| We consider a Bose Einstein condensate trapped in a symmetric double well potential whose tunnelling rate is controlled by an electrostatic qubit. We show that determination of the number of tunnelled atoms amounts to a quantum meaurement of the duration the qubit has spent in the state for which the barrier is lowered. The problem admits a simple solution. Reduced density matrices of the qubit and the condensate are expressed as quadratures. Strong and weak measrement regimes, finite time Zeno effect and decoherence-free subspaces are discussed. |
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