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Opening the gates to scalable quantum computing

Technical update
Opening the gates to scalable quantum computing
Setiawan Setiawan
25 April, 2023

Errors in two-qubit gates are a major hurdle to realise practical quantum computers. These errors often arise from imperfect control fields and environmental noise.  

In a recent research paper, I worked with two academic researchers to propose a novel approach to design two-qubit gates that are simpler, faster and more resource efficient than previous proposals.  

These proposed two-qubit gates are both fast enough to mitigate the unwanted effects of noise and robust enough to overcome variations in the control pulses – helping us take one step closer to scalable qubit control. Plus, these gates could be used for both superconducting and neutral atom qubit platforms. 

Existing protocols for realising robust geometric two-qubit gates are hard to implement experimentally as the required resources are extremely demanding, requiring a large number of control pulses.  

Plus, they can only be realised experimentally in atomic systems with specific kind of qubit-qubit interactions. And, since they rely only on purely adiabatic protocols, the gate time is significantly long which renders them susceptible to the effects of dissipation and noise. 

As a result, existing approaches will fail as quantum computers continue to scale, taking too long and requiring too many control pulses to perform any useful calculations. 

In this paper, we propose a novel approach to designing geometric two-qubit quantum gates that circumvents the above limitations.  

We introduce and analyse a powerful new geometric approach to two-qubit gates based on STIRAP (Stimulated Raman Adiabatic Passage).  STIRAP is perhaps the best known and most powerful approach for using non-trivial dark state physics to enable state transfer operations and single qubit gates between two isolated levels that only interact via an intermediary excited state (the famous Lambda system configuration).   

The idea of adapting STIRAP for two qubit gates was previously explored in both trapped ions and Rydberg atoms, as reported in high-profile theory papers in Science 292, 1695 (2001) and Physical Review Letters 100, 170504 (2008). These protocols are however resource inefficient, requiring many independent control fields and strong, specific kinds of qubit-qubit interactions (phonon mediated interactions for ions, Rydberg blockade for Rydberg atoms).  This makes them difficult (if not impossible) to implement in most systems.   

Our new protocol overcomes these limitations.  We describe a completely new way of harnessing STIRAP for geometric two qubit gates that only requires two time-dependent control fields. We also show that this can be achieved without requiring any sort of strong static qubit-qubit interaction. Instead, we only require a dispersive interaction between one of the qubits and an intermediary coupler system.   

This makes our approach suitable for a large number of platforms.  It also alleviates a common problem in gate design, namely the inability to fully turn off interactions between the two qubits.  Yet another virtue of our approach is that it is directly compatible with modular computing architectures where one needs to do gates between remote qubits that only interact via an intermediary bus mode.   We also show that our approach can be accelerated using a particular kind of “shortcuts to adiabaticity” method, allowing for fast gates.  

In addition to the basic workings and philosophy of our gate, we also present detailed simulation results for an implementation in a leading superconducting circuit platform comprised of two fluxonium qubits. By including realistic levels of dissipation (and all relevant coherent error channels), we found excellent fidelity performance and gate times.   

To summarise, our approach is both resource efficient, requiring fewer control pulses, and accelerates the adiabatic protocols. 

Most importantly, our scheme can be applied to a wide variety of different qubit platforms in superconducting circuits and neutral atoms. It is also compatible with modular architectures where multiple qubits are coupled via tuneable interactions to a shared auxiliary mode (such as a bus or coupler).  

This work, therefore, paves the way towards robust and scalable quantum control across multiple quantum architectures. 

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