Gate-Tunable Qubit Capacitor to Cut Dielectric Loss and Cross-Talk
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Solution Overview
Problem
Existing tunable couplers for superconducting qubits face challenges in achieving high coherence due to dielectric losses, particularly in III-V semiconductor materials like GaAs, which contribute to higher losses at single photon powers and millikelvin temperatures, and stray electric and magnetic fields lead to cross-talk between qubits.
Innovation Solution
A gate-tunable capacitive coupling element using a pair of fixed air-gap capacitors with metal-oxide gates to deplete a two-dimensional electron gas (2DEG) in an InAs/InGaAs heterostructure, reducing stray fields and dielectric losses, and replacing flux-biased SQUID-based inductive couplers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If flux-biased SQUID-based inductive couplers are used to achieve fast, tunable coupling between superconducting qubits, then two-qubit gate fidelities are improved, but dielectric losses increase due to piezoelectricity in III-V semiconductor materials
Solution Approach 1:
The patent changes the coupling mechanism from inductive (flux-biased SQUID) to capacitive (gate-tunable), fundamentally altering the electrical parameters of the system. This parameter change eliminates the piezoelectric loss mechanism while maintaining tunability through voltage control of the capacitive coupling between qubits
Solution Approach 2:
The patent replaces the magnetic field-based inductive coupling mechanism with an electric field-based capacitive coupling mechanism. This substitution eliminates the need for flux biasing and the associated dielectric losses in III-V materials, while achieving comparable or improved coupling performance
2Ease of manufacture
If III-V semiconductor materials are used in gate-tunable Josephson junctions to enable electric field tuning, then fabrication flexibility is improved, but coherence is degraded due to applied electric potential depleting the superconductor-semiconductor-superconductor junction
Solution Approach 1:
The patent segments the coupling function from the qubit structure itself, using separate gate electrodes to control the capacitive coupling between qubits. This segmentation allows the use of III-V materials in the qubits while using different materials (avoiding piezoelectric substrates) in the coupling region, thus maintaining coherence
Solution Approach 2:
The patent introduces air-gap capacitors as intermediary elements between the qubits and the gate electrodes. These air-gap capacitors provide electrical coupling control without requiring the gate electrodes to be in direct contact with the superconductor-semiconductor-superconductor junction, thereby preventing depletion and maintaining coherence
3Adaptability or versatility
If metal-oxide gates are used to deplete the two-dimensional electron gas in InAs/InGaAs heterostructure, then coupling tunability is improved, but stray electric fields cause cross-talk between qubits
Solution Approach 1:
The patent uses air-gap capacitors with thin dielectric films to confine the electric fields generated by the metal-oxide gates. These thin films act as field confining structures that prevent stray fields from extending to neighboring qubits, thereby reducing cross-talk while maintaining full coupling tunability
Solution Approach 2:
The patent employs guard electrodes as counterbalancing elements that generate opposing electric fields to cancel out stray fields from the control gates. This counterweight approach actively compensates for the harmful stray fields, reducing cross-talk between qubits
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The proposed solution achieves a high on/off ratio of over two orders of magnitude, minimizing dielectric losses and reducing cross-talk, thereby improving qubit coherence and scalability in quantum computing systems.
Implementation Method 1
a single, effective capacitance that is tuned by a collection of gates (e.g., metal oxide) that deplete a two dimensional electron gas (2DEG) in an InAs/InGaAs semiconductor heterostructure
Implementation Method 2
The capacitive coupling element may include a pair of fixed air-gap capacitors with a single, effective capacitance
Data Source
AI summary
An exemplary tundable capacitor in a quantum system includes a pair of qubits, and a capacitive coupling element coupled between the pair of qubits. The capacitive coupling element includes a plurality of gate terminals. The capacitive coupling element is configured to receive a respective gate voltage at each of the plurality of gate terminals and to adjust a capacitance of the capacitive coupling element in response to the respective gate voltage received at each of the plurality of gate terminals. The capacitance of the capacitive coupling element is configured to control a coupling strength between the pair of qubits.