In the past 20 years, impressive progress has been made both experimentally and theoretically in superconducting quantum circuits, which provide a platform for manipulating microwavephotons. This emerging field of superconducting quantum microwave circuits has been driven by many new interesting phenomena in microwave photonics and quantum information processing. For instance, the interaction between superconducting quantum circuits and single microwave photons can reach the regimes of strong, ultra-strong, and even deep-strong coupling. Many higher-order effects, unusual and less familiar in traditional cavity quantum electrodynamics with natural atoms, have been experimentally observed, e.g., giant Kerr effects, multi-photon processes, and single-atom induced bistability of microwave photons. These developments may lead to improved understanding of the counterintuitive properties of quantum mechanics, and speed up applications ranging from microwave photonics to superconducting quantum information processing. In this article, we review experimental and theoretical progress in microwave photonics with superconducting quantum circuits. We hope that this global review can provide a useful roadmap for this rapidly developing field.
We study experimentally a vacuum induced Aulter-Townes doublet in a superconducting three-level artificial atom strongly coupled to a coplanar waveguide resonator and simultaneouslyto a transmission line. The Aulter-Townes splitting is observed in the reflection spectrum of the three-level atom when the transition between two excited states is resonant with the resonator. By varying an amplitude of the driving field applied to the resonator, we observe quantum-to-classical transition of the Aulter-Townes splitting. Our results may pave the way for the control of microwaves by single photons.
The engineering of quantum devices has reached the stage where we now have small scale quantum processors containing multiple interacting qubits within them. Simple quantum circuitshave been demonstrated and scaling up to larger numbers is underway. However as the number of qubits in these processors increases, it becomes challenging to implement switchable or tunable coherent coupling among them. The typical approach has been to detune each qubit from others or the quantum bus it connected to, but as the number of qubits increases this becomes problematic to achieve in practice due to frequency crowding issues. Here, we demonstrate that by applying a fast longitudinal control field to the target qubit, we can turn off its couplings to other qubits or buses (in principle on/off ratio higher than 100 dB). This has important implementations in superconducting circuits as it means we can keep the qubits at their optimal points, where the coherence properties are greatest, during coupling/decoupling processing. Our approach suggests a new way to control coupling among qubits and data buses that can be naturally scaled up to large quantum processors without the need for auxiliary circuits and yet be free of the frequency crowding problems.
Electromagnetically induced transparency (EIT) has been extensively studied in various systems. However, it is not easy to observe in superconducting quantum circuits (SQCs), becausethe Rabi frequency of the strong controlling field corresponding to EIT is limited by the decay rates of the SQCs. Here, we show that EIT can be achieved by engineering decay rates in a superconducting circuit QED system through a classical driving field on the qubit. Without such a driving field, the superconducting qubit and the cavity field are approximately decoupled in the large detuning regime, and thus the eigenstates of the system are approximately product states of the cavity field and qubit states. However, the driving field can strongly mix these product states and so-called polariton states can be formed. The weights of the states for the qubit and cavity field in the polariton states can be tuned by the driving field, and thus the decay rates of the polariton states can be changed. We choose a three-level system with Λ-type transitions in such a driven circuit QED system, and demonstrate how EIT and ATS can be realized in this compound system. We believe that this study will be helpful for EIT experiments using SQCs.
Using different configurations of applied strong driving and weak probe fields, we find that only a single three-level superconducting quantum circuit (SQC) is enough to realize amplification,attenuation and frequency conversion of microwave fields. Such a three-level SQC has to possess Δ-type cyclic transitions. Different from the parametric amplification (attenuation) and frequency conversion in nonlinear optical media, the real energy levels of the three-level SQC are involved in the energy exchange when these processes are completed. We mainly show the efficiencies of the amplification and the frequency conversion for different types of driving fields. Our study provides a new method to amplify (attenuate) microwave, realize frequency conversion, and also lays a foundation for generating single or entangled microwave photon states using a single three-level SQC.
Stimulated Raman adiabatic passage (STIRAP) offers significant advantages for coherent population transfer between un- or weakly-coupled states and has the potential of realizing efficientquantum gate, qubit entanglement, and quantum information transfer. Here we report on the realization of STIRAP in a superconducting phase qutrit – a ladder-type system in which the ground state population is coherently transferred to the second-excited state via the dark state subspace. The result agrees well with the numerical simulation of the master equation, which further demonstrates that with the state-of-the-art superconducting qutrits the transfer efficiency readily exceeds 99% while keeping the population in the first-excited state below 1%. We show that population transfer via STIRAP is significantly more robust against variations of the experimental parameters compared to that via the conventional resonant π pulse method. Our work opens up a new venue for exploring STIRAP for quantum information processing using the superconducting artificial atoms.
It has been shown that there are extbf{}not only transverse but also longitudinal couplings between microwave fields and a superconducting qubit with broken inversion symmetry of thepotential energy. Using multiphoton processes induced by longitudinal coupling fields and frequency matching conditions, we design a universal algorithm to produce arbitrary superpositions of two-mode photon states of microwave fields in two separated transmission line resonators, which are coupled to a superconducting qubit. Based on our algorithm, we analyze the generation of evenly-populated states and NOON states. Compared to other proposals with only single-photon process, we provide an efficient way to produce entangled microwave states when the interactions between superconducting qubits and microwave fields are in the ultrastrong regime.
A single superconducting artificial atom provides a unique basis for coupling electromagnetic fields and photons hardly achieved with a natural atom. Bringing a pair of harmonic oscillatorsinto resonance with transitions of the three-level atom converts atomic spontaneous processes into correlated emission dynamics. We demonstrate two-mode correlated emission lasing on harmonic oscillators coupled via the fully controllable three-level artificial atom. Correlation of two different color emissions reveals itself as equally narrowed linewiths and quench of their mutual phase-diffusion. The mutual linewidth is more than four orders of magnitude narrower than the Schawlow-Townes limit. The interference between the different color lasing fields demonstrates the two-mode fields are strongly correlated.
Besides the conventional transverse couplings between superconducting qubits (SQs) and electromagnetic fields, there are additional longitudinal couplings when the inversion symmetryof the potential energies of the SQs is broken. We study nonclassical-state generation in a SQ which is driven by a classical field and coupled to a single-mode microwave field. We find that the classical field can induce transitions between two energy levels of the SQs, which either generate or annihilate, in a controllable way, different photon numbers of the cavity field. The effective Hamiltonians of these classical-field-assisted multiphoton processes of the single-mode cavity field are very similar to those for cold ions, confined to a coaxial RF-ion trap and driven by a classical field. We show that arbitrary superpositions of Fock states can be more efficiently generated using these controllable multiphoton transitions, in contrast to the single-photon resonant transition when there is only a SQ-field transverse coupling. The experimental feasibility for different SQs is also discussed.
We theoretically study the transparency and amplification of a weak probe field applied to the cavity in hy- brid systems formed by a driven superconducting circuit QED system and amechanical resonator, or a driven optomechanical system and a superconducting qubit. We find that both the mechanical resonator and the su- perconducting qubit can result in the transparency to a weak probe field in such hybrid systems when a strong driving field is applied to the cavity. We also find that the weak probe field can be amplified in some parameter regimes. We further study the statistical properties of the output field via the degrees of second-order coherence. We find that the nonclassicality of the output field strongly depends on the system parameters. Our studies show that one can control single-photon transmission in the optomechanical system via a tunable artificial atom or in the circuit QED system via a mechanical resonator.