Cavity QED and strong coupling in superconducting qubits

Short answer: Researchers have proposed and demonstrated architectures where superconducting qubits couple strongly to microwave photons in transmission lines or resonators. This regime enables the study of light-matter interaction at the quantum level, including spontaneous emission modification and dispersive interactions.

Defining the Strong Coupling Regime

Circuit quantum electrodynamics (cQED) investigates the interaction between superconducting qubits and quantized electromagnetic fields. The field emerged from the realization that superconducting qubits can strongly and controllably interact with microwave photons. While atomic cavity QED inspired early developments, circuit QED has become an independent field of research. It allows the study and control of light-matter interaction in unprecedented detail.

A specific "superstrong" coupling regime has been identified in one-dimensional vacuum systems. This occurs when a high wave impedance superconducting transmission line is directly wired to a transmon qubit. In this configuration, the transmon’s spontaneous emission linewidth can greatly exceed the discrete transmission line modes spacing. This condition defines a regime where many frequency-resolved vacuum modes hybridize with a single atom. It is distinct from standard cavity QED, which typically describes a reversible exchange of an excitation between a single resonant mode and a qubit at the vacuum Rabi frequency.

Experimental Realization and Measurement

Researchers have established the superstrong coupling regime by observing the spontaneous emission line of a transmon. This observation was revealed through the mode-by-mode measurement of the vacuum’s density of states. The vacuum fluctuations fundamentally affect the atom by inducing a finite excited state lifetime and a Lamb shift of its transition frequency. The reported experiments demonstrate the reverse effect: the modification of vacuum modes by a single atom.

The physical behavior in this regime is accurately described by a Caldeira–Leggett model. In this model, the transmon’s quartic nonlinearity is treated as a perturbation. The atom-induced dispersive photon-photon interaction is also captured by this framework. Nonperturbative modification of the vacuum, including inelastic scattering of single photons, can be enabled by replacing the transmon with more anharmonic qubits. This approach has broad implications for simulating quantum impurity models of many-body physics.

Proposed Architectures and Control

Earlier proposals suggested using one-dimensional transmission line resonators to reach the strong coupling limit in superconducting electrical circuits. These architectures are attractive for quantum computing and control because they provide strong inhibition of spontaneous emission. This inhibition potentially leads to greatly enhanced qubit lifetimes. Such systems also allow high-fidelity quantum non-demolition measurements of the state of multiple qubits.

Furthermore, these architectures offer a natural mechanism for the entanglement of qubits separated by centimeter distances. They would allow the production of microwave photon states of fundamental importance for quantum communication. The vacuum Rabi frequency for the coupling of cavity photons to quantized excitations of an adjacent electrical circuit can easily exceed the damping rates of both the cavity and the qubit. One-dimensional cavities offer practical advantages in reaching the strong coupling limit over previous proposals.

Role in Quantum Information Processing

Circuit QED plays an essential role in all current approaches to gate-based digital quantum information processing with superconducting circuits. It provides a framework for the study of hybrid quantum systems, such as quantum dots, magnons, Rydberg atoms, surface acoustic waves, and mechanical systems interacting with microwave photons. The coherent coupling of superconducting qubits to microwave photons in high-quality oscillators is central to this field.

The physics of the Jaynes-Cummings model, its dispersive limit, and the different regimes of light-matter interaction are key areas of review. Coupling of superconducting circuits to their environment is necessary for coherent control and measurements in circuit QED. However, this coupling also invariably leads to decoherence. Dispersive qubit readout, a central ingredient in almost all circuit QED experiments, relies on these interactions. The field continues to evolve from fundamental studies of open systems to engineering quantum states.

FAQ

What defines the superstrong coupling regime?

The superstrong coupling regime is defined when a transmon’s spontaneous emission linewidth greatly exceeds the discrete transmission line modes spacing. This occurs in high-impedance superconducting transmission lines directly wired to the qubit, causing many vacuum modes to hybridize with the atom.

How is this regime experimentally verified?

Researchers verify the regime by observing the transmon’s spontaneous emission line. This is achieved through mode-by-mode measurement of the vacuum’s density of states. The resulting data allows accurate description via a Caldeira–Leggett model, treating the transmon’s nonlinearity as a perturbation.

What are the proposed benefits for qubit control?

Proposed architectures using transmission line resonators offer strong inhibition of spontaneous emission, potentially enhancing qubit lifetimes. They also enable high-fidelity quantum non-demolition measurements and provide a natural mechanism for entangling qubits separated by centimeter distances, supporting quantum communication applications.

How does circuit QED differ from atomic cavity QED?

While atomic cavity QED inspired early developments, circuit QED is now an independent field. It uses superconducting circuits to study light-matter interaction at the quantum level. It supports hybrid systems and gate-based digital quantum information processing, distinct from the original atomic cavity contexts.

What role does the environment play in these systems?

Coupling superconducting circuits to their environment is necessary for coherent control and measurements. However, this coupling invariably leads to decoherence. Dispersive qubit readout, a central ingredient in experiments, relies on managing these environmental interactions within the circuit QED framework.