Spin-Resonance Linewidths of Bismuth Donors in Silicon Coupled to Planar Microresonators

Abstract

Ensembles of bismuth-donor spins in silicon are promising storage elements for microwave quantum memories due to their long coherence times, which exceed seconds. The operation of an efficient quantum memory requires the achievement of critical coupling between the spin ensemble and a suitable highquality factor resonator-this in turn requires a thorough understanding of the line shapes for the relevant spin-resonance transitions, particularly considering the influence of the resonator itself on line broadening. Here, we present pulsed electron-spin-resonance measurements of ensembles of bismuth donors in natural silicon, above which niobium superconducting resonators have been patterned. By studying spin transitions across a range of frequencies and fields, we identify distinct line-broadening mechanisms and, in particular, those that can be suppressed by operating at magnetic-field-insensitive "clock transitions." Given the donor concentrations and resonator used here, we measure a cooperativity C ∼ 0.2 and based on our findings we discuss a route to achieve unit cooperativity, as required for a quantum memory.

Spin coherence of near-surface ionised $^{125}$Te$^+$ donors in silicon

“…4A, consistent with inhomogeneous broadening from 29 Si nuclear spins, as is commonly seen for donors in nat Si. The linewidth reaches a minimum value of ∼0.6 MHz, which is close to the pulse bandwidth limit (π pulse duration 140 ns), but approximately equal to that measured for a clock transition in Bi: nat Si doped at similar concentration [21]. The lineshape is well fit by a single Gaussian (see Fig.…”

“…For measurements at S-band (∼ 3.5 GHz), we used a home-built spectrometer (similar in design to that described in Ref. [21]) and 3.5-4.5 GHz loop-gap resonators, wire abraded using oxygenfree copper, which were measured in transmission (see Supplementary Materials for details). The loop-gap resonators had as-fabricated quality factors of ∼ 200, reducing to ∼ 50 when the frequency was tuned by inserting a small (approximately 2x4 mm) piece of intrinsic 275 µm float zone Si wafer into the capacitive gap, giving a window of about 100 MHz for experiments at different microwave frequencies, and had a typical π-pulse duration of ∼ 700 ns when using a 30 W solid-state power amplifier.…”

Identification of different types of high-frequency defects in superconducting qubits

“…Using this method, we succeeded in the unambiguous detection of critical-current-fluctuation TLS defects that remained elusive until now. The described technique should be also suitable for detection of other types of high-frequency defects, such as spin defects [67][68][69][70][71]. We envision that the reported method will become a standard protocol for systematic studies of high-frequency defects in both flux-tunable and fixed-frequency superconducting qubits, revealing new insights into the microscopic origin of TLS defects and their mitigation strategies.…”

Reducing strain in heterogeneous quantum devices using atomic layer deposition

“…It is then coated by a 50 nm layer of Al 2 O 3 deposited at 150 • C by ALD immediately after RCA cleaning. A 100 nm thick niobium resonator is then fabricated on top of the sample by lift off as in [20]. We compare this to a resonator with similar frequency fabricated without the ALD layer.…”

“…The spin transitions are measured at 100 mK by performing echo detected field sweeps (EDFS) using CPMG averaging using a home built electron spin resonance spectrometer as in Ref. [20]. We measure the spin linewidth at high drive power in all three device configurations in two limits: (i) close to a magnetic clock transition where ∂f /∂B 0 ∼ −0.1γ e and ∂f /∂A∼ −5 (Fig.…”