# 2019

**Quantum Effects in a Mechanically Modulated Single-Photon Emitter** – M. Abdi and M. B. Plenio, *Physical Review Letters, 122, 023602* (2019)

DOI: doi.org/10.1103/PhysRevLett.122.023602

**Improving the precision of frequency estimation via long-time coherences** – A. Smirne, A. Lemmer, M. B. Plenio, and S. F. Huelga, *Quantum Science and Technology, 4, 025004* (2019)

DOI: doi.org/10.1088/2058-9565/aaf43d

#### The gist of it

One of the most promising routes to exploit quantum mechanics in broad impact technologies is indeed the possibility to use quantum features to determine the value of some unknown parameter (energy splitting, external field, and so on) as precise as possible, possibly overcoming classical standards also in realistic conditions.

In recent years, several estimation strategies have been formulated to deal with the presence of noise, typically relying on the use of quantum entanglement between the sensing probes and on measurements at shorter and shorter time scales with the increasing of the number of probes. Such strategies have been shown to be optimal in the asymptotic limit in the number of probes, but the preparation of a high number of entangled probes and the access to short interrogation times is certainly too demanding in several situations of interest. In this paper, we present a different approach to frequency estimation, which relies on the presence of quantum coherence in the state of each sensing particle in the long time limit, the so-called coherence-trapping phenomenon. First, by means of a commonly used master equation, we show that coherence trapping is obtained by engineering the environment, adding a two-level system properly interacting with it [see also the figure]. After that, we show that our estimation strategy can overcome the precision achievable with entanglement-based strategies for a finite number of probes. Furthermore, we discuss a possible implementation of the scheme in a realistic setup that uses trapped ions as quantum sensors.

**Modulated Continuous Wave Control for Energy-Efficient Electron-Nuclear Spin Coupling** – J. Casanova, E. Torrontegui, M. B. Plenio, J. J. García-Ripoll, and E. Solano, *Physical Review Letters, 122, 010407* (2019)

DOI: doi.org/10.1103/PhysRevLett.122.010407

#### The gist of it

In order to manipulate nuclear spins by means of NV-centers in diamond we need to control the electronic degree-of-freedom of the NV by means of microwaves to ensure energetic resonance. With standard protocols, in the presence of high magnetic fields, this requires high Rabi frequencies and hence high field intensities which are hard to generate and may which also lead to increased levels of absorption in target materials. Here we develop an alternative approach where we keep the intensity low but implement rapid phase or amplitude modulation in order to obtain an energetic resonance between electron and nucleus in a rotating frame. We demonstrate that for the same action, e.g. nuclear spin polarisation, our new scheme require much less energy and power.

**Noise-resilient architecture of a hybrid electron-nuclear quantum register in diamond** – M. Perlin, Z. Wang, J. Casanova and M. B. Plenio, *Quantum Science and Technology, 4 015007 (2019) *DOI: https://doi.org/10.1088/2058-9565/aade5c

#### The gist of it

A major obstacle to the development of quantum technologies is reconciling their need to have two opposing features: reliable controls (strong external coupling) and isolation from environmental noise (weak external coupling). These features are required because quantum processes tend to be extremely fragile to errors. This work provides an architecture for a quantum memory register that has both necessary features.

Specifically, in diamond, pairs of carbon-13 nuclei at lattice sites in certain symmetric configurations with respect to a highly controllable nitrogen-vacancy (NV) centre provide natural hardware for a noise-resilient quantum register. Due to the identical spin precession frequencies (i.e. spectral indistinguishability) of such nuclei, this architecture has a decoherence-free subspace (DFS) of nuclear spin states, or a set of states that are immune to the dominant noise from the NV-bound electron as well as fluctuating stray magnetic fields. However, previously existing methods could not access this DFS because of the inability to individually control spectrally indistinguishable nuclear spins. We develop an explicit protocol to store, manipulate, and extract quantum information in such nuclei’s noise-resilient subspace. Our protocol overcomes the obstacle to individual spin control by combining two mature experimental techniques, namely (i) dynamical decoupling, which can selectively couple the NV electron to only a pair of spectrally indistinguishable nuclear spins, and (ii) radio-frequency nuclear spin control, which breaks the symmetry of these nuclear spins with respect to the NV centre, thereby making them distinguishable and allowing for individual spin addressing.

In addition to storing quantum information to allow for quantum sensing, collections of our hybrid registers can be used to perform large-scale quantum communication and computing tasks via existing experimental techniques.

**Coherence and non-classicality of quantum Markov processes** – A. Smirne, D. Egloff, M. G. Diaz, M. B. Plenio, and S. F. Huelga, *Quantum Science and Technology, 4, 01LT01* (2019)

DOI: doi.org/10.1088/2058-9565/aaebd5

#### The gist of it

The distinction between the classical and the quantum description of physical systems has been a central issue from the birth of quantum mechanics itself. Recently, this topic has been attracting a renewed interest, not only within the purely foundational context, but also due to the experimental capabilities which have led to the observation of possible quantum features in regimes so far unexplored from this point of view; one can think, for example, to transport processes in molecular complexes or super-classical efficiencies of quantum thermal machines. In all these contexts it is clear that quantum coherence does represent the key feature of the quantum description of several physical phenomena, but a precise and unambiguous connection between quantum coherence and non-classicality is still missing.

In this paper, we take some relevant steps towards a rigorous link between quantum coherence and nonclassicality, proving that a Markovian multi-time statistics obtained from repeated measurements of a non-degenerate observable cannot be traced back to a classical statistics if and only if the dynamics generates coherences and subsequently turns them into populations. In this way, on the one hand we identify the relevant property of quantum coherence connected with nonclassicality and on the other hand we clarify when and to which extend the link can be established. This is further supported by a first investigation of the non-Markovian regime, where we show that there can be a genuinely non-classical statistics associated with the measurements of an observable without that any quantum coherence of such observable is present at any time in the state of the measured system.

# News

Congratulations to Ish Dhand for winning a "Foschungsbonus"

Martin Plenio is selected as Highly Cited Researcher again in the 2018!

# Most Recent Papers

**Quantum Effects in a Mechanically Modulated Single-Photon Emitter**, *Physical Review Letters, 122, 023602* (2019)

**Improving the precision of frequency estimation via long-time coherences**, *Quantum Science and Technology, 4, 025004* (2019)

**Modulated Continuous Wave Control for Energy-Efficient Electron-Nuclear Spin Coupling**, *Physical Review Letters, 122, 010407* (2019)

# Contact

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