Never Meet Again by Algis Budrys

When a disillusioned quantum physicist activates his invention to escape from totalitarian rule in 1958 Berlin, he plunges headfirst into a nightmare reality where Germany lost the war. Shocked to discover Soviet forces occupying a bombed-out shell of a city, Professor Kempfer struggles to survive, haunted by persecution from power-hungry government agents determined to claim his technology for themselves.

Just as all seems lost, a bittersweet reunion sparks hope for Kempfer to rebuild his life with his long lost love. But sinister agendas continue to endanger the scientist, forcing him to make an impossible choice between personal happiness and moral conscience.