Transmon qubit in a magnetic field: Evolution of coherence and transition frequency

Abstract

We report on spectroscopic and time-domain measurements on a fixed-frequency concentric transmon qubit in an applied in-plane magnetic field to explore its limits of magnetic field compatibility. We demonstrate quantum coherence of the qubit up to field values of B = 40 mT, even without an optimized chip design or material combination of the qubit. The dephasing rate Γϕ is shown to be not affected by the magnetic field in a broad range of the qubit transition frequency. For the evolution of the qubit transition frequency, we find the unintended second junction created in the shadow angle evaporation process to be non-negligible and deduce an analytic formula for the field-dependent qubit energies. We discuss the relevant field-dependent loss channels, which can not be distinguished by our measurements, inviting further theoretical and experimental investigation. Using well-known and well-studied standard components of the superconducting quantum architecture, we are able to reach a field regime relevant for quantum sensing and hybrid applications of magnetic spins and spin systems.

Sideband-resolved resonator electromechanics based on a nonlinear Josephson inductance probed on the single-photon level

“…This coupling rate does not yet satisfy the singlephoton strong coupling condition. However, improved microwave decoherence rates of up to 400 kHz as reported for superconducting quantum circuits 42 in combination with an optimized critical current I c of the SQUID will allow to reach the strong-coupling regime (see Supplementary Note 9). Nevertheless, ground-state cooling for n r ¼ 1 is expected to be within reach for this device using moderate fields of B ext = 20 mT applied in parallel to the superconducting layer.…”

Two-level systems in superconducting quantum devices due to trapped quasiparticles

“…It is becoming increasingly evident that the most coherence-limiting two-level systems (TLS) reside outside the qubit junctions on the surface of metals and dielectrics surrounding them (1)(2)(3)(4)(5) and their slowly fluctuating dynamics poses a serious challenge for quantum computation (1,(5)(6)(7)(8). A wide range of techniques has been developed to study such TLS by developing qubits and resonators into probes of a wide range of material properties (2,5,(9)(10)(11)(12)(13)(14)(15). At the same time, charged surface TLS and paramagnetic impurities also result in a stochastic and locally varying backdrop for quasiparticles (QPs) in the superconductor itself.…”

Implementation of a Transmon Qubit Using Superconducting Granular Aluminum

“…The success of superconducting circuits is linked to the availability of nonlinear elements with high intrinsic coherence, namely Josephson junctions (JJs) fabricated by thermal oxidation from thin film aluminum (Al) [14]. However, their applicability in hybrid systems [15,16] is limited by the low critical field of Al [17] and by the emergence of quantum interference effects in the JJs, even for magnetic fields aligned in plane [18]. Here we show that the JJ can be replaced by a small volume of granular aluminum (grAl) [19] providing enough nonlinearity to implement a superconducting transmon qubit [20], which we operate in magnetic fields up to ∼0.1 T.…”