When will quantum computers (with a useful number of qubits) be built? Depends. The time it takes to travel from location A to location B depends on how fast you are willing to drive.
Example of Fast Driving:
Look at this timeline of the Manhattan Project.
- 1939 August 2: Albert Einstein signs a letter authored by physicist Leo Szilard addressed to President Franklin D. Roosevelt, advising him to fund research into the possibility of using nuclear fission as a weapon in the event that Nazi Germany may also be conducting such research.
- 1945 August 6: “Little Boy”, a gun-type uranium-235 weapon, is used against the city of Hiroshima, Japan.
- 1945 August 9: “Fat Man”, an implosion-type plutonium-239 weapon, is used against the city of Nagasaki, Japan.
That’s 6 years between the time when nobody had the faintest idea of how to build an atomic bomb, or purify U-235 or Plutonium-239 (Plutonium hadn’t even been discovered yet. That happened in Feb 1941), to the time when 2 very different bomb designs were demonstrated to work all too well.
To be honest, this is an unfair comparison. These people didn’t have lasers or transistors or integrated circuits or personal computers or the internet. Not even TVs. Their laboratory tools were quite primitive by today’s standards. Building a quantum computer with our modern tools should be much easier.
Example of Very Very Slow Driving:
The current global efforts to build a quantum computer, which are expected to take about 200 years to achieve a product.