Various embodiments of the present invention are directed to compact systems for generating polarization-entangled photons. A mixture of metal oxides and carbon is introduced into the furnace, the metal oxide is reduced by the carbon below the slag at the metalslag interface, and the resulting gases bring about the foaming of the slag by bubble formation. Queues are created by chaining one or more memory segments together. An image is printed at the color printer in accordance with the print image data. Then, the communication apparatus generates and transmits a command of the second protocol to the second device, in accordance with the statuses of the first device obtained before and after the reception of the message.