Universal, high-fidelity quantum gates based on superadiabatic, geometric phases on a solid-state spin-qubit at room temperature

Geometric phases and holonomies (their non-commuting generalizations) are a promising resource for the realization of high-fidelity quantum operations in noisy devices, due to their intrinsic fault-tolerance against noise and experimental imperfections. Despite their conceptual appeal and proven fault-tolerance, for a long time their practical use in quantum computing was limited to proof of principle demonstrations. Only in 2012 Sj\"oqvist et al. formulated a strategy to generate non-Abelian (i.e. holonomic) quantum gates through non-adiabatic transformation. Successful experimental demonstrations of this concept followed on various physical qubit systems and proved the feasibility of this fast, holonomic quantum gate concept. Despite these successes, the experimental implementation of such non-Abelian quantum gates remains experimentally challenging since in general the emergence of a suitable holonomy requires encoding of the logical qubit within a three (or higher) level system being driven by two (or more) control fields. A very recent proposal by Liang et al. offers an elegant solution generating a non-Abelian, geometric quantum gate on a simple, two-level system driven by one control field. Exploiting the concept of transitionless quantum driving it allows the generation of universal geometric quantum gates through superadiabatic evolution. This concept thus generates fast and robust phase-based quantum gates on the basis of minimal experimental resources. Here, we report on the first such implementation of a set of non-commuting single-qubit superadiabatic geometric quantum gates on the electron spin of the negatively charged nitrogen vacancy center in diamond. The realized quantum gates combine high-fidelity and fast quantum gate performance. This provides a promising and powerful tool for large-scale quantum computing under realistic, noisy experimental conditions

Narrow-band few photon filter and phase lock control for EIT with Cs in a nanofiber dipole trap

Nicht angegeben.This Master thesis was performed around an experiment aiming at the investigation of the optical properties of laser cooled Cesium (Cs) atoms dipole trapped in the evanescent field of an optical nanofiber. Two parts of the total experiment are covered in this thesis. The focus of the first part is the beam preparation of the EIT control and probe lasers which ensures a phase stable joint performance of both beams necessary for the implementation of EIT. This is achieved by an optical phase-locked loop (OPLL) locking the probe to the control laser. The performance of this OPLL is examined with an out-of-loop phase noise measurement. The second part of this thesis concerns the efficient detection of the prospective few-photon EIT probe signal, which will be immersed in a broadband noise background. While the weak EIT probe signal (<pW power) is expected to be extremely narrow-band (<kHz) the noise has a power of ~5pW within a wavelength window of 10nm around the probe wavelength. Conventional optical filters fail in efficiently separating the signal from the fluorescence. Here, two strategies are elaborated theoretically aiming at a reasonable solution for narrow-band few-photon filtering: one employing a diffraction grating and another based on homodyne detection. Experimental proposals for filters based on both options are made, followed by an experimental realization and analysis of a compact test setup, a homodyne saturation spectroscopy

Measuring environmental quantum noise exhibiting a non-monotonous spectral shape

Understanding the physical origin of noise affecting quantum systems is important for nearly every quantum application. Quantum noise spectroscopy has been employed in various quantum systems, such as superconducting qubits, NV centers and trapped ions. Traditional spectroscopy methods are usually efficient in measuring noise spectra with mostly monotonically decaying contributions. However, there are important scenarios in which the noise spectrum is broadband and non-monotonous, thus posing a challenge to existing noise spectroscopy schemes. Here, we compare several methods for noise spectroscopy: spectral decomposition based on the Carr-Purcell-Meiboom-Gill (CPMG) sequence, the recently presented DYnamic Sensitivity COntrol (DYSCO) sequence and a modified DYSCO sequence with a Gaussian envelope (gDYSCO). The performance of the sequences is quantified by analytic and numeric determination of the frequency resolution, bandwidth and sensitivity, revealing a supremacy of gDYSCO to reconstruct non-trivial features. Utilizing an ensemble of nitrogen-vacancy centers in diamond coupled to a high density $^{13}$C nuclear spin environment, we experimentally confirm our findings. The combination of the presented schemes offers potential to record high quality noise spectra as a prerequisite to generate quantum systems unlimited by their spin-bath environment

Il bizzarro mondo dei quanti

Scritto prima dell'esame di maturità da una giovane di eccezionale talento, questo libro colma il vuoto esistente tra la letteratura divulgativa sulla fisica quantistica, che normalmente evita ogni formula matematica, e la letteratura specialistica, ben farcita, invece, di matematica avanzata. L'autrice, appena diciannovenne, con l'ausilio della sola matematica della scuola superiore, introduce il lettore ai principi della fisica dei quanti. Se ne ricava uno sguardo profondo sul microcosmo, il regno affascinante delle particelle elementari: oggetti il cui comportamento si distingue in modo drastico e fondamentale da tutto ciò a cui è avvezzo il nostro umano buonsenso. "Un libro... che avrei desiderato avere a 17 anni". Silvia Arroyo Camejo "In modo assolutamente preciso dal punto di vista fisico, l'autrice spiega con grande passione e divertimento i fondamenti della moderna fisica quantistica ... " Prof. Reinhold A. Bertlmann "Un libro stupefacente di un'autrice straordinaria! Si avverte il suo entusiasmo per gli enigmi e le stranezze del microcosmo in ogni paragrafo". Prof. H. Dieter Ze

Room temperature high-fidelity holonomic single-qubit gate on a solid-state spin.

At its most fundamental level, circuit-based quantum computation relies on the application of controlled phase shift operations on quantum registers. While these operations are generally compromised by noise and imperfections, quantum gates based on geometric phase shifts can provide intrinsically fault-tolerant quantum computing. Here we demonstrate the high-fidelity realization of a recently proposed fast (non-adiabatic) and universal (non-Abelian) holonomic single-qubit gate, using an individual solid-state spin qubit under ambient conditions. This fault-tolerant quantum gate provides an elegant means for achieving the fidelity threshold indispensable for implementing quantum error correction protocols. Since we employ a spin qubit associated with a nitrogen-vacancy colour centre in diamond, this system is based on integrable and scalable hardware exhibiting strong analogy to current silicon technology. This quantum gate realization is a promising step towards viable, fault-tolerant quantum computing under ambient conditions

Dynamical sensitivity control of a single-spin quantum sensor.

The Nitrogen-Vacancy (NV) defect in diamond is a unique quantum system that offers precision sensing of nanoscale physical quantities at room temperature beyond the current state-of-the-art. The benchmark parameters for nanoscale magnetometry applications are sensitivity, spectral resolution, and dynamic range. Under realistic conditions the NV sensors controlled by conventional sensing schemes suffer from limitations of these parameters. Here we experimentally show a new method called dynamical sensitivity control (DYSCO) that boost the benchmark parameters and thus extends the practical applicability of the NV spin for nanoscale sensing. In contrast to conventional dynamical decoupling schemes, where π pulse trains toggle the spin precession abruptly, the DYSCO method allows for a smooth, analog modulation of the quantum probe's sensitivity. Our method decouples frequency selectivity and spectral resolution unconstrained over the bandwidth (1.85 MHz-392 Hz in our experiments). Using DYSCO we demonstrate high-accuracy NV magnetometry without |2π| ambiguities, an enhancement of the dynamic range by a factor of 4 · 103, and interrogation times exceeding 2 ms in off-the-shelf diamond. In a broader perspective the DYSCO method provides a handle on the inherent dynamics of quantum systems offering decisive advantages for NV centre based applications notably in quantum information and single molecule NMR/MRI

Superresolution optical magnetic imaging and spectroscopy using individual electronic spins in diamond.

Nitrogen vacancy (NV) color centers in diamond are a leading modality for both superresolution optical imaging and nanoscale magnetic field sensing. In this work, we address the key challenge of performing optical magnetic imaging and spectroscopy selectively on multiple NV centers that are located within a diffraction-limited field-of-view. We use spin-RESOLFT microscopy to enable precision nanoscale mapping of magnetic field patterns with resolution down to ~20 nm, while employing a low power optical depletion beam. Moreover, we use a shallow NV to demonstrate the detection of proton nuclear magnetic resonance (NMR) signals exterior to the diamond, with 50 nm lateral imaging resolution and without degrading the proton NMR linewidth