# Building blocks for the computer of the future

**While quantum computers will not replace classical computers, they could enable calculations that require more computing power than classical computers are ever likely to possess. This opens the way for revolutionary applications, such as the resolution of complex optimization challenges, or the prediction, simulation and modelling of the behavior of molecules, catalysts and new materials.**

Realizing the promise of quantum computing requires the development of different layers of hardware and software, together referred to as the quantum computing stack (pictured below). This is what we do at QuTech. The basis of the stack, referred to as the ‘quantum chip’, contains the qubits. We are studying different types of qubits for this part of the stack, each with its own advantages.

We are also developing the system architecture that connects and translates quantum algorithms to the low-level pulses that operate on the qubits of quantum processors.

## Spin qubits

Long-term goal:

To build a modular spin-qubit processor for scalable quantum information processing.

### 2019 Research Highlights:

- We
**controllably loaded an 8 quantum dot register**. Thanks to novel tuning concepts and improvements in our measurement acquisition, tuning an 8-dot device from scratch takes only 1-2 days (*npj Quantum Information*). - We performed a powerful, platform-independent
**scaling analysis**(*MICPRO*) and reviewed the state-of-the-art in Physics Today. We also presented novel ideas on**scalable tiles**based on sparse spin qubit arrays (IEDM 2019). - We achieved an improvement in
**gate-based dispersive spin readout**by two orders of magnitude in signal-to-noise ratio and measurement time compared to earlier work, via an on-chip superconducting resonator (*Nature Nanotechnology*)*.*We published our experiment on**randomized benchmarking of two-qubit gates**in*Phys. Rev. X*. Furthermore, we showed that it is possible to control the coupling between single electrons in silicon (*Nano Letters*__).__ - Our work on Ge-based spin qubits continued to progress at a remarkable pace, with the first demonstration of coherently controlling a single-hole spin qubit (
*arXiv*) and the realization of**universal two-qubit control**(*Nature*). An important advantage of Ge-based qubit is that no external micromagnets or striplines are needed for qubit control, thanks to the comparatively large spin-orbit interaction, and that germanium can be coupled to superconductors (*Phys. Rev. B*). We continued our characterization and optimization of the**Ge quantum wells**, which are increasingly used by groups around the world (*Phys. Rev. B*,*Adv. Funct. Mat.*) - We took our work on quantum simulation of Fermi-Hubbard physics to the realm of
**quantum magnetism**(*Nature*). Using a 2x2 quantum dot array, we observed for the first time in any system a form of magnetism predicted by Nagaoka in 1966, a powerful illustration of the potential of quantum dot systems for studying novel phenomena in an engineered quantum system.

## Transmon qubits

Long-term goal:

To realize an error-protected logical qubit with a 17-qubit superconducting circuit and a flexible control stack enabling NISQ applications.

### 2019 Research Highlights:

- Focusing on improving the two-qubit gating scheme, we developed a method for
**sampling control flux pulses on-chip**, turning the qubit into a precision oscilloscope, or*cryoscope*. The results were featured on the cover of*Appl. Phys. Lett*. - We developed a
**new pulse shape**which is insensitive to long-timescale distortions (*Phys. Rev. Lett*.), allowing us to demonstrate a fast (40 ns), high-fidelity (99.1%) low-leakage (0.1%) two-qubit conditional phase gate, pushing the state of the art. - We next implemented a
**three-qubit quantum circuit**that produces and stabilizes two-qubit entanglement through repeated non-demolition quantum parity measurements, an essential step for quantum error correction (*Science Advances*). - We completed the
**integration of our control stack**by incorporating the central controller (developed by TNO) necessary for control of the 7 and 17-qubit surface-code circuits with which we target quantum error detection and correction of a logical qubit, respectively. We fabricated the quantum circuits using lateral interconnections, which is easier than using vertical interconnections. These developments contributed to meeting critical milestones, and ensured that the QuSurf consortium (led by TU Delft/QuTech) remained part of the IARPA program LogiQ. - We applied our quantum hardware and control stack targeting error correction simultaneously to
**noisy intermediate-scale quantum (NISQ) applications**. In the domain of quantum chemistry we used the tools to calculate ground-state energies (*Phys. Rev. A*) and energy derivatives (*npj Quantum Information*).

## Majorana qubits

Long-term goal:

To develop the first qubit based on Majorana bound states, which is topologically protected against decoherence of the superposition state.

### 2019 Research Highlights:

- To realize their full potential, Majoranas must be engineered on a scalable platform where several complex elements need to come together to construct a functional quantum bit.
**new hybrid platform**, coupling high-quality 2D semiconductors to superconductors (*Nature Communications*). - Controlling a Majorana qubit requires circuitry that allows manipulating and reading out quantum states stored in Majorana bound states. We developed
**superconducting circuits coupled to semiconductor nanowires**that are compatible with the conditions for creating Majoranas, i.e. a large magnetic field (*arXiv*). The work was part of the master’s project of Marta Pita Vidal who was named*Best Graduate of Applied Sciences 2019*for this work. - Combining experimental measurements and theoretical simulations, we confirmed the
**physical origin of Majorana bound states**, namely the interplay between magnetic fields, spin-orbit coupling and induced superconductivity (*Phys. Rev. Lett.*) The good agreement between experiments and theory gives confidence that the fundamental principles are well understood. - Together with colleagues from Tsinghua university we laid out a
**roadmap for Majorana experiments**that are within reach in the upcoming years using existing technology (*Nature Communications*). The roadmap describes experiments with gradually increasing complexity to investigate fundamental physical properties of Majorana bound states, leading up to devices that can serve as topological qubits.

## Quantum computing architecture stack

Long-term goal:

To develop a scalable quantum computing control system stack that bridges the gap between quantum applications and quantum devices.

### 2019 Research Highlights:

- We added the
**Qmap mapper**to the OpenQL compiler, which is an evolving quantum software platform in which new features are constantly required. Qmap makes quantum algorithms executable by adapting them to satisfy the quantum processor constraints (*arXiv*). - We proposed a flag-bridge approach to implement
**fault-tolerant quantum error correction**on near-term quantum devices (*Phys. Rev. A*). In addition, we showed that lattice surgery and code deformation can be expressed as special cases of gauge fixing, permitting a simple and rigorous test for fault tolerance together with simple guiding principles for the implementation of fault tolerant logical operations (*New J. Phys.*) - We proposed and defined a
**control microarchitecture**that can efficiently support fault-tolerant quantum computations (*MICPRO*), and compared neural network based decoders for the surface code, and created a scalable distributed neural network decoder (*IEEE Trans. Comp.**Quantum Mach. Intell.*).

## Cryogenic electronics

Long-term goal: To replace the room-temperature electronics controlling quantum processors with cryogenic integrated electronics operating in close proximity to the qubits, to facilitate large-scale quantum computers.

### 2019 Research Highlights:

- We carried out the first systematic study of the
**impact of any non-ideality**of classical electrical control signals on the fidelity of single-electron spin qubits (*Phys. Rev. Appl.*) This will help us ensure qubit performance while minimizing power consumption, as required to stay within the strict power budget of existing refrigerators. - We designed, fabricated, and demonstrated several
**cryo-CMOS circuits**for the electrical interface of quantum processors, including voltage references (ESSCIRC 2019), high-frequency oscillators (ISSCC 2020) and circulators (RFIC 2019, in collaboration with EPFL). - In collaboration with Intel, we demonstrated the
**most****complex cryo-CMOS integrated system**ever built. Fabricated using the Intel 22 nm FinFET CMOS process, it operates at 3 K and is able to drive up to 128 qubits. Demonstrating both proper operation of such a complex cryo-CMOS circuit and, for the first time, the ability to drive a spin qubit with a cryo-CMOS circuit represents a major milestone towards the full cryogenic control of large-scale quantum computers. Read more

## Theory for assessing the performance of quantum error correction for superconducting and spin qubits

Long-term goal:

To provide analyses and ideas towards implementing and assessing the performance of quantum error correction for superconducting and spin qubits

### 2019 Research Highlights:

- We proposed a simple method for
**assessing the performance of a few-qubit gate**which is SPAM-insensitive and captures noise including correlated noise and leakage (*npj Quantum Information*). - We developed a scheme for
**simulating an optomechanical coupling**between two resonators in circuit-QED, for the purpose of preparing so-called GKP qubit states (*arXiv*). - We modeled the
**two-qubit CZ gate**, minimizing leakage and error, and studied the performance of Surface-17 in the presence of leakage, detecting leakage without quantum overhead (*Phys. Rev. Lett*. and*arXiv*). - We proposed and numerically studied the performance of a
**1D repetition code**for an array of GaAs singlet-triplet spin qubits, showing that a sub-threshold experiment lies within experimental reach (*arXiv*). - Our results on
**stoquastic 'sign-free' Hamiltonians**revealed that it is NP-complete to determine whether local basis changes exist which cure the sign for Hamiltonians with 2-qubit interactions and 1-local fields. We presented an efficient algorithm to find these basis changes or decide that they don't exist for purely 2-local Hamiltonians (*arXiv*).