The first Sloika design test, RDS-6s, was detonated in 1953 with a yield equivalent to ( from fusion). Attempts to use a "Sloika" design to achieve megaton-range results proved unfeasible. After the United States tested the "Ivy Mike" thermonuclear device in November 1952, proving that a multimegaton bomb could be created, the Soviets searched for an alternative design. The "Second Idea", as Sakharov referred to it in his memoirs, was a previous proposal by Ginzburg in November 1948 to use lithium deuteride in the bomb, which would, in the course of being bombarded by neutrons, produce tritium and free deuterium. In late 1953 physicist Viktor Davidenko achieved the first breakthrough, that of keeping the "primary" and "secondary" parts of the bombs in separate pieces ("staging"). The next breakthrough was discovered and developed by Sakharov and Yakov Zel'dovich, that of using the X-rays from the fission bomb to compress the "secondary" before fusion ("radiation implosion"), in early 1954. Sakharov's "Third Idea", as the Teller–Ulam design was known in the USSR, was tested in the shot "RDS-37" in November 1955 with a yield of .