Searching for the phrase "electron reactor" and "alien" I turned up a possibility. Does this sound familiar?
A Signal Shattered
by Eric S. Nylund
Thanks to the treachery of an all-but-omnipotent alien known as Wheeler -- mathematician Jack Potter's former business partner in the trade of alien and human technologies -- Earth has become the graveyard of billions. When Jack refused to find new worlds for Wheeler to despoil, the unscrupulous alien terminated their relationship...along with every living thing on Earth. But Jack and a handful of others escaped the holocaust by using an alien technology known as the gateway that allows instantaneous transmission of matter over vast distances. Now these few survivors are all that's left of the human race. Meanwhile, Wheeler is out to finish what he started...and Jack's only hope is the gateway. If he can hack into the device and decrypt its alien coding, he may learn the secret of a power great enough to destroy even Wheeler -- if it doesn't destroy Jack first.
The above is a sequel. The first book was Signal To Noise.
Jack Potter puts computer cryptography to work for the highest bidder: sometimes for private corporations, sometimes for the government. Sometimes the work is legal; if not, Jack simply raises his price. But one day, Jack discovers something cloaked in the hiss of background radiation streaming past the Earth from deep space: a message from an alien civilization. One that's eager to do business with humanity -- and its representative.
Before he knows it, Jack has entered into a partnership that will open a Pandora's Box of potential profit and loss. The governments, the multinationals, and mysterious players more powerful still, all want a piece of the action -- and they're willing to kill, even wage war, to get it. Now Jack is entangled shifting web of deceit and intrigue in which no one, not even his closest friends, can be trusted. For Earth's cloak-and-dagger business practices are writ large in the heavens...and hostile takeovers are just as common across light years as they are across boardroom tables.
Some excerpts from the book, courtesy of the Internet Archive:
Jack took the paper, opened it. Written upon it was:
Wheeler examined the burnt square of plastic-coated semiconductor. He made a tsking noise and shook his head. "It is an electron reactor." He held up a single manicured nail. "One electrong, no more."
Tempest wavefunction. One of Wheeler's other choices.
A synonym for the electron reactor.
Jack flipped a switch and let the reactor interact with the quaternary quantum logic cells that surrounded it. The sea lapped the edges, filled the logic cells, flipped their metastable configurations between four topologies: ones and zeroes, nulls and zeds, back and forth, without patter. Random and logic. Random logic.
Wheeler's other choice that wasn't a choice.
Wheeler had given him is pick of three names for the same technology. Jack flushed with anger, not at Wheeler, though. He should have known better and seen it coming.