Next generation of qubits for quantum computing

The number one quantum computing challenge is that qubits (the basic units in quantum computers) are extremely fragile. Qubits are easily influenced by many things, like light and temperature. This is referred to as quantum decoherence, meaning that a quantum system gradually loses its special quantum behaviour over time.


To overcome this challenge, we study new types of qubits that are by design protected from outside influences. These new qubits are referred to as ‘protected qubits’ and have the potential to outperform established technologies. We aim to understand, develop and demonstrate protected qubits. In our efforts we combine material science, quantum theory and novel device design.

New avenues for Majorana qubits

Long-term goals: 

To develop a platform for topologically protected qubits with long coherence times and fault-tolerant quantum operations.

Highlights:

  • Two-dimensional electron gases (2DEGs) coupled to superconductors have low levels of disorder and offer scalability and flexibility in Majorana device design. We developed and studied a new hybrid system consisting of InSbAs 2DEGs coupled to aluminium with properties that surpass those of many of the platforms traditionally used to study Majorana physics (Nano Lett.).

  • We theoretically studied devices with ferromagnetic insulators to induce a Zeeman splitting. Using experimental input for the boundary conditions of our simulations, we showed how device geometry can help to induce a topological phase (Phys. Rev. B).

  • Making use of shadow masks and angled evaporation, we developed a new technique to fabricate a device with multiple leads without breaking vacuum. This method allows much larger flexibility on the complexity of the desired device (Nature).

Retraction paper

This year did not only include highlights, but also set-backs in the pursuit of Majorana bound states. In 2018, a paper was published on the supposed observation of a quantized Majorana conductance, the strongest evidence for Majorana bound states thus far. Unfortunately, this paper had to be retracted in 2021 (Nature 2021). The TU Delft Research Integrity Committee launched an investigation, and this procedure had not been finalized within 2021. The RIC consulted four independent international experts who wrote down their findings. As seen in the public version of the expert report, the central claim of the paper was found to be invalid due to a combination of technical errors uncovered by the authors and inappropriate data selection. An extended set of data was republished in 2021 (arXiv:2101.11456), and the authors concluded that the observed data may be also compatible with disorder- or potential-induced trivial Andreev bound states. Moreover, the authors of a related publication on material growth (Nature 2017) have alerted the editors of Nature to potential problems of the data processing in this paper, and an expression of concern has been posted on the website of Nature. As QuTech, we try to learn from past mistakes and do everything we can to ensure our published work is reliable. Concrete steps we have taken include having discussion meetings to raise awareness and hiring our own data steward.

Next generation of probes for protected qubits

Long-term goals: 

To establish the best benchmarks for the characterization of protected qubits and topological quantum devices.

Highlights:

  • We systematically studied the entire phase space of charge periodicity of Coulomb blockade conductance oscillations in a nanowire device, using a data-driven approach. Using our new approach, selection bias was systematically avoided and we identified which phase space regions could be of promise for topological states (Phys. Rev. B).

  • To improve our ability to rapidly explore a wide range of device parameters, we developed radio frequency techniques using superconducting resonators for fast device readout (Phys. Rev. Applied).

  • Using these new radio frequency techniques, we measured the capacitance of nanoscale semiconducting devices in field-effect transistor configurations with a sensitivity below attofarad, which sets the stage for fast and precise exploration of future Majorana candidate devices (arXiv).

Novel superconducting and hybrid qubit architectures

Long-term goals: 

To develop new qubits based on superconducting circuits with intrinsic protection against noise and with gate fidelities outperforming conventional qubit architectures.

Highlights:

  • Using a hybrid nanowire within a transmon qubit, we studied quasiparticle dynamics in magnetic fields, which is a critical aspect for understanding the robustness of potential protected qubits (arXiv).

  • Using a nanowire transmon, we embedded a quantum dot into a Josephson junction, realizing a single-impurity Anderson model. The transmon-quantum dot system proved to be an excellent probe of a 0-pi phase transition in the Josephson junction and may facilitate the future realization of protected superconducting qubits (arXiv).