## Koen Groenland

## About me

I am a* ***quantum innovation office r **at the

*University of Amsterdam*and

*QuSoft*. My goal is to accelerate the development of valuable

*quantum technology*by Netherlands-based companies.

We accomplish this by teaching courses and workshops, and by building the Quantum Amsterdam hub that stimulates collaboration within the ecosystem.

I am also involved in scientific research on quantum computers, in particular on the physical stimuli that implement useful *logical operations *on a piece of quantum hardware. In other words, we find input signals that make a quantum computer perform the correct calculations (more).

## Efficient circuits for Trapped Ion quantum computers

January 2020

We find a striking connection between the physics of quantum computers that use trapped ions, and the emerging field of quantum signal processing. This allows us to perform difficult quantum gates in less steps, relying only on the most simple entangling operation a trapped ion computer can perform.

(Update July 2020) This result is now published as follows:

Koen Groenland, Freek Witteveen, Kareljan Schoutens, Rene Gerritsma, *Signal processing techniques for efficient compilation of controlled rotations in trapped ions, *New Journal of Physics, Volume 22 (2020)

## Difficult quantum gates can be performed in a single step

November 2019

Together with Stig Rasmussen and Nikolaj Zinner from Aarhus University, we find that the notoriously hard Toffoli quantum gate can be performed using a surprisingly simple protocol. We require an all-to-all Ising type interaction between the qubits, and a resonant field on a single special qubit. After throwing away the special qubit, a Toffoli occurred on the remaining qubits.

(Update Februari 2020) This result is now published as:

S. E. Rasmussen, KG, R. Gerritsma, K. Schoutens, N. T. Zinner, *Single-step implementation of high fidelity n-bit Toffoli gates, *Phys. Rev. A **101**, 022308 (2020)* (*without paywall: arXiv:1911.07548)

## Popular state transfer protocols now work in more cases

September 2019

Certain experimental protocols, named with acronyms STIRAP or CTAP, turn out to work on many more systems than was previously known. We find that they naturally generalize to bipartite graphs.

KG, Carla Groenalnd, Reinier Kramer, *Adiabatic transfer of amplitude using STIRAP-like protocols generalizes to many bipartite graphs, *Journal of Mathematical Physics **61**, 072201 (2020); * * arXiv:1904.09915

## Transferring a quantum state over a network of coupled spins

January 2019

With the advent of advanced quantum information processing, it is of increasing importance to transport quantum information over a physical medium (think of a wire, or a network of wires). We consider the case where the information is encoded in a spin degree of freedom (think of an electron whose "rotation axis" can point either up or down), and the medium is made up of spins that are all pinned in place.

**It turns out that the repulsive forces between the spins can be used to delocalize the information over the whole network, and then localize it again at some other place. **This was known for mediums that form a perfect line. I generalize this to more general configurations, finding that information can be sent over many networks that look like a bipartite graph.

My article is planned for publication in** SciPost Physics** (DOI: 10.21468/scipostphys.6.1.011)

## Many-body strategies for multi-qubit gates

April 2018

Quantum computers, just like their classical counterparts, may use a *universal *gate set consisting of local gates, in order to approximate *any* possible operation on it's qubits. Typically, one chooses a two-qubit gate such as the *CNOT* together with a set of single-qubit gates.

However, we asked ourselves the question: **If N qubits are coupled by some interaction of our choice, can we construct interesting gates that act on all qubits at the same time? **

For this to work, we look at the so-called Krawtchouk chain, which is special because all of it's eigenvalues are integer numbers. Because this system is well understood, we can apply condensed-matter many-body techniques, resulting in two surprising new contributions:

- The
**eigengate**, which maps computational states into eigenstates of the coupling Hamiltonian. **Resonant driving,**which, together with knowledge of the simple spectrum, allows us to select precisely 2 our of 2^N (and no more!) to undergo a transition.

Our article was recently published in PRA (https://journals.aps.org/pra/abstract/10.1103/PhysRevA.97.042321). Find the version without paywall at ArXiv or my GDrive.