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6CCVD Research

Quantum nonlinear spectroscopy of single nuclear spins

At a Glance

Section titled ā€œAt a Glanceā€
MetadataDetails
Publication Date2022-09-09
JournalNature Communications
AuthorsJonas Meinel, Vadim Vorobyov, Ping Wang, Boris Yavkin, Matthias Pfender
InstitutionsMax Planck Institute for Solid State Research, Chinese University of Hong Kong
Citations21
AnalysisFull AI Review Included

Executive Summary

Section titled ā€œExecutive Summaryā€
  • Core Innovation: Demonstration of Quantum Nonlinear Spectroscopy (QNS) using a Nitrogen-Vacancy (NV) center in diamond as a quantum sensor, enabling the extraction of arbitrary types and orders of correlations in a quantum system.
  • Key Achievement (Correlation Order): Successful measurement of fourth-order correlations of single nuclear spins (13C), a feat inaccessible by conventional nonlinear spectroscopy relying on classical electromagnetic probes.
  • Methodology: Utilizes sequential weak measurement combined with Knill Dynamical Decoupling (KDD-XY5) and the 14N nuclear spin as a quantum memory to enhance readout fidelity (single-shot readout capability).
  • Noise Differentiation: Higher-order correlations (specifically the third moment 2D spectrum) provide unique ā€œfingerprint featuresā€ that unambiguously differentiate quantum spins from classical noise sources (e.g., Gaussian noise or random-phased AC fields), which are often indistinguishable in standard second-order correlations.
  • Quantified Spin Counting: The QNS method allows for the discrete determination of the number of uniformly coupled nuclear spins (N), similar to g(2) measurements in photon statistics, by analyzing the relative height (Ī·) of specific spectral peaks.
  • Quantum Foundation Testing: This work establishes a platform for testing fundamental quantum mechanics, such as higher-order Leggett-Garg inequalities, and studying quantum many-body physics.

Technical Specifications

Section titled ā€œTechnical Specificationsā€
ParameterValueUnitContext
Sensor SystemNV Center in DiamondN/AElectron spin (spin-1) used as sensor.
Target Spin13C Nuclear SpinN/ASingle target spin detected.
Diamond Substrate12C-enriched (99.995%)N/A(111)-oriented, 2 mm x 2 mm x 80 Āµm slice.
External Magnetic Field (B0)250mTAligned parallel to the NV axis.
NV Transition Frequency~4.1GHzBetween the
Electron Spin Rabi Frequency7MHzAt full microwave pulse amplitude.
NV Electron Spin T1 Lifetime~50ĀµsMeasured by Ramsey interference.
NV Electron Spin T2 Coherence~300ĀµsMeasured by spin echo.
Nuclear Zeeman Frequency (Ī½0)~2.6795MHzFrequency of the 13C spin under B0.
Dynamical Decoupling SequenceKDD-XY5N/AKnill pulse sequence used for sensing.
Number of KDD Pulses (Np)100N/AUsed during the interrogation phase.
Interaction Strength (Ī±)0.189piWeak, tuneable entanglement strength.
Inter Pulse Time (Ļ„c)186.68nsIncludes 68.67 ns for the pi pulse length.
Readout Repetitions (M)40N/ALimit imposed to mitigate 13C decoherence during laser readout.
CNOTe Duration~4ĀµsWeak MW pulse for electron-14N SWAP.
CNOTn Duration~50ĀµsConditional RF pulse for electron-14N SWAP.
Laser Pulse Duration (Readout)0.3ĀµsUsed during each readout repetition.

Key Methodologies

Section titled ā€œKey Methodologiesā€

The experiment employs a sequential weak measurement protocol on the NV electron spin, utilizing the 14N nuclear spin as a quantum memory to enhance readout fidelity.

  1. Initialization:

    • The NV center electron spin is optically pumped into the |0> state using a 532 nm laser pulse (green block).
    • The sensor spin is prepared into the superposition state |x> = (| +> + |->)/āˆš2 using a microwave (MW) pi/2 pulse.
    • SWAP gates are used to polarize the 14N nuclear spin, which acts as the quantum memory.

  2. Sensing (Interrogation):

    • The NV electron spin interacts with the target 13C nuclear spin during the interrogation time (tc).
    • A Knill Dynamical Decoupling (KDD-XY5) sequence, consisting of Np = 100 pulses, is applied to modulate the interaction, inducing weak, tuneable entanglement (Ī± ā‰ˆ 0.189 pi).
    • The KDD sequence effectively filters the hyperfine interaction, making the quantum field B(t) from the 13C spin appear as B(t) āˆ [Ixcos(Ī½0t) - Iysin(Ī½0t)].

  3. Weak Measurement:

    • A rotation (MW pulse) is applied to the sensor spin, changing the measurement axis (eĪø) to the z-axis. This realizes the weak measurement of the spin component ĻƒĪø.
    • The measurement angle Īø is set to approximately 54.0037Ā° to maximize the amplitude of the third-order correlation signal.

  4. Readout (Enhanced Fidelity):

    • The sensor spin state is transferred to the 14N nuclear spin memory using a SWAP gate (consisting of CNOTe and CNOTn pulses).
    • The 14N spin state is repeatedly read out (M=40 times) via a CNOT gate and spin-dependent fluorescence of the NV electron spin.
    • The statistical moments (Si, Sij, Sijk) are reconstructed from the collected photon counts.

  5. Correlation Analysis:

    • The measured third-order correlation (Sijk) is analyzed in the 2D Fourier transform domain (S(Ī½ij, Ī½jk)).
    • The resulting spectra show four distinct peaks for the quantum spin, contrasting sharply with the six peaks observed for a classical random-phased AC field, confirming the quantum nature of the noise.

Commercial Applications

Section titled ā€œCommercial Applicationsā€

The demonstrated Quantum Nonlinear Spectroscopy (QNS) technique, enabled by NV center quantum sensors, has significant implications for high-precision metrology and quantum technology development:

  • Quantum Sensing and Metrology:

    • Classical Noise Screening: Isolating and detecting quantum objects (like single nuclear spins) by filtering out classical noise backgrounds, leading to ultrasensitive detection capabilities.
    • Noise Characterization: Providing detailed fingerprint features for unambiguous differentiation between various noise types (Gaussian, AC fields, quantum spins), crucial for optimizing quantum device performance.
    • Nanoscale NMR/ESR: Achieving high-resolution magnetic resonance spectroscopy of single spins or small spin clusters, applicable in structural biology and material science.

  • Quantum Computing and Information:

    • Qubit Characterization: Helping characterize and optimally suppress decoherence sources (noise) in solid-state qubits (like NV centers or superconducting circuits) by providing higher-order noise statistics.
    • Quantum Memory Development: The use of the 14N spin as a quantum memory demonstrates a robust architecture for enhancing readout fidelity in solid-state quantum registers.

  • Fundamental Physics Research:

    • Quantum Foundation Testing: Enabling experimental verification of higher-order quantum correlations, such as testing higher-order Bell inequalities or Leggett-Garg inequalities, with reduced interpretation loopholes.
    • Many-Body Physics: Studying non-Gaussian fluctuations and dynamics in mesoscopic quantum many-body systems (e.g., cold atom systems or spin baths).

Tech Support

Section titled ā€œTech Supportā€

Original Source

Section titled ā€œOriginal Sourceā€

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