* generations of microwave cat states have been demonstrated using superconducting circuits by two different approaches, one of which is based on conditional operations using a superconducting quantum bit (qubit) [@Vlastakis2013a] and the other is based on two-photon driving and two-photon loss larger than one-photon loss [@Leghtas2015a; @Touzard2018a]. By extending the former approach to a two-cavity case, entangled coherent states in two cavities have been observed experimentally [@Wang2016a]. Note that these microwave cat states are confined *inside* cavities. Recently, the cat state generated inside 