Concentric transmon qubit featuring fast tunability and an anisotropic magnetic dipole moment

Abstract

We present a planar qubit design based on a superconducting circuit that we call concentric transmon. While employing a straightforward fabrication process using Al evaporation and lift-off lithography, we observe qubit lifetimes and coherence times in the order of 10 µs. We systematically characterize loss channels such as incoherent dielectric loss, Purcell decay and radiative losses. The implementation of a gradiometric SQUID loop allows for a fast tuning of the qubit transition frequency and therefore for full tomographic control of the quantum circuit. Due to the large loop size, the presented qubit architecture features a strongly increased magnetic dipole moment as compared to conventional transmon designs. This renders the concentric transmon a promising candidate to establish a site-selective passive directẐ coupling between neighboring qubits, being a pending quest in the field of quantum simulation.Quantum bits based on superconducting circuits are leading candidates for constituting the basic building block of a prospective quantum computer. A common element of all superconducting qubits is the Josephson junction. The nonlinearity of Josephson junctions generates an anharmonic energy spectrum in which the two lowest energy states can be used as the computational basis 1,2 . Over the last decade there has been a two order of magnitude increase in coherence times of superconducting qubits. This tremendous improvement allowed for demonstration of several major milestones in the pursuit of scalable quantum computing, such as the control and entanglement of multiple qubits 3,4 . Further increases in coherence times will eventually allow for building a fault tolerant quantum computer with a reasonable overhead in terms of error correction, as well as implementing novel quantum simulation schemes by accessing wider experimental parameter ranges 5 . While superconducting qubits embedded in a 3D cavity 6 have shown coherence times in excess of 100 µs 7 , this approach may impose some constraint on the scalability of quantum circuits. Since the Josephson junction itself does not limit qubit coherence 6 , comparably long lifetimes can also be achieved in a planar geometry by careful circuit engineering.In this paper, we present the design and characterization of a superconducting quantum circuit comprising a concentric transmon qubit 8 , schematically depicted in Fig. 1(a). The two capacitor pads forming the transmon's a) Electronic mail: jochen.braumueller@kit.edu. large shunt capacitance are implemented by a central disk island and a concentrically surrounding ring. The two islands are interconnected by two Josephson junctions forming a gradiometric SQUID. A 50 Ω impedance matched on-chip flux bias line located next to the qubit allows for fast flux tuning of the qubit frequency due to the imposed asymmetry. This guarantees high experimental flexibility and enables full tomographic control. The gradiometric flux loop design reduces the sensitivity to external uniform magnetic fields and thus to external flux ...

Micromachined Integrated Quantum Circuit Containing a Superconducting Qubit

“…As a further example, the addition of a second junction and flux-bias-line would constitute a frequency tunable device inspired by the concentric transmon in Ref. [22]. More sophisticated on-chip input/output circuitry, such as quantum limited amplifiers [23][24][25], circulators [26,27], and switching elements [28,29], will also be required for practical quantum information processing.…”

Displacemon Electromechanics: How to Detect Quantum Interference in a Nanomechanical Resonator

“…This flux difference is tuned primarily by means of a variable perpendicular field ΔB ⊥ ðtÞ, while mechanical coupling is flux mediated using a static in-plane field B jj [45][46][47]. This type of concentric transmon is insensitive to uniform magnetic fields, which has two advantages for this proposal: The qubit can be operated coherently away from a flux sweet spot [37], and any misaligned static flux will not perturb the energy levels. Both these facts are favorable for strong nanomechanical coupling.…”

“…Here, we have neglected the qubit inductance, which makes a small contribution on the energy levels [37]. In the transmon limit E J ≫ E C , we can approximate Eq.…”

“…Qubit rotations, initialization, and projective measurement are now well-established techniques, using capacitive coupling to a microwave cavity in a circuit quantum electrodynamics architecture [37,53].…”

“…We propose a design that is robust against electrical and magnetic noise while still achieving strong mechanical coupling. We envisage a superconducting qubit of the concentric transmon design [37] in which at least one of the junctions is a vibrating nanotube (Fig. 1).…”