The detection and modulation of single quantum states and molecular scale imaging technology are important directions in the development of precision spectroscopy instruments. With the in-depth exploration of magnetic detection technology, CIQTEK independently produced and developed a quantum diamond single spin spectrometer, based on the spectroscopic technology of nitrogen-vacancy center in diamond, which has super high magnetic detection instinct and has wide and important application prospects in different disciplines such as physics, chemistry, biology, materials, and medicine [1-11].
Development of Magnetometry Technology
Spin magnetic resonance technology is by far one of the most developed and widely used conventional techniques. Magnetic detection-related spectrometers have a long history of development, and there are different methods to achieve magnetic resonance detection which have their own advantages and disadvantages. Figure 1 visualizes the distribution of several general technical means such as Hall sensors, SQUID detectors, and the spin magnetic resonance in terms of sensitivity and resolution [12]. Compared with the conventional magnetometry techniques, the diamond-based magnetic resonance method has a large improvement in both core metrics, which provides a strong reference for the development of a quantum diamond single-spin spectroscopy.
Figure 1: Comparison of the indicators of various magnetometry techniques
Hall sensors have been commonly used in laboratory magnetic field measurements since the 1950s. These detectors are based on the Hall effect for direct measurements of external magnetic fields [13]. When the direction of the magnetic field is different from the direction of the current in the loop, the electrons in the conductor are deflected due to the Lorentz force, and a potential difference is generated, through which the magnitude of the magnetic field is directly measured. Magnetic field probes have mainly consisted of semiconductor crystals that are able to be made into monolithic integrated circuits, which are shock resistant and easy to use but are not accurate enough.
Superconducting quantum interferometer (SQUID) is a magnetic flux sensor based on Josephson junctions [14], which can measure weak magnetic signals using the variation of the voltage across the Josephson junction with the external magnetic flux in the closed loop.In the 1960s, Robert et al. successfully developed SQUID.Such magnetometry techniques have high magnetic detection sensitivity, but the instrument needs to operate in a low-temperature environment and expensive.
Microscopic magnetic detection based on the diamond system is the emerging method for magnetic resonance detection. The technique combines the optical detection magnetic resonance technique (ODMR) and the point defects of nitrogen-vacancy (NV) color centers in diamond, which works by preparing NV color centers as quantum interferometers and using dual resonance techniques to achieve highly sensitive and spatially resolved magnetic signal detection. This technique does not require low temperature and high vacuum extreme chemical conditions to work properly, and it has higher commercial applications compared to the previous several magnetometry techniques.
High-resolution and high-sensitivity measurements of magnetic fields are of great value in the field of engineering technology. The currently available detection means can no longer meet the needs of microscopic magnetic resonance for the development of high-resolution and high-sensitivity technology, for example, in the imaging of microscopic scale, the spatial resolution and probe size of techniques such as atomic force microscopy (AFM) and scanning tunneling microscopy (STM) are comparable. Therefore, to achieve high spatial resolution, a single atom is the best choice, and the use of quantum interferometry, which converts the weak magnetic signal into phase, can achieve high-sensitivity magnetic signal detection.
Development of Magnetometry Technology
According to the literature, the spatial resolution of the NV color-centered single-spin system can reach less than 5 nm [15] and the maximum magnetometric sensitivity can reach [16], which makes the NV color-centered system a strong candidate for high-resolution magnetic detection. The diamond NV color center can have a coherence time of the order of ms at room temperature, can be localized to an accuracy of less than 10 nm, the electron spins are very sensitive to external magnetic fields, and the distance between the NV color center and the sample can be less than 5 nm. Therefore, NV centers can be made into a very powerful single-quantum sensor.
The NV center has a multi-electron state energy level structure [17], and the NV center in the excited state energy level has two competing deexcitation paths: spontaneous radiative leap back to the ground state and inter-system crossing relaxation to the ground state. While the probability of occurrence of these two reaction paths depends on the spin state of the NV center ground state, the probability of the spin state |ms = 0⟩ can be read out by collecting the fluorescence signal, and the NV center can be initialized by optical resonance excitation. More importantly, when the electron spin is in the superposition state, the kinetic evolution in the presence of an external magnetic field accumulates the relative phase, thus correlating the collected fluorescence signal with the magnetic field magnitude.
In 2008, Lukin's group and Wrachtrup's group discovered almost simultaneously that NV centers have excellent magnetic field sensing ability and proposed that the NV center system can be used for high-resolution and high-sensitivity magnetic measurements [18-19]. In 2012, Wrachtrup et al. experimentally verified the principle of single-core spin detection [20]. In 2013, the literature reported 5 nm microscopic NMR using diamond NV centers as probes for proton detection in organic samples [21]. Therefore, the application of diamond NV-centered single-spin system in sensing and detection is gradually developing to be realistically feasible as an emerging technology in the history of magnetic detection, and the development of relevant spectroscopic instruments is imminent.
Development of Magnetometry Technology
Figure 2: Current Status of Commercial Spectrometers
As shown in Figure 2, the related MR products developed and produced by the leading global technology companies in the market, such as Bruker, Siemens, and Philips, are based on traditional MR technologies, such as NMR (nuclear magnetic resonance), EPR (electron paramagnetic resonance), MRI (nuclear resonance imaging), and other MR spectrometers. However, there is no commercially available MR spectrometer based on the principle of diamond NV center single spin system in the market.
Figure 3: CIQTEK Quantum Diamond Single Spin Spectroscopy
At present, CIQTEK has mastered the core technology based on the NV center system and has a mature manufacturing process, and successfully developed the quantum diamond single spin spectrometer, the physical picture of the spectrometer appearance is shown in Figure 3. It realizes quantum manipulation and readout of the spins of NV-centered luminescence defects of diamonds by controlling the basic physical quantities of light, electricity, and magnetism using ODMR technology. Compared with conventional paramagnetic resonance and nuclear magnetic resonance, it has the following features:
1. The initial state is a quantum pure state, easy to initialize, manipulate and read out. The base electron spin state of the NV center can be initialized and read out by optical leap, and the quantum state can be manipulated by using the microwave.
2. The long spin-quantum coherence time. The long coherence time can ensure longer coherent manipulation and optical signal accumulation.
3. Ultra-high sensitivity and ultra-high resolution. Due to the optical properties of NV centers and their electronic wave function characteristics, the sensitivity of the prepared single-quantum interferometer to measure magnetic fields can reach the order of 10-9 T. The NV center system synthesis even reaches the order of 10-13 T. The spatial resolution of magnetic field measurement can reach the sub-nanometer.
4. It can be operated under room temperature atmospheric conditions and has good compatibility for biological samples.
5. With high fidelity quantum spin state modulation technology, the self-developed 50 ps time precision pulse generator and broadband high-power microwave modulation device can realize the low noise, efficient and fast quantum coherent manipulation of spin. Figure 4 shows the topology of the device. The spectrometer is equipped with highly intelligent control and signal acquisition software, which can realize automatic optical path adjustment, automatic magnetic field adjustment, and long-time unattended automatic sample measurement experiments.
Figure 4: Schematic Diagram of the Instrument System Architecture
CIQTEK R&D team has a perfect preparation process for high-quality diamond probes, and can independently prepare diamond probes with long coherence time and high stability, which can achieve a higher technical index than similar products.
Based on the various advantages of the above NV center system, this technology has more mature applications in quantum computing, magnetic detection, electrical detection, and biological detection. In the field of quantum computing, NV centers can be used as very good room-temperature solid single-spin materials for quantum information storage and modulation [1-5]. For example, the D-J algorithm, large number decomposition algorithm, etc. have been demonstrated using the NV center system, which brings great help to the improvement of computational efficiency.
In the field of precision measurement, the precision measurement technology based on diamond nitrogen-vacancy color centers enables the precision measurement of physical quantities such as electric field, magnetic field, temperature, and stress, and empowers various industries such as scientific research, education, energy, security, health, and industry. For example, in the field of biomedicine, magnetic field [6], temperature detection [7], and nerve unit potential detection [8] of living cells; in the field of materials science, ODMR technology is used to realize the study of optical properties and geometric structure of different materials [9-11].
Diamond NV center as the core of quantum diamond single spin spectrometer emerged in the field of magnetic detection to meet the future commercial demand for high resolution and high sensitivity of magnetic resonance imaging. With the development of micro and nano processing technology and the further improvement of spectrometer performance, more and more interdisciplinary applications are being explored. It is believed that the quantum precision measurement technology of NV center will be widely promoted at home and abroad in the near future, and the prospect is expected.
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