# Markus HennrichStockholm University | SU · Department of Physics

Markus Hennrich

Dr. rer. nat. habil.

## About

102

Publications

9,997

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6,848

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Introduction

Quantum Simulation with Trapped Rydberg Ions (QuaSIRIO)
In this project we focus on the realization of trapped Rydberg ions. This will bring together two quantum systems: trapped ions and Rydberg atoms. Joining them will form a novel quantum system with advantages from both sides. Trapped Rydberg ions are a promising system for applications in quantum information processing and quantum simulation.

Additional affiliations

January 2013 - January 2015

**Universität Innsbruck**

Position

- Professor (Associate)

Description

- Quantum information and quantum optics experiments with trapped ions

Education

June 2007 - May 2012

April 1999 - January 2004

October 1992 - June 1998

## Publications

Publications (102)

We propose a scheme to dissipatively produce steady-state entanglement in a two-qubit system, via an interaction with a bosonic mode. The system is driven into a stationary entangled state, while we compensate the mode dissipation by injecting energy via a coherent pump field. We also present a scheme which allows us to adiabatically transfer all t...

We minimize the stray electric field in a linear Paul trap quickly and accurately, by applying interferometry pulse sequences to a trapped ion optical qubit. The interferometry sequences are sensitive to the change of ion equilibrium position when the trap stiffness is changed, and we use this to determine the stray electric field. The simplest pul...

We simulate the dissipative dynamics of a mesoscopic system of long-range interacting particles which can be mapped into non-Hermitian spin models with a $\mathcal{PT}$ symmetry. We find rich $\mathcal{PT}$-phase diagrams with $\mathcal{PT}$-symmetric and $\mathcal{PT}$-broken phases. The dynamical regimes can be further enriched by modulating tuna...

We observe an atomic ion's response to an electric quadrupole field to second and higher orders; this arises from the ion's electric quadrupole polarizability and hyperpolarizabilities. We probe a single Sr+88 ion which is confined in the electric fields of a Paul trap and excited to Rydberg states. The quadrupolar trapping fields cause atomic ener...

We minimize the stray electric field in a linear Paul trap quickly and accurately, by applying interferometry pulse sequences to a trapped ion optical qubit. The interferometry sequences are sensitive to the change of ion equilibrium position when the trap stiffness is changed, and we use this to determine the stray electric field. The simplest pul...

Conical intersections between electronic potential energy surfaces are paradigmatic for the study of nonadiabatic processes in the excited states of large molecules. However, since the corresponding dynamics occurs on a femtosecond timescale, their investigation remains challenging and requires ultrafast spectroscopy techniques. We demonstrate that...

Conical intersections between electronic potential energy surfaces are paradigmatic for the study of non-adiabatic processes in the excited states of large molecules. However, since the corresponding dynamics occurs on a femtosecond timescale, their investigation remains challenging and requires ultrafast spectroscopy techniques. We demonstrate tha...

Trapped Rydberg ions represent a flexible platform for quantum simulation and information processing that combines a high degree of control over electronic and vibrational degrees of freedom. The possibility to individually excite ions to high-lying Rydberg levels provides a system where strong interactions between pairs of excited ions can be engi...

DOI:https://doi.org/10.1103/PhysRevA.102.029902

In this chapter, we present an overview of experiments with trapped Rydberg ions and outline the advantages and challenges of developing applications of this new platform for quantum computing, sensing, and simulation. Trapped Rydberg ions feature several important properties, unique in their combination: they are tightly bound in a harmonic potent...

Highly excited Rydberg states are usually extremely polarizable and exceedingly sensitive to electric fields. Because of this Rydberg ions confined in electric fields have state-dependent trapping potentials. We engineer a Rydberg state that is insensitive to electric fields by coupling two Rydberg states with static polarizabilities of opposite si...

Trapped Rydberg ions represent a flexible platform for quantum simulation and information processing which combines a high degree of control over electronic and vibrational degrees of freedom. The possibility to individually excite ions to high-lying Rydberg levels provides a system where strong and long-range interactions between pairs of excited...

The response of matter to fields underlies the physical sciences, from particle physics to astrophysics, and from chemistry to biophysics. We observe an atom's response to an electric quadrupole field to second- and higher orders; this arises from the atom's electric quadrupole polarizability and hyperpolarizabilities. We probe a single atomic ion...

Generating quantum entanglement in large systems on timescales much shorter than the coherence time is key to powerful quantum simulation and computation. Trapped ions are among the most accurately controlled and best isolated quantum systems¹ with low-error entanglement gates operated within tens of microseconds using the vibrational motion of few...

In this chapter, we present an overview of experiments with trapped Rydberg ions, outline the advantages and challenges of developing applications of this new platform for quantum computing, sensing and simulation. Trapped Rydberg ions feature several important properties, unique in their combination: they are tightly bound in a harmonic potential...

The existence of ideal quantum measurements is one of the fundamental predictions of quantum mechanics. In theory, an ideal measurement projects a quantum state onto the eigenbasis of the measurement observable, while preserving coherences between eigenstates that have the same eigenvalue. The question arises whether there are processes in nature t...

Usually the influence of the quadratic Stark effect on an ion’s trapping potential is minuscule and only needs to be considered in atomic clock experiments. In this work we excite a trapped ion to a Rydberg state with polarizability ∼8 orders of magnitude higher than a low-lying electronic state; we find that the highly polarizable ion experiences...

Generating quantum entanglement in large systems on time scales much shorter than the coherence time is key to powerful quantum simulation and computation. Trapped ions are among the most accurately controlled and best isolated quantum systems with low-error entanglement gates operated via the vibrational motion of a few-ion crystal within tens of...

Usually the influence of the quadratic Stark effect on an ion's trapping potential is minuscule and only needs to be considered in atomic clock experiments. In this work we excite a trapped ion to a Rydberg state with polarizability $\sim$~eight orders of magnitude higher than a low-lying electronic state; we find that the highly-polarizable ion ex...

The existence of ideal quantum measurements is one of the fundamental predictions of quantum mechanics. In theory the measurement projects onto the eigenbasis of the measurement observable while preserving all coherences of degenerate eigenstates. The question arises whether there are dynamical processes in nature that correspond to such ideal quan...

Trapped Rydberg ions are a promising novel approach to quantum computing and simulations. They are envisaged to combine the exquisite control of trapped ion qubits with the fast two-qubit Rydberg gates already demonstrated in neutral atom experiments. Coherent Rydberg excitation is a key requirement for these gates. Here, we carry out the first coh...

Trapped Rydberg ions are a promising new system for quantum information processing. They have the potential to join the precise quantum operations of trapped ions and the strong, long-range interactions between Rydberg atoms. Technically, the ion trap will need to stay active while exciting the ions into the Rydberg state, else the strong Coulomb r...

Single atoms or atom-like emitters are the purest source of on-demand single photons, they are intrinsically incapable of multi-photon emission. To demonstrate this degree of purity we have realized a tunable, on-demand source of single photons using a single ion trapped at the common focus of high numerical aperture lenses. Our trapped-ion source...

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A century on from the development of quantum theory, the interpretation of a
quantum state is still discussed. If a physicist claims to have produced a
system with a particular wave function, does this represent directly a physical
wave of some kind, or is the wave function merely a summary of knowledge, or
information, about the system? A recent n...

Trapped ion crystals have proved to be one of the most viable physical
implementations of quantum registers and a promising candidate for a scalable
realization of quantum networks. The latter will require the development of an
efficient interface between trapped ions and photons. We describe two research
directions that are currently investigated...

We propose a quantum algorithm in an embedding ion-trap quantum simulator for
the efficient computation of N-qubit entanglement monotones without the
necessity of full tomography. Moreover, we discuss possible realistic scenarios
and study the associated decoherence mechanisms.

The construction of a quantum computer remains a fundamental scientific and technological challenge because of the influence
of unavoidable noise. Quantum states and operations can be protected from errors through the use of protocols for quantum
computing with faulty components. We present a quantum error-correcting code in which one qubit is enco...

Quantum computers hold the promise to solve certain problems exponentially
faster than their classical counterparts. Trapped atomic ions are among the
physical systems in which building such a computing device seems viable. In
this work we present a small-scale quantum information processor based on a
string of $^{40}$Ca${^+}$ ions confined in a ma...

Entanglement in a quantum system can be demonstrated experimentally by
performing the measurements prescribed by an appropriate entanglement witness.
However, the unavoidable mismatch between the implementation of measurements in
practical devices and their precise theoretical modelling generally results in
the undesired possibility of false-positi...

A trapped ion quantum computer usually uses the motional modes of the ion crystal to exchange quantum information and to generate entanglement. As the number of ions in a trap is increased, the motional modes become progressively more complicated and their energy spacing is reduced. This makes the entangling operation increasingly difficult. An alt...

Emission and absorption of single photons by single atoms is a fundamental limit of matter–light interaction, manifesting its quantum mechanical nature. As a controlled process, it is also a key tool in quantum optical information technology 1, 2, 3. Controlled single-photon emission is well advanced 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14; for contro...

Dynamical maps describe general transformations of the state of a physical system—their iteration interpreted as generating a discrete time evolution. Prime examples include classical nonlinear systems undergoing transitions to chaos. Quantum mechanical counterparts show intriguing phenomena such as dynamical localization on the single-particle lev...

A scheme for entangling distant atoms is realized, as proposed in the seminal
paper by Cabrillo et al. [Phys. Rev. A 59, 1025 (1999)]. The protocol is based
on quantum interference and detection of a single photon scattered from two
effectively one meter distant laser-cooled and trapped atomic ions. The
detection of a single photon heralds entangle...

We report on the implementation of a quantum process tomography technique known as direct characterization of quantum dynamics applied on coherent and incoherent single-qubit processes in a system of trapped ^{40}Ca^{+} ions. Using quantum correlations with an ancilla qubit, direct characterization of quantum dynamics reduces substantially the numb...

We investigate the quantum electrodynamic (QED) properties of an atomic electron close to the focus of a spherical mirror. We first show that the spontaneous emission and excited-state level shift of the atom can be fully suppressed with mirror-atom distances of many wavelengths. A three-dimensional theory predicts that the spectral density of vacu...

Light controlled on- and off-switching of intramolecular energy and charge transfer is demonstrated in donor-switch-acceptor (D-S-A) supermolecules. The supermolecules are designed based on our experience on donor-bridge-acceptor (D-B-A) systems. Switching function is achieved by incorporating optically bistable photochromic fulgides and diarylethe...

With trapped ions quantum information can be encoded in various two-level systems or quantum bits (qubits). Here, we present an overview on qubit encoding with Ca + and several state-of-the-art operations involving two and three qubits. The use of decoherence-free subspaces and encoding logical qubits using two physical qubits may offer an advantag...

We report on the detection of single, slowly moving Rubidium atoms using laser-induced fluorescence. The atoms move at 3 m/s while they are detected with a time resolution of 60 [mu]s. The detection scheme employs a near-resonant laser beam that drives a cycling atomic transition, and a highly efficient mirror setup to focus a large fraction of the...

Emission and absorption of single photons by single atoms is a fundamental limit of matter–light interaction, manifesting its quantum mechanical nature. As a controlled process, it is also a key tool in quantum optical information technology. Controlled single-photon emission is well advanced; for controlled single-photon absorption by a single ato...

The dynamics of fulgide photoswitches has been investigated applying femtosecond transient absorption spectroscopy. The switching performance of differently substituted fulgides is equally improved in polymer (PMMA) environment as compared to liquid solution (toluene). In the polymer a single pathway for the ring-closure reaction, i.e., without the...

We observe the interaction of a single trapped calcium ion with single photons produced by a narrow-band, resonant down-conversion source [A. Haase et al., Opt. Lett. 34, 55 (2009)], employing a quantum jump scheme. Using the temperature dependence of the down-conversion spectrum and the tunability of the narrow source, absorption of the down-conve...

We present a scheme for stabilizing multiple lasers at wavelengths between
795 and 866 nm to the same atomic reference line. A reference laser at 852 nm
is stabilized to the Cs D2 line using a Doppler-free frequency modulation
technique. Through transfer cavities, four lasers are stabilized to the
relevant atomic transitions in 40Ca+. The rms linew...

Entanglement -- once only a subject of disputes about the foundation of quantum mechanics -- has today become an essential issue in the emerging field of quantum information processing, promising a number of applications, including secure communication, teleportation and powerful quantum computation. Therefore, a focus of current experimental work...

In general, a quantum measurement yields an undetermined answer and alters
the system to be consistent with the measurement result. This process maps
multiple initial states into a single state and thus cannot be reversed. This
has important implications in quantum information processing, where errors can
be interpreted as measurements. Therefore,...

Efficient self-interference of single-photons emitted by a sideband-cooled
Barium ion is demonstrated. First, the technical tools for performing efficient
coupling to the quadrupolar transition of a single $^{138}$Ba$^{+}$ ion are
presented. We show efficient Rabi oscillations of the internal state of the ion
using a highly stabilized 1.76 $\mu m$...

We report absorption experiments from a single trapped atom. We focus a weak and narrow band Gaussian light beam onto an optically cooled
138
Ba
+
ion using a high numerical aperture lens. Extinction of this beam is observed with measured values of up to 1.35%. Using this result, we then demonstrate Electromagnetically‐Induced‐Transparency (EI...

By tightly focusing a laser field onto a single cold ion trapped in front of a far-distant dielectric mirror, we could observe a quantum electrodynamic effect whereby the ion behaves as the optical mirror of a Fabry-Pérot cavity. We show that the amplitude of the laser field is significantly altered due to a modification of the electromagnetic mode...

A digital quantum simulator is an envisioned quantum device that can be programmed to efficiently simulate any other local system. We demonstrate and investigate the digital approach to quantum simulation in a system of trapped ions. With sequences of up to 100 gates and 6 qubits, the full time dynamics of a range of spin systems are digitally simu...

By tightly focussing a probe field onto an ion trapped in front of a distant mirror, we observe a modulation of the vacuum Rabi constant and demonstrate the operation of a single-ion as an optical mirror.

The computational potential of a quantum processor can only be unleashed if errors during a quantum computation can be controlled and corrected for. Quantum error correction works if imperfections of quantum gate operations and measurements are below a certain threshold and corrections can be applied repeatedly. We implement multiple quantum error...

We report the creation of Greenberger-Horne-Zeilinger states with up to 14 qubits. By investigating the coherence of up to 8 ions over time, we observe a decay proportional to the square of the number of qubits. The observed decay agrees with a theoretical model which assumes a system affected by correlated, Gaussian phase noise. This model holds f...

The control of quantum systems is of fundamental scientific interest and promises powerful applications and technologies. Impressive progress has been achieved in isolating quantum systems from the environment and coherently controlling their dynamics, as demonstrated by the creation and manipulation of entanglement in various physical systems. How...

For scalable quantum computation errors occurring during storage and processing need to be continuously corrected. Here we report on the experimental realization of repetitive quantum error correction with trapped ions.

We report an absorption spectroscopy experiment and the observation of
electromagnetically induced transparency from a single trapped atom. We focus a
weak and narrowband Gaussian light beam onto an optically cooled Barium ion
using a high numerical aperture lens. Extinction of this beam is observed with
measured values of up to 1.3 %. We demonstra...

Multiparticle entanglement leads to richer correlations than two-particle entanglement and gives rise to striking contradictions with local realism, inequivalent classes of entanglement and applications such as one-way or topological quantum computing. When exposed to decohering or dissipative environments, multiparticle entanglement yields subtle...

The emission and absorption of single photons by single atomic particles is a fundamental limit of matter-light interaction, manifesting its quantum mechanical nature. At the same time, as a controlled process it is a key enabling tool for quantum technologies, such as quantum optical information technology [1, 2] and quantum metrology [3, 4, 5, 6]...

We will report on the realisation of high-fidelity Schroedinger- Cat states with more than six qubits in a string of ^40Ca^ + ions stored in a linear ion trap. We achieved fidelities with the target states exceeding 95% for up to four ions and 88% for six ions. These high fidelities allow to investigate decoherence of highly entangled quantum state...