Superconducting qubit–oscillator circuit beyond the ultrastrong-coupling regime

A circuit that pairs a flux qubit with an LC oscillator via Josephson junctions pushes the coupling between light to matter to uncharted territory, with the potential for new applications in quantum technologies. The interaction between an atom and the electromagnetic field inside a cavity1,2,3,4,5,6 has played a crucial role in developing our understanding of light–matter interaction, and is central to various quantum technologies, including lasers and many quantum computing architectures. Superconducting qubits7,8 have allowed the realization of strong9,10 and ultrastrong11,12,13 coupling between artificial atoms and cavities. If the coupling strength g becomes as large as the atomic and cavity frequencies (Δ and ωo, respectively), the energy eigenstates including the ground state are predicted to be highly entangled14. There has been an ongoing debate15,16,17 over whether it is fundamentally possible to realize this regime in realistic physical systems. By inductively coupling a flux qubit and an LC oscillator via Josephson junctions, we have realized circuits with g/ωo ranging from 0.72 to 1.34 and g/Δ ≫ 1. Using spectroscopy measurements, we have observed unconventional transition spectra that are characteristic of this new regime. Our results provide a basis for ground-state-based entangled pair generation and open a new direction of research on strongly correlated light–matter states in circuit quantum electrodynamics.

[1]  J Casanova,et al.  Deep strong coupling regime of the Jaynes-Cummings model. , 2010, Physical review letters.

[2]  Jean-Michel Raimond,et al.  Cavity Quantum Electrodynamics , 1993, Quantum Dynamics of Simple Systems.

[3]  C. Ciuti,et al.  No-go theorem for superradiant quantum phase transitions in cavity QED and counter-example in circuit QED. , 2010, Nature communications.

[4]  E. Jaynes,et al.  Comparison of quantum and semiclassical radiation theories with application to the beam maser , 1962 .

[5]  E. Solano,et al.  Circuit quantum electrodynamics in the ultrastrong-coupling regime , 2010 .

[6]  Dreyer,et al.  Quantum Rabi oscillation: A direct test of field quantization in a cavity. , 1996, Physical review letters.

[7]  J. Raimond,et al.  Manipulating quantum entanglement with atoms and photons in a cavity , 2001 .

[8]  F. Marquardt,et al.  Superradiant phase transitions and the standard description of circuit QED. , 2011, Physical review letters.

[9]  A. A. Abdumalikov,et al.  Ultrastrong coupling regime of cavity QED with phase-biased flux qubits , 2009, 0906.1383.

[10]  J. Hopfield.  a Quantum-Mechanical Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals. , 1958 .

[11]  A. Doherty,et al.  Cavity Quantum Electrodynamics: Coherence in Context , 2002, Science.

[12]  Michel Devoret,et al.  Circuit‐QED: How strong can the coupling between a Josephson junction atom and a transmission line resonator be? * , 2007, Annalen der Physik.

[13]  Orlando,et al.  Josephson Persistent-Current Qubit , 2022 .

[14]  E Solano,et al.  Observation of the Bloch-Siegert shift in a qubit-oscillator system in the ultrastrong coupling regime. , 2010, Physical review letters.

[15]  F. Nori,et al.  Qubit-oscillator systems in the ultrastrong-coupling regime and their potential for preparing nonclassical states , 2009, 0912.4888.

[16]  K. Chambers.  The most distant radio galaxies , 1993 .

[17]  I. Rabi.  Space Quantization in a Gyrating Magnetic Field , 1937 .

[18]  Willis E. Lamb,et al.  Fine Structure of the Hydrogen Atom by a Microwave Method , 1947 .

[19]  M. A. Yurtalan,et al.  Ultrastrong coupling of a single artificial atom to an electromagnetic continuum in the nonperturbative regime , 2016, Nature Physics.

[20]  Michel Devoret,et al.  Superconducting quantum bits , 2005 .

[21]  K. Rzążewski,et al.  Phase Transitions, Two-Level Atoms, and the A 2 Term , 1975 .

[22]  Maira Amezcua,et al.  Quantum Optics , 2012 .

[23]  Koichi Shimoda,et al.  Further Aspects of the Theory of the Maser , 1956 .

[24]  D. Braak,et al.  Integrability of the Rabi model. , 2011, Physical review letters.

[25]  Elliott H. Lieb,et al.  On the superradiant phase transition for molecules in a quantized radiation field: the dicke maser model , 1973 .

[26]  D. Wineland,et al.  Spectroscopy of a single Mg+ ion , 1981 .

[27]  P. Bertet,et al.  Coherent dynamics of a flux qubit coupled to a harmonic oscillator , 2004, Nature.

[28]  S. Girvin,et al.  Strong coupling of a single photon to a superconducting qubit using circuit quantum electrodynamics , 2004, Nature.

[29]  Y. Pashkin,et al.  Coherent control of macroscopic quantum states in a single-Cooper-pair box , 1999, Nature.