Quantum two-level systems (TLSs) are known to limit the performance of superconducting qubits and resonators by absorbing energy from oscillations. However, their destructive behavior can be controlled, for instance, by applying an AC field and a time-varying bias. In a novel regime, TLS population inversion can change loss to gain, resulting in lasing instability. This inversion can be identified by a red shift in the cavity resonance frequency. This research provides a new tool for studying TLSs in quantum devices and offers valuable insights for further development in the field.
Introduction to Quantum Two-Level Systems (TLSs) and Their Impact on Superconducting Qubits
Quantum two-level systems (TLSs) are known to limit the performance of superconducting qubits and superconducting and optomechanical resonators. These systems absorb energy from oscillations, breaking down coherence and affecting the overall performance of these devices. TLSs are believed to exist in the Josephson junctions of superconducting qubits as well as the surface and barrier layers of superconducting and optomechanical resonators. The existence of TLSs in practically all disordered materials and their quantitative universality makes their destructive effect seemingly unavoidable.
Controlling TLS Destructive Behavior
Despite the challenges posed by TLSs, it is possible to control their destructive behavior. For instance, TLS dielectric losses, which are relevant for superconducting qubit performance, can be controlled by simultaneously applying an AC field with a frequency ω0 and a time-varying bias. In the absence of a bias sweep, the TLS loss tangent has a maximum at small field amplitude and decreases at larger fields where the TLS Rabi frequency exceeds its relaxation rate.
TLS Population Inversion and Its Effects
In the adiabatic Landau-Zener regime, the TLS population inversion takes place at TLS energies below resonant energy. This population inversion changes loss to gain in a certain frequency domain that can result in lasing instability. This novel regime of inverted TLS populations at all energies smaller than the pump field quantization energy can be reached using a single pump field. In this regime, the coherent oscillations at any frequency below the pump field frequency will not be absorbed but enhanced by TLSs due to their inverted populations.
Red Shift of Cavity Resonance as a Signature of Population Inversion
In this novel regime, the cavity resonance should be substantially red-shifted because the population inversion pushes downwards the cavity resonance due to quantum mechanical level repulsion. Thus, a red shift in the cavity resonance frequency presents a direct signature of the population inversion. The nonmonotonic dependence of the frequency shift on AC field and bias field sweep rate was found, with the frequency first decreasing with increasing the AC field and then increasing back towards its zero field value.
Implications for the Study of TLSs in Quantum Devices
The position of the frequency minimum allows the determination of the typical TLS relaxation times and dipole moments based on the experimentally observed dependence. Thus, this work presents a new tool for the study of TLSs in quantum devices and new insights into the effect of TLSs on quantum devices in and out of equilibrium conditions.
Model Formulation and Corrections to the Resonant Frequency
Two-level systems can be characterized by their tunneling amplitudes, asymmetry energies, energy splitting, dipole moment, and relaxation and decoherence times. In this work, the frequency shift within the standard tunneling model was considered, ignoring TLS-TLS interactions which are usually small.
Conclusion
The study of TLSs and their impact on superconducting qubits and resonators is crucial for the advancement of quantum devices. The ability to control TLS destructive behavior and the understanding of TLS population inversion and its effects on cavity resonance provide valuable insights for further research and development in this field. The red shift of cavity resonance serves as a direct signature of TLS population inversion, offering a new tool for the study of TLSs in quantum devices.
The article titled “Red shift of the superconductivity cavity resonance in Josephson junction qubits as a direct signature of TLS population inversion” was published on January 28, 2024. The authors of this article are Alexander L. Burin, Moshe Schechter, Daniel Tennant, and Yaniv Rosen. The article was sourced from arXiv, a repository managed by Cornell University. The study explores the red shift phenomenon in the superconductivity cavity resonance in Josephson junction qubits.
