Low-frequency spin qubit energy splitting noise in highly purified 28Si/SiGe

Abstract

We identify the dominant source for low-frequency spin qubit splitting noise in a highly isotopically-purified silicon device with an embedded nanomagnet and a spin echo decay time T echo 2 = 128 µs. The power spectral density (PSD) of the charge noise explains both, the clear transition from a 1/f 2-to a 1/f-dependence of the splitting noise PSD as well as the experimental observation of a decreasing time-ensemble spin dephasing time, from T Ã 2 % 20 µs, with increasing measurement time over several hours. Despite their strong hyperfine contact interaction, the few 73 Ge nuclei overlapping with the quantum dot in the barrier do not limit T Ã 2 , likely because their dynamics is frozen on a few hours measurement scale. We conclude that charge noise and the design of the gradient magnetic field are the key to further improve the qubit fidelity in isotopically purified 28 Si/SiGe.

Controllable freezing of the nuclear spin bath in a single-atom spin qubit

“…A 1/ω noise spectrum has been observed in multiple recent experiments on isotopically enriched silicon devices, each including deliberate magnetic field gradients generated by locally fabricated micromagnets ( 43 , 44 ). As these studies verify, in these cases, the 1/ω noise results from device electric-field noise transduced into magnetic-field noise due to the strong gradient.…”

Exploiting a Single‐Crystal Environment to Minimize the Charge Noise on Qubits in Silicon

“…The magnetic noise can be decreased with silicon isotopic purification methods to remove 29 Si (with I = 1/2) atoms, thereby resulting in a nuclear spin‐free environment, with 29 Si concentrations as low as 60 ppm. [ 4–7 ] Charge noise, however, is more difficult to control since its origins are not fully understood and seems to be specific to the qubit‐hosting material platform. The pervasive nature of charge noise has several implications for quantum computing implementations in silicon.…”

“…First, it limits the gate fidelities of single spin qubits by coupling to the electron g ‐factor [ 8 ] and to the electric drive pulse in the presence of a micromagnet. [ 6,7 ] Second, charge noise hinders spin‐cavity coupling through the charge degree of freedom, [ 9–11 ] posing a challenge for designing efficient long‐range interconnects between qubits. Finally, for two‐qubit gates where the coupling between two electron spin qubits is achieved via the exchange interaction with strength J , [ 3,12–18 ] the charge noise results in J fluctuations during the two‐qubit gate operation limiting the overall gate fidelity.…”

“…In nanoelectronic devices, the power spectral density of charge noise S ε has a frequency dependence of S ε ( f ) = S 0 / f α , where S 0 is the power spectral density at 1 Hz and the spectral exponent α is typically ≈1 with reported values between 0.5 and 2. [ 6–8,20,22–24,27–33 ] The power‐law nature of charge noise can be explained by an ensemble of thermally activated two‐level fluctuators (TLFs) where each TLF switches between two metastable states. [ 34,35 ] The level of charge noise in a device is typically quantified by the S 0 value obtained via charge sensor measurements using Coulomb blockade.…”

“…[ 34,35 ] The level of charge noise in a device is typically quantified by the S 0 value obtained via charge sensor measurements using Coulomb blockade. [ 7,20–24,28,31,36 ] Alternatively, the S 0 value can be measured via a superconducting cavity coupled to a double quantum dot, where the microwave signal transmitted through the resonator is sensitive to the charge environment of the double quantum dot. [ 36,37 ] Reported values of the 1 Hz noise level S 0 in silicon using these two techniques range from 0.11 to 12 μeV 2 Hz −1 [ 7,22–24,27,28,31,36 ] (see Table 1 ).…”