Improving Blind Spot Denoising for Microscopy

Many microscopy applications are limited by the total amount of usable light and are consequently challenged by the resulting levels of noise in the acquired images. This problem is often addressed via (supervised) deep learning based denoising. Recently, by making assumptions about the noise statistics, self-supervised methods have emerged. Such methods are trained directly on the images that are to be denoised and do not require additional paired training data. While achieving remarkable results, self-supervised methods can produce high-frequency artifacts and achieve inferior results compared to supervised approaches. Here we present a novel way to improve the quality of self-supervised denoising. Considering that light microscopy images are usually diffraction-limited, we propose to include this knowledge in the denoising process. We assume the clean image to be the result of a convolution with a point spread function (PSF) and explicitly include this operation at the end of our neural network. As a consequence, we are able to eliminate high-frequency artifacts and achieve self-supervised results that are very close to the ones achieved with traditional supervised methods.Comment: 15 pages, 4 figure

Enhanced solid-state multi-spin metrology using dynamical decoupling

We use multi-pulse dynamical decoupling to increase the coherence lifetime (T2) of large numbers of nitrogen-vacancy (NV) electronic spins in room temperature diamond, thus enabling scalable applications of multi-spin quantum information processing and metrology. We realize an order-of-magnitude extension of the NV multi-spin T2 for diamond samples with widely differing spin environments. For samples with nitrogen impurity concentration <~1 ppm, we find T2 > 2 ms, comparable to the longest coherence time reported for single NV centers, and demonstrate a ten-fold enhancement in NV multi-spin sensing of AC magnetic fields

Narrowband Biphotons: Generation, Manipulation, and Applications

In this chapter, we review recent advances in generating narrowband biphotons with long coherence time using spontaneous parametric interaction in monolithic cavity with cluster effect as well as in cold atoms with electromagnetically induced transparency. Engineering and manipulating the temporal waveforms of these long biphotons provide efficient means for controlling light-matter quantum interaction at the single-photon level. We also review recent experiments using temporally long biphotons and single photons.Comment: to appear as a book chapter in a compilation "Engineering the Atom-Photon Interaction" published by Springer in 2015, edited by A. Predojevic and M. W. Mitchel

Fourier Magnetic Imaging with Nanoscale Resolution and Compressed Sensing Speed-up using Electronic Spins in Diamond

Optically-detected magnetic resonance using Nitrogen Vacancy (NV) color centres in diamond is a leading modality for nanoscale magnetic field imaging, as it provides single electron spin sensitivity, three-dimensional resolution better than 1 nm, and applicability to a wide range of physical and biological samples under ambient conditions. To date, however, NV-diamond magnetic imaging has been performed using real space techniques, which are either limited by optical diffraction to 250 nm resolution or require slow, point-by-point scanning for nanoscale resolution, e.g., using an atomic force microscope, magnetic tip, or super-resolution optical imaging. Here we introduce an alternative technique of Fourier magnetic imaging using NV-diamond. In analogy with conventional magnetic resonance imaging (MRI), we employ pulsed magnetic field gradients to phase-encode spatial information on NV electronic spins in wavenumber or k-space followed by a fast Fourier transform to yield real-space images with nanoscale resolution, wide field-of-view (FOV), and compressed sensing speed-up.Comment: 31 pages, 10 figure

Robust Dynamical Decoupling for Arbitrary Quantum States of a Single NV Center in Diamond

Dynamical decoupling is a powerful technique for extending the coherence time (T$_2$) of qubits. We apply this technique to the electron spin qubit of a single nitrogen-vacancy center in type IIa diamond. In a crystal with natural abundance of $^{13}$C nuclear spins, we extend the decoherence time up to 2.2 ms. This is close to the T$_1$ value of this NV center (4 ms). Since dynamical decoupling must perform well for arbitrary initial conditions, we measured the dependence on the initial state and compared the performance of different sequences with respect to initial state dependence and robustness to experimental imperfections.Comment: EPL Accepte

Fourier magnetic imaging with nanoscale resolution and compressed sensing speed-up using electronic spins in diamond

Optically-detected magnetic resonance using Nitrogen Vacancy (NV) color centres in diamond is a leading modality for nanoscale magnetic field imaging,1-3 as it pro-vides single electron spin sensitivity,4 three-dimensional resolution better than 1 nm,5 and applicability to a wide range of physical6-8 and biological9 samples under ambient conditions. To date, however, NV-diamond magnetic imaging has been performed using “real space” techniques, which are either limited by optical diffrac-tion to ≈250 nm resolution10 or require slow, point-by-point scanning for nanoscale resolution, e.g., using an atomic force microscope,11 magnetic tip,5 or super-resolution optical imaging.12 Here we introduce an alternative technique of Fourier magnetic imaging using NV-diamond. In analogy with conventional magnetic reso-nance imaging (MRI), we employ pulsed magnetic field gradients to phase-encode spatial information on NV electronic spins in wavenumber or “k-space”13 followed by a fast Fourier transform to yield real-space images with nanoscale resolution, wide field-of-view (FOV), and compressed sensing speed-up.Physic

Suppression of spin-bath dynamics for improved coherence of multi-spin-qubit systems

multi-qubit systems are crucial for the advancement and application of quantum science. such systems require maintaining long coherence times while increasing the number of qubits available for coherent manipulation. For solid-state spin systems, qubit coherence is closely related to fundamental questions of many-body spin dynamics. Here we apply a coherent spectroscopic technique to characterize the dynamics of the composite solid-state spin environment of nitrogen-vacancy colour centres in room temperature diamond. We identify a possible new mechanism in diamond for suppression of electronic spin-bath dynamics in the presence of a nuclear spin bath of sufficient concentration. This suppression enhances the efficacy of dynamical decoupling techniques, resulting in increased coherence times for multispin-qubit systems, thus paving the way for applications in quantum information, sensing and metrology. U nderstanding and controlling the coherence of multispin-qubit solid-state systems is crucial for quantum information science [1][2][3] , basic research on quantum many-body dynamics 4 and quantum sensing and metrology [5][6][7] The paradigm of a central spin coupled to a spin environment has been studied intensively for many years (see for example refs 16 and 17), and quantum control methods have been developed to extend the spin coherence lifetime by reducing the effective interaction with the environment. In particular, dynamical decoupling techniques pioneered in nuclear magnetic resonance have recently been applied successfully to extend the effective T 2 of single NV-diamond electronic spins by more than an order of magnitude Here we study experimentally the spin environment of NV colour centres in room temperature diamond and their multispin-qubit FOM. We apply a spectral decomposition technique [21] Results Spectral decomposition technique. Owing to coupling of the NV spins to their magnetic environment where S(ω) is the spectral function describing the coupling of the system to the environment. Equation (1) holds in the approximation of weak coupling of the NV spins to the environment, which is appropriate for systems with (dominantly) electronic spin baths 17 , as is the case with the diamond samples discussed here (see below). S(ω) can be determined from straightforward decoherence measurements of the NV spin qubits using a spectral decomposition . Therefore, by measuring the time dependence of the qubit coherence C(t) when subjected to such a spectral δ-function modulation, we can extract the spin bath&apos;s spectral component at frequency ω 0 : This procedure can then be repeated for different values of ω 0 to provide complete spectral decomposition of the spin environment. A close approximation to the ideal spectral filter function F t (ω) described above can be provided by a variation on the well-known Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence for dynamical decoupling of a qubit from its environment 27 Spectral function of a spin bath. The composite solid-state spin environment in diamond is dominated by a bath of fluctuating N electronic spin (S = 1/2) impurities, which causes decoherence of the probed NV electron-spin qubits through magnetic dipolar interactions. In the regime of low external magnetic fields and room temperature (relevant to the present experiments), the N bath spins are randomly oriented, and their flip-flops (spin-state exchanges) can be considered as random uncorrelated events 17 . Therefore, the resulting spectrum of the N bath&apos;s coupling to the NV spins can be assumed to be Lorentzian 25 : This spin-bath spectrum is characterized by two parameters: ∆ is the average coupling strength of the N bath to the probed NV spins, and τ c is the correlation time of the N bath spins with each other, which is related to their characteristic flip-flop time. In general, the coupling strength ∆ is given by the average dipolar interaction energy between the bath spins and the NV spins, and the correlation time τ c is given by the inverse of the dipolar interaction energy between neighbouring bath-spins. As such spin-spin interactions scale as 1/r 3 , where r is the distance between spins, it is expected that the coupling strength scales as the N bath spin density n spin (that is, ∆∝n spin ), and the correlation time scales as the inverse of this density (that is, τ c ∝n spin ). Note also that the multi-pulse CPMG sequence used in the spectral decomposition technique extends the NV spin coherence lifetime by suppressing the time-averaged coupling to the fluctuating spin environment. In general, the coherence lifetime T 2 increases with the number of pulses n used in the CPMG sequence. For a Lorentzian bath, in the limit of very short correlation times (τ c T 2 ), the sequence is inefficient and T 2 ∝n 0 (no improvement with number of pulses). In the opposite limit of very long correlation times τ c T 2 , the scaling is T 2 ∝n 2/3 (refs 29-31; see also recent work on quantum dots 32 ). In the following we apply spectral decomposition to study the spin-bath dynamics and resulting scaling of T 2 with n for NV centres in diamond. Experimental application of spectral decomposition. The NV centre is composed of a substitutional nitrogen atom and a vacancy on adjacent lattice sites Experimentally, we manipulate the |0〉 − |1〉 spin manifold of the NV triplet electronic ground state using a static magnetic field and resonant MW pulses, and use a 532-nm laser to initialize and provide optical readout of the NV spin states The measured coherence is then used to extract the corresponding spin-bath spectral component S n (ω) as described above. We applied the spectral decomposition technique to extract the spin-bath parameters ∆ and τ c as well as the NV multi-qubit coherence T 2 and FOM for three diamond samples with differing NV densities and concentrations of electronic and nuclear spin impurities The results for the 12 C sample are summarized in We repeated the spectral decomposition analysis for the two other diamond samples, with the results for all three samples summarized in Discussion We explain this suppression of spin-bath dynamics as a result of random, relative detuning of electronic spin energy levels due to interactions between proximal electronic (N) and nuclear ( 13 C) spin impurities (similar to processes identified by Bloembergen 33 and Portis 34 for other solid-state spin systems). The ensemble average effect of such random electronic-nuclear spin interactions is to induce an inhomogeneous broadening ∆E of the resonant electronic spin transitions in the bath, which reduces the electronic spin flip-flop rate R (~1/τ c ) given by where ∆ N is the dipolar interaction between N electronic spins. In this physical picture, ∆E is proportional to the concentration of 13 C impurities and to the N-13 C hyperfine interaction energy, whereas ∆ N is proportional to the N concentration. Given the magnetic moments and concentrations of the N and 13 C spin impurities, and the large N-13 C hyperfine interaction in diamond 37,38 , we estimate ∆E~10 MHz and ∆ N~1 MHz for the Apollo and HPHT samples. These values imply an order of magnitude suppression of R compared with the bare electronic spin flip-flop rate ignoring N-13 C interactions (R bare~∆N ), which is consistent with our experimental results for τ c~1 /R Methods Single NV confocal microscope. Single NV measurements (HPHT sample) were performed using a custom-built confocal microscope. Optical excitation was provided by a 300 mW 532 nm diode pumped solid-state laser (Changchun New Industries Optoelectronics Tech, MLLIII532-300-1), focused onto the sample using a ×100, NA = 1.3 oil-immersion objective (Nikon CFI Plan Fluor ×100 oil). NV fluorescence was collected through the same optics, and separated from the excitation beam using a dichroic filter . The light was additionally filtered (Semrock LP02-633RS-25) and focused onto a single-photon counting module (Perkin-Elmer, SPCM-ARQH-12). The excitation laser was pulsed by focusing it through an acousto-optic modulator (Isomet 1205C-2). Microwaves were delivered to the sample through a 20-µm thick wire soldered across it. The wire was driven by an amplified (Mini-circuits ZHL-16W-43-S + ) MW synthesizer (Agilent E4428C). Phase modulation of the MW pulses was achieved using an in-phase/quadrature (IQ) mixer (Marki IQ-1545). MW and optical pulses were controlled using a computer-based digital delay generator (SpinCore PulseBlaster ESR400). A static magnetic field of ≈50 G was applied to the sample to lift the degeneracy of the m s = ± 1 levels. NV wide-field fluorescence microscope. NV multi-spin measurements (for samples 12 C and Apollo) were performed using a custom-built wide-field fluorescence microscope NV fluorescence was collected by the objective, filtered, and imaged onto a cooled charge-coupled device camera (Starlight Xpress SXV-H9). As the duration of a single measurement is shorter than the minimum exposure time of the camera, the measurement was repeated for several thousand averages within a single exposure and synchronized to an optical chopper placed before the camera in order to block fluorescence from the optical preparation pulse In the measurements described here, the loop of wire delivered 3.07 GHz MW pulses to the sample, resonant with the NV |0〉 − |1〉 spin transition for the applied static magnetic field ≈70 G, to manipulate the NV spin coherence and implement CPMG spin-control pulse sequences. Spectral decomposition deconvolution procedure. The coherence of a two-level quantum system can be related to the magnitude of the off-diagonal elements of the system&apos;s density matrix. Specifically, we deal here with NV electronic spin qubits in a finite external magnetic field, which can be treated as effective two-level spin systems with quantization (z) axis aligned with the NV axis. When the NV spins are placed into a coherent superposition of spin eigenstates, for example, aligned with the x axis of the Bloch sphere, the measurable spin coherence is given by C t Tr t S x ( ) = [ ( ) ] r . The filter function for the n-pulse CPMG control sequence F CPMG (ω) covers a narrow frequency region (given by π/t, where t is total length of the sequence), which is centred at ω 0 = πn/t, and is given by 43 : We note that the narrow-band feature of the CPMG filter essentially defines the effectiveness of this sequence for dynamical decoupling. The filter function F t n CPMG ( ) w associated with the CPMG pulse sequence deviates from a δ-function by the finite width of the main spectral peak and by the presence of higher harmonics ( Owing to the high frequency components of the filter function and the monotonically decreasing nature of the spectral function, a naive reconstruction of the spectral function assuming a δ-function form of the filter function produces results that are biased to lower values by ~15%. This bias is removed using the reconstruction algorithm presented above. Reference

Dressed-State Resonant Coupling between Bright and Dark Spins in Diamond

Under ambient conditions, spin impurities in solid-state systems are found in thermally mixed states and are optically “dark”; i.e., the spin states cannot be optically controlled. Nitrogen-vacancy (NV) centers in diamond are an exception in that the electronic spin states are “bright”; i.e., they can be polarized by optical pumping, coherently manipulated with spin-resonance techniques, and read out optically, all at room temperature. Here we demonstrate a scheme to resonantly couple bright NV electronic spins to dark substitutional-nitrogen (P1) electronic spins by dressing their spin states with oscillating magnetic fields. This resonant coupling mechanism can be used to transfer spin polarization from NV spins to nearby dark spins and could be used to cool a mesoscopic bath of dark spins to near-zero temperature, thus providing a resource for quantum information and sensing, and aiding studies of quantum effects in many-body spin systems.National Institute of Standards and Technology (U.S.)National Science Foundation (U.S.)United States. Defense Advanced Research Projects Agenc