Two decay measurements are reported on the same qubit: a Ramsey sequence gives a…

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Quantum Information and Technology, Ch What a qubit must do: requirements, tomography and benchmarking

Problem

Two decay measurements are reported on the same qubit: a Ramsey sequence gives a

1/e

decay time of

18\,\mu\mathrm{s}

, and a Hahn echo gives a

1/e

decay time of

95\,\mu\mathrm{s}

. Someone argues: 'Both sequences let the qubit dephase freely between two

\pi/2

pulses, so both measure the same physical process and must give the same decay time. Since they disagree by more than a factor of five, one of the two measurements has to be wrong.' Find the error in this argument, and state what physical process the echo removes that the plain Ramsey sequence does not.

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