The Soviets demonstrated

The Soviets demonstrated the power of the "staging" concept in October 1961, when they detonated the massive and unwieldy Tsar Bomba, a 50 megaton hydrogen bomb that derived almost 97% of its energy from fusion. It was the largest nuclear weapon developed and tested by any country. HP Compaq HSTNNIB12 Battery

In 1954 work began at Aldermaston to develop the British fusion bomb, with Sir William Penney in charge of the project. British knowledge on how to make a thermonuclear fusion bomb was rudimentary, and at the time the United States was not exchanging any nuclear knowledge because of the Atomic Energy Act of 1946. HP Compaq HSTNN-IB16 Battery

However, the British were allowed to observe the American Castle testsand used sampling aircraft in the mushroom clouds, providing them with clear, direct evidence of the compression produced in the secondary stages by radiation implosion. HP Compaq HSTNN-IB18 Battery

Because of these difficulties, in 1955 British prime minister Anthony Eden agreed to a secret plan, whereby if the Aldermaston scientists failed or were greatly delayed in developing the fusion bomb, it would be replaced by an extremely large fission bomb. HP Compaq HSTNN-IB28 Battery

In 1957 the Operation Grapple tests were carried out. The first test, Green Granite was a prototype fusion bomb, but failed to produce equivalent yields compared to the Americans and Soviets, only achieving approximately 300 kilotons. HP Compaq HSTNN-IB51 Battery

The second test Orange Herald was the modified fission bomb and produced 700 kilotons—making it the largest fission explosion ever. At the time almost everyone (including the pilots of the plane that dropped it) thought that this was a fusion bomb. This bomb was put into service in 1958. HP Compaq HSTNN-IB52 Battery

A second prototype fusion bomb Purple Granite was used in the third test, but only produced approximately 150 kilotons.

A second set of tests was scheduled, with testing recommencing in September 1957. The first test was based on a "… new simpler design. HP Compaq HSTNN-IB55 Battery

A two stage thermonuclear bomb which had a much more powerful trigger". This test Grapple X Round C was exploded on November 8 and yielded approximately 1.8 megatons. On April 28, 1958 a bomb was dropped that yielded 3 megatons—Britain's most powerful test. HP Compaq HSTNN-IB62 Battery

Two final air burst tests on September 2 and September 11, 1958, dropped smaller bombs that yielded around 1 megaton each.

American observers had been invited to these kinds of tests. After their successful detonation of a megaton-range device HP Compaq HSTNN-LB05 Battery

(and thus demonstrating their practical understanding of the Teller–Ulam design "secret"), the United States agreed to exchange some of their nuclear designs with the United Kingdom, leading to the1958 US–UK Mutual Defence Agreement. HP Compaq HSTNN-LB08 Battery

Instead of continuing with their own design, the British were given access to the design of the smaller American Mk 28 warhead and were able to manufacture copies.

The People's Republic of China detonated its first H-Bomb using a Yu–Deng design June 17, 1967 ("Test No. 6"),HP Compaq HSTNN-LB0E Battery

a mere 32 months after detonating its first fission weapon (the shortest fission-to-fusion development in history), with a yield of 3.31 Mt.

The Yu–Deng design is different from the Teller–Ulam design. It doesn't use X-ray reflector, but refraction lens to achieve similar effect. HP Compaq HSTNN-LB11 Battery

India's first nuclear test occurred on May 18, 1974, which initially surprised the world. The first test, codename Smiling Buddha, was not a thermonuclear device according to the Bhabha Atomic Research Centre.^[19] On May 11, 1998, India reportedly detonated a thermonuclear bomb in its Operation Shakti tests ("Shakti-1", specifically). HP Compaq HSTNN-LB51 Battery

Dr. Samar Mubarakmand asserted that Shakti-1 was a successful test, but if it was a thermonuclear device as claimed, then it failed to produce certain results that were to be expected of a thermonuclear device.^[19]

Director for the 1998 test site preparations, Dr. K. Santhanam, HP Compaq HSTNN-LB52 Battery

reported the yield of thermonuclear explosion was lower than expected, although his statement has been disputed by other Indian scientists involved in the test.^[20] Indian sources, using local data and citing a US Geologic Survey report compiling seismic data from 125 IRIS stations across the world, HP Compaq HSTNN-MB05 Battery

argue that the magnitudes suggested a combined yield of up to 60 kilotonnes, consistent with the Indian announced total yield of 56 kilotonnesHowever, several independent experts have reported lower yields for the nuclear test and remained skeptical about the claims,^[19] and others have argued that even the claimed 50 kiloton yield was low for confirmation of a thermonuclear design. HP Compaq HSTNN-OB06 Battery