arXiv subjects

Boris Naydenov

Publications and source records attributed to Boris Naydenov.

32 records · Page 2Linked to original sources

arXiv2017

Autonomous Calibration of Single Spin Qubit Operations

Fully autonomous precise control of qubits is crucial for quantum information processing, quantum communication, and quantum sensing applications. It requires minimal human intervention on the ability to model, to predict and to anticipate the quantum dynamics [1,2], as well as to precisely control and calibrate single qubit operations. Here, we demonstrate single qubit autonomous calibrations via closed-loop optimisations of electron spin quantum operations in diamond. The operations are examined by quantum state and process tomographic measurements at room temperature, and their performances against systematic errors are iteratively rectified by an optimal pulse engineering algorithm. We achieve an autonomous calibrated fidelity up to 1.00 on a time scale of minutes for a spin population inversion and up to 0.98 on a time scale of hours for a Hadamard gate within the experimental error of 2%. These results manifest a full potential for versatile quantum nanotechnologies.

quant-ph↗

arXiv2016

Quantum metrology enhanced by repetitive quantum error correction

The accumulation of quantum phase in response to a signal is the central mechanism of quantum sensing, as such, loss of phase information presents a fundamental limitation. For this reason approaches to extend quantum coherence in the presence of noise are actively being explored. Here we experimentally protect a room-temperature hybrid spin register against environmental decoherence by performing repeated quantum error correction whilst maintaining sensitivity to signal fields. We use a long-lived nuclear spin to correct multiple phase errors on a sensitive electron spin in diamond and realize magnetic field sensing beyond the timescales set by natural decoherence. The universal extension of sensing time, robust to noise at any frequency, demonstrates the definitive advantage entangled multi-qubit systems provide for quantum sensing and offers an important complement to quantum control techniques. In particular, our work opens the door for detecting minute signals in the presence of high frequency noise, where standard protocols reach their limits.

quant-ph↗

arXiv2016

Optically induced dynamic nuclear spin polarisation in diamond

The sensitivity of Magnetic Resonance Imaging (MRI) depends strongly on nuclear spin polarisation and, motivated by this observation, dynamical nuclear spin polarisation has recently been applied to enhance MRI protocols (Kurhanewicz, J., et al., Neoplasia 13, 81 (2011)). Nuclear spins associated with the 13 C carbon isotope (nuclear spin I = 1/2) in diamond possess uniquely long spin lattice relaxation times (Reynhardt, E.C. and G.L. High, Prog. in Nuc. Mag. Res. Sp. 38, 37 (2011)) If they are present in diamond nanocrystals, especially when strongly polarised, they form a promising contrast agent for MRI. Current schemes for achieving nuclear polarisation, however, require cryogenic temperatures. Here we demonstrate an efficient scheme that realises optically induced 13 C nuclear spin hyperpolarisation in diamond at room temperature and low ambient magnetic field. Optical pumping of a Nitrogen-Vacancy (NV) centre creates a continuously renewable electron spin polarisation which can be transferred to surrounding 13 C nuclear spins. Importantly for future applications we also realise polarisation protocols that are robust against an unknown misalignment between magnetic field and crystal axis.

quant-ph↗

arXiv2015

Accelerated 2D magnetic resonance spectroscopy of single spins using matrix completion

Two dimensional nuclear magnetic resonance (NMR) spectroscopy is one of the major tools for analysing the chemical structure of organic molecules and proteins. Despite its power, this technique requires long measurement times, which, particularly in the recently emerging diamond based single molecule NMR, limits its application to stable samples. Here we demonstrate a method which allows to obtain the spectrum by collecting only a small fraction of the experimental data. Our method is based on matrix completion which can recover the full spectral information from randomly sampled data points. We confirm experimentally the applicability of this technique by performing two dimensional electron spin echo envelope modulation (ESEEM) experiments on a two spin system consisting of a single nitrogen vacancy (NV) centre in diamond coupled to a single 13C nuclear spin. We show that the main peaks in the spectrum can be obtained with only 10 % of the total number of the data points. We believe that our results reported here can find an application in all types of two dimensional spectroscopy, as long as the measured matrices have a low rank.

quant-ph↗

arXiv2015

Stabilizing shallow color centers in diamond created by nitrogen delta-doping using SF$_6$ plasma treatment

Here we report the fabrication of stable, shallow (< 5 nm) nitrogen-vacancy (NV) centers in diamond by nitrogen delta doping at the last stage of the chemical vapor deposition (CVD) growth process. The NVs are stabilized after treating the diamond in $SF_6$ plasma, otherwise the color centers are not observed, suggesting a strong influence from the surface. X-Ray photoelectron spectroscopy measurements show the presence of only fluorine atoms on the surface, in contrast to previous studies, and suggests very good surface coverage. We managed to detect hydrogen nuclear magnetic resonance signal from protons in the immersion oil, revealing a depth of the NVs of about 5 nm

cond-mat.mes-hall↗

arXiv2015

A Germanium-Vacancy Single Photon Source in Diamond

Color centers in diamond are widely recognized as a promising solid state platform for quantum cryptography and quantum information processing. For these applications, single photon sources with a high intensity and reproducible fabrication methods are required. Here, we report a novel color center in diamond, composed of a germanium (Ge) and a vacancy (V) and named the GeV center, which has a sharp and strong photoluminescence band with a zero-phonon line at 602 nm at room temperature. We demonstrate this new color center works as a single photon source. Both ion implantation and chemical vapor deposition techniques enabled fabrication of GeV centers in diamond. A first-principles calculation revealed the atomic crystal structure and energy levels of the GeV center.

cond-mat.mtrl-sci↗

arXiv2014

Multiple intrinsically identical single photon emitters in the solid-state

Emitters of indistinguishable single photons are crucial for the growing field of quantum technologies. To realize scalability and increase the complexity of quantum optics technologies, multiple independent yet identical single photon emitters are also required. However typical solid-state single photon sources are inherently dissimilar, necessitating the use of electrical feedback or optical cavities to improve spectral overlap between distinct emitters. Here, we demonstrate bright silicon-vacancy (SiV-) centres in low-strain bulk diamond which intrinsically show spectral overlap of up to 91% and near transform-limited excitation linewidths. Our results have impact upon the application of single photon sources for quantum optics and cryptography, and the production of next generation fluorophores for bio-imaging.

quant-ph↗

arXiv2014

Precise ultra fast single qubit control using optimal control pulses

Ultra fast and accurate quantum operations are required in many modern scientific areas - for instance quantum information, quantum metrology and magnetometry. However the accuracy is limited if the Rabi frequency is comparable with the transition frequency due to the breakdown of the rotating wave approximation (RWA). Here we report the experimental implementation of a method based on optimal control theory, which does not suffer these restrictions. We realised the most commonly used quantum gates - the Hadamard (π/2 pulse) and NOT (πpulse) gates with fidelities ($F^{\mathrm{exp}}_{π/2}=0.9472\pm0.01$ and $F^{\mathrm{exp}}_π=0.993\pm0.016$), in an excellent agreement with the theoretical predictions ($F^{\mathrm{theory}}_{π/2}=0.9545$ and $F^{\mathrm{theory}}_π=0.9986$). Moreover, we demonstrate magnetic resonance experiments both in the rotating and lab frames and we can deliberately "switch" between these two frames. Since our technique is general, it could find a wide application in magnetic resonance, quantum computing, quantum optics and broadband magnetometry.

quant-ph↗

arXiv2013

Strongly Coupled Diamond Spin Qubits by Molecular Nitrogen Implantation

Ionized nitrogen molecules ($^{15}$N$_{2}^+$) are used as efficient point sources for creating NV$^-$ pairs in diamond with nanoscale spatial separation and up to 55 kHz magnetic coupling strength. Co-implantation of $^{12}$C$^+$ increased the yield of pairs, and a $^{13}$C-depleted diamond allowed 0.65 ms coherence times to be obtained. Further coupling to a third dark spin provided a strongly coupled three spin register. These results mark an important step towards realization of multi-qubit systems and scalable NV$^-$ quantum registers.

quant-ph↗

arXiv2013

Detection of a single fundamental charge with nanoscale resolution in ambient conditions using the NV$^-$ center in diamond

Single charge detection with nanoscale spatial resolution in ambient conditions is a current frontier in metrology that has diverse interdisciplinary applications. Here, such single charge detection is demonstrated using two nitrogen-vacancy (NV) centers in diamond. One NV center is employed as a sensitive electrometer to detect the change in electric field created by the displacement of a single electron resulting from the optical switching of the other NV center between its neutral (NV$^0$) and negative (NV$^-$) charge states. As a consequence, our measurements also provide direct insight into the charge dynamics inside the material.

cond-mat.mes-hall↗

arXiv2012

Room temperature entanglement between distant single spins in diamond

Entanglement is the central yet fleeting phenomena of quantum physics. Once being considered a peculiar counter-intuitive property of quantum theory it has developed into the most central element of quantum technology providing speed up to quantum computers, a path towards long distance quantum cryptography and increased sensitivity in quantum metrology. Consequently, there have been a number of experimental demonstration of entanglement between photons, atoms, ions as well as solid state systems like spins or quantum dots, superconducting circuits and macroscopic diamond. Here we experimentally demonstrate entanglement between two engineered single solid state spin quantum bits (qubits) at ambient conditions. Photon emission of defect pairs reveals ground state spin correlation. Entanglement (fidelity = 0.67 \pm 0.04) is proven by quantum state tomography. Moreover, the lifetime of electron spin entanglement is extended to ms by entanglement swapping to nuclear spins, demonstrating nuclear spin entanglement over a length scale of 25 nm. The experiments mark an important step towards a scalable room temperature quantum device being of potential use in quantum information processing as well as metrology.

quant-ph↗

arXiv2010

Increasing the coherence time of single electron spins in diamond by high temperature annealing

Negatively charged Nitrogen-Vacancy (NV$^-$) centers in diamond produced by ion implantation often show properties different from NVs created during the crystal growth. We observe that NVs created from nitrogen ions with four different energies (30-300 keV) show much shorter electron spin coherence time $T_2$ compared to the "natural" NVs and we could find any dependence of $T_2$ on the implantation energy. Moreover, we also find out that about 10% of them show switching from NV$^-$to NV$^0$. We develop a method to increase $T_2$ of single NV$^-$ centers by annealing the sample at $t=1200 {\circ}$C whereas at the same time the fraction of the NVs showing discharging is greatly reduced.

cond-mat.mtrl-sci↗

arXiv2010

Dynamical Decoupling of a single electron spin at room temperature

Here we report the increase of the coherence time T$_2$ of a single electron spin at room temperature by using dynamical decoupling. We show that the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence can prolong the T$_2$ of a single Nitrogen-Vacancy center in diamond up to 2.44 ms compared to the Hahn echo measurement where T$_2 = 390 μ$s. Moreover, by performing spin locking experiments we demonstrate that with CPMG the maximum possible $T_2$ is reached. On the other hand, we do not observe strong increase of the coherence time in nanodiamonds, possibly due to the short spin lattice relaxation time $T_1=100 μ$s (compared to T$_1$ = 5.93 ms in bulk). An application for detecting low magnetic field is demonstrated, where we show that the sensitivity using the CPMG method is improved by about a factor of two compared to the Hahn echo method.

quant-ph↗

arXiv2010

High sensitivity magnetic imaging using an array of spins in diamond

We present a solid state magnetic field imaging technique using a two dimensional array of spins in diamond. The magnetic sensing spin array is made of nitrogen-vacancy (NV) centers created at shallow depths. Their optical response is used for measuring external magnetic fields in close proximity. Optically detected magnetic resonance (ODMR) is readout from a 60x60 $μ$m field of view in a multiplexed manner using a CCD camera. We experimentally demonstrate full two-dimensional vector imaging of the magnetic field produced by a pair of current carrying micro-wires. The presented widefield NV magnetometer offers in addition to its high magnetic sensitivity of 20 nT/$\sqrt{Hz}$ and vector reconstruction, an unprecedented spatio-temporal resolution and functionality at room temperature.

cond-mat.mes-hall↗