# 2018

**Toward Hyperpolarization of Oil Molecules via Single Nitrogen Vacancy Centers in Diamond** – P. Fernández-Acebal, O. Rosolio, J. Scheuer, C. Müller, S. Müller, S. Schmitt, L.P. McGuinness, I. Schwarz, Q. Chen, A. Retzker, B. Naydenov, F. Jelezko, and M.B. Plenio, *Nano Letters, 18, 1882* (2018)

**Non-additive dissipation in open quantum networks out of equilibrium** – M. T. Mitchison and M. B. Plenio, *New Journal of Physics, 20, 033005* (2018) | ArXiv

#### The gist of it

Many fundamental and practical problems in physics reduce to studying a network of coupled quantum systems driven out of equilibrium by external noise. This broad paradigm incorporates biological transport networks, critical phenomena in dissipative many-body systems, and next-generation mesoscopic electronic devices. However, the theoretical description of these systems is difficult and typically requires approximations of questionable validity. Here, we shed some light on this problem by studying a simple non-equilibrium network comprising just two coupled subsystems, each of which interacts with an external heat reservoir. This model has some very nice symmetry properties, which help us to solve its dynamics exactly. We show that, when the reservoirs are not in thermal equilibrium with each other, they +ACI-interfere+ACI with each other in the sense that their effect on the network cannot be described as a sum of independent contributions. This interference is absent from standard descriptions based on master equations and can even lead to highly non-Markovian dynamics. Remarkably, the effect persists even when the reservoirs are spectrally unstructured and weakly coupled to the open system. Our research contributes to a body of recent work highlighting the subtle interplay between multiple noise sources in dissipative quantum networks.

**Dissipative phase transition in the open quantum Rabi model** – M.-J. Hwang, P. Rabl, and M. B. Plenio, *Physical Review A, 97, 013825* (2018) | ArXiv

#### The gist of it

In open quantum systems, a delicate interplay between coherent interactions and dissipations can bring a system into a non-equilibrium steady state that undergoes an abrupt and non-analytical change, marked by a so-called dissipative phase transition (DPT). In this work, we provide arguably the simplest model system, consisting of a damped harmonic oscillator and a qubit described by the so-called open quantum Rabi model, that exhibits a DPT, by extending the notion of finite-component system phase transitions to an open quantum system. We propose a trapped-ion experiment where the predicted DPT can be realised. Our proposal allows one to control the strength of dissipation and even turn on and off the dissipation so that one can switch from realizing a quantum phase transition to dissipative phase transition; this unique feature of our proposal opens a door for exploring the dynamics of dissipative phase transition in a highly controlled fashion.

**Magnetic field fluctuations analysis for the ion trap implementation of the quantum Rabi model in the deep strong coupling regime** – R. Puebla, J. Casanova, and M. B. Plenio,

*J. Mod. Opt. (Special Issue: Quantum optics, cooling and collisions of ions and atoms), 603-611* (2017) | ArXiv

#### The gist of it

Light-matter interactions are of paramount importance for several applications, such as in quantum-based technologies. In particular, triggered by an impressive experimental progress and theoretical works, which have pointed out the suitability of these systems for quantum information processing, sensing and simulation, the study of light-matter systems has experienced a renewed interest. In this regard, the quantum Rabi model (QRM) appears as the simplest, yet fundamental, system describing the interaction between a spin (matter) and a bosonic mode (light).

In this work we have extended previous works [New J. Phys. 18, 113039 (2016) and Phys. Rev. A, 95, 063844 (2017)] where a noise-resilient trapped-ion realization of QRMs based on continuous dynamical decoupling techniques was proposed. While trapped ions are prone to different imperfections, magnetic-field fluctuations are considered as the main source of noise, and hence its suppression is highly desired to properly retrieve QRM’s physics. Nevertheless, depending on the desired and targeted parameter regime of the QRM, one may resort to different dynamical decoupling schemes. As an example, we show here how to attain a QRM in the striking deep-strong coupling regime while suppressing the effects of the magnetic-field fluctuations – an inaccessible regime relying on the previously proposed schemes.

**Probabilistic low-rank factorization accelerates tensor network simulations of critical quantum many-body ground states** – L. Kohn, F. Tschirsich, M. Keck, M. B. Plenio, D. Tamascelli, and S. Montangero, *Physical Review E, 97, 013301* (2018) | ArXiv

#### The gist of it

Tensor Networks(TN) methods are indispensable tools in simulating quantum and classical many-body problems by providing an efficient parametrization of the wave function in the many-body phase space. In order to find such efficient parametrization, truncated Singular Value Decomposition (SVD) is widely used to compress states into their respective TN manifold. The SVD lies therefore at the heart of many TN methods, but also represents the most time-consuming part of a wide class of TN algorithms.

In this work we demonstrate that a randomized version of SVD (RSVD), which was proven to reduce the complexity of the Time-Evolving-Block-Decimation TN algorithm [D. Tamascelli, R. Rosenbach, and M. B. Plenio, Physical Review E 91, 063306 (2015)], can be applied to a relevant class of many-body systems, namely systems undergoing a quantum phase transition. Such regime is much more challenging regime since long-range correlations are building up and risk to compromise the effectiveness of the RSVD compression. We provide evidence that RSVD delivers the same accuracy as standard SVD routines with speed-up that can go up to 24 times. We show that the accuracy of the results is not influenced by the speedups and discuss the impact of techniques typical for TN studies.

# News

Korbinian and Pelayo participated in IQST’s Quantum Science Slam

Martin Plenio is listed as Highly Cited Researcher 2017

A 4-year long project bears fruit with a publication in Nature Physics

ITP work is chosen as Editor’s Suggestion in PRL

# Most Recent Papers

**Toward Hyperpolarization of Oil Molecules via Single Nitrogen Vacancy Centers in Diamond**,*Nano Letters,*(2018)**18**, 1882**Non-additive dissipation in open quantum networks out of equilibrium**,*New Journal of Physics,*(2018)**20**, 033005**Dissipative phase transition in the open quantum Rabi model**,*Physical Review A,*(2018)**97**, 013825

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