Strong coupling of a qubit to shot noise

We perform a nonperturbative analysis of a charge qubit in a double-quantum-dot structure coupled to its detector. We show that a strong detector-dot interaction tends to slow down and halt coherent oscillations. The transitions to a classical and a low-temperature quantum overdamping (Zeno) regime are studied. In the latter, the physics of the dissipative phase transition competes with the effective shot noise.

Figure 1

(Color online) Schematic view of the double-dot system analyzed; see, e.g., Refs. 4, 5. The QPC and rf-SET detectors can be used alternatively; both options are discussed in the paper.

Figure 2

(Color online) Quantum limit: imaginary parts of the numerically determined poles as a function of the QPC transmission $D$. The other parameters are $eV={10}^2\hslash \Delta$ and ${\omega }_c={10}^{12}\Delta$. Inset: real parts of the poles as a function of the QPC transmission $D$.

Figure 3

(Color online) Quantum limit: relaxation rate ${\Gamma }_r$ as a function of the QPC transmission $D$. The other parameters are $\epsilon =10\Delta$, $\Delta =1.524\times {10}^{9}1∕\mathrm{s}$, $eV={10}^2\hslash \Delta$, and ${\omega }_c={10}^{12}\Delta$. Inset: relaxation rate ${\Gamma }_r$ as a function of the qubit bias $\epsilon$. Other parameters as above, but with $D=0.5$.