Abstract:
Long spin coherence lifetimes are realized for ensembles of electronic spin impurities in solid state spin systems, for example NV color centers in diamond, by using spin-control RF pulse sequences to provide dynamic decoupling of the ensembles of spin impurities from environmental sources of decoherence such as dipolar and hyperfine interactions with proximal spin and other paramagnetic impurities in diamond. In this way, the measurement sensitivity of the coherent evolution of ensembles of solid state spin impurities are increased. Using the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence, the spin coherence lifetimes of NV ensembles can be extended to more than 2 ms in room temperature diamond, and sensitivity of magnetometry that uses NV ensembles can be increased.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is based upon, and claims the benefit of priority under 35 U.S.C. §119, to co-pending United States Provisional Patent Application No. 61/496,511 (the “&#39;511 provisional application”), filed Jun. 13, 2011 and entitled “Dynamic Decoupling of Spin Ensembles in Solid State Magnetometry.” The content of the &#39;511 provisional application is incorporated herein by reference in its entirety as though fully set forth. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    This invention was made with government support under contract number 60NANB10D002 awarded by NEST (National Institute Of Standards And Technology). The government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0003]    The negatively-charged nitrogen-vacancy (NV) color center in diamond possesses many useful properties, including without limitation: long electronic spin coherence times at room temperature; optical mechanisms for initializing and detecting their spin states; and electron spin resonance (ESR) techniques that allow for coherent spin manipulation. The NV color center has generated much interest for scalable applications in quantum information and metrology, such as sensitive detection of electric and magnetic fields. 
         [0004]    Dynamical decoupling techniques have been used to reduce the effective interaction of single NV spins with other spin impurities in the environment, enabling significant improvements in the NV single-spin coherence lifetime. 
         [0005]    For multi-spin systems, for example ensembles of NV color centers in diamond containing large densities of NV spins, realizing long spin coherence lifetimes remains a challenge. Likewise, there is a need for increasing the sensitivity of magnetometry and other multi-spin metrology that involves ensembles of NVs or other spin impurities. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The drawings disclose illustrative embodiments. They do not set forth all embodiments. Other embodiments may be used in addition or instead. 
           [0007]      FIG. 1  is a schematic block diagram of a system for implementing multi-pulse dynamical decoupling of an ensemble of NV centers in diamond. 
           [0008]      FIG. 2A  illustrates an NV color center in a diamond lattice. 
           [0009]      FIG. 2B  illustrates the energy level structure of negatively charged NV center shown in  FIG. 2A . 
           [0010]      FIG. 3  illustrates a number of different spin control pulses that can be used for dynamic decoupling of multi-spin ensembles, in accordance with some embodiments of the present application. 
           [0011]      FIG. 4  plots the measurements of NV multi-spin coherent evolution using an n-pulse CPMG control sequence, for a diamond sample having an NV density of ˜60 ppb, and a spin-bath environment consisting of 100 ppm nitrogen atoms (N), and 1.1% concentration of  13 C. 
           [0012]      FIG. 5  illustrates examples of measured normalized fluorescence signals as functions of AC field magnitude B ac  using a Hahn Echo sequence and multi-pulse XY sequences with different numbers of control pulses. 
           [0013]      FIG. 6  provides a comparison of calculated and measured AC magnetic field sensitivity, using dynamical decoupling control pulses. 
       
    
    
     DESCRIPTION 
       [0014]    Illustrative embodiments are discussed in this disclosure. Other embodiments may be used in addition or instead. 
         [0015]    Before the present invention is described in further detail, it should be understood that the invention is not limited to the particular embodiments described, as such may vary. It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims. 
         [0016]    Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention. 
         [0017]    Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, a limited number of the exemplary methods and materials are described herein. 
         [0018]    In the present disclosure, methods and systems are described relating to the use of multi-pulse dynamical decoupling to increase the coherence lifetime (T 2 ) of large numbers of spin impurities in solid state spin systems, including without limitation NV electronic spins in room temperature diamond. 
         [0019]      FIG. 1  is a schematic block diagram of a system  100  for implementing multi-pulse dynamical decoupling of solid-state multi-spin systems, which in the illustrated embodiment is an ensemble of NV centers in room temperature diamond. In other embodiments, multi-pulse dynamical decoupling may be performed on solid-state multi-spin systems other than NV centers in diamond, including without limitation phosphorous donors in silicon. 
         [0020]    In the illustrated embodiment, the system  100  is a wide-field fluorescence microscope. The system  100  includes a pulsed microwave source  130  configured to generate a series of microwave control pulses and apply them to a sample  110 . In the illustrated embodiment, the microwave source  130  is a loop antenna designed to generate a homogeneous B 1  field over the sample detection volume, thereby generating resonant microwave control pulses for coherent manipulation of the NV spin states. 
         [0021]    In the illustrated embodiment, the sample  110  is a diamond crystal containing an ensemble of NV centers. The spin-bath environment comprises  13 C (carbon 13) nuclear spin impurities and N (nitrogen atom) electronic spin impurities. The diamond samples may consist of an NV-rich layer grown by chemical vapor deposition on a non-fluorescing diamond substrate. In some embodiments, the sample  100  may include an ensemble of spin impurities having a density of at least about 10 2 /cm 3 . 
         [0022]    The system  100  further includes an optical source  120  configured to generate optical excitation radiation that prepares the NV centers into m s =0 ground states and drives spin-dependent fluorescence detection from the NV centers. In the illustrated embodiment, the optical source  120  is a switched 3-Watt 532-nm laser that can provide optical excitation of NV centers within a 10 μm-diameter cross-section of each sample. An AOM (acousto-optic modulator)  132 , for example an Isomet M1133-aQ80 L-H, pulses the excitation laser  120  with precise timing in order to prepare and read out the NV spin states. 
         [0023]    The system  100  further includes a detector  140  configured to detect output optical radiation from the spin impurities after being exposed to the optical excitation signal and the series of microwave control pulses. The detector  140  may be an optical fluorescence detector, for example. Many other types of detectors may be used, including without limitation CCD (charge coupled device) arrays and photodiodes. 
         [0024]    NV spin state-dependent fluorescence is collected by a microscope objective  122 , and is imaged onto the optical fluorescence detector  140  after being separated from the excitation beam by a dichroic mirror  124  and filtered by red filters  128 . An optical chopper  126  is synched such that the initialization pulse is blocked from the detector  140 , while the readout pulse is recorded. The optical signal is shuttered by the optical chopper  126 , allowing spatially resolved ensemble measurements. 
         [0025]    A processing system may be integrated with the system  100  described in  FIG. 1 . The processing system is configured to implement the methods, systems, and algorithms described in the present application. The processing system may include, or may consist of, any type of microprocessor, nanoprocessor, microchip, or nanochip. The processing system may be selectively configured and/or activated by a computer program stored therein. It may include a computer-usable medium in which such a computer program may be stored, to implement the methods and systems described above. The computer-usable medium may have stored therein computer-usable instructions for the processing system. The methods and systems in the present application have not been described with reference to any particular programming language; thus it will be appreciated that a variety of platforms and programming languages may be used to implement the teachings of the present application. 
         [0026]      FIG. 2A  schematically illustrates the crystal structure of an NV center  200  in a diamond lattice. As seen in  FIG. 2A , the NV center is an empty position or vacancy resulting from a missing carbon atom in the diamond lattice. The NV impurity is based in the lattice of carbon atoms  210 , where two adjacent sites are altered, because one carbon atom is replaced with a nitrogen atom  220  and the other space is left vacant. The vacancies may interact with interstitial atoms such as nitrogen  220 , and may act as color centers by absorbing visible light. NV centers are visible as red spots when illuminated by laser. Applying a static field (B 0 ˜70 Gauss) along one of the four diamond crystallographic axes selected approximately one quarter of the NV centers to be resonant with the microwave pulses. 
         [0027]      FIG. 2B  shows the electronic structure of an NV center in diamond. As shown in  FIG. 2B , the electronic structure of the negatively charged state of the NV center has a spin-triplet ground state, where the m s ±1 levels are shifted from the m s =0 level by ˜2.87 GHz due to the dipolar spin-spin interaction. Application of an external static magnetic field along the NV axis Zeeman shifts the m s ±1 levels and allows one to treat the spin manifold as an effective two-level system. 
         [0028]    The NV spin environment, i.e., spin bath, is dominated by  13 C and N impurities, randomly distributed in the diamond crystal. These spin impurities create time-varying local magnetic fields at each NV spin, which can be approximated as an average local magnetic field that fluctuates on a timescale set by the mean interaction between spins in the bath, inducing rapid dephasing of freely precessing NV spins on a timescale T 2 *˜1 μs for typical spin impurity concentrations. 
         [0029]      FIG. 3  illustrates some examples of spin control pulses for dynamic decoupling of multi-spin systems, in one or more embodiments of the present application. An AC magnetic field b(t) is applied, where b(t) has a time dependence represented by: 
         [0000]        b ( t )=b ac sin[(2 πτ ac ) t+φ],  
 
         [0030]    and the phase φ is chosen such that the nodes of the AC magnetic field b(t) coincide with the microwave π pulses. In particular, the Hahn Echo, CPMG-2, CPMG-N, and XY-N control pulse sequences are shown. 
         [0031]    By applying a single resonant microwave π pulse to refocus the dephasing, the Hahn Echo sequence decouples NV spins from bath field fluctuations that are slow compared to the free precession time. Application of additional control pulses, as in n-pulse CPMG (CPMG-N) and n-pulse XY sequences, have been shown to decouple single NV spins from higher frequency bath fluctuations. 
         [0032]    In the present application, these additional control pulses are applied to large ensembles of NV spins, resulting in dynamic decoupling of the NV spins from magnetic field fluctuations that are slow compared to the time between the pulses. As discussed below, an order-of-magnitude extension of the coherence lifetime T 2  can be realized for diamond samples with widely differing spin environments, using multi-pulse dynamic decoupling. 
         [0033]      FIG. 4  plots the measurements of NV multi-spin coherent evolution using an n-pulse CPMG control sequence, for a diamond sample having an NV density of ˜60 ppb (as measured by NV fluorescence intensity), an N concentration of about ˜100 ppm (measured by secondary ion mass spectroscopy), and ˜1.1% natural abundance  13 C concentration. The high N concentration dominated NV decoherence in this sample, limiting the measured Hahn Echo multi-spin coherence time to T 2 ≈2 μs. 
         [0034]    The diamond sample used in the measurements illustrated in  FIG. 4  is an NV-rich layer with a thickness of about 16 μm. CPMG-n sequences were applied to the sample, and the NV multi-spin coherence time was determined as a function of the number of pulses. T 2   (n)  was determined from the 1/e decay of the spins&#39; coherent evolution as a function of the total CPMG-n evolution period. 
         [0035]    The data points in  FIG. 4  are representative measurements of NV multi-spin coherence decay for the above-described sample, with T 2   (n)  extended by a factor &gt;10 for n=128. It was found that T 2   (n)  exhibited a power-law dependence on n:T 2   (n) αn s , with s=0.65±0.02 for the sample. These results demonstrate that the inhomogeneities in the spin bath do not limit the effectiveness of dynamical decoupling for extending solid-state multi-spin coherence times by at least an order of magnitude. 
         [0036]    When the Hahn echo pulse sequences were applied to two other samples with much lower NV and N concentrations, namely NV ˜0.2 ppb and N ˜0.1 ppm for one sample, and NV ˜0.6 ppb and N ˜1 ppm for the other sample, similar Hahn Echo NV multi-spin coherence times (T 2 ˜300 μs) were measured. By applying n-pulse CPMG dynamical decoupling sequences, the multi-spin T 2   (n)  was extended to approximately 2 ms (milliseconds) for both samples, which is comparable to the longest coherence time reported for dynamical decoupling applied to single NV centers. 
         [0037]    In some embodiments of the present application, dynamical decoupling as described above can be applied to improve the sensitivity of NV multi-spin magnetometry. In a standard AC magnetometry measurement utilizing a Hahn Echo sequence, an oscillating magnetic field, b(t)=b ac sin[(2 πτ ac ) t+φ], induces a net phase accumulation of the NV spin coherence, which is maximized when the full time of the Hahn Echo sequence is equivalent to the period of the AC magnetic field (τ ac ) and the phase offset φ is such that the control pulses coincide with nodes in the magnetic field. Under these conditions, the field amplitude b ac  can be extracted from the measurement of accumulated NV spin phase with optimum sensitivity, where an approximate expression for the sensitivity is given by: 
         [0000]    
       
         
           
             
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         [0038]    In the above expression, C is a parameter that encompasses the measurement contrast, optical collection efficiency, and number of NV spins contributing to the measurement. The contrast is modified by NV decoherence over the course of the measurement, described phenomenologically by an exponential factor with power p. The value of p is found to be sample dependent, in the range of 1 to 2.5, and is related to the dynamics of the spin environment and to ensemble inhomogeneous broadening. 
         [0039]    In an AC magnetometry measurement utilizing n-pulse dynamical decoupling, the sensitivity is given approximately by the with two modifications: The measurement time is increased by τ ac →n/ 2  τ ac , and the NV multi-spin coherence time is extended by T 2 →T 2 n s . The resulting sensitivity is given by: 
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         [0040]    Because the measurement time increases linearly with the number of control pulses n, whereas the coherence time increases sub-linearly, there is an optimum number of pulses n opt  that yields the most sensitive measurement of an AC magnetic field of period τ ac  given a set of sample-determined parameters: 
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         [0041]    For a given sample, all the parameters except τ ac  are fixed, so that the above equation can be simplified to: n opt α( 1 /τ ac ) 1/p(1−s) . Thus, at higher AC frequencies, more pulses are needed to reach the optimum sensitivity. The high frequency regime corresponds to short time intervals between control pulses during which time there is very little contrast lost due to decoherence, i.e. τ ac  is much less than T 2 . More pulses increase the sensitivity by allowing for a longer measurement time and subsequently more phase accumulation per measurement. Therefore, multi-pulse sequences are more effective at enhancing magnetometry sensitivity in the high-frequency regime, where the Hahn Echo scheme provides relatively poor magnetic field sensitivity. 
         [0042]    In one embodiment of the present disclosure, the AC magnetic field sensitivity was measured for a 30 μm 3  sensing volume of a diamond sample with NV ˜0.6 ppb and N ˜1 ppm, with ˜10 3  sensing NV spins.  FIG. 5  illustrates examples of measured normalized fluorescence signals as functions of AC field magnitude b ac  using a Hahn Echo sequence, and using multipulse XY sequences with 28 and 54 control pulses. As seen in  FIG. 5 , the uncertainty in the measured signal (δS) limits the uncertainty in the extracted magnetic field magnitude (δB). The sine behavior of the signal with respect to b ac  is achieved by shifting the phase of the last microwave pulse by 90° from what is shown in  FIG. 3 . 
         [0043]      FIG. 6  provides a comparison of calculated (lines) and measured (points) sensitivity, at several AC magnetic field frequencies. As seen in  FIG. 6 , NV multi-spin measurements confirm that multi-pulse dynamical decoupling outperform the Hahn Echo scheme over a wide range of AC magnetic field frequencies, in agreement with theoretical expectations. The enhancement in magnetic field sensitivity provided by multi-pulse dynamical decoupling is especially pronounced at frequencies higher than the Hahn Echo 1/T 2  coherence. 
         [0044]    In summary, methods and systems have been described for the application of dynamical decoupling to multi-spin systems, in particular large ensembles of NV centers. Multi-pulse dynamical decoupling sequences have been disclosed that can extend the coherence lifetime of large numbers of NV electronic spins in room temperature diamond, by about an order of magnitude, for samples with widely differing NV densities and spin environments. 
         [0045]    Using the methods and systems described in the present application, NV multi-spin coherence time greater than about 2 ms can be realized. This is comparable to the best results from application of dynamical decoupling to single NV centers. Multi-pulse dynamical decoupling improves NV multi-spin AC magnetic field sensitivity relative to the Hahn Echo scheme, with about a ten-fold enhancement for higher frequency fields. 
         [0046]    Nothing that has been stated or illustrated is intended to cause a dedication of any component, step, feature, object, benefit, advantage, or equivalent to the public. While the specification describes particular embodiments of the present disclosure, those of ordinary skill can devise variations of the present disclosure without departing from the inventive concepts disclosed in the disclosure. 
         [0047]    While certain embodiments have been described, it is to be understood that the concepts implicit in these embodiments may be used in other embodiments as well. In the present disclosure, reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” All structural and functional equivalents to the elements of the various embodiments described throughout this disclosure, known or later come to be known to those of ordinary skill in the art, are expressly incorporated herein by reference.