Quantum Fourier transform for nanoscale quantum sensing
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2021-08-09 |
| Journal | npj Quantum Information |
| Authors | Vadim Vorobyov, Sebastian Zaiser, Nikolas Abt, Jonas Meinel, Durga Bhaktavatsala Rao Dasari |
| Institutions | University of Stuttgart, Center for Integrated Quantum Science and Technology |
| Citations | 36 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled āExecutive SummaryāThe research details the implementation of the Quantum Fourier Transform (QFT) algorithm to significantly enhance the performance of a diamond Nitrogen-Vacancy (NV) center quantum sensor, primarily for nanoscale Nuclear Magnetic Resonance (NMR) spectroscopy.
- Core Achievement: Successful implementation of the QFT algorithm within a hybrid solid-state quantum register consisting of an NV electron spin (sensor) and three nuclear spins (14N qutrit and two 13C qubits).
- Performance Enhancement: QFT is used as a quantum digitizer, converting acquired phase information into a population basis, which exponentially increases the sensorās dynamic range (proportional to 2n) while maintaining high sensitivity.
- Nanoscale NMR Capability: The QFT-enhanced correlation spectroscopy protocol achieved high spectral resolution (ā70 Hz or 10 ppm) and successfully demultiplexed the signals of two distinct 13C nuclear spins.
- Hybrid System Functionality: The 12-level register (equivalent to ng ā 3.5 effective qubits) allows for the efficient digitization of complex, time-varying signals containing multiple frequency components.
- Algorithm Validation: The protocol overcomes the traditional dynamic range-sensitivity trade-off inherent in single-qubit sensing, making it suitable for saturated sensor regimes (where BrmsĪ³eT2 is greater than 2Ļ).
- Efficiency: The QFT gate sequence adds a relatively small time overhead (ā300 Āµs) compared to the long correlation times (Tc up to 5 ms) required for high-resolution spectroscopy.
Technical Specifications
Section titled āTechnical Specificationsā| Parameter | Value | Unit | Context |
|---|---|---|---|
| Sensor System | NV Center Electron Spin | N/A | Used for phase acquisition (sensing). |
| Electron Spin Coherence Time (T2) | 430 | Āµs | Measured using Hahn echo at ambient conditions (B0 = 0.7 T). |
| Register Composition | 1 Qutrit (14N) + 2 Qubits (13C) | N/A | Hybrid 12-level quantum system (ng ā 3.5 effective qubits). |
| 13C Hyperfine Coupling (Azz) | 414 and 90 | kHz | Stronger and weaker coupled 13C spins, respectively. |
| NMR Spectral Resolution | ā70 (or ā10) | Hz (or ppm) | Achieved in correlation spectroscopy of two target 13C spins. |
| Maximum Detectable Field (Bmax) | 7.2 | ĀµT | Theoretical limit for a 4-qubit register (T2 = 500 Āµs) before saturation. |
| QFT Gate Time Overhead | ā300 | Āµs | Additional time required for QFT gates. |
| Correlation Time (Tc) | 10-20 | ms | Typical time used for high-resolution correlation spectroscopy. |
| Dynamic Range Scaling (QFT) | Exponential (proportional to 2n) | N/A | Improvement over Standard Quantum Limit (SQL) protocols. |
Key Methodologies
Section titled āKey MethodologiesāThe experiment utilizes a modified correlation spectroscopy protocol incorporating the Quantum Fourier Transform (QFT) and its inverse (QFTā ) to function as a quantum analog-to-digital converter (ADC).
- System Initialization: The NV electron spin sensor is initialized to the |0> state. The nuclear spin register is prepared in a uniform superposition state using local Hadamard and Chrestenson gates applied to the initial |000> state.
- Phase Encoding (First Step): A controlled-NOT (CKNOTe) gate entangles the electron spin with the nuclear register. The sensor interacts with the target nuclear spins for interrogation time Ļ, acquiring phase Ī¦1 proportional to the target field.
- QFT Digitization: The acquired phase state of the register (|ĪØ1>) is mapped from the phase basis to the population (bit representation) basis using the QFT algorithm. This step efficiently and unambiguously digitizes the phase.
- Correlation Interval (Tc): The register stores the digitized phase during the long correlation time Tc (up to ā5 ms), during which the target spins undergo free evolution (e.g., Ramsey sequence).
- Phase Acquisition (Second Step): A second interrogation step acquires phase Ī¦2. The inverse QFT (QFTā ) is applied, resulting in a state where the net phase difference ĪĪ¦ = Ī¦1 - Ī¦2 is encoded.
- Readout: A final QFTā operation converts the phase difference back into the population basis (Iz) for single-shot projective measurement, allowing the demultiplexing of multiple target spin signals onto separate register outputs.
- QFT Implementation: The QFT circuit was realized using optimal control techniques, employing generalized qutrit-controlled rotational gates and local Chrestenson gates tailored for the hybrid 12-level NV-nuclear spin system.
Commercial Applications
Section titled āCommercial ApplicationsāThe QFT-enhanced quantum sensing protocol has direct relevance across several high-tech and scientific sectors, particularly those requiring ultra-high sensitivity and wide dynamic range measurements.
- Nanoscale Chemical Analysis:
- Zeptoliter NMR: Enabling chemically resolved NMR spectroscopy on extremely small sample volumes (zeptoliters), critical for analyzing single molecules, proteins, or materials in limited supply.
- In-Situ Correlation Spectroscopy: High-resolution analysis of spin dynamics in solid-state materials and liquid volumes at the nanoscale.
- Quantum Computing and Metrology:
- Quantum Algorithm Benchmarking: The NV-nuclear spin system serves as a robust, room-temperature platform for testing and validating complex quantum algorithms like QFT, which are fundamental building blocks for larger quantum computers.
- High Dynamic Range Sensors: Development of next-generation quantum sensors capable of measuring time-varying signals with high precision across a wide range of amplitudes, overcoming saturation limits.
- Advanced Materials Science:
- Solid-State Defect Characterization: High-resolution magnetic and electric field sensing used to characterize defects and impurities in diamond and other wide bandgap semiconductors.
- Biomedical Imaging and Diagnostics:
- Nanoscale MRI/NMR: Potential for developing highly localized magnetic resonance techniques for cellular or sub-cellular imaging where sample size and coherence time are limiting factors.