| circuit QED | |
|---|---|
| Name | Circuit quantum electrodynamics |
| Field | Quantum mechanics; Quantum optics |
| Introduced | 2004 |
| Founders | Robert J. Schoelkopf; Andrei Blais; Alexandre Wallraff |
| Institutions | Yale University; University of Sherbrooke; ETH Zurich; IBM; Google; D-Wave Systems |
| Related | Superconducting qubit; Cavity quantum electrodynamics; Transmon (qubit); Josephson junction |
circuit QED
Circuit QED is the study of the quantum interaction between electromagnetic modes in electrical circuits and artificial atoms formed by superconducting qubits. It adapts concepts from cavity quantum electrodynamics to on‑chip circuits, enabling strong and ultrastrong coupling regimes between microwave resonators and nonlinear quantum elements; this platform is central to contemporary experimental Quantum information science and scalable approaches to quantum computing.
Circuit QED emerged in the early 2000s from efforts to integrate quantum optics phenomena into lithographically fabricated electrical circuits. Seminal theoretical proposals by Andrei Blais and experimental demonstrations by groups led by Robert J. Schoelkopf and Alexandre Wallraff established on‑chip coplanar waveguide resonators coupled to Josephson junction-based qubits such as the Cooper pair box. The development of the transmon in 2007 by John M. Martinis's collaborators and Michel Devoret's school reduced charge noise sensitivity and accelerated practical circuit QED experiments. Major research centers include Yale University, University of Sherbrooke, ETH Zurich, IBM, and industrial efforts by Google and D-Wave Systems have driven progress toward multi‑qubit processors and quantum annealing interfaces.
The core theoretical description uses a quantum circuit Hamiltonian combining harmonic modes for resonators and anharmonic two‑level (or multilevel) systems for qubits. Typical models include the Jaynes–Cummings model for resonant coupling and its generalizations: the Rabi model, the dispersive approximation, and the Tavis–Cummings model for multimode or multi‑qubit configurations. Quantization procedures follow the methods developed by Yuriy Makhlin, G. Schön, and A. Shnirman for superconducting circuits and by Giovanni Falci and Michel Devoret for circuit element quantization. Key parameters are coupling strength g, resonator frequency ω_r, qubit transition frequency ω_q, and decoherence rates (T1, T2). Circuit QED allows access to regimes such as strong coupling (g > κ, γ) and ultrastrong coupling (g/ω ≳ 0.1), where counter‑rotating terms and nonperturbative effects become significant.
Superconducting qubits used in circuit QED include the Cooper pair box, transmon, flux qubit, and phase qubit. These devices rely on nonlinear inductance of the Josephson junction to create anharmonic energy spectra. Resonators are implemented as coplanar waveguide or three‑dimensional microwave cavities fabricated from superconducting metals (e.g., aluminium, Niobium). Quality factors and mode engineering determine photon lifetimes; three‑dimensional cavities demonstrated by Andreas Wallraff's and Robert J. Schoelkopf's groups extended coherence times. Coupling architectures include capacitively and inductively coupled geometries, Purcell filters to mitigate spontaneous emission, and multimode bus resonators for qubit‑qubit interactions.
Circuit QED reproduces and extends many quantum optics phenomena at microwave frequencies: vacuum Rabi splitting, photon blockade, quantum non‑demolition (QND) readout, and single‑photon generation and detection. The platform enables exploration of itinerant microwave photon scattering, strong nonlinearities at the single‑photon level, and engineered dissipation for steady‑state entanglement. Circuit implementations have realized analogues of electromagnetically induced transparency, squeezed states, the Mollow triplet, and studies of open quantum systems linking to quantum trajectory and quantum measurement theory.
Experiments operate at millikelvin temperatures using dilution refrigerators to suppress thermal photons. Readout employs dispersive measurement of resonator transmission or reflection with parametric amplifiers such as the Josephson parametric amplifier (JPA) and Josephson traveling-wave parametric amplifier (JTWPA) to achieve near‑quantum‑limited detection. Fabrication techniques use electron‑beam lithography, shadow evaporation for Josephson junctions, and planar superconducting circuit processing. Calibration and control leverage microwave pulse shaping, optimal control theory, and cryogenic wiring standards. Notable demonstrations include two‑qubit gates, high‑fidelity single‑shot readout, and bosonic codes using cavity modes (e.g., cat codes).
Circuit QED provides a dominant architecture for superconducting quantum processors used in gate‑based quantum computing and error‑correcting codes. It supports fast single‑ and two‑qubit gates, quantum nondemolition readout for measurement‑based protocols, and modular network proposals combining resonators and waveguide quantum electrodynamics (waveguide QED). Companies and national labs (e.g., IBM, Google, Rigetti Computing) use circuit QED primitives for cloud quantum processors and demonstrations of quantum advantage. Additionally, circuit QED platforms are employed for quantum simulation of many‑body models and hybrid quantum systems coupling to spins, mechanical resonators, or optical transducers.
Remaining challenges include improving coherence times, scaling connectivity while controlling crosstalk, and implementing fault‑tolerant quantum error correction at scale. Materials science issues (dielectric loss, quasiparticles) and microwave packaging constrain performance. Future directions pursue error‑corrected logical qubits via bosonic encodings, integration with cryogenic classical control, exploration of ultrastrong and deep‑strong coupling regimes, and hybridization with optical interfaces for long‑distance quantum communication. Continued collaboration among universities (e.g., Yale University, University of Sherbrooke), national labs, and industry partners (e.g., IBM, Google) will shape the roadmap toward practical quantum processors and new tests of quantum electrodynamics in engineered circuits.