# Experimental Quantum Optics Chair

Welcome to the Experimental Quantum Optics Chair of Prof. Dr. Ch. Wunderlich at the University of Siegen.

Our experimental and theoretical work concentrates on the development and exploration of new schemes for quantum information processing using individual atoms and open fundamental questions related to quantum physics.

# News

## The Quantumrevolution - ARTE sciencemagazine visited our group

09/05/2017

ARTE sciencemagazine Xenius visited our group for it's newest episode Die Quantenrevolution: Wie sie unsere digitale Welt verändert.

For their journalistic research on tap-proof communications, high performance computers, that outperform todays super-computers, and GPS systems accurate on the centimeter scale, the team asked itself *"What is this new quantum technology and why the world-wide race for it's realization?"*.

To get answers the magazine visited the research groups of Prof. Rainer Blatt in Innsbruck, Prof. Anton Zeilinger in Wien and Prof. Christof Wunderlich in Siegen.

Prof. Christof Wunderlich explained clearly how a quantum computer is able to perform tremendous amounts of parallel computations by using qu(antum)bits instead of classical bits and presented our research regarding miniaturization and streamlining of quantum technology.

The (german) episode is airing on 08.09.2017, 16:50h on ARTE and will be available online.

- Online: Die Quantenrevolution ― Xenius - Das Wissensmagazin auf ARTE (german)
- TV: 08.09.2017 16:50h - ARTE

## Machine learning on an ion trap quantum processor

09/05/2017

In collaboration with the University of Innsbruck (Austria), IQOQI Vienna (Austria) and the Max-Planck-Institute for Quantum Optics in Munich, we realized a proof-of-principle experiment that combines novel
concepts from artificial intelligence with the potential of ion trap
quantum computers.

In the very first experimental
demonstration of a quantum-enhanced reinforcement learning system, we investigate a quantum learning agent in a rapidly changing
environment. In the framework of the projective simulation model of
reinforcement learning, we demonstrate a generic speed-up in the agent's
decision-making/deliberation time in a system of two
radiofrequency-driven trapped ion qubits.

The quantum algorithm
underlying the deliberation process is essentially a Grover-like
algorithm, which we implemented efficiently using single-qubit rotations
and two-qubit conditional quantum dynamics.

Within experimental
uncertainties, our results confirm that the decision-making process of
the quantum learning agent is quadratically faster compared to
classical learning agents.

This experiment highlights the
potential of a scalable ion trap quantum computer in the field of
quantum-enhanced learning and artificial intelligence.

- Speeding-up the decision making of a learning agent using an ion trap quantum processor, T. Sriarunothai et al., arXiv: 1709.01366 (2017)

## Insights on Quantum Dynamics in Magnetic Field Gradients

08/21/2017

Novel ion traps that employ magnetic gradient fields allow for the application of conditional quantum logic, the essential prerequisite for quantum computing, by the use of radio-frequency radiation that can be generated by off-the-shelf electronics.

We show that the Hamiltonian describing the necessary couplings in the presence of a resonant dynamic gradient, is identical, in a dressed state basis, to the Hamiltonian in the case of a static gradient.
The coupling strength is in both cases described by the same effective Lamb-Dicke parameter.

Our insights can be used to overcome demanding experimental requirements when using a dynamic gradient field for state-of-the-art experiments with trapped ions, for example, in quantum information science.

At the same time, we show new experimental perspectives by way of using a single dynamic gradient field, inducing long-range coupling, for conditional multi-qubit dynamics.

## Investigation of the anomalous heating of trapped ions

08/10/2017

A major obstacle in the way of miniaturisation of ion traps,
desired for many aspects of trapped ion quantum information
processing, is anomalous heating. This is heating of the ion's
motion via electric field fluctuations of the trap electrodes.
These fluctuations are orders of magnitude stronger than
expected from Johnson noise. The reason for that is mostly
associated with surface contamination/oxidation, however the
actual mechanism behind this phenomenon is not understood so
far, hence the heating is called *"anomalous"*.

One of the ways to shed light onto the anomalous heating
mechanism is the study of the ion's heating rate dependence on
the ion-electrode distance.

We have built a unique planar ion trap with tuneable
ion-surface separation and measured this dependence directly
for the first time. Our measurements yield a dependence in
reasonable agreement with a power law of exponent -4 that is
predicted by some theories.

- Measuring anomalous heating in a planar ion trap with variable ion-surface separation, I. Boldin et al., arXiv: 1708.03147 (2017)

## Kickoff for the optIclock pilot project

05/01/2017

On May 1st 2017 started the first pilot project Optische Einzelionenuhr für Anwender (optIclock) (user-operable optical single ion clock) in the QUTEGA initiative of Germany's Federal Ministry of Education and Research (BMBF).
With a network of scientific researchers from Physikalisch-Technischen Bundesanstalt Braunschweig, Universität Bonn and Ferdinand-Braun-Institut Berlin and industry partners High Finesse GmbH, Menlo Systems GmbH, QUARTIQ GmbH, Qubig GmbH, TOPTICA Photonics AG and Vacom GmbH, it is our goal to realize a demonstrator for an optical single ion clock within the next three years.
optIclock (optical Ion clock) will be designed for precision of 10^{-15} bis 10^{-17} and thus surpass every commercially available frequency standard. Yet it is meant to be transportable, easy to use and therefore end-user operable - in contrast to the current optical clocks operated by research facilities.
In the optIclock a single charged atom is held in an electrodynamical trap, laser-cooled down to a few thousands of a degree Celsius above absolute zero and interrogated by a so-called clock-laser, that is stabilized to an optical transition of the atom for a high-precision frequency measurement.
Usability for an end-user is of particular importance for this pilot project and will be realized through miniaturization, automatization and integration of all device's single components into a profound system architecture.

- Project website: www.opticlock.de
- Website of the BMBF quantumtechnology initiative: www.qutega.de

## High-fidelity transport of quantum information using trapped ions

04/07/2017

A promising scheme for scalable trapped-ion quantum simulators and processors rely on assembling a larger system from smaller modules between which quantum information is exchanged. This exchange can be realized by physical transport of the ions as carriers of quantum information between those modules. The transport operations have to be performed with high-fidelity to meet the requirements of efficient error correction that is needed for scalability.

We report for the first time physical transport of quantum information encoded into trapped ion qubits with a fidelity 99.9994%, that is, well above a commonly accepted error correction threshold.

In our experiment we embedded ion transport into Ramsey measurements and compared the results for different numbers of transport operations.
During the measurements a single Ytterbium ion was transported up to 4000 times over a distance of 280 micrometer in a microstructured ion trap.

- High-fidelity preservation of quantum information during trapped ion transport, P. Kaufmann et al., arXiv: 1704.02141 (2017)

## A Quantum Computer Blueprint

02/01/2017

A large scale quantum computer is best constructed using a modular approach. Joining with researchers from the University of Sussex (UK), Google (USA), Aarhus University (Denmark) and RIKEN (Japan), we present the blueprint for an ion trap based scalable quantum computer module which makes it possible to create an arbitrarily large quantum computer architecture powered by long-wavelength radiation. This quantum computer module controls all operations as a stand-alone unit, is constructed using silicon microfabrication techniques and within reach of current technology. To perform the required quantum computations, the module makes use of long-wavelength-radiation quantum gate technology and relies only on a vacuum environment and global laser and microwave fields. Scaling this microwave quantum computer architecture beyond one module can be done by connecting arbitrarily many identical modules for a large scale architecture.

- Blueprint for a microwave trapped ion quantum computer, B. Lekitsch et al.,Science Advances 3 (2017)
- Interview with Prof. Dr. Chr. Wunderlich
- Media coverage by Nature, BBC, Financial Times

## Analog Quantum Simulation of a QED with Trapped Ions

09/16/2016

The prospect of quantum-simulating lattice gauge theories opens exciting possibilities for understanding fundamental forms of matter. Together with colleagues from the University of Innsbruck (Austria), we show that trapped ions represent a promising platform in this context when simultaneously exploiting internal pseudospins and external phonon vibrations. We illustrate our ideas with two complementary proposals for simulating lattice-regularized quantum electrodynamics (QED) in (1+1) space-time dimensions. Both schemes work on energy scales significantly larger than typical decoherence rates in experiments, thus enabling the investigation of phenomena such as string breaking, Coleman's quantum phase transition, and false-vacuum decay. The underlying ideas of the proposed analog simulation schemes may also be adapted to other platforms, such as superconducting qubits.

## Coherent Quantum Fourier Transform using Versatile Microwave Trapped Ion Spin Systems

07/08/2016

Using trapped atomic ions, we demonstrate a tailored and versatile effective spin system suitable for quantum simulations and universal quantum computation. By microwave control, the sign and effective strength of spin-spin couplings can be changed even during the course of a quantum algorithm. Spins can be selected to serve as quantum memories, not participating in the simultaneously perfomed conditional quantum dynamics of other spins. Using the simultaneous long-range coupling between three spins, we realize a coherent quantum Fourier transform — an essential building block for many quantum algorithms. This approach, using microwave manipulation of the spins only, opens a new route to overcoming technical and physical challenges in the quest for a quantum simulator and a quantum computer.

## Ultrasensitive Single Atom Magnetometer

06/16/2016

Precision sensing, and in particular high precision magnetometry, is a central goal of research into quantum technologies. For magnetometers, often trade-offs exist between sensitivity, spatial resolution, and frequency range. In a collaboration with theorists from Huazhong University of Science and Technology (China), The Hebrew University of Jerusalem (Israel) and University of Ulm, we adapted a dynamical decoupling scheme that improves phase coherence by orders of magnitude and merged it with a magnetic sensing protocol. This allowed us to achieve a measurement sensitivity close to the standard quantum limit, even for high frequency fields. Using a single atomic ion as a sensor, we experimentally attain a sensitivity of 4.6 pT/√Hz for an alternating-current magnetic field near 14 MHz. This unprecedented sensitivity combined with spatial resolution in the nanometer range and tunability from direct current to the gigahertz range could be used for magnetic imaging in as of yet inaccessible parameter regimes.