Universal quantum logic in hot silicon qubits

Abstract

In the last decade silicon has emerged as a potential material platform for quantum information. The main attraction comes from the fact that silicon technologies have been developed extensively in the last semiconductor revolution, and this gives hope that quantum dots can be fabricated one day with the same ease transistors are made today. However, building a large-scale quantum computer presents also complications that go beyond fabrication. The heat-dissipation challenge is one of these. As many other qubit platforms, also quantum dot qubits are cooled down at temperatures close to absolute zero in order to overcome the problem of decoherence. While this can be advantageous in few-qubit experiments, it becomes soon impractical as the qubit number increases. The first part of the thesis describes a series of experiments that demonstrate how Si- MOS quantum dot qubits can be successfully operated beyond one Kelvin, where the increase in cooling power is substantial. The first step is to demonstrate that electrons have sufficiently large energy scales to be properly isolated and controlled at these high temperatures. In the first experimental chapter of the thesis we demonstrate a highly uniform double quantum dot system at the temperature of 0.5 K. The on-chip single-electron-transistor (SET) shows very regular oscillations and an exceptional sensitivity to dot-reservoir and interdot transitions. The electrons in the quantum dot can also be completely decoupled from the reservoir, resulting in a fully isolated system. In order to performquantumoperations it is not only crucial to isolate electrons, but also to couple them. While this is routinely achieved in Si-SiGe heterostructures, it is usually more challenging in Si-MOS due to the larger disorder at the Si-SiO2 interface. However, we find that in the same device we can control the tunnel coupling between the electrons, in a range from below 1 Hz up to 13 GHz. This would allow to isolate the electrons for single-qubit operations and to couple them for two-qubit gates or readout using Pauli spin blockade. Part of the challenges concerning operation of ‘hot’ spin qubits lies in the temperature dependence of two parameters: the spin lifetime and the charge noise, which are thoroughly studied in chapter 4. The spin lifetime is usually very long in silicon, due to a weak spin-orbit coupling, and it can approach seconds at low magnetic fields. However, the temperature increases the excitations in the phonon bath and activates two-phonon transitions, which have a steep temperature dependence. These processes, which we experimentally find to start around 500 mK, can ultimately limit qubit performances. However, the spin lifetime can be significantly improved by working in a low magnetic field and high valley splitting regime. Si-MOS quantum dot qubits have a large valley splitting, usually of several hundreds of &eV, and a lowmagnetic field can be set by reading out the qubits with Pauli spin blockade. This guarantees that useful spin lifetimes can still be found at temperatures close to one Kelvin. In particular, in chapter 4 we measure values exceeding 1 ms at 1.1 K, and discuss how they can be further improved in case of a larger valley splitting.