2019

Randomization of Pulse Phases for Unambiguous and Robust Quantum Sensing – Z.-Y. Wang, J. E. Lang, S. Schmitt, J. Lang, J. Casanova, L. McGuinness, T. S. Monteiro, F. Jelezko, and M. B. Plenio, Physical Review Letters, 122, 200403 (2019)
DOI: doi.org/10.1103/PhysRevLett.122.200403

The gist of it

Dynamical decoupling control—an important technique in quantum information processing and quantum sensing—becomes more accurate when randomized phases are used in the control pulses, according to the theory and experiment in this paper. Quantum systems are sensitive to noise and errors, including those from external control. Two important types of errors are imperfections in control pulses and spurious responses due to limited control power and environmental perturbation. The latter leads to false signal identification in quantum sensing, e.g., the misidentification of carbon nuclei for proton nuclei.

This paper demonstrates that instead of a deterministic application, the randomization of pulse phases suppresses both these errors simultaneously. This method is universal and can be directly incorporated into existing dynamical decoupling pulse sequences, allowing unambiguous and robust quantum sensing and quantum control for any physical realization of qubits.

Quantifying Operations with an Application to Coherence – T. Theurer, D. Egloff, L. Zhang, and M. B. Plenio, Physical Review Letters, 122, 190405 (2019)
DOI: doi.org/10.1103/PhysRevLett.122.190405

The gist of it

Resource theories offer the mathematical tools to investigate nonclassical quantum effects and their potential for applications in technology. Until very recently, resource theories were mainly used to determine the nonclassicality of quantum states, which are static resources. Ultimately, however, when speaking about operational advantages of quantum technologies, one is interested in dynamical resources which correspond to nonclassical quantum operations.

In this work, we present a method to determine how nonclassical quantum operations are and apply these finding to the detection of coherence.

Initialization and Readout of Nuclear Spins via a Negatively Charged Silicon-Vacancy Center in Diamond – M. H. Metsch, K. Senkalla, B. Tratzmiller, J. Scheuer, M. Kern, J. Achard, A. Tallaire, M. B. Plenio, P. Siyushev, and F. Jelezko, Physical Review Letters, 122, 190503 (2019)
DOI: doi.org/10.1103/PhysRevLett.122.190503

The gist of it

Silicon vacancy (SiV) centres in diamond are promising platforms for scalable quantum processors and quantum repeaters due to high spectral stability and efficient photon collection.  A main drawback is the limited coherence time, which can be avoided by storing the information on a nearby nuclear spin.

In this work we demonstrate initialization and readout of a 13C nuclear spin with a SiV centre and show that its coherence time is limited by the SiV relaxation time.

Blueprint for Nanoscale NMR – I. Schwartz, J. Rosskopf, S. Schmitt, B. Tratzmiller, Q. Chen, L.P. McGuinness, F. Jelezko, and M.B. Plenio, Sci. Rep., 9, 6938 (2019)
*This work is licensed under Creative Commons Attribution 4
DOI: doi.org/10.1038/s41598-019-43404-2

The gist of it

The detection and characterization of molecules on the nanometric scale is a holy grail of may scientific fields. Recently nitrogen vacancy (NV) centers in diamond have been used as ultrasensitive magnetometers to perform nuclear magnetic resonance (NMR) spectroscopy of statistically polarized samples at 1–100 nm length scales, providing an opportunity to achieve this goal. However, the spectral linewidth is typically limited to the kHz level, both by the NV sensor coherence time and by rapid molecular diffusion, removing the crucial information of the molecule and making distinguishing different molecules not feasible. Here we provide a blueprint supported by detailed theoretical analysis for a set-up that combines a sensitivity sufficient for detecting NMR signals from nano- to micron-scale samples with a spectral resolution that is limited only by the nuclear spin coherence, i.e. comparable to conventional NMR.

We leverage novel lock-in detection techniques, diamond-based optical hyperpolarization as well as Bayesian inference models for signal processing, thereby making nano/microscale NMR spectroscopy feasible on realistic on sample concentrations, obtaining several orders of magnitude better sensitivity than the current state of the art.

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

The gist of it

Shake The Emitter, Get a Comb

Frequency combs with fine and orderly teeth are generated out of the single photons emitted from a jiggling atom (color center). When the electronic degrees of freedom of a quantum emitter strongly couple to its motion the radiative transitions get modulated so strongly that the outgoing photons start forming a regulatory framework in the frequency domain; a comb. We show that this can indeed happen for a color center in a freestanding hexagonal boron nitride membrane where the coupling is provided by the Casimir effect.

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.

Most Recent Papers

Randomization of Pulse Phases for Unambiguous and Robust Quantum Sensing, Physical Review Letters, 122, 200403 (2019)

Quantifying Operations with an Application to Coherence, Physical Review Letters, 122, 190405 (2019)

Initialization and Readout of Nuclear Spins via a Negatively Charged Silicon-Vacancy Center in Diamond, Physical Review Letters, 122, 190503 (2019)

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