Quantum interference device for controlled two-qubit operations

At a Glance

Metadata	Details
Publication Date	2020-05-29
Journal	npj Quantum Information
Authors	Niels Jakob Søe Loft, Morten Kjaergaard, Lasse Bjørn Kristensen, Christian Kraglund Andersen, Thorvald W. Larsen
Institutions	ETH Zurich, Quantum Devices (United States)
Citations	15
Analysis	Full AI Review Included

Technical Documentation & Analysis: Quantum Interference Device for Controlled Two-Qubit Operations

This document analyzes the research paper “Quantum interference device for controlled two-qubit operations” (npj Quantum Information (2020)6:47) to extract critical technical specifications and align them with 6CCVD’s advanced MPCVD diamond material capabilities, specifically targeting engineers and scientists developing high-fidelity quantum computing architectures.

Executive Summary

The research successfully demonstrates a novel four-qubit gate architecture—the “diamond gate”—implemented using capacitively coupled superconducting transmon qubits. This architecture is crucial for scalable, high-fidelity quantum computation.

Universal Gate Set: The device natively implements four distinct controlled two-qubit operations: two entangling SWAP/Phase gates, a Parity Phase gate, and an Identity (idle) gate, enabling universal quantum computation on the target qubits.
High Fidelity: Achieved an average total gate fidelity of F ≈ 0.9923 (Parameter Set 1) and an idle gate fidelity of F_ψ- ≈ 0.9968, meeting the stringent requirements for fault-tolerant surface codes (F > 0.99).
Ultra-Fast Operation: Simulated gate times are exceptionally fast, operating at 59.3 ns (Set 1) and demonstrating potential for 31.5 ns operation (Set 2), minimizing susceptibility to decoherence.
Scalable Architecture: The diamond gate serves as a building block for an extensible 2D lattice quantum computer, allowing for parallel processing and entanglement spreading.
Leakage Mitigation: A passive, robust method was demonstrated using engineered crosstalk (J_T) to induce destructive interference, effectively canceling unwanted leakage to higher-energy transmon states (qutrit spectrum).
Robustness: Simulations confirmed high stability against common system infidelities, including random coupling noise, control state preparation errors, and realistic decoherence rates (γ = 0.01 MHz).

Technical Specifications

The following hard data points were extracted from the numerical simulations and analytical predictions, primarily based on Parameter Set 1, which achieved the highest fidelity.

Parameter	Value (Set 1)	Value (Set 2)	Unit	Context
Target-Control Coupling (J/2π)	65	45	MHz	Exchange interaction strength.
Control-Control Coupling (J_c/2π)	20	20	MHz	Required for control Bell state preparation.
Detuning (Δ/2π)	2	0.5	GHz	Frequency difference between target and control qubits.
Decoherence Rate (γ)	0.01	0.01	MHz	State-of-the-art T₁/T₂ coherence time (100 µs).
Predicted Gate Time (t_g)	59.2	30.9	ns	Calculated via π
Simulated Gate Time (t_g)	59.3	31.5	ns	Time required to maximize total average fidelity.
Total Average Fidelity (F)	0.9923	0.9637	-	Overall performance across arbitrary input states.
ZZ-CZ-SWAP Fidelity (F₀₀)	0.9943	0.9662	-	Fidelity when control state is
-CZ-SWAP Fidelity (F₁₁)	0.9931	0.9668	-	Fidelity when control state is
Identity Gate Fidelity (F_ψ-)	0.9968	0.9983	-	Highest fidelity, limited primarily by decoherence.
Optimal Crosstalk (J_T^opt/2π)	-3.66	-	MHz	Value required to cancel unwanted leakage processes.

Key Methodologies

The experiment relies on advanced theoretical modeling and numerical simulation of superconducting transmon circuits to achieve controlled quantum interference.

Architecture: Utilized a diamond-shaped geometry of four capacitively coupled transmon qubits (C1, C2, T1, T2), where the control qubits (C) are detuned from the target qubits (T).
Hamiltonian Derivation: The system dynamics were modeled using a four-qubit Hamiltonian (H) simplified via the Rotating Wave Approximation (RWA), assuming large detuning (Δ >> J).
Unitary Time-Evolution: The effective unitary operation U was derived using the Magnus expansion within Floquet theory, which is valid when the qubit detuning (Δ) is much larger than the coupling strengths (J, J_c).
Performance Quantification: Gate performance was quantified by solving the Lindblad master equation numerically using the QuTiP toolbox, incorporating realistic decoherence rates (γ) for state-of-the-art superconducting qubits.
Fidelity Calculation: Gate quality was measured using the average gate fidelity (F), which compares the simulated quantum map E(ρ) to the target unitary U_target over a uniform distribution of input states.
Qutrit Analysis & Leakage Control: The model was extended to a four-qutrit system (including the second excited state |2>) to analyze leakage. Leakage was passively mitigated by engineering a specific crosstalk coupling (J_T) between target qubits to induce destructive interference.

6CCVD Solutions & Capabilities

The successful implementation of high-fidelity, ultra-fast quantum gates using superconducting circuits is fundamentally dependent on the quality and purity of the underlying substrate. 6CCVD specializes in providing the advanced MPCVD diamond materials necessary to meet the stringent requirements of next-generation quantum processors.

The proposed diamond gate architecture, especially when scaled into a 2D lattice (Fig. 2), demands substrates with exceptional dielectric properties and large dimensions—areas where 6CCVD’s MPCVD diamond excels.

Research Requirement/Challenge	6CCVD Solution & Capability	Technical Advantage for Quantum Circuits
Ultra-Low Loss Substrates	High-Purity MPCVD Polycrystalline Diamond (PCD)	PCD offers superior thermal conductivity and an extremely low dielectric loss tangent, minimizing energy dissipation and maximizing the T₁ and T₂ coherence times required for F > 0.99 operation.
Scalable 2D Lattice Integration	Custom Dimensions up to 125mm Diameter	We provide large-area PCD plates (up to 125mm) necessary for fabricating the extensive 2D diamond-plaquette arrays proposed for extensible quantum computers.
Minimizing Surface Decoherence	Precision Polishing Services	Our proprietary polishing achieves surface roughness Ra < 5 nm on inch-size PCD wafers, drastically reducing surface defects that contribute to qubit decoherence and loss.
Alternative Qubit Platforms	Optical Grade Single Crystal Diamond (SCD)	For researchers exploring similar gate logic using solid-state qubits (e.g., NV centers or Silicon QDs), we supply high-purity SCD wafers in thicknesses from 0.1 µm to 500 µm, with ultra-smooth surfaces (Ra < 1 nm).
Integrated Circuit Fabrication	In-House Metalization Capabilities	We offer custom deposition of standard contact and bonding metals (Au, Pt, Pd, Ti, W, Cu) directly onto diamond substrates, streamlining the fabrication process for superconducting circuits.
Material Optimization	Expert Engineering Support	6CCVD’s in-house PhD team provides consultation on material selection, doping levels (BDD for conductive layers), and optimal substrate thickness (up to 10 mm) to ensure maximum performance for [Superconducting Qubit] projects.

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

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41534-020-0275-3