Decoherence mitigation by real-time noise acquisition
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-08-04 |
| Journal | Journal of Applied Physics |
| Authors | G. Braunbeck, Maximilian Kaindl, A. M. Waeber, Friedemann Reinhard |
| Institutions | University of Rostock, Technical University of Munich |
| Citations | 1 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Core Value Proposition: The research introduces Quasi-Feedforward (QFF) decoupling, a scheme that neutralizes the dephasing effect of classical noise on qubits by combining dynamical decoupling with real-time noise acquisition.
- Methodology: QFF measures the integral of classical noise (e.g., current fluctuations) during the qubit evolution and conditions the phase of the final projection pulse accordingly. This correction was implemented via post-selection rather than active real-time control.
- Performance Achievement: The method successfully recovered the full intrinsic coherence time (T2) of a Nitrogen-Vacancy (NV) center qubit, achieving a sevenfold increase in T2 (e.g., from 55 ns to 366 ns) under strong current noise.
- Gate Fidelity: Single-qubit gates were demonstrated with an infidelity of approximately 10-2, even when driven by noisy sources. The theoretical limit, set by the 14-bit digitization hardware, is estimated to be 10-5.
- Benchmark Results: QFF extended the maximum coherent phase accumulation by 38% in benchmark tests, enabling 38% more full oscillations compared to uncorrected measurements.
- Hardware Requirements: The scheme relies on fast acquisition hardware (500 MS/s, 14-bit resolution) to record noise simultaneously with qubit evolution.
- Anticipated Impact: The technique is expected to find widespread use in experiments requiring fast control pulses driven by strong currents, particularly in nanoscale Magnetic Resonance Imaging (MRI), which demands 100 mA peak currents and 100 MHz bandwidth.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Achieved Gate Infidelity | ~10-2 | N/A | General performance across tested schemes. |
| Estimated Ultimate Infidelity Limit | 10-5 | N/A | Set by 14-bit digitization noise. |
| Intrinsic Coherence Time (T2, Sample 1) | 1.4 ± 0.1 | ”s | Unperturbed Hahn echo (Figure 2). |
| Coherence Time (T2, No Correction) | 55 ± 13 | ns | With random current pulse noise (Figure 3). |
| Coherence Time (T2, QFF Corrected) | 366 ± 133 | ns | Sevenfold increase, recovering intrinsic T2 (Figure 3). |
| Coherence Time (T2, Benchmark Uncorrected) | 12.7 ± 0.5 | ”s | Fixed 32 ”s free evolution (Figure 6). |
| Coherence Time (T2, Benchmark Corrected) | 17.6 ± 0.7 | ”s | 38% increase in coherent oscillations (Figure 6). |
| Qubit-Current Coupling (Î0) | 9.48 ± 0.08 | rad/(mA·”s) | Calibration factor. |
| Current Fluctuation (Noise) | 1 | mArms | Shortens T2 to 50 ns without mitigation. |
| Rabi Frequency (ΩÏ) | Up to 300 | MHz | Achieved using 100 mA current. |
| Required Peak Current (Nanoscale MRI) | 100 | mA | Target application requirement. |
| Required Bandwidth (Nanoscale MRI) | 100 | MHz | Target application requirement. |
| Classical Driver Bandwidth Limit | Less than 1 | MHz | Limit of low-noise drivers (e.g., Libbrecht-Hall). |
| Acquisition Hardware Resolution | 14 | bit | Oscilloscope digitization accuracy. |
| Acquisition Rate | 500 | MS/s | Oscilloscope speed (Spectrum M4i.4451-x8). |
| Current Path Uncertainty (Estimated) | 25 | nm | Potential source of residual 10-2 infidelity. |
Key Methodologies
Section titled âKey MethodologiesâThe Quasi-Feedforward (QFF) decoupling scheme was implemented using a single NV center qubit coupled to a microfabricated gold wire structure.
- Qubit and Coupling: A single NV center in diamond was placed in close (”m) proximity to a gold wire. Current in the wire generates a strong magnetic field, enabling high Rabi frequencies (up to 300 MHz) but also strong coupling to electronic current noise.
- Pulse Sequence: A modified Hahn Echo sequence (Init - Ï/2 - Ï - Ï - Ï - Ï/2 - Readout) was used. A current pulse of fixed amplitude but varying or random duration (T1) was applied during the second free evolution period (Ï).
- Noise Acquisition Setup:
- Current flow was measured using a differential probe (Yokogawa 701920) monitoring the voltage drop across a 50 Ω series resistor.
- The voltage signal was recorded by a high-speed oscilloscope (Spectrum M4i.4451-x8) operating at 500 MS/s with 14-bit digital resolution.
- QFF Implementation (Post-Selection):
- For each experimental repetition, the integrated current (⫠I(t) dt) was measured, which directly yields the accumulated phase shift (Ί) on the qubit.
- The phase of the final Ï/2 projection pulse was discretized into four quadrants (x+, y+, x-, y-).
- Instead of real-time control, post-selection was used: only measurements where the measured noise-induced phase (Ί) matched the required phase shift of the final projection pulse (to maximize the echo signal) were retained.
- Self-Learning Calibration: An advanced scheme was demonstrated where the qubit-current coupling constant (Î0) was determined post-measurement by recording the integrated current and free evolution time (Ï) simultaneously, eliminating the need for pre-calibration steps.
Commercial Applications
Section titled âCommercial ApplicationsâThe QFF decoupling technique is highly relevant for applications where strong control signals must be applied near qubits, leading to significant noise coupling.
- Nanoscale Magnetic Resonance Imaging (MRI):
- Requirement: High-resolution imaging demands strong magnetic field gradients generated by high-current pulses (100 mA) with high bandwidth (100 MHz).
- Benefit: QFF mitigates the decoherence caused by the inherent noise of these high-power drivers, allowing for the required long coherent exposure times necessary for high spatial resolution (e.g., 25 nm resolution for Fourier imaging).
- Quantum Computing and Control:
- High-Fidelity Gates: Enables the implementation of robust single-qubit gates using noisy electronic sources, bypassing the technical difficulty of building ultra-low-noise, high-bandwidth control electronics.
- Hybrid Qubit Systems: Crucial for protocols involving simultaneous manipulation of multiple qubit species (e.g., NV electron spin and nuclear qubits), where magnetic crosstalk from strong nuclear control pulses (MHz regime) causes electron spin dephasing.
- Solid-State Qubit Architectures:
- Applicable to any solid-state system (like superconducting circuits or quantum dots) where conductors are placed in sub-”m proximity to achieve strong coupling (e.g., for high Rabi frequencies or Cavity-QED), but suffer from electronic noise decoherence.
- Quantum Sensing and Metrology:
- Improves the sensitivity of Hahn echo and related sensing protocols by removing the phase corruption caused by recordable classical noise (like current fluctuations), leaving only the phase gathered by the desired signal.
View Original Abstract
We present a scheme to neutralize the dephasing effect induced by classical noise on a qubit. The scheme builds upon the key idea that this kind of noise can be recorded by a classical device during the qubit evolution, and that its effect can be undone by a suitable control sequence that is conditioned on the measurement result. We specifically demonstrate this scheme on a nitrogen-vacancy center that strongly couples to current noise in a nearby conductor. By conditioning the readout observable on a measurement of the current, we recover the full qubit coherence and the qubitâs intrinsic coherence time T2. We demonstrate that this scheme provides a simple way to implement single-qubit gates with an infidelity of 10â2 even if they are driven by noisy sources, and we estimate that an infidelity of 10â5 could be reached with additional improvements. We anticipate this method to find widespread adoption in experiments using fast control pulses driven from strong currents, in particular, in nanoscale magnetic resonance imaging, where control of peak currents of 100 mA with a bandwidth of 100 MHz is required.