This 2015 Wired article claims it is possible to use a remote device to determine a key used during decryption by sampling the radio emissions from the processor at 100kHz
They also claim that by:
tricking the target into decrypting a carefully chosen message, they were able to “twist the algorithm’s arm” into leaking more sensitive information, creating more clues in the leaked emanations for their PITA radio to pick up.
Even with this condition, it seems unlikely for a modern processor to emit enough radio noise such that a device can get meaningful info. But I'm not an electrical engineer