# 2020

**Optimized auxiliary oscillators for the simulation of general open quantum systems** – F. Mascherpa, A. Smirne, A. D. Somoza, P. Fernández-Acebal, S. Donadi, D. Tamascelli, S. F. Huelga, and M. B. Plenio, *Physical Review A, 101, 052108* (2020)

DOI:https://doi.org/10.1103/PhysRevA.101.052108

**Enhancing the Robustness of Dynamical Decoupling Sequences with Correlated Random Phases** – Z. Wang, J. Casanova, and M. B. Plenio, *Symmetry 2020, 12, 730* (2020)

DOI:https://doi.org/10.3390/sym12050730

#### The gist of it

This paper demonstrates that the performance of dynamical decoupling sequences can be improved by imposing correlated random phases to the basic pulse units. These correlated random phases cancel the leading order errors in the basic pulse units such as the static fluctuation in the Rabi frequency and frequency detuning of the control field. Furthermore, these correlated random phases suppress spurious responses, which is due to limited control power and environmental perturbation and can lead to false signal identification in quantum sensing, e.g., the misidentification of carbon nuclei for proton nuclei.

**Efficient simulation of open quantum systems coupled to a fermionic bath** – A. Nüßeler, I. Dhand, S. F. Huelga, and M. B. Plenio, *Physical Review B, 101, 155134* (2020)

DOI:https://doi.org/10.1103/PhysRevB.101.155134

#### The gist of it

The problem of open quantum systems coupled to fermionic environments arises in a variety of fields ranging from quantum dots to quantum transport at zero and finite temperature. This work presents and analyzes a tensor-network based time integration scheme for this problem, which enables simulations of such systems with controllable and certified error. Furthermore, it is shown that performing simulations in the Heisenberg picture can lead to significant efficiency gains.

**Limited-control metrology approaching the Heisenberg limit without entanglement preparation** – B. Tratzmiller, Q. Chen, I. Schwartz, S. F. Huelga, and M. B. Plenio, *Physical Review A, 101, 032347* (2020)

DOI:https://doi.org/10.1103/PhysRevA.101.032347

#### The gist of it

A basic question in quantum metrology is how precise a quantity (e.g. magnetic field strength) can be measured given a number of particles M and a total time T. The ultimate precision limit is the Heisenberg limit, which requires entanglement between the particles involved. In this work we present a scheme that approaches the Heisenberg limit without requiring the preparation of an entangled state by using nuclear spins which are read out by a nearby NV center. We compare the scheme to other approaches and discuss applications.

**Hybrid Microwave-Radiation Patterns for High-Fidelity Quantum Gates with Trapped Ions** – I. Arrazola, M.B. Plenio, E. Solano, and J. Casanova, *Physical Review Applied, 13, 024068* (2020)

DOI: doi.org/10.1103/PhysRevApplied.13.024068

#### The gist of it

Quantum processors based on trapped ions are one of the leading candidates to build quantum simulators and computers, due to their high controllability and long coherence times. Although laser control has been extremely successful achieving high-fidelity quantum gates, the scaling up of these devices requires the precise control of several laser sources, which is a hard technological challenge. Alternatives to laser control that are easier to scale up involve microwave radiation and magnetic field gradients. In this work, we propose microwave radiation patterns that lead to high-fidelity two-qubit gates while being robust against errors in the magnetic or microwave fields. Our method combines continuous dynamical decoupling techniques with phase modulated drivings, phase-flips and refocusing pi pulses, leading to high-fidelity quantum gates in realistic experimental scenarios.

**Quantum coherence and state conversion: theory and experiment** – K.-D. Wu, T. Theurer, G.-Y. Xiang, C.-F. Li, G.-C. Guo, M. B. Plenio, and A. Streltsov, *npj Quantum Information, 6, 22* (2020)

DOI: doi.org/10.1038/s41534-020-0250-z

#### The gist of it

The resource theory of coherence studies the operational value of superpositions in quantum technologies. A key question in this theory is which resources, i.e., quantum states showing coherence, can be obtained from a given initial state using only operations that are unable to create coherence, so called incoherent operations. This determines the relative value of states for quantum technologies: a state that can be obtained from another one via incoherent operations contains less coherence and is hence less valuable. Here, we solve this question completely for qubit states by determining the optimal single-shot probabilities for mixed-state conversions via stochastic incoherent operations. Extending the discussion to distributed scenarios, we introduce and address the task of assisted incoherent state conversion, where the process is enhanced by making use of correlations with a second party. Building on these results, we demonstrate experimentally that the optimal state-conversion probabilities can be achieved in a linear optics setup. This paves the way towards real world applications of coherence transformations in current quantum technologies.

**Protecting Quantum Spin Coherence of Nanodiamonds in Living Cells** – Q.-Y. Cao, P.-C. Yang, M.-S. Gong, M. Yu, A. Retzker, M.B. Plenio, C. Müller, N. Tomek, B. Naydenov, L.P. McGuinness, F. Jelezko, and J.-M. Cai, *Physical Review Applied, 13, 024021* (2020)

DOI: doi.org/10.1103/PhysRevApplied.13.024021

#### The gist of it

Because of its excellent coherent and optical properties at room temperature, the nitrogen-vacancy (NV) center in diamond, especially when located in nanodiamonds, represents a promising tool for sensing applications in biological environments. However, in nanodiamonds the relatively short NV electron spin coherence times require microwave control to decouple the electron spin of the NV center from its noisy environment. In a biological environment this needs to be achieved with minimal microwave power in order to reduce possible heating effects. Building on earlier work from our group here we demonstrate energy-efficient protection of NV spin coherence in nanodiamonds using concatenated continuous dynamical decoupling. When this is applied to nanodiamonds in living cells, we are able to extend the spin coherence time by an order of magnitude to the T1 limit of 30 μs. Further analysis demonstrates concomitant improvements of sensing performance, which shows that our results provide an important step toward in vivo quantum sensing using NV centers in nanodiamond.

# News

Our work on Quantum Physics and Biology featured Local and National Radio

Ludovico Lami wins a Humboldt Fellowship to continue his work in our group for another 2 years

Martin Plenio is selected as Highly Cited Researcher for 2019!

Second ERC Synergy grant for the group

# Most Recent Papers

**Optimized auxiliary oscillators for the simulation of general open quantum systems**, *Physical Review A, 101, 052108* (2020)

**Enhancing the Robustness of Dynamical Decoupling Sequences with Correlated Random Phases**, *Symmetry 2020, 12, 730* (2020)

**Efficient simulation of open quantum systems coupled to a fermionic bath**, *Physical Review B, 101, 155134* (2020)

# Contact

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