DEVICE AND METHOD BASED ON DIAMOND NV CENTERS

Abstract

The invention generally concerns an enhanced process for detecting spin states of nitrogen vacancy centers in diamonds.

Claims

1. A process for enhancing sensitivity in measuring spin state in nitrogen vacancy (NV) centers in a diamond sample, the process comprising applying an optical excitation radiation to a diamond having at least one nitrogen vacancy (NV) center, the radiation comprising light having a wavelength between 400 and 638 nm, illuminating the sample with light having a wavelength between 700 and 1042 nm, and detecting, measuring and/or counting photons emitted from the at least one NV center.

2. A process for enhancing sensitivity in measuring spin state in nitrogen vacancy (NV) centers in a diamond sample, the process comprising: irradiating a diamond having at least one nitrogen vacancy (NV) center with a light having a wavelength between 400 and 638 nm, to thereby excite the NV centers, irradiating the diamond with a light having a wavelength between 700 and 1042 nm, and detecting photons emitted from the at least one NV centers, at wavelengths ranging between 700 and 1050 nm.

3. The process according to claim 1, wherein the step of detecting photons emitted from the at least one NV centers is at wavelengths between 1040 and 1050 nm.

4. The process according to claim 2, further comprising a step of enhancing the fluorescence emission signal.

5. The process according to claim 4, wherein said enhancing fluorescence emission comprises coupling a singlet transition emission to a photonic structure.

6. The process according to claim 5, wherein the photonic structure is an optical antenna, a plasmonic antenna, a hyperbolic metamaterial (HMM) or a photonic crystal cavity.

7. The process according to claim 1, wherein the optical excitation with light in a wavelength between 400 and 638 nm is for a duration between 1 and 3 us.

8. The process according to claim 1, wherein the illuminating with light in a wavelength between 700 and 1042 nm is for a duration between 1 ns and 5 ms or between 1 ns and 1 ms.

9. A device comprising a diamond sample comprising at least one nitrogen vacancy (NV) center, a first illumination source configured and operable to illuminate the diamond sample at a wavelength in a spectral range between 400 and 638 nm, a photon counter, and a second illumination source configured and operable to illuminate the diamond sample at a wavelength in a spectral range between 700 and 1042 nm.

10. A magnetometer device comprising a diamond having at least one nitrogen vacancy (NV) center comprising one or more electronic spins, wherein the electronic spins are configured to align with the diamond crystallographic axis in response to optical excitation radiation applied to the at least one NV center; and a photon counter configured to detect output optical radiation at the IR range correlated with the electronic spins when subjected to an optical enhancement.

11. The device according to claim 9, wherein the photons counter is a device comprising a single-photon detector (SPD).

12. The device according to claim 11, wherein the photon counter is selected from a photodiode, a single photon detector, a superconducting nanowire, a photomultiplier, a Geiger counter, a single-photon valance diode, a transition edge sensor, a scintillation counters and a charge-coupled device.

13. The device according to claim 12, wherein the photons counter is a device comprising a single-photon detector (SPD).

14. The device according to claim 12, wherein the photon counter is selected from a photodiode, a single photon detector, a superconducting nanowire, a photomultiplier, a Geiger counter, a single-photon valance diode, a transition edge sensor, a scintillation counters and a charge-coupled device.

15. The device according to claim 9, further comprising a microwave radiation element, a polarization control element, a light modulation device, a lock-in amplifier, a time tagging element, a data acquisition, a processing device, a sequence generation device, a magnetic field generation element, or an optical element.

16. The device according to claim 10, further comprising a microwave radiation element, a polarization control element, a light modulation device, a lock-in amplifier, a time tagging element, a data acquisition, a processing device, a sequence generation device, a magnetic field generation element, or an optical element.

17. The device according to claim 9, being a magnetometer.

18. The device according to claim 9, being a quantum communication device or a spintronic device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0098] In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0099] FIG. 1 provides an exemplary device for NV centers spin state readout according to some embodiments of the invention.

[0100] FIG. 2 is an energy-level diagram and relevant transitions for the neutral and negatively charged NV center. Green excitation is depicted with green arrows, red fluorescence is depicted with downward red arrows, IR excitation and fluorescence are depicted with purple and red arrows, respectively, nonradiative decay is depicted with blue arrows, ionization and recombination transitions depicted with dashed black arrows.

[0101] FIGS. 3A-D provide graphs of SNR as a function of green excitation power and pulse duration for bulk (FIGS. 3A and 3C) and surface (FIGS. 3B and 3D) NVs, without (FIGS. 3A and 3B) and with (FIGS. 3C and 3D) time normalization.

[0102] FIGS. 4A-E are graphs of IR fluorescence spin-state readout. FIG. 4A depicts the pulse sequence for the IR fluorescence spin readout. FIGS. 4B-E shows SNR as a function of IR excitation power and pulse duration for bulk NV (FIGS. 4B and 4D) and surface (FIGS. 4C and 4E) NVs, with (FIGS. 4D and 4E) and without (FIGS. 4B and 4C) normalization.

[0103] FIGS. 5A-E provide schematic drawings of one of the suggested experimental setups and electric field energy density of the photonic crystal structure. FIG. 5A is a schematic drawing of one of the suggested experimental setups. FIG. 5B is presents photonic crystal structure and electric field near-field energy density. FIG. 5C presents electric field far-field energy level density, showing highly directed emission from the cavity.

[0104] FIGS. 6A-D provide graphs of expected spin state SNR, fidelity and number of photons emitted as a function of Purcell factor under 1 W excitation (inside the cavity), 1 μs readout duration and an optimized delay duration τ=10 ns. FIGS. 6A-B shows spin-state readout SNR (FIG. 6A) and fidelity (FIG. 6B) for bulk (blue lined) and surface (red lines) NVs. The black line illustrates the highest SNR/fidelity possible for bulk NV using red fluorescence. FIGS. 6C-D shows number of photons emitted during the singlet excitation for ms=0 (black lines) and ms=±1 (green lines) for bulk (FIG. 6C) and surface (FIG. 6D) NVs.

[0105] FIGS. 7A-D present graphs of the expected spin-readout SNR and fidelity as a function of Purcell factor and laser power. FIGS. 7A-B are graphs of the expected spin state SNR for bulk (FIG. 7A) and surface (FIG. 7B) NV. FIGS. 7C-D are graphs of the expected fidelity for (FIG. 7A) bulk and (FIG. 7B) surface NVs.

DETAILED DESCRIPTION OF EMBODIMENTS

[0106] In the below examples there is provided a novel calculation of the red fluorescence-based spin state readout, a novel method of reading the NV center's spin state, using the weak fluorescence emitted in the singlet manifold, and calculation of the expected signal-to-noise ratio (SNR) by numerically solving the master equation, for both surface and bulk NVs. From these results, there is described a regime of excitation parameters in which a significant increase of the NV's spin state readout SNR is expected. Finally, an example of utilization of a photonic crystal structure to increase the radiative coupling of the singlet transition is described, which shows that a dramatic enhancement of the spin state SNR can be achieved using this or similar structures, towards a single-shot readout.

Example 1: Calculating the Spin Readout SNR

[0107] The negatively charged NV center consists of 2 adjacent lattice sites occupied by a nitrogen atom and a vacancy inside a diamond crystal. The electronic ground state of the NV center is a spin triplet with a 2.87 GHz zero-field splitting between spin projections m.sub.s=0 and m.sub.s=1. The electronic excited states contain a spin triplet with a strong radiative coupling and a spin singlet with a much weaker radiative coupling. FIG. 2 depicts a simplified energy level diagram of NV.sup.− and NV.sup.0, together with the main transitions. In red fluorescence-based spin state readout scheme, an NV in the triplet ground state (.sup.3A) is excited to the triplet excited state (.sup.3E) using green light, and the red fluorescence during the decay back to the ground state is collected. The number of photons collected from each of the spin states dictates the SNR, which is defined as:

[00001]$\begin{array}{cc}\mathrm{SNR}=\frac{.Math.N_0-N_1.Math.}{\sqrt{N_0+N_1}}& \left(1\right)\end{array}$

where N.sub.1 denotes the number of photons collected when the NV is initialized to its m.sub.s=|i> state, where i can be 0 or 1.

[0108] Herein, the spin readout SNR is calculated using green excitation and red fluorescence (but can also be calculated using green excitation alone), as a function of readout duration and excitation power for a confocal system, for both surface and bulk NVs, assuming perfect collection and detection efficiencies. In addition, fluorescence from NV.sup.0 is ignored, although it overlaps with the NV's. The SNR is calculated numerically, using an eight level model, over a wide range of parameters. The rate equations dictating the populations for FIG. 3, as well as for FIG. 4, are the following:

P.sub.g,0.sup.−=−K.sub.e.sup.−P.sub.e,0.sup.−+K.sub.j.sup.−P.sub.e,0.sup.−+K.sub.sg,0P.sub.s,g+½(Kr.sub.G+Kr.sub.IB)P.sub.e.sup.0

P.sub.g,1.sup.−=−K.sub.e.sup.−P.sub.e,1.sup.−+K.sub.j.sup.−P.sub.e,1.sup.−+K.sub.sg,1P.sub.s,g+½(Kr.sub.G+Kr.sub.IB)P.sub.e.sup.0

P.sub.e,0.sup.−=−(K.sub.j.sup.−+K.sub.es,0+Ki.sub.G+Ki.sub.IR)P.sub.e,0.sup.−+K.sub.e.sup.−P.sub.g,0

P.sub.e,1.sup.−=−(K.sub.j.sup.−+K.sub.es,0+Ki.sub.G+Ki.sub.IR)P.sub.e,1.sup.−+K.sub.e.sup.−P.sub.g,1

P.sub.s,s=−K.sub.ssP.sub.ss+K.sub.ss,0(P.sub.e,0.sup.−+P.sub.e,1.sup.−)+K.sub.sP.sub.x,g

P.sub.s,g=−(K.sub.sg,0+K.sub.sg,1)P.sub.x,g−K.sub.sP.sub.x,g+K.sub.ssP.sub.x,e

P.sub.g.sup.0=K.sub.e.sup.0P.sub.g.sup.0+K.sub.j.sup.0P.sub.x.sup.0+(K.sub.i.sub.G+K.sub.i.sub.IR)(P.sub.e,0.sup.−+P.sub.e,1.sup.−)

P.sub.e.sup.0=−(K.sub.f.sup.0+Kr.sub.G+Kr.sub.IR)P.sub.x.sup.0+K.sub.e.sup.0P.sub.g.sup.0

[0109] In the above equations, P.sup.−.sub.g,0 and P.sup.−.sub.g,1 represent the population in the m.sub.s=0 and m.sub.s=±1 triplet ground states of the negatively charged NV, respectively, P.sup.−.sub.e,0 and P.sup.−.sub.e,1 represent the population in the m.sub.s=0 and m.sub.s=±1 triplet excited states of the negatively charged NV, respectively, P.sup.0.sub.g and P.sup.0.sub.e represent the populations of the neutral charge NV ground and excited states, respectively, and P.sub.s,g and P.sub.s,e represent the populations in the ground and excited singlet states of the negatively charged NV, respectively. K.sup.−.sub.e and K.sup.0.sub.e represent the green laser-induced excitation rates of NV.sup.− and NV.sup.0 ground states to the excited states, respectively, K.sup.−.sub.s represents the IR laser-induced excitation rate from the ground singlet state to the excited singlet state, K.sup.−.sub.f and K.sup.0.sub.f represent the fluorescence rate from the NV.sup.− and NV.sup.0 excited states to their ground states, respectively, K.sup.−.sub.ss, rad and K.sup.−.sub.ss, nonrad represent the radiative and nonradiative decay rates from the excited singlet state to the ground singlet state, respectively, F.sub.p represents the Purcell factor, which enhances the radiative rate, K.sup.−.sub.es,0 and K.sup.−.sub.es,1 represent the decay rates from the triplet excited states to the excited singlet state, respectively, K.sup.−.sub.sg,0 and K.sup.−.sub.sg,1 represent the decay rates from the ground singlet state to the NV.sup.−m.sub.s=0 and m.sub.s=±1 triplet ground states, respectively, K.sub.iG and K.sub.iIR represent the green and IR excitation-induced ionization rates, respectively, and K.sub.rG and K.sub.rIR represent the green and IR excitation-induced recombination rates, respectively.

[0110] The achievable red fluorescence spin-readout SNR, assuming 100% collection and perfect detection without external noise sources (such as dark counts) is illustrated in FIG. 3, where FIGS. 3A and 3B depict the absolute SNR, described in Eq. (1) for bulk and surface NVs, respectively, over a wide range of green excitation powers and readout durations. FIGS. 3C and 3D present the bulk (FIG. 3C) and surface (FIG. 3D) NV SNRs for the same power and duration regimes normalized by the square root of the pulse duration in μs. The significant difference in SNR between bulk and surface NVs stems from differences in the ionization cross section of the .sup.3E.sub.2 levels.

Example 2: Description of IR Fluorescence-Based Spin Readout Scheme

[0111] The pulsed sequence, depicted in FIG. 4A, starts with a short and strong green laser pulse, exciting the NV from the ground state (3A.sub.2) to the excited state (3E.sub.2) and populating the singlet ground state (.sup.1E.sub.1) of almost only m.sub.s=±1 polarized NVs. Next, a short delay (represented by τ) is introduced to avoid undesired ionization from the excited triplet state, followed by a strong and long 980 nm pulse that excites the NV from the ground singlet state (.sup.1E.sub.1) to the singlet excited state (.sup.1A.sub.1) while collecting the emitted 1042-nm fluorescence. Due to the fact that the IR laser does not excite the triplet ground state, no mixing processes are expected, enabling a relatively long measurement. By carefully tuning the green laser pulse power and duration, the sequence can be repeated three times before significant mixing (via the singlet manifold or ionization/recombination processes) takes place, thus enhancing the signal.

[0112] Despite the poor radiative coupling between the .sup.1A.sub.1 and .sup.1E.sub.1 levels, the fast decay rate from the .sup.1A.sub.1 state together with the relatively long shelving time in the .sup.1E.sub.1 state, enable a large number of cycles before the NV decays back to the .sup.3A.sub.2 ground state without risking photo-ionization, allowing for a large enough number of photons to be collected during a single measurement, for high enough excitation powers.

[0113] FIG. 4 depicts the IR fluorescence spin-readout SNR as a function of IR laser power and pulse duration of bulk and surface NVs, with delay duration τ=10 ns (optimized with respect to the excited state lifetime). The laser power and pulse duration are scaled logarithmically to cover all of the relevant parameter space. Perfect collection and detection efficiencies are assumed for comparison with the results shown in FIG. 3. IR-induced ionization is neglected from the singlet state, for which the cross section is currently unknown (but assumed to be small), and consider a radiative to nonradiative coupling ratio of 1/1000. FIGS. 4B and 4C present the calculated absolute SNR for bulk and surface NVs, showing an expected significant enhancement of the spin-state readout SNR compared to the red fluorescence spin-readout scheme for high enough IR power. In addition, in this scheme the SNR grows monotonically with readout duration due to the absence of spin mixing. FIGS. 4D and 4E present the calculated normalized SNR for bulk and surface NVs for the IR fluorescence method, showing that the normalized SNR can reach higher values than that of the red fluorescence spin-readout SNR for bulk and surface NVs, for strong excitation powers.

Example 3: Means for Improving IR Fluorescence Spin-Readout SNR

[0114] To further improve the spin-readout SNR shown in FIG. 4 while reducing the necessary IR excitation power, overcoming the weak fluorescence signal resulting from the nonradiative nature of the .sup.1A.fwdarw..sup.1E decay is needed. Thus, utilization of optical/plasmonic antennas, hyperbolic metamaterials or a photonic crystal cavity is proposed to strengthen the radiative coupling between the .sup.1A and .sup.1E states and thus increase the singlet fluorescence signal.

[0115] Photonic crystal structures with small mode volumes (V≈(λ/n).sup.3) and high-quality factors (high frequency-to-bandwidth ratio in the resonator) are now within reach, and together with the relatively narrow IR fluorescence spectral width, are expected to provide high Purcell factors, especially for nano-diamonds and diamond films, but also potentially for bulk diamonds.

[0116] The Purcell factor, an enhancement of the spontaneous emission rate from the excited state due to radiative coupling, depends on the quality factor and mode volume in the following way:

[00002]$\begin{array}{cc}{F}_P=\frac{3}{4{\pi }^2}{\left(\frac{\lambda }{n}\right)}^3\frac{Q}{V},& \left(3\right)\end{array}$

where λ represents the wavelength, Q represents and quality factor, n represents the refractive index, and V represents the mode volume. In terms of the rate equations, the radiative part of the decay rate is multiplied by the Purcell factor. The fact that only approximately 0.1% of the decay results in photon emission holds great potential for enhancing the signal level and thus the SNR. In addition, the high emission directionality induced by a photonic crystal structure may dramatically increase the collection efficiency, and thus the number of photons detected.

[0117] One of the suggested experimental systems is depicted in FIG. 5A. Green and detuned IR lasers excite the triplet (.sup.3A.sub.2) and singlet (.sup.1E.sub.1) ground states, respectively, while Acousto-Optic Modulators (AOMs) modulate them. Two dichroic mirrors with proper cutoff wavelengths (533 nm-979 nm and 981 nm-1041 nm for the green and IR lasers, respectively) direct the lasers onto the objective and enable fluorescence collection on a single-photon counter module (SPCM), after the unwanted red fluorescence and reflected green and IR lasers are filtered out. The objective focuses the light onto the diamond sample, here illustrated as a nano-diamond, to reach the high intensity IR excitation needed for driving the singlet transition efficiently. FIGS. 5B and 5C illustrate the electric field near-field and far-field energy densities, as well as the photonic crystal cavity structure, optimized for nano-diamonds. The cavity structure is a 250-nm-thick silicone-nitride hexagonal PHC L3 cavity with five neighboring hole positions shifted. For this structure, the refractive index is 2, the lattice constant, a, is 450 nm and hole radius is 125 nm, and the positions of the holes were shifted by 0.315a, 0.35a, 0.118a, 0.205a, and 0.284a. The far-field energy density enables approximately 45% collection efficiency with numerical aperture of 0.95, while the near-field simulations predict a quality factor of about 2650 for this structure. Considering the small mode volume of this structure, 0.27 (λ/n).sup.3, the resulting Purcell factor according to Eq. (3), which is manifested by K.sup.−s in FIG. 2, can reach up to 2343, and thus significantly enhance the emission and the number of photons collected. Simulation over a wide range of wavelengths was performed to verify the existence of a single resonant mode in the spectral range of 500 nm-1100 nm, ensuring no enhancement of the radiative decay of the main transition (.sup.3E to .sup.3A). Similar calculations for diamond membranes and bulk diamonds predict quality factors of up to 13,300 and 790 with mode volumes of 0.38 (λ/n).sup.3 and 0.8 (λ/n).sup.3, respectively, resulting in Purcell factors of up to 8355 for diamond membranes and 235 for bulk diamonds.

[0118] The expected spin-readout SNR and fidelity under 1W of IR excitation (inside the cavity) and a short readout duration (1 ns), as a function of Purcell factor for both surface (red line) and bulk (blue line) NVs are illustrated in FIG. 6. For this calculation, the Purcell factor was manifested by the radiative part of the rate K.sup.−.sub.s in FIG. 2. Based on the figure, the scheme provides a fivefold enhancement of the spin-readout SNR for a feasible Purcell factor of 40, which was already achieved for silicon-vacancy centers, and more than an order of magnitude enhancement for F.sub.p=300 and F.sub.p=1000 (which are significantly lower than the Purcell factors calculated for nano-diamonds and diamond membranes) for bulk and surface NVs, respectively, thus exceeding the single-shot readout threshold. The SNR can reach even higher values for readout duration >1 μs and higher excitation powers. Thus, the magnetic field sensitivity, which obeys the following relation:

[00003]$\begin{array}{cc}\eta \propto \delta B\sqrt{T}\propto \frac{1}{\mathrm{SNR}},& \left(4\right)\end{array}$

could be reduced by more than an order of magnitude as well (where T is the measurement time and δB is the minimum magnetic field that can be measured during this time).

[0119] Presented in FIGS. 6B and 6C is a calculation of the number of photons emitted from the m.sub.s=0 and m.sub.s=±1 spin states as a function of the Purcell factor for the same excitation power during the 1 μs readout duration, showing that a higher number of photons is expected to be emitted during the readout sequence, while the contrast between the two spin states is sustained. FIG. 7 depicts the interplay between the IR excitation power and the Purcell factor in terms of their impact on the spin-readout SNR and fidelity. This is calculated both for bulk and surface NVs, over a wide range of parameters. Although the excitation power and Purcell factor change different parameters in the rate equations, low laser power can be compensated for by a higher Purcell factor and vice versa. Thus, a measurement with 100 mW of laser excitation still produces a sixfold enhancement in the readout SNR for F.sub.p≈1000, according to the simulation.