Ultrastrong coupling of a single artificial atom to an electromagnetic continuum in the nonperturbative regime

The interaction between an atom and the electromagnetic field inside a cavity 1-6 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 qubits 7,8 have allowed the realization of strong 9,10 and ultrastrong 11-13 coupling between artificial atoms and cavities. If the coupling strength g becomes as large as the atomic and cavity frequencies (∆ and ω o , respectively), the energy eigenstates including the ground state are predicted to be highly entangled 14 . There has been an ongoing debate 15-17 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/ω o ranging from 0.72 to 1.34 and g/∆ 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.We begin by describing the Hamiltonian of each component in the qubit-oscillator circuit, which comprises a superconducting flux qubit and an LC oscillator inductively coupled to each other by sharing a tunable inductance L c , as shown in the circuit diagram in Fig. 1a.The Hamiltonian of the flux qubit can be written in the basis of two states with persistent currents flowing in opposite directions around the qubit loop 18 , |L q and |R q , as H q = − (∆σ x + εσ z )/2, where ∆ and ε = 2I p 0 (n φq − n φq0 ) are the tunnel splitting and the energy bias between |L q and |R q , I p is the maximum persistent current, and σ x, z are Pauli matrices. Here, n φq is the normalized flux bias through the qubit loop in units of the superconducting flux quantum, 0 = h/2e, and n φq0 = 0.5 + k q , where k q is the integer that minimizes |n φq − n φq0 |. The macroscopic nature of the persistent-current states enables strong coupling to other circuit elements. Another important feature of the flux qubit is its strong anharmonicity: the two lowest energy levels are well isolated from the higher levels.The Hamiltonian of the LC oscillator can be written asC is the resonance frequency, L 0 is the inductance of the superconducting lead, L qc ( L c ) is the inductance across the qubit and coupler (see Supplementary Section 2), C is the capacitance, andâ (â † ) is the oscillator's annihilation (creation) operator. Figure 1b shows a laser microscope image of the lumped-element LC oscillator, where L 0 is designed to be as small as possible to maximize the zeropoint fluctuations in the currentand hence achieve strong coupling to the flux qubit, while C is adjusted so as to achieve a desired value of ω o . The freedom of choosing L 0 for large I zpf is one of the advantages of lumped-element LC oscillators over coplanar-waveguide ...

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“…Note added in proof: After acceptance of our paper, we became aware of a related manuscript 27 taking a different approach to the same theme.…”

Section: Lettersmentioning

confidence: 99%

Superconducting qubit–oscillator circuit beyond the ultrastrong-coupling regime

et al. 2016

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“…Pioneered by the experiments of Refs. [12,13], experiments in the DSC regime have now been achieved with flux qubits coupled to resonators [14] as well as an electromagnetic continuum [15]. Additionally, the U/DSC coupling regime of the Rabi model was the subject of recent analog quantum simulations [16,17].…”

Section: Introductionmentioning

confidence: 99%

Approaching ultrastrong coupling in transmon circuit QED using a high-impedance resonator

et al. 2017

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In this experiment, we couple a superconducting transmon qubit to a high-impedance 645 microwave resonator. Doing so leads to a large qubit-resonator coupling rate g, measured through a large vacuum Rabi splitting of 2g 910 MHz. The coupling is a significant fraction of the qubit and resonator oscillation frequencies ω, placing our system close to the ultrastrong coupling regime (ḡ = g/ω = 0.071 on resonance). Combining this setup with a vacuum-gap transmon architecture shows the potential of reaching deep into the ultrastrong couplingḡ ∼ 0.45 with transmon qubits.

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“…These terms that do not conserve the excitation number are negligible with naturally occurring coupling rates (g ω c ) in NMR, cavity QED, or in standard experiments in circuit QED. With an innovative sample design, the ultrastrong coupling can be reached in a circuit QED setups [111,[212][213][214]. In addition to direct engineering, the ultrastrong coupling can be achieved with a quantum simulation [215] in a JC system with two properly chosen external fields.…”

Section: Modulated Jaynes-cummings Hamiltonianmentioning

confidence: 99%

Quantum systems under frequency modulation

et al. 2017

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Abstract. We review the physical phenomena that arise when quantum mechanical energy levels are modulated in time. The dynamics resulting from changes in the transition frequency is a problem studied since the early days of quantum mechanics. It has been of constant interest both experimentally and theoretically since, with the simple two-state model providing an inexhaustible source of novel concepts. When the transition frequency of a quantum system is modulated, several phenomena can be observed, such as Landau-Zener-Stückelberg-Majorana interference, motional averaging and narrowing, and the formation of dressed states with the presence of sidebands in the spectrum. Adiabatic changes result in the accumulation of geometric phases, which can be used to create topological states. In recent years, an exquisite experimental control in the time domain was gained through the parameters entering the Hamiltonian, and high-fidelity readout schemes allowed the state of the system to be monitored non-destructively. These developments were made in the field of quantum devices, especially in superconducting qubits, as a well as in atomic physics, in particular in ultracold gases. As a result of these advances, it became possible to demonstrate many of the fundamental effects that arise in a quantum system when its transition frequencies are modulated. The purpose of this review is to present some of these developments, from two-state atoms and harmonic oscillators to multilevel and many-particle systems.

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Ultrastrong coupling of a single artificial atom to an electromagnetic continuum in the nonperturbative regime

Cited by 479 publications

References 30 publications

Superconducting qubit–oscillator circuit beyond the ultrastrong-coupling regime

Superconducting qubit–oscillator circuit beyond the ultrastrong-coupling regime

Approaching ultrastrong coupling in transmon circuit QED using a high-impedance resonator

Quantum systems under frequency modulation

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