Ab Initio Modelling: A Predictive Tool for Optimising Quantum Processor Development

Ab initio modelling of quantum dot qubits is a predictive modelling method used in the development of quantum processors. It is based on real-space grids and does not make assumptions related to device topology. The method can determine the exchange coupling between quantum dot qubits and model the full evolution of a SWAP gate, predicting qubit loss and infidelity rates for various voltage profiles. It also explores the impact of unwanted charge defects and tests robust pulse sequences. This technique can identify promising device designs for fabrication and bespoke control sequences, aiding in the development of quantum computers.

What is Ab Initio Modelling of Quantum Dot Qubits?

Ab initio modelling of quantum dot qubits is a method of predictive modelling that is used in the development of quantum processors (QPs). This method is based on real-space grids and does not make assumptions related to device topology, making it widely applicable. Given an electrode geometry, this method can determine the exchange coupling between quantum dot qubits and model the full evolution of a SWAP gate. This allows for the prediction of qubit loss and infidelity rates for various voltage profiles.

The method also explores the impact of unwanted charge defects, both static and dynamic, in the environment and tests robust pulse sequences. For instance, it can exhibit a sequence correcting both systematic errors and unknown charge defects, observing an order of magnitude boost in fidelity. This technique can thus identify the most promising device designs for fabrication as well as bespoke control sequences for each such device.

How Does Ab Initio Modelling Aid in the Development of Quantum Computers?

Electron spins in gated-defined semiconductor quantum dots are a strong candidate for qubit implementation. Due to their small size, fast operations, and compatibility with current industry standards, silicon spin qubits hold a great promise for the development of full-scale, mature-era quantum computers. However, the vast design space provided by semiconductor foundries makes the identification of promising chip architecture challenging.

The iterative nature of chip development, coupled with the time and cost of a chip design cycle, makes it highly desirable to have accurate predictive modelling tools to speed up the discovery of scalable designs. Given a candidate chip layout defined using standard commercial software as employed for traditional chips, such tools would extract key metrics and enable one to optimise the design before committing to fabrication. Ab initio modelling serves as one such tool, providing predictive modelling without assumptions related to device topology.

What are the Key Considerations in Optimising the Fidelity of Two-Qubit Gates?

A key consideration in optimising the fidelity of two-qubit gates is the sensitivity of the exchange coupling to charge noise. Accounting for the qubits’ environment is thus necessary for a predictive tool to be practically useful. The ab initio approach taken in this study makes very few assumptions regarding the anticipated physics but sees the key properties emerge once suitable parameters are found.

The method uses a uniform real-space grid method which finds the low-lying two-electron states for any reasonably smooth potential landscape without requiring that it be approximately locally harmonic or that eigenstates be close to any given analytic form. This numerical modelling is fairly demanding but can tackle quite general scenarios.

How Does Ab Initio Modelling Handle the Dynamics of a SWAP Gate?

The ab initio modelling method directly models the dynamics of a SWAP gate as the confining potential continuously deforms, propagating the full two-electron state forward with a split-operator method. At gate completion, the method reassesses the validity of the qubit representation, i.e., whether there is an increase in the 2-0-0-2 configurations which correspond to qubit loss, and reports this separately from the fidelity of the gate operation within the 1-1 subspace.

The method observes the effect of non-adiabatic voltage ramp on this loss probability and verifies that a suitable ramp introduces negligible loss. It then proceeds to assess the impact of unwanted trapped charges in the environment, either as static entities or as fluctuators that switch charge position during the gate operation.

How Does Ab Initio Modelling Explore Options for Robust Pulse Sequences?

By extracting the behavior of the full model into a reduced basis, the ab initio modelling method can rapidly explore options for robust pulse sequences, i.e., a multi-stage gate that corrects for intrinsic axis-error as well as charges in the environment. Adapting methods from the NMR literature, the method can exhibit a 9-pulse sequence with remarkable robustness against both systematic errors and unknown charge defects.

This ability to identify the most promising device designs for fabrication as well as bespoke control sequences for each such device makes ab initio modelling a valuable tool in the development of quantum processors.