Quantum Coherence in Germanium Hole Spin Qubits

The ability to reliably store quantum information is an underpinning assumption for the operation of a quantum computer. This ability is compromised when qubits exchange energy with external systems in a process known as decoherence. In spin qubit systems, decoherence can be quantified by the time the transeverse component of a spin state takes to dephase (T2). For hole spin qubits in planar germanium (Ge/SiGe), pure dephasing times (T2*) have been limited to a few hundred nanoseconds.

 

Here we study the magnetic field dependence of T2* for hole qubits in Ge/SiGe. We find a 1/B dependence suggesting charge noise is the dominant decohering process, and observe T2* as high as 1.8 μs at 0.1 T, extendable to 500 μs using refocussing techniques. Furthermore, we  observe the decohering effect of the residual 73Ge nuclear spins on hole spin qubits. We confirm that hole spin qubits are sensitive to the nuclear spin bath, and extract the amplitude of the fluctuating Overhauser field to be 34.4 kHz, that we predict to limit spin dephasing times to T2*,hf = 6.54 μs. These results simultaneously represent a milestone in spin coherence in hole qubits, while also demonstrating their sensitivity to hyperfine coupling, providing concrete evidence of the necessity of isotopic purification.