Equivalent circuit of a charge-phase qubit with the parameters , , , , , and (a). The image in an electron microscope (SEM) of the region of the “island” with two tunnel contacts and charge gate (b).
Scheme of the dependences of the ground-state |0⟩ and excited-state |1⟩ energy levels of charge-phase qubit versus the external magnetic field. The arrows show Rabi-type oscillations with frequency of the occupation probability of levels near exact resonance with an external microwave field, (a). Curves of the effective quantum inductance in the ground and excited states versus the external magnetic flux (b). It is evident that during part of the period of the low-frequency oscillations of the reactive parameter of the qubit assumes negative values. Computed parameters , , .
Block diagram of the experimental investigations of the characteristics of a charge-phase qubit. The dashed lines enclose temperature regions of the dilution refrigerator and the elements located within them. The polarization charge on the island was produced from a voltage generator coupled with the charge gate of the system of cooled filters. The qubit is enclosed in a lead screen (not shown in the figure), which for microwave frequencies is a cylindrical resonator. The microwave generator excited, through a coaxial cable with large damping and a cooled attenuator, a resonator at the frequency . Excitation of the circuit at the frequency and detuning of the qubit with respect to the magnetic field were performed from rf and constant-current generators. The signal from the resonant circuit, coupled with the qubit by the mutual inductance , was amplified by a cooled amplifier and measured with a vector voltmeter. 1, 2, 3—powder (CuO) filter. 1) Voltage generator 2) Microwave generator, 3) rf generator, 4) Current generator 5) Vector voltmeter 6) Attenuator 7) rf amplifier
Curves of the phase signal on the resonant circuit coupled with a charge-phase qubit versus the external magnetic flux near : 1—no filtering of the measuring channel; 2—with cooling to with a powder filter in the chain of the resonant circuit (a). Curves of the amplitude of the superconducting current circulating in the qubit, constructed on the basis of the characteristics (1) and (2), versus the external flux (b).
Charge-phase qubit with in a microwave resonance field with frequency . Family of curves of the phase signal characteristics coupled with a qubit resonant circuit versus the external magnetic flux . The parameter of the family is the output power of the microwave generator.
Signal characteristics of the charge-phase qubit-detector in a microwave field with frequency at the constant power of the microwave generator . The phase signal in the resonant circuit for the values of the polarization charge and as a function of the external magnetic flux (a). The dependence of the change of the voltage on the resonant circuit and versus the magnetic flux. The family was obtained with the minimum detuning of the generator frequency from the resonance frequency of the circuit and (b). 1) , rad 2) , arb. units
Phase difference (a) and amplitude difference (b) for the signal characteristics of qubit-detector which are shown in Fig. 6.
Family of phase signal characteristics charge-phase qubit in a microwave field with frequency with variation of the gate charge by in a neighborhood of (a, b) and (c, d). The input power for all characteristics.
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