Will Lawrie

PhD Student , TU Delft

Spin Qubit Arrays in Germanium

The spin state of an elementary charge is a well-established candidate for quantum information processing. Multiple semiconductor platforms are presently under considerable study to determine their viability as host materials for spin qubits, in particular their suitability as hosts of scalable qubit unit cells. Germanium quantum wells in planar heterostructures (Ge/SiGe) have recently emerged as a highly promising platform in which to host hole-based spin qubits. Demonstrations of the first quantum dots, two qubit logic, fast single qubit operations with fault tolerant fidelities, and even the operation of two-dimensional arrays have occurred within the relatively short time span of 3 years.

This talk will examine three recent experimental efforts on a 2x2 germanium quantum processor.

First, we operate a four-qubit quantum processor. We perform one, two, three, and four qubit logic for all qubit combinations, realizing a compact and high-connectivity circuit. By extending quantum coherence using refocusing techniques, we perform a quantum circuit executed on the full four-qubit system.

The ability to drive multiple qubits simultaneously will be of import for quantum processors, to reduce total operation time and in particular, the idling time of qubits. However driving simultaneously can introduce frequency crowding effects that can lead to systematic errors. Here we characterize single qubit fidelities in our four-qubit quantum processor while driving individually, two and four qubits simultaneously using randomized benchmarking. We find a strong dependence of single qubit fidelities on the driving speed, corresponding to a tradeoff between decoherence and crosstalk as the primary error mechanisms. Our characterization leads to single qubit fidelities as high as 99.9899(4) %, 99.904(4) % and 99.04 % while simultaneously driving one, two and four qubits respectively.

Finally, we study the magnetic field dependence of quantum coherence and spin relaxation times of T1 = 32 ms, spin dephasing times T2* = 2 us, extendable to T2 = 500 us with refocusing techniques, constituting the longest ever measured in the platform. We also use CPMG pulse sequences as notch filter functions to probe the non-zero 73Ge nuclear spin present in natural Ge and find that holes are still sensitive to hyperfine related dephasing. We estimate the relative strength of charge noise and hyperfine noise limiting coherence in our system to be within tthe same order of magnitude.

Zoom Link: https://ucph-ku.zoom.us/j/68683364059