Quantum pattern recognition on real quantum processing units

At a Glance

Metadata	Details
Publication Date	2023-04-04
Journal	Quantum Machine Intelligence
Authors	Sreetama Das, Jingfu Zhang, Stefano Martina, Dieter Suter, Filippo Caruso
Institutions	University of Florence, TU Dortmund University
Citations	23
Analysis	Full AI Review Included

Technical Documentation & Analysis: Quantum Pattern Recognition via Destructive Swap Test

This document analyzes the research paper “Quantum pattern recognition on real quantum processing units” (Das et al., 2023), focusing on the material science requirements for the Nitrogen Vacancy (NV) center experiments and aligning them with 6CCVD’s advanced MPCVD diamond capabilities.

Executive Summary

The research validates the Destructive Swap Test (DST) as a robust protocol for quantum pattern recognition, specifically addressing the high noise inherent in Noisy Intermediate-Scale Quantum (NISQ) devices.

Noise Mitigation: The DST protocol significantly reduces the number of required quantum gates, leading to improved performance and noise robustness compared to the standard swap test, particularly for states encoded using three or more qubits.
Quantum Platform Validation: The protocol was successfully demonstrated on both cloud-based superconducting qubits (IBMQ) and an experimental setup utilizing Nitrogen Vacancy (NV) centers in diamond.
Material Limitation Identified: Experimental fidelity in the NV center setup was limited by the electron spin dephasing time ($T_2^* \approx 35$ µs), highlighting the critical need for ultra-high-purity diamond substrates.
Application Potential: DST was shown to be effective for distinguishing patterns in binary images, grayscale MNIST numbers, and complex biomedical images (MRI of human blood vessels), opening pathways for quantum machine learning in specialized fields.
6CCVD Value Proposition: 6CCVD specializes in the high-purity Single Crystal Diamond (SCD) required to maximize NV center coherence times ($T_2$ and $T_2^*$), directly addressing the primary material constraint identified in the study.

Technical Specifications

The following hard data points were extracted from the experimental and simulation results, focusing on the NV center platform and general protocol performance.

Parameter	Value	Unit	Context
Maximum Qubit Count (Swap Test)	2	qubits	Maximum for reliable performance on NISQ devices
Maximum Qubit Count (DST)	3	qubits	Improved performance demonstrated using DST
NV Center Electron Spin States	$m_s = 0, -1$	State	Qubit 1 in the diamond NV setup
NV Center Coupled Spin	¹³C	Spin	Qubit 2 in the diamond NV setup
MW Pulse Rabi Frequency	2	MHz	Used for spin manipulation in NV experiment
Maximum Total MW Pulse Duration	16.3	µs	NV experiment pulse sequence length
NV Center Electron Spin Dephasing Time ($T_2^*$)	$\approx 35$	µs	Material limitation affecting experimental fidelity
Readout Error (Simulation Baseline)	0.01	Fraction	Typical error magnitude in real quantum processors
Lowest Gate Error (Real Processors)	10^-3	Fraction	Order of magnitude for lowest gate errors
SCD Polishing Requirement (6CCVD Standard)	< 1	nm	Surface roughness (Ra) for optimal NV performance

Key Methodologies

The pattern recognition protocol relies on quantum state comparison, implemented via the noise-robust Destructive Swap Test (DST).

Image Encoding (QPIE): Classical 2D images (grayscale or binary) are encoded into quantum states using Quantum Probability Image Encoding (QPIE), where pixel positions correspond to basis vectors and pixel values correspond to the state coefficients.
Protocol Selection: The standard swap test was rejected due to high noise for n > 2 qubits. The DST was adopted, requiring fewer gates (CNOT and Hadamard only) and eliminating the auxiliary qubit, significantly reducing circuit depth and noise sensitivity.
Scaling via Segmentation: To handle larger images (e.g., 32 x 32 pixels) on limited 5-7 qubit processors, images were divided into small segments (e.g., 2 x 2 pixels). The average overlap ($I_{avg}$) across all segment pairs was used as the distance metric.
NV Center Implementation: The DST was realized experimentally using a two-qubit system in a diamond NV center: the electron spin ($m_s = 0, -1$) and a coupled ¹³C nuclear spin. The circuit was implemented using precise Microwave (MW) pulse sequences and detected via laser readout.
Noise Analysis: Robustness was studied against depolarizing, amplitude damping, and phase damping noise channels, confirming that DST remains faithful for pattern recognition despite NISQ noise levels.

6CCVD Solutions & Capabilities

The successful implementation of quantum pattern recognition using NV centers in diamond underscores the critical role of high-quality MPCVD diamond materials. 6CCVD provides the specialized substrates necessary to advance this research, particularly by mitigating the decoherence limitations observed in the study ($T_2^* \approx 35$ µs).

Research Requirement	6CCVD Material Solution	Technical Specification & Sales Advantage
Ultra-Low Defect Density (Maximizing $T_2^*$)	Optical Grade Single Crystal Diamond (SCD)	SCD substrates are grown with ultra-low nitrogen content (< 1 ppm), minimizing paramagnetic defects that cause decoherence. This directly extends the $T_2$ and $T_2^*$ coherence times required for complex DST pulse sequences.
Extended Coherence Time (Beyond 35 µs)	Isotopically Purified SCD	We offer diamond with high isotopic purity (e.g., < 1% ¹³C). Removing the ¹³C nuclear spin bath is essential for achieving $T_2$ times in the millisecond range, enabling higher fidelity and more complex quantum algorithms.
Custom Qubit Platform Integration	Custom Dimensions & Thickness	6CCVD provides SCD plates from 0.1 µm to 500 µm thick, and substrates up to 10 mm. We support custom dimensions up to 125 mm (PCD) for scalable quantum processor arrays.
On-Chip Microwave Control	Advanced Custom Metalization	We offer in-house deposition of critical metals (Au, Pt, Pd, Ti, W, Cu). This capability is vital for researchers needing to fabricate integrated microwave antennas and control lines directly onto the diamond surface for high-speed spin manipulation.
Optimal Optical/MW Coupling	Precision Polishing (Ra < 1 nm)	Our SCD surfaces are polished to an atomic smoothness (Ra < 1 nm). This minimizes surface scattering and defects, ensuring efficient optical collection of NV fluorescence and stable microwave coupling, crucial for high-fidelity readout.

Applicable Materials

To replicate or extend the NV center research detailed in Section 6, 6CCVD recommends:

Optical Grade SCD: Essential for creating high-performance NV centers with long coherence times.
Isotopically Purified SCD: Necessary for pushing the $T_2^*$ limit beyond the 35 µs reported, enabling the execution of longer, more complex quantum circuits required for higher-dimensional pattern recognition.

Customization Potential

The study implies the need for precise integration of control electronics (MW pulses). 6CCVD offers:

Custom Metalization Stacks: We can deposit multi-layer metal stacks (e.g., Ti/Pt/Au) tailored for specific microwave circuit designs and bonding requirements.
Precision Laser Cutting: Custom substrate shapes and precise dimensions can be achieved via our laser cutting services, ensuring seamless integration into existing cryogenic or optical setups.

Engineering Support

6CCVD’s in-house PhD team specializes in the material science of quantum defects. We can assist researchers in selecting the optimal diamond specifications (purity, orientation, thickness, and surface termination) for similar quantum pattern recognition and NV-based quantum sensing projects, ensuring the material platform does not become the limiting factor in experimental fidelity.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1007/s42484-022-00093-x