δ Δ ^F IG. 1: (a) Schematics of the interface: Atoms (sphere), in a 1D lattice of the length L, are electrically (magnetically) coupled to light in the nanofiber (the superconducting waveguide), respectively. The quantum microwave (optical) field with Rabi frequencyÊM (Êo) is manipulated using a classical control radio-frequency (optical) field with Rabi frequency ΩM (Ωo), respectively. Trapping lights are not shown in the figure. (b) The quantum (ÊM) and control (ΩM) electromagnetic fields arrive while the atomic system is in the ground state. (c) The quantum field is stored as an atomic spin excitation (S(z)). (d) Internal level structure of a 87 Rb atom and transitions induced by the four electromagnetic fields. δ, ∆2 are two-photon and one-photon detunings, respectively. (e) Dimensions of an example LC resonator. (f) Magnetic field profile viewed from the top, lighter colors show higher fields.level atoms [4, 5], based on electric-dipole coupling in a Λ-system, allow for efficient storage and retrieval of optical photons into atomic excitations from an ensemble of atoms. We adapt this approach to also store and retrieve MW photons. We investigate the effect of finite bandwidth of MW photons on the storage-retrieval process arXiv:1110.3537v2 [quant-ph]
We present results on a circuit QED experiment in which a separate transmission line is used to address a quasilumped element superconducting microwave resonator which is in turn coupled to an Al/AlO(x)/Al Cooper-pair box charge qubit. With our device, we find a strong correlation between the lifetime of the qubit and the inverse of the coupling between the qubit and the transmission line. At the smallest coupling we measured, the lifetime of the Cooper-pair box was T₁=200 μs, which represents more than a twentyfold improvement in the lifetime of the Cooper-pair box compared with previous results. These results imply that the loss tangent in the AlO(x) junction barrier must be less than about 4×10⁻⁸ at 4.5 GHz, about 4 orders of magnitude less than reported in larger area Al/AlO(x)/Al tunnel junctions.
We have observed a few distinct anomalous avoided level crossings and voltage dependent transitions in the excited state spectrum of an Al/AlOx/Al Cooper-pair box (CPB). The device was measured at 40 mK in the 15 - 50 GHz frequency range. We find that a given level crosses the CPB spectrum at two different gate voltages; the frequency and splitting size of the two crossings differ and the splitting size depends on the Josephson energy of the CPB. We show that this behavior is not only consistent with the CPB being coupled to discrete charged "two-level" quantum systems which move atomic distances in the CPB junctions but that the spectra provide new information about the fluctuators, which is not available from phase qubit spectra of anomalous avoided levels. In particular by fitting a model Hamiltonian to our data, we extract microscopic parameters for each fluctuator, including well asymmetry, tunneling amplitude, and the minimum hopping distance for each fluctuator. The tunneling rates range from less than 3.5 to 13 GHz, which represent values between 5% and 150% of the well asymmetry, and the dipole moments give a minimum hopping distance of 0.3 to 0.8 Anstrom. We have also found that these discrete two-level systems have a pronounced effect on the relaxation time (T1) of the quantum states of the CPB and hence can be a source of dissipation for superconducting quantum bits
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