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We propose an efficient quantum memory scheme with controlled reversible inhomogeneous broadening in an optical cavity.When the absorption per cavity round trip exactly matches the transmission of the coupling mirror(impedance matching condition),we show that the input light field can be completely absorbed by an inhomogeneous broadening atomic ensemble.From calculation we obtain that either a forward or a backward retrieval process the quantum memory efficiency can reach unity even for a small optical depth of the atomic system.If there is no cavity,100% efficiency is obtained only for extremely large optical depth.A high efficiency for the quantum memory at a small optical depth is easily achieved in practice.Therefore our proposal offers promising possibilities for the practical realization of high quantum memory efficiency.

We propose a one-step method to prepare multi-qubit GHZ and W states with transmon qubits capacitively coupled to a superconducting transmission line resonator (TLR). Compared with the scheme firstly introduced by Wang et al. [Phys. Rev. B 81 (2010) 104524], our schemes have longer dephasing time and much shorter operation time because the transmon qubits we used are not only more robust to the decoherence and the unavoidable parameter variations, but also have much stronger coupling constant with TLR. Based on the favourable properties of transmons and TLR, our method is more feasible in experiment.