How to Measure the Decoherence of a Micromaser Field under Well Controlled Conditions

We discuss a possible realization of a quantum register with controllable decoherence in terms of $|0⟩$ and $|1⟩$ photon number states of a micromaser field. It is shown how to create in the Jaynes-Cummings model a superposition state of $|0⟩$ and $|1⟩$ photon number states inside a closed micromaser cavity. The loss of phase coherence between these two states can subsequently be measured by a second probe atom monitoring the decoherence of the field. A technique is proposed for forming the superposition of number states $|0⟩$ and $|1⟩$ using the time structure of the Rabi oscillation. The proposed method avoids problems with stray fields at the cavity holes, which disturb the coherence of the atomic superposition, and offers a way to study how the coupling strength to the environment influences the decoherence rate, displaying the robustness of physical qubits and the fidelity of quantum computations.

Figure 1

Scheme for preparing the cavity. The two upper energy levels are the maser levels $63P$ and $61D$, the lowest-lying energy level denotes the final state $5P$ (black circle) of the atom. The red (light grey) circle denotes the system state $|e,0⟩$, the red and blue (light grey and dark grey) ellipse the superposition state $\alpha (t_0)|e,0⟩+\beta (t_0)|g,1⟩$. $t_0$ is the time when the two laser pulses $L_1$ and $L_2$ are applied with the pulse duration being $\tau$ (gray shaded area). $t_0$ is chosen so that the atom is in the cavity when the laser pulses are applied.