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Register a new userFlux qubit noise spectroscopy using Rabi oscillations under strong driving conditions
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We infer the high-frequency flux noise spectrum in a superconducting flux qubit by studying the decay of Rabi oscillations under strong driving conditions. The large anharmonicity of the qubit and its strong inductive coupling to a microwave line enabled high-amplitude driving without causing significant additional decoherence. Rabi frequencies up to 1.7 GHz were achieved, approaching the qubits level splitting of 4.8 GHz, a regime where the rotating-wave approximation breaks down as a model for the driven dynamics. The spectral density of flux noise observed in the wide frequency range decreases with increasing frequency up to 300 MHz, where the spectral density is not very far from the extrapolation of the 1/f spectrum obtained from the free-induction-decay measurements. We discuss a possible origin of the flux noise due to surface electron spins.
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We demonstrate theoretically the noise-stimulated enhancement of quantum coherence in a superconducting flux qubit. First, an external classical noise can increase the off-diagonal components of the qubit density matrix. Second, in the presence of noise, the Rabi oscillations survive for times significantly longer than the Rabi decay time in a noiseless system. These Rabi oscillations appear as a modulation of the forced response of the qubit to the ac driving field. These effects can be considered as a manifestation of quantum stochastic resonance and are relevant to experimental techniques, such as Rabi spectroscopy.
We have observed the coherent exchange of a single energy quantum between a flux qubit and a superconducting LC circuit acting as a quantum harmonic oscillator. The exchange of an energy quantum is known as the vacuum Rabi oscillations: the qubit is oscillating between the excited state and the ground state and the oscillator between the vacuum state and the first excited state. We have also obtained evidence of level quantization of the LC circuit by observing the change in the oscillation frequency when the LC circuit was not initially in the vacuum state.
Nonlinear effects in mesoscopic devices can have both quantum and classical origins. We show that a three-Josephson-junction (3JJ) flux qubit in the _classical_ regime can produce low-frequency oscillations in the presence of an external field in resonance with the (high-frequency) harmonic mode of the system, $omega$. Like in the case of_quantum_ Rabi oscillations, the frequency of these pseudo-Rabi oscillations is much smaller than $omega$ and scales approximately linearly with the amplitude of the external field. This classical effect can be reliably distinguished from its quantum counterpart because it can be produced by the external perturbation not only at the resonance frequency $omega$ and its subharmonics ($omega/n$), but also at its overtones, $nomega$.
Solid state devices for quantum bit computation (qubits) are not perfect isolated two-level systems, since additional higher energy levels always exist. One example is the Josephson flux qubit, which consists on a mesoscopic SQUID loop with three Josephson junctions operated at or near a magnetic flux of half quantum. We study intrinsic leakage effects, i.e., direct transitions from the allowed qubit states to higher excited states of the system during the application of pulses for quantum computation operations. The system is started in the ground state and rf- magnetic field pulses are applied at the qubit resonant frequency with pulse intensity $f_p$. A perturbative calculation of the average leakage for small $f_p$ is performed for this case, obtaining that the leakage is quadratic in $f_p$, and that it depends mainly on the matrix elements of the supercurrent. Numerical simulations of the time dependent Schrodinger equation corresponding to the full Hamiltonian of this device were also performed. From the simulations we obtain the value of $f_p$ above which the two-level approximation breaks down, and we estimate the maximum Rabi frequency that can be achieved. We study the leakage as a function of the ratio $alpha$ among the Josephson energies of the junctions of the device, obtaining the best value for minimum leakage ($alphaapprox0.85$). The effects of flux noise on the leakage are also discussed.
We report on radio-frequency measurements of the charge-phase qubit being under continuous microwave irradiation in the state of weak coupling to a radio-frequency tank circuit. We studied the rf impedance dependence on the two important parameters such as power of microwave irradiation whose frequency is close to the gap between the two lowest qubit energy levels, and temperature of the internal heat bath. We have found that backaction effects of the qubit on the rf tank, and vice versa, tank on the qubit, lead to a negative as well as a positive real part of the qubit impedance Re$Z(omega)$ seen by the tank. We have implemented noise spectroscopy measurements for direct impedance readout at the extreme points corresponding to maximum voltage response and obtained absolute values of about 0.017 $Omega$ for the negative and positive Re$Z(omega)$. Our results demonstrate the existence and persistence of the coherent single- and multi-photon Rabi dynamics of the qubit with both negative and positive dynamic resistance inserted into the tank in the temperature range of 10 to 200 mK.
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