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arXiv:0908.2728 · Physical Review B

Multiphoton excitations and inverse population in a system of two flux qubits

Evgeni V. Il’ichev, Sergey N Shevchenko, S. H. W. van der Ploeg, M. Grajcar, E. A. Temchenko, A. N. Omelyanchouk, H.‐G. Meyer

Published Jan 20, 2010• 20 citations
arXivHTMLPDF

Abstract

We study the multiphoton spectroscopy of artificial solid-state four-level quantum system. This system is formed by two coupled superconducting flux qubits. When multiple driving frequency of the applied microwaves matches the energy difference between any two levels, the transition to the upper level is induced. We demonstrate two types of the multiphoton transitions: direct transitions between two levels and ladder-type transitions via an intermediate level. Our calculations show, that for the latter transitions, in particular, the inverse population of the excited state with respect to the ground one is realized. These processes can be useful for the control of the level population for the multilevel scalable quantum systems.

DOI
10.1103/physrevb.81.012506

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We study the multiphoton spectroscopy of artificial solid-state four-level quantum system. This system is formed by two coupled superconducting flux qubits. When multiple driving frequency of the applied microwaves matches the energy difference between any two levels, the transition to the upper level is induced. We demonstrate two types of the multiphoton transitions: direct transitions between two levels and ladder-type transitions via an intermediate level. Our calculations show, that for the latter transitions, in particular, the inverse population of the excited state with respect to the ground one is realized. These processes can be useful for the control of the level population for the multilevel scalable quantum systems.
The quantum level population behavior of two coupled flux qubits depending on the external driving field characteristics is studied. Explicit expressions for the multiphoton transition probabilities at an arbitrary control field amplitude are obtained for the case of small tunnel splitting energies. We describe the controllable features of their formation and thereby the creation or destruction of entanglement on the direct interlevel transition and during the transition through intermediate states by system bias tuning. We found a feature of the qubit population inversion that results in the independence of the resonance positions from the qubit coupling strength. Using the Floquet-Markov equation, we numerically demonstrate that the positions of multiphoton resonances are stable to dissipative processes.
Two basic physical models, a two-level system and a harmonic oscillator, are realized on the mesoscopic \nscale as coupled qubit and resonator. The realistic system includes moreover the electronics for controlling the \ndistance between the qubit energy levels and their populations and to read out the resonator's state, as well as the \nunavoidable dissipative environment. Such rich system is interesting both for the study of fundamental quantum \nphenomena on the mesoscopic scale and as a promising system for future electronic devices. We present recent \nresults for the driven superconducting qubit–resonator system, where the resonator can be realized as an LC \ncircuit or a nanomechanical resonator. Most of the results can be described by the semiclassical theory, where a \nqubit is treated as a quantum two-level system coupled to the classical driving field and the classical resonator. \nApplication of this theory allows to describe many phenomena for the single and two coupled superconducting \nqubits, among which are the following: the equilibrium-state and weak-driving spectroscopy, Sisyphus damping \nand amplification, Landau–Zener–Stückelberg interferometry, the multiphoton transitions of both direct and ladder-type \ncharacter, and creation of the inverse population for lasing.
Multiphoton emitters are a sought-after resource in quantum photonics. Nonlinear interactions between a multilevel atomic system and a coherent drive can lead to resonant two-photon emission, but harvesting light from this process has remained a challenge due to the small oscillator strengths involved. Here we present a study of two-photon resonance fluorescence at microwave frequencies, using a superconducting, ladder-type artificial atom, a transmon, strongly coupled to a waveguide. We drive the two-photon transition between the ground and second-excited states at increasingly high powers and observe a resonance fluorescence peak whose intensity becomes comparable to single-photon emission until it splits into a Mollow-like triplet. We measure photon correlations of frequency-filtered spectral lines and find that while emission at the fundamental frequency stays antibunched, the resonance fluorescence peak at the two-photon transition is superbunched. Our results provide a route towards the realization of multiphoton sources in the microwave domain.
Two basic physical models, a two-level system and a harmonic oscillator, are realized on the mesoscopic scale as coupled qubit and resonator. The realistic system includes moreover the electronics for controlling the distance between the qubit energy levels and their populations and to read out the resonator’s state, as well as the unavoidable dissipative environment. Such rich system is interesting both for the study of fundamental quantum phenomena on the mesoscopic scale and as a promising system for future electronic devices. We present recent results for the driven superconducting qubit–resonator system, where the resonator can be realized as an LC circuit or a nanomechanical resonator. Most of the results can be described by the semiclassical theory, where a qubit is treated as a quantum two-level system coupled to the classical driving field and the classical resonator. Application of this theory allows to describe many phenomena for the single and two coupled superconducting qubits, among which are the following: the equilibrium-state and weak-driving spectroscopy, Sisyphus damping and amplification, Landau–Zener–Stückelberg interferometry, the multiphoton transitions of both direct and ladder-type character, and creation of the inverse population for lasing.
Resonant driving enables coherent control of quantum systems, including single and coupled qubits. From a complementary perspective, transitions of a quantum system can be exploited for the detection of microwave photons. In this work, we theoretically investigate resonant multiphoton excitations in a system of qubits. When the energy of K photons matches the energy splitting of the qubit system, the absorption of these photons leads to collective excitation of the qubits. We focus on the case of two coupled qubits and analyze the quantum dynamics of both excitation and relacation processes. In the particular case where only a single qubit is relevant and the remaining qubits can be neglected, the dynamics admits an analytical treatment. We examine multiphoton resonances, the Bloch-Siegert shift, and population inversion, phenomena that are central to both coherent quantum control and microwave photon detection.
Resonant driving enables coherent control of quantum systems, including single and coupled qubits. From a complementary perspective, transitions of a quantum system can be exploited for the detection of microwave photons. In this work, we theoretically investigate resonant multiphoton excitations in a system of qubits. When the energy of K photons matches the energy splitting of the qubit system, the absorption of these photons leads to collective excitation of the qubits. We focus on the case of two coupled qubits and analyze the quantum dynamics of both excitation and relaxation processes. In the particular case where only a single qubit is relevant, and the remaining qubits can be neglected, the dynamics admits analytical treatment. We examine multiphoton resonances, the Bloch–Siegert shift, and population inversion, phenomena that are central to both coherent quantum control and microwave photon detection.