Coupling a single nitrogen-vacancy center with a superconducting qubit via the electro-optic effect

At a Glance

Metadata	Details
Publication Date	2018-05-15
Journal	Physical review. A/Physical review, A
Authors	Changhao Li, Fuli Li
Institutions	Xi’an Jiaotong University
Citations	13
Analysis	Full AI Review Included

6CCVD Technical Analysis: Hybrid NV-SC Qubit Coupling via Electro-Optic Effect

This document analyzes the technical requirements and achievements detailed in the research paper, “Coupling a single NV center with a superconducting qubit via the electro-optic effect,” and maps them directly to 6CCVD’s advanced Monocrystalline Plasma Chemical Vapor Deposition (MPCVD) diamond solutions.

Executive Summary

This paper presents a robust theoretical scheme for integrating a single Nitrogen Vacancy (NV) spin qubit (quantum memory) with a Superconducting (SC) qubit (quantum processor) using a coherent electro-optic interface.

Hybrid Qubit Architecture: The system couples an NV center (in diamond) to an optical WGM micro-cavity and an SC qubit to a microwave resonator, with the two resonators bridged by the electro-optic effect.
High-Fidelity Quantum State Transfer (QST): By utilizing red-sideband laser driving, the protocol achieves QST with a simulated fidelity of up to 0.94.
Entanglement Generation: Blue-sideband laser driving induces two-mode squeezing, generating highly entangled states between the NV center and the SC qubit with a concurrence of up to 0.77.
Critical Diamond Requirement: Successful implementation relies on high-quality Single Crystal Diamond (SCD) that hosts NV centers with extremely long coherence times ($T_1$).
Cavity Performance: Requires state-of-the-art optical cavity Q factors (Q > 10¹⁰), demanding atomically smooth diamond surfaces (Ra < 1nm) for optimal evanescent coupling.
Practicality: The scheme is achievable with current experimental techniques, requiring only standard millikelvin temperatures (10 mK) for the SC qubit and avoiding additional ground state cooling.

Technical Specifications

The feasibility of this quantum communication scheme is critically dependent on achieving high performance in both the SCD NV material and the integrated resonator components.

Parameter	Value	Unit	Context
Maximum QST Fidelity	0.94	(Unitless)	Achieved at optimal coupling/time ($t = 2.98 / g$)
Maximum Entanglement Concurrence	0.77	(Unitless)	Achieved at optimal coupling/time ($t = 2.92 / g$)
Required Optical Q Factor ($\kappa_1$)	10¹⁰ to 10¹²	(Unitless)	WGM Cavity Quality Factor for low photon loss
Target Photon Loss Rate ($\kappa_1 / 2\pi$)	0.5	MHz	Lowest reported rate achievable with slow-light enhancement
SC Qubit Decay Rate ($\gamma_2 / 2\pi$)	3.5	kHz	Based on reported 44 µs energy relaxation time
NV Zero-Field Splitting ($D_g$)	2.87	GHz	Natural splitting used to define spin states
EOM Electro-Optic Coefficient ($n^3 r$)	300	pm/V	Characteristic of required materials (e.g., Lithium Niobate)
EOM Thickness ($d$)	10	µm	Estimated dimension for electro-optic coupling
Operating Temperature	10	mK	Required for suppressing thermal microwave photon occupation in GHz range

Key Methodologies

The proposed protocol uses a highly controlled hybrid setup relying on precision material interfaces and tailored laser driving protocols.

System Setup: A single NV center is secured onto the exterior surface of a WGM optical micro-cavity. A Superconducting (SC) qubit is coupled to a microwave resonator.
Inter-Resonator Coupling: The optical cavity and microwave resonator are coupled via an Electro-Optic Material (EOM) embedded within the WGM structure (similar to optomechanical coupling via radiation pressure).
NV Center Level Control: The NV center is modeled as a three-level ($\Lambda$-level) system (two ground states $|0\rangle, |1\rangle$ and one excited state $|e\rangle$), with the excited state adiabatically eliminated under large detuning to leverage the long coherence time of the ground spin states.
Quantum State Transfer (QST) Protocol:
- Utilizes red-sideband laser driving ($\Delta = \omega_a - \omega_L = \omega_b$).
- The resulting effective Hamiltonian assumes a beam-splitter form interaction term, enabling efficient photon hopping and state transfer between the qubits.
Entanglement Generation Protocol:
- Utilizes blue-sideband laser driving ($\Delta = \omega_a - \omega_L = -\omega_b$).
- The resulting effective Hamiltonian includes a two-mode squeezing form term, enabling simultaneous creation or annihilation of photons/quanta, leading to entanglement.
Coupling Enhancement: The weak intrinsic electro-optic coupling ($g_i$) is enhanced by strong laser driving through the linearization approximation ($a \to a_a + \delta a$), achieving effective coupling strength $G_i$ on the order of MHz, comparable to the NV-cavity and SC-resonator strengths.

6CCVD Solutions & Capabilities

Replicating and extending this high-coherence, hybrid quantum system requires diamond materials that meet stringent specifications in purity, surface finish, and integration readiness. 6CCVD is uniquely positioned to supply the necessary foundation materials and technical support.

Applicable Materials

To achieve the necessary coherence and coupling characteristics detailed in this research, Optical Grade Single Crystal Diamond (SCD) is mandatory.

Optical Grade SCD: Essential for minimizing nitrogen and defect concentrations (< 1 ppb N), which is critical for maximizing the $T_1$ and $T_2$ coherence times of the integrated single NV center. The low strain inherent in 6CCVD SCD promotes stable spin properties.
Substrate Quality: SCD wafers must be provided with excellent crystal orientation control for consistent NV formation and minimal surface defects, ensuring high-Q factor cavity fabrication.

Customization Potential

The experimental setup requires integrating the NV diamond material with complex WGM micro-cavity structures and metal electrodes for the SC qubit interface.

Research Requirement	6CCVD Engineering Capability	Technical Advantage
Ultra-Smooth Surface (WGM Coupling)	Precision Polishing (Ra < 1 nm): SCD wafers are polished to achieve atomic-level smoothness, essential for minimizing light scatter and maximizing the optical Q factor (target Q > 10¹⁰).	Ensures efficient evanescent field coupling between the NV center and the WGM cavity mode.
Electrodes & Device Interface	Custom Metalization: We offer internal deposition of standard superconducting (Ti, W) and contact (Pt, Au, Cu) metal stacks required for creating the top- and ground-electrodes for the EOM capacitor and interfacing with the SC qubit’s MW resonator.	Providing integrated, ready-to-use substrates, reducing fabrication steps and interface risks for the client.
Unique Geometry (Micro-disk/WGM)	Custom Dimensions and Shaping: Our capability supports plates/wafers up to 125 mm (PCD) and SCD up to large scales, with precision laser cutting for creating custom pre-forms necessary for WGM micro-disk fabrication.	Enabling scaling from proof-of-concept components to integrated, inch-size quantum chips.
Material Thickness Control	Precise Thickness Control (0.1 µm - 500 µm): Deliver SCD materials at exact, engineer-specified thicknesses required for optimizing optical coupling lengths and integration geometry.	Supports stringent design constraints inherent in WGM and electro-optic device geometries (e.g., matching the implied 10 µm EOM thickness).

Engineering Support

6CCVD’s in-house team of material scientists and PhD engineers are specialized in defect engineering and surface preparation for quantum applications. We can assist researchers with:

Optimizing NV creation recipes (implantation or growth) based on the supplied SCD material specifications.
Consultation on surface etching techniques to prepare diamond for low-loss bonding or direct WGM fabrication.
Selecting the ideal diamond purity and strain characteristics for high-coherence NV spin qubit projects, ensuring compatibility with millikelvin superconducting circuits.

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

View Original Abstract

We propose an efficient scheme for transferring quantum states and generating\nentangled states between two qubits of different nature. The hybrid system\nconsists a single nitrogen vacancy (NV) center and a superconducting (SC)\nqubit, which couple to an optical cavity and a microwave resonator,\nrespectively. Meanwhile, the optical cavity and the microwave resonator are\ncoupled via the electro-optic effect. By adjusting the relative parameters, we\ncan achieve high fidelity quantum state transfer as well as highly entangled\nstates between the NV center and the SC qubit. This protocol is within the\nreach of currently available techniques, and may provide interesting\napplications in quantum communication and computation with single NV centers\nand SC qubits.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physreva.97.052319