We present a method for in situ temperature measurement of superconducting quantum circuits, by using the first three levels of a transmon device to which we apply a sequence of p gates. Our approach employs projective dispersive readout and utilizes the basic properties of the density matrix associated with thermal states. This method works with an averaging readout scheme and does not require a single-shot readout setup. We validate this protocol by performing thermometry in the range of 50-200 mK, corresponding to a range of residual populations 1%-20% for the first excited state and 0.02%-3% for the second excited state.
The probability amplitudes of the processes related to the transfer of the excited state from one qubit to another are calculated using an indirect interaction in an open waveguide. The system consists of two qubits located at an arbitrary distance from each other. The non-Hermitian effective Hamiltonian approach used herein makes it possible to bypass using the Markovchain approximation method. Analytic expressions describing the probability of transfer of the excited state from one qubit to another under different initial states of the system were obtained.
In this paper, the scattering of a single photon in a waveguide–resonator–qubit system is studied. An open waveguide is connected to two resonators, located at an arbitrary distance from each other and containing a single qubit each. The scattering of a single photon makes it possible to describe the behavior of the system completely quantum mechanically. We show the existence of Fano resonance, which is a direct manifestation of the interference between the incident photon and virtual photons associated with transitions between the states of the system. The obtained expressions for the transmission coefficients allowed us to take into account the influence of the incident photon frequency on the resonances and their widths.
Quantum phase estimation is a paradigmatic problem in quantum sensing and metrology. Here we show that adaptive methods based on classical machine learning algorithms can be used to enhance the precision of quantum phase estimation when noisy non-entangled qubits are used as sensors. We employ the Differential Evolution (DE) and Particle Swarm Optimization (PSO) algorithms to this task and we identify the optimal feedback policies which minimize the Holevo variance. We benchmark these schemes with respect to scenarios that include Gaussian and Random Telegraph fluctuations as well as reduced Ramsey-fringe visibility due to decoherence. We discuss their robustness against noise in connection with real experimental setups such as Mach–Zehnder interferometry with optical photons and Ramsey interferometry in trapped ions, superconducting qubits and nitrogen-vacancy (NV) centers in diamond.
We analyze a photon transport through an 1D open waveguide side coupled to the N -photon microwave cavity with embedded artificial two-level atom (qubit). The qubit state is probed by a weak signal at the fundamental frequency of the waveguide. Within the formalism of projection operators and non-Hermitian Hamiltonian approach we develop a one-photon approximation scheme to obtain the photon wavefunction which allows for the calculation of the probability amplitudes of the spontaneous transitions between the levels of two Rabi doublets in N -photon cavity. We obtain analytic expressions for the transmission and reflection factors of the microwave signal through a waveguide, which contain the information of the qubit parameters. We show that for small number of cavity photons the Mollow spectrum consists of four spectral lines which is a direct manifestation of quantum nature of light. The results obtained in the paper are of general nature and can be applied to any type of qubits. The specific properties of the qubit are only encoded in the two parameters: the energy Ω of the qubit and its coupling λ to the cavity photons.
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