The present invention provides a novel method for fabricating high-performance superconducting qubits featuring integrated on-chip photonic crystal shielding structures. This method effectively addresses the limitations of conventional noise mitigation techniques by enabling the seamless co-fabrication of high-coherence superconducting circuits and precisely engineered photonic crystals directly on the same substrate using process flows compatible with superconducting quantum circuit manufacturing. This seamless, integrated approach facilitates tailored, localized, and frequency-selective noise suppression, directly tackling significant scaling challenges posed by on-chip noise and crosstalk. The invention leverages standard microfabrication techniques common in superconducting circuit fabrication, such as thin-film deposition (e.g., Al, Nb, TiN), various lithography methods (including advanced electron-beam lithography), and etching processes (wet and dry), integrating them with steps for creating the periodic structures that form the photonic crystals. A key advantage is the flexibility in integration timing, allowing the definition of photonic crystal structures at various points in the standard superconducting qubit fabrication process flow—before, during, or after the fabrication of the primary superconducting circuit elements—optimizing their placement and effect based on the target noise mechanism and materials stack. These integrated structures, typically realized as two-dimensional (2D) periodic arrays of features such as holes, pillars, or anti-dots defined in low-loss dielectric layers (e.g., SiNx, SiO2), superconducting or normal metallic layers, or the substrate itself, are specifically designed through electromagnetic simulation to exhibit photonic bandgaps or strong attenuation bands at target microwave frequencies. By strategically positioning these photonic crystal structures adjacent to or surrounding sensitive qubit components, along critical transmission lines, within ground planes, or in the substrate, they function as highly localized, frequency-selective filters or barriers, significantly attenuating detrimental electromagnetic fields and noise excitations. This effectively reduces decoherence (improving T1 and T2), enhances gate fidelity, and minimizes unwanted crosstalk between circuit elements. Key advantages include providing highly localized and frequency-selective noise suppression directly on-chip, reduced reliance on external filtering, a compact on-chip footprint enabling denser integration, and inherent scalability with standard microfabrication processes. The method encompasses specific design parameters for engineering the photonic crystal lattices based on electromagnetic simulations and detailed co-fabrication process flows designed to minimize impact on superconducting material properties and qubit performance. Furthermore, the invention provides the superconducting qubit device fabricated according to this method, exhibiting enhanced performance characteristics attributable to the integrated photonic crystal shielding. #### Cross-Reference to Related Applications [To be added if applicable] #### Statement Regarding Federally Sponsored Research or Development [To be added if applicable] #### Background of the Invention Superconducting qubits represent a leading platform for building fault-tolerant quantum computers. Achieving and maintaining long qubit coherence times (T1 and T2) and high gate fidelity remains a central challenge, limited by various on-chip and off-chip noise sources and parasitic interactions. While external noise can be mitigated, scaling up quantum processors exacerbates on-chip noise mechanisms, such as stray microwave radiation, thermal noise, quasiparticle poisoning, dielectric losses, material defects, and electromagnetic crosstalk, which are difficult to address with global techniques. Conventional on-chip mitigation techniques like patterned ground planes or lumped-element filters offer limited effectiveness; patterned ground planes lack frequency selectivity, and lumped-element filters require significant chip area and may introduce loss. Spurious electromagnetic modes, including substrate-guided modes and surface waves, propagate within the chip and contribute significantly to crosstalk, posing a major challenge for scaling due to their pervasive nature and difficulty in suppression by conventional planar filtering. The increasing density of qubits and associated wiring in scaled processors severely exacerbates crosstalk and sensitivity to localized on-chip noise. Thus, there is a critical need for integrated, on-chip shielding solutions providing highly localized, frequency-selective noise suppression directly at the qubit level, compatible with existing fabrication processes, maintaining low loss, and being scalable. Photonic crystals (PCs), capable of creating forbidden frequency bands (photonic bandgaps) for electromagnetic wave propagation, offer a promising paradigm for tailored electromagnetic control. Integrating PCs directly onto the superconducting qubit chip holds significant promise for targeted shielding. However, the successful co-fabrication of precise, low-loss PC structures operating at relevant microwave frequencies (1-20 GHz) alongside highly sensitive superconducting qubits presents materials science and fabrication challenges requiring careful co-optimization of process flows, material properties, and electromagnetic design to avoid degrading qubit performance. Overcoming these challenges is essential for realizing the full potential of superconducting qubits for large-scale quantum computing. The present invention provides a method for fabricating high-performance superconducting qubits with integrated on-chip photonic crystal shielding structures that overcomes these limitations by enabling seamless, compatible co-fabrication. #### Brief Description of the Several Views of the Drawing(s) Figure 1 illustrates a representative step-by-step process flow for fabricating a superconducting qubit device incorporating integrated on-chip photonic crystal shielding structures, highlighting integration points for PC definition relative to superconducting circuit fabrication. Figure 2 shows a schematic top view of a multi-qubit superconducting circuit layout, illustrating strategic placement and configurations of integrated photonic crystal structures surrounding qubits, shielding elements, along transmission lines, or within ground planes/substrate. Figure 3 provides a conceptual cross-sectional view of the integrated device, depicting the substrate, superconducting/dielectric layers, and integrated PC features within a layer or the substrate, illustrating vertical integration schemes. Figure 4 illustrates various planar configurations and detailed designs for integrated photonic crystal structures, such as different 2D lattice types, scatterer shapes, and their placement relative to circuit components, along with key design parameters. Figure 5 depicts simulated electromagnetic response spectra (transmission, reflection, band structure) for designed photonic crystal structures, demonstrating engineered photonic bandgaps at target microwave noise frequencies and transmission at desired operational frequencies. #### Detailed Description of the Invention The present invention provides a detailed method for fabricating superconducting qubits with integrated on-chip photonic crystal shielding structures to significantly enhance qubit coherence and fidelity by mitigating noise and crosstalk. A central inventive aspect is the **seamless co-fabrication** of these frequency-selective photonic crystals directly alongside sensitive superconducting circuit elements using process flows fully compatible with high-quality superconducting quantum circuit manufacturing. This integrated approach directly addresses the scaling challenges of on-chip noise and crosstalk by creating localized, frequency-selective electromagnetic barriers. The method achieves this by carefully interleaving the precise definition of periodic PC structures with standard superconducting circuit fabrication steps, minimizing detrimental impact on delicate components like Josephson junctions. The fabrication process is adaptable to different qubit architectures, substrates, and superconducting films, operating effectively at microwave frequencies (1 GHz to 20 GHz, typically focused on 4-8 GHz). **Substrate Selection and Preparation:** The method begins with selecting and preparing a suitable substrate with ultra-low dielectric loss (tan δ < 10^-6, preferably < 10^-8 at 10 GHz and 20 mK), minimal defects, high thermal conductivity, and compatibility with processing. Exemplary substrates include high-resistivity silicon, c-plane sapphire, semi-insulating GaAs, or SiC. Rigorous cleaning (solvent, chemical, plasma) and precise surface treatments (H passivation, high-temperature annealing) are crucial to remove contaminants and defects, minimize dangling bonds and trap states (sources of 1/f noise and TLS), and ensure atomic-level cleanliness and flatness (< 0.5 nm RMS roughness) for optimal interface quality. Pristine interfaces are paramount for minimizing interface losses and variability, particularly T1 loss from substrate surface TLS and quasiparticle generation. Characterization using AFM, SEM, XPS, or SIMS verifies surface quality. **Superconducting Circuit Fabrication:** Core superconducting qubit circuit elements are fabricated using established, high-yield procedures. This involves depositing thin films of superconducting materials (Al, Nb, TiN, Ta, V, MoRe, multilayers) via sputtering, evaporation, or ALD to achieve low loss, appropriate Tc, Jc, and kinetic inductance. Film thickness (10-300 nm) and deposition parameters (temperature, pressure, rate, purity) are controlled to manage stress, grain structure, stoichiometry, defect density, and surface roughness, all impacting superconducting quality, TLS density, and coherence. Amorphous films (MoSi, NbSi) may reduce TLS noise. Circuit patterns (qubit islands, capacitive/inductive elements, resonators, transmission lines, ground planes) are defined by lithography. DUV photolithography suits larger features, while EBL provides nanometer precision for critical sub-micron features like Josephson junction contacts and overlaps, coupling structures, or high-Q resonators. Optimized single/multi-layer resist systems (PMMA, ZEP, HSQ) and advanced techniques (multi-pass EBL, hierarchical layout, high-precision stitching, PEC) improve feature definition and reduce proximity effects/stitching errors. Patterns are transferred into superconducting films using anisotropic/isotropic etching (RIE, ICP etching with fluorine/chlorine plasmas for Nb, TiN, Ta, V; wet or dry etching for Al). Etching is optimized for vertical profiles, smooth sidewalls, minimal undercut, and reduced damage/redeposition. Wet etching is generally isotropic and less suitable for high-density or precise features. Josephson junctions (Al/AlOx/Al, Nb/AlOx/Nb) are typically fabricated using advanced shadow evaporation (Dolan method) with EBL-defined suspended masks and controlled *in situ* oxidation to form the tunnel barrier (0.5-2 nm AlOx). Barrier properties (thickness, uniformity, composition, trap density) are critical for qubit frequency, anharmonicity, and coherence (T1, flux noise). Precise control of *in situ* oxidation is paramount for low-defect barriers, as defects near the barrier/electrodes are major sources of 1/f charge noise and TLS. Alternative junction techniques exist. Any damage or contamination after fabrication degrades performance. Rigorous post-fabrication cleaning (solvent rinses, gentle plasma ash, dilute acid/base dips, *in situ* cleaning) removes residues and contaminants. Characterization includes measuring Tc, Jc, sheet resistance, and low-temperature I-V characteristics (Ic, Rn) and uniformity. **Seamless Integration of Photonic Crystal Shielding Structures:** The seamless co-fabrication of PC structures, interleaved with standard steps, is a central inventive aspect enabling on-chip noise control without compromising qubit performance. This integration is achieved by carefully optimizing PC processes for compatibility, designing parameters (plasma power, gas, etch time, temperature, thermal budget, ion energy, wet etch chemistry) to maintain low loss and high coherence, and meticulously controlling lithography, etching, deposition, and cleaning to minimize damage, contamination, and stress. The timing and method of PC integration depend on the structure type, materials, qubit architecture, and stack. While flexibility in PC placement allows tailoring shielding to target specific noise mechanisms, a key challenge addressed by this method is performing PC fabrication steps without degrading sensitive structures (JJs, films, interfaces). PCs can be defined at key points in the qubit fabrication flow, with specific process considerations and mitigation strategies for each approach: 1. **Substrate-First Integration (Figure 1a):** PC features are etched into the bare or prepared substrate *before* significant layer deposition. This allows for deep features (trenches, holes) via DRIE into Si, sapphire, SiC (tens-hundreds µm deep, potentially through-wafer) to mitigate substrate-guided modes, BAWs, and provide acoustic isolation. A key advantage is minimizing potential damage to later superconducting layers. *Process Considerations & Mitigation:* DRIE (Bosch, cryogenic) is used for high aspect ratios. Conformal deposition over topography requires ALD for dielectrics or optimized sputtering/evaporation for metals. Careful post-etch cleaning (solvents, megasonic, plasma) removes residues. Annealing can repair damage, and passivation (ALD Al2O3, HfO2, chemical) minimizes trap states and TLS at the substrate-film interface. Process parameters must avoid re-entrant profiles, mitigate plasma charging, and ensure complete resist removal. 2. **Intermediate-Layer Integration (Figure 1b):** PC structures are defined within an intermediate layer (e.g., a dielectric on a ground plane, a non-critical superconducting layer, or a normal metal layer). This is effective for mitigating modes confined to specific layers or metal interfaces (e.g., surface waves, ground plane modes). Requires highly selective etching processes. *Process Considerations & Mitigation:* Etch processes must minimize redeposition and be highly selective to the PC layer over underlying or adjacent layers. Thermal budget must be compatible with existing JJs (< 150-200°C for Al/AlOx/Al). Low-power, low-ion-energy dry etching, *in situ* treatments, and precise deposition temperatures are employed. Careful control of interfaces created at this stage is critical to prevent the introduction of defects or contamination. 3. **Top-Layer or Post-Fabrication Integration (Figure 1c):** PC features are defined in a top dielectric passivation layer or etched into an exposed substrate or ground plane *after* primary qubit and resonator fabrication (including defining JJs). This approach offers excellent alignment to sensitive features and is effective for localizing shielding (e.g., near qubits or junctions), suppressing surface waves, or patterning the ground plane. *Process Considerations & Mitigation:* While minimizing risk to underlying structures by being a later step, this requires extreme control to avoid damage or contamination (e.g., plasma damage, charging, residues) to exposed, sensitive surfaces. Low-power/low-bias dry etching, gentle ion milling, optimized wet etching, downstream plasma cleaning, and UV-Ozone treatments are used. Complete, non-damaging resist stripping is essential. Low-temperature deposition is preferred for top dielectrics (e.g., PECVD ~200-300°C, ALD <100°C). Mitigate plasma charging with dissipation layers or process control. Regardless of the specific timing chosen, successful co-fabrication requires addressing general microfabrication challenges inherent to processing alongside sensitive superconducting circuits: * **High-Resolution Lithography:** Crucial for defining periodic structures with precision (nanometer placement, <10 nm LER, <10 nm overlay accuracy) and scalability over large areas. *Challenges & Mitigation:* Achieving this precision, especially for sub-micron PC features operating at GHz frequencies, requires optimized resists (PMMA, ZEP, HSQ), multi-pass exposures, hierarchical layout, high-precision stitching, and proximity effect correction (PEC). Scaling EBL to large chips can be time-consuming; combining with optical lithography (DUV, immersion, EUV) or nanoimprint lithography (NIL) can improve throughput if compatible and meet resolution/overlay requirements. * **Controlled Etching:** Achieving desired anisotropic profiles, depth uniformity, and minimal damage/redeposition when transferring patterns into dielectric, metallic, or substrate layers. *Challenges & Mitigation:* Requires careful tuning of dry etch parameters (power, pressure, gas mix, bias, duration, temperature) and plasma type to achieve desired vertical profiles and smooth sidewalls without excessive undercut or damage to underlying/sidewall materials. Low-power, low-bias processes, optimized chemistries, pulsed plasmas, and sacrificial layers are employed to reduce damage. Post-etch cleaning (solvent, dilute acid/base, plasma) removes residues and helps repair damage. Wet etching is less controlled for high-aspect ratio PCs. Cryogenic silicon etching provides anisotropic, smooth profiles in Si. Mitigate plasma charging effects. * **High-Quality Deposition:** Ensuring high-quality, low-loss film deposition over complex topography created by PC patterns. *Challenges & Mitigation:* Non-conformal coverage can cause defects and loss. Use ALD for dielectrics, optimize sputtering/evaporation parameters (e.g., oblique angle, substrate rotation, temperature), and consider planarization techniques (CMP, resist reflow) if compatible with the process flow. Manage film stress (by tuning deposition parameters or using stress-compensating stacks) to prevent wafer bowing or cracking. * **Preserving Interface Quality:** Maintaining pristine interfaces (substrate-film, film-dielectric) which are major sources of TLS and loss. *Challenges & Mitigation:* Use meticulous pre-deposition cleaning, *in situ* treatments (plasma, annealing), ALD for low-defect dielectric interfaces, and compatible low-temperature post-processing anneals (e.g., forming gas). Passivation layers can help if deposited with low damage. Characterize interfaces with XPS, SIMS, TEM, and AFM. * **Stress Management:** Managing stress from deposited layers and patterned PCs to prevent wafer bowing or cracking, which can affect yield and performance. *Challenges & Mitigation:* Optimize deposition parameters, use stress-compensating multilayer stacks, and design PC patterns for even stress distribution. Low-temperature anneals can be used if compatible. Finite element analysis can model stress distribution. * **Integration Complexity:** Interleaving PC fabrication steps with delicate qubit fabrication steps requires careful process flow design, considering thermal budgets, chemical compatibility, and cumulative potential for damage. *Challenges & Mitigation:* Mitigation involves detailed process flow design and optimization, testing on test structures before full wafer processing. Multi-wafer bonding can integrate complex 3D PCs or pre-fabricated PC layers but introduces alignment and interface challenges (bonding strength, interface quality, thermal contraction mismatch). The seamless co-fabrication method, by addressing these challenges through careful process design, optimization, and integration timing flexibility, provides a flexible and powerful framework for engineering the electromagnetic environment of superconducting qubits, crucial for enhancing performance and yield. **Electromagnetic Design Considerations for Photonic Crystal Structures:** Precise electromagnetic design is critical to create PC structures with bandgaps or strong attenuation bands at desired microwave frequencies, tailored to the specific noise environment of the superconducting circuit. This involves noise identification, simulation, parameter optimization, and validation. Dominant on-chip noise sources, spurious modes, and crosstalk mechanisms are identified (e.g., through spectral analysis, cryogenic characterization, and EM simulation of the circuit layout). Their frequency spectra, spatial distribution, polarization, and coupling mechanisms inform the target frequency range for suppression, required attenuation depth, polarization sensitivity, spatial extent, and optimal placement of the PCs. Sophisticated EM simulation tools (FEM - HFSS, COMSOL; FDTD - Lumerical, CST; PWE) are used to model PC designs within the multilayer stack and cryogenic package environment (Figure 5). FEM and FDTD simulations are used to simulate transmission and reflection (S-parameters) and field distributions in realistic layouts. Plane wave expansion (PWE) is used to calculate the band structure of infinite periodic lattices, identifying bandgaps, bandwidth, and completeness (for different polarizations and propagation directions relative to the Brillouin zone). Acoustic simulations can predict SAW/BAW attenuation properties of patterned structures. Simulations predict the band structure (passbands and bandgaps) based on the wave vector (k). The first bandgap typically occurs near the guided wavelength (λ_g) related to the lattice constant ('a') and effective refractive index (n_eff), approximately λ_g ≈ 2a/n_eff. Key design parameters are iteratively adjusted in simulations to achieve a bandgap with sufficient depth (> 20-40 dB over multiple periods, > 60 dB preferred) and bandwidth covering target noise frequencies, while verifying unimpeded transmission (low insertion loss < -15 to -20 dB magnitude, < -30 dB preferred; minimal reflection/return loss S11 < -15 dB, < -20 dB preferred) of desired control and readout signals in the passband. Parameters influencing bandgap properties include: * **Lattice Type:** The geometric arrangement of scatterers (e.g., square, hexagonal, honeycomb). Influences the isotropy and completeness of the bandgap. Hexagonal lattices often provide larger, more isotropic bandgaps for planar modes. Can enable directional filtering. * **Lattice Constant ('a'):** The spatial period of the structure (typically tens to hundreds of µm for GHz frequencies). Scales the bandgap frequency. A smaller 'a' shifts the bandgap to higher frequencies. The guided wavelength λ_g is approximately proportional to 'a'. The effective index n_eff is crucial. * **Scatterer Size and Shape ('d'):** The dimensions and geometry of the periodic features (typically 0.1a to 0.9a). Strongly influences the fill factor, perturbation strength, and the width, depth, and location of the bandgap. The d/a ratio is a crucial design parameter. Larger material contrast (e.g., dielectric vs. air/vacuum or superconducting vs. normal metal) generally leads to wider and deeper bandgaps. The shape can be tailored for polarization dependence, directionality, or coupling strength to specific modes. * **Material Dielectric/Metallic Contrast:** The ratio of dielectric constants (ε_r) for dielectric PCs, or the impedance/kinetic inductance contrast for metallic PCs. Larger contrast typically results in wider and deeper bandgaps. Material loss (dielectric tan δ, conductor loss) is critical; low-loss materials are necessary to maintain qubit coherence and high-Q resonators. Superconducting PCs leverage the large kinetic inductance effect at microwave frequencies. * **Number of Periods (N):** The number of unit cells that the electromagnetic wave traverses. Attenuation increases exponentially with N (>20-40 dB attenuation is achievable with 3-10 periods). Impacts the required chip footprint and potential passband loss. The PC structure acts like a distributed Bragg reflector. * **Layer Stack Configuration and Placement:** The vertical position of the PC layer within the multilayer circuit stack and its lateral placement relative to sensitive components. Dictates which electromagnetic modes the PC structure interacts with (e.g., substrate modes, surface waves, CPW modes). Interaction is maximized where the targeted mode's field is concentrated or where surface currents are strongest. Strategic lateral placement (Figure 2) directs the shielding effect. The thickness of the PC layer relative to the guided wavelength or period also matters. Simulations account for the multilayer structure, substrate properties, dispersion (including kinetic inductance in superconductors), losses (dielectric, conductor, radiation), and package effects. S-parameter analysis quantifies attenuation (S21) and reflection (S11). Modal analysis identifies spurious modes and their characteristics. Full-chip simulations can provide insight into global mode behavior. Time-domain simulations can assess the impact on control pulses. This iterative design process ensures that the PCs function as selective barriers, blocking unwanted noise while transmitting desired signals. The design balances noise suppression effectiveness, minimal insertion loss and reflection, required chip footprint, and fabrication complexity. Design considerations include the impact on characteristic impedance and resonator frequency if PCs are placed near these elements. Polarization dependence is considered for planar modes (TM-like electric field normal to plane, TE-like electric field in plane). **Embodiments of Integrated Photonic Crystal Structures:** Integrated PCs can be realized using various materials and configurations compatible with superconducting circuits: * **Dielectric Photonic Crystals:** Patterned features defined within a low-loss dielectric layer (e.g., SiNx, SiO2, AlOx, HfO2, TaOx, low-loss polymers) deposited on the substrate or etched directly into the substrate material. Dielectric films can be deposited via PECVD, LPCVD, ALD, thermal oxidation, sputtering, or spin-coating. ALD is often preferred for conformal, low-defect films, especially over topography. Layer thickness (typically tens to hundreds of nm) is chosen based on the desired bandgap properties. 2D periodic arrays of holes, pillars, or more complex shapes are defined using high-resolution lithography (primarily EBL) requiring sub-micron periodicity and nanometer precision in feature size and placement. Anisotropic etching (e.g., RIE, ICP with fluorine-based plasmas like SF6/C4F8, SF6/O2, or CHF3/O2) is used to transfer the lithographic patterns into the dielectric layer or the substrate. Deep reactive ion etching (DRIE, e.g., Bosch or cryogenic processes) is used for creating deep features in the substrate (e.g., for acoustic isolation). Etching processes are optimized for desired profile control, minimal damage to surrounding materials, and low surface roughness. Critical parameters (a, d, fill factor, lattice type, N, ε_contrast) are engineered via simulation (Figure 5) to produce a bandgap at target noise frequencies (e.g., 1-4 GHz, 6-8 GHz for crosstalk or modes) while ensuring high transmission (< 0.1 dB/period insertion loss) at operational frequencies (e.g., 5-6 GHz, 7-8 GHz) in the passband. The bandgap arises from Bragg scattering due to the periodic variation in dielectric constant. Deep etching into the substrate can also disrupt acoustic waves, creating acoustic bandgaps or strong scattering. The depth and profile of the etched features are critical for effective mode interaction. Air-bridge structures can be used to create very high dielectric contrast. * **Metallic Photonic Crystals:** Formed by patterning a metallic material, which can be a superconducting layer (e.g., Al, Nb, TiN, MoRe below Tc) or a normal metal layer (e.g., Cu, Au, W, Pt). Examples include patterning anti-dots (etched holes), pillars, or slots in a ground plane, a shielding layer, or other metallic circuit elements. These structures can act as periodic metamaterials or modify the current distribution and impedance. They exhibit bandgaps or strong attenuation based on Bragg scattering, impedance mismatch, localized resonances, and strong attenuation/reflection for electric fields and surface currents. Their behavior scales with the guided wavelength or skin depth. They can provide broadband or narrowband suppression. Metallic PCs are useful as reflective boundaries. Fabrication involves depositing the metal film (sputtering, evaporation, ALD), defining patterns using lithography (photolithography or EBL), and transferring the patterns via etching (wet etching, dry etching, ion milling, or RIE/ICP etching for superconducting films). The choice of metal and patterning method is guided by the substrate material, desired bandgap properties, process compatibility, and the type of noise source being targeted. Metallic PCs patterned within a superconducting ground plane are particularly effective against surface waves, ground plane modes, and magnetic field noise by disrupting current paths and creating variations in kinetic inductance or magnetic permeability. Kinetic inductance is significant at microwave frequencies and plays a key role in superconducting PC behavior. Superconducting PCs offer low loss below their critical temperature (Tc). Above Tc, they behave as normal metals (with loss). Combining metallic and dielectric PCs in a multilayer stack (hybrid PCs) offers greater flexibility in targeting different noise types and enhancing bandgap properties. * **Hybrid and Multilayer Photonic Crystals:** This approach combines PC features in multiple layers or uses a combination of metallic and dielectric materials within the same PC structure or stack. This allows for achieving enhanced or tailored bandgap properties. Multilayer PCs can create more complex band structures, including complete bandgaps, and can be designed to independently target different modes localized in specific layers or interfaces. For example, a dielectric PC can be placed on top to suppress surface waves and electric fields, while a metallic PC is placed in a buried ground plane layer to mitigate substrate modes, ground plane resonances, and magnetic fields. 3D PC structures (potentially realized via FIB, multi-layer deposition, or wafer bonding) offer the potential for complete bandgaps, providing robust and broadband shielding, but typically involve more complex fabrication processes. **Mechanisms of Noise Mitigation through Strategic PC Placement and Design:** Precise placement, configuration, and dimensions relative to circuit elements are paramount for effective, targeted shielding without interfering with desired qubit operation. PCs are strategically placed based on electromagnetic simulations (Figure 5) and analysis of noise pathways (Figure 2), functioning as frequency-selective barriers or filters by placing a forbidden band along the propagation path of noise excitations. The specific mitigation mechanism depends on the PC's placement, material composition, design parameters, and the nature of the noise: * **Suppression of Substrate-Guided Modes and Surface Waves:** PCs etched deep into the substrate (e.g., in substrate-first integration, using dielectric holes/trenches or metallic features in etched wells) or patterned in dielectric layers near substrate interfaces. These structures scatter and attenuate modes propagating within the substrate bulk or along surfaces/interfaces via Bragg scattering and destructive interference. Substrate modes and surface waves are major sources of long-range crosstalk and energy loss. PCs can be used as isolation barriers, perimeter shielding, surrounding coupling structures, or patterned within the substrate itself to define shielded zones. Deep etching is effective for bulk modes, while PCs in upper dielectric or metallic ground plane layers are effective for surface waves and modes confined to those layers. This suppression is crucial for reducing parasitic coupling, isolating circuits, and preventing energy loss into the substrate or surface waves, thereby increasing T1 and reducing frequency crowding and crosstalk. * **Filtering of Transmission Lines:** PCs can be placed along control and readout lines (e.g., using intermediate or top-layer integration, as dielectric features in or under a dielectric layer, or as metallic features in the ground plane adjacent to the line). These act as frequency filters, with the PC bandgap covering unwanted noise frequencies (e.g., harmonics, environmental noise, spurious tones, on-chip noise) while the passband allows desired signals (control pulses, readout tones) to pass unimpeded. Attenuating out-of-band noise before it reaches sensitive nodes improves the signal-to-noise ratio, prevents spectral crowding (e.g., AC Stark shifts, off-resonant driving, spurious transitions), which in turn improves gate fidelity and reduces dephasing (T2). This requires careful EM design and potentially impedance matching to the transmission line (e.g., 50 Ohm) to minimize reflections in the passband that can distort pulses or create standing waves. * **Localized Shielding of Sensitive Elements:** PCs can be placed immediately adjacent to or surrounding sensitive qubit components (e.g., qubit pads, Josephson junctions, coupling capacitors, resonators) using top-layer or intermediate-layer integration, with dielectric or metallic features. This creates a localized "quiet" zone. The PC bandgap is designed to target frequencies of near-field crosstalk (capacitive or inductive coupling from neighboring elements or lines) or local noise sources (e.g., TLS, other on-chip components). They attenuate electromagnetic fields before they reach the qubit's sensitive parts (e.g., JJ or pads). PCs in the ground plane surrounding a qubit (metallic) suppress ground plane modes and currents near the qubit. PCs in dielectric layers above or below (dielectric) scatter or block electric fields. Patterning features directly within qubit pads (e.g., metallic anti-dots, dielectric holes) can provide localized modification of the electromagnetic environment. This is effective against localized noise sources and variations and helps improve both T1 and T2. The shielding effect can reduce the coupling of noise from other parts of the chip, even if the PC material itself contains some TLS. * **Suppression of Radiative Loss and Package Resonances:** Metallic PCs acting as reflective boundaries, or dielectric PCs with high impedance contrast, can confine electromagnetic fields and reduce radiation loss or coupling to package modes. This can be implemented at the chip edges, around I/O pads, or within the ground plane. This reduces the ingress of external noise and mitigates chip and package resonances, which can act as loss channels, spurious coupling paths, or contribute to frequency crowding. Metallic PCs are effective reflective barriers, acting as on-chip mirrors or even absorbing boundaries if lossy materials are intentionally used (though less common in high-coherence circuits). Suppressing radiative loss increases T1. * **Mitigation of Surface Acoustic Waves (SAWs) and Bulk Acoustic Waves (BAWs):** Certain PC structures, particularly those involving etching into the substrate or patterned features in surface layers, can scatter or attenuate SAWs and BAWs. The periodicity is designed to Bragg scatter acoustic waves at relevant frequencies (typically GHz). This reduces the interaction of acoustic phonons with qubits, which can cause dephasing (T2) and energy relaxation (T1). Acoustic noise can originate from vibration, on-chip heating, or absorption of radiation or particles. It couples to qubits via mechanisms like parameter modulation (e.g., strain, piezoelectric effects) or phonon-assisted processes. Deeply etched substrate PCs or PCs in surface layers that interact strongly with the acoustic modes are effective. Addressing acoustic noise provides a complementary noise mitigation channel. Acoustic PC design is analogous to EM PC design but uses acoustic impedance and sound velocities. * **Suppression of Quasiparticle-Induced Loss:** While not a direct shielding mechanism in the same way as EM or acoustic shielding, PCs can indirectly contribute to reducing quasiparticle-induced loss. Quasiparticle poisoning (non-equilibrium quasiparticles) is a major source of T1 loss, often generated by the absorption of stray radiation (>2Δ photons), energetic particles, or high-energy phonons. By suppressing stray electromagnetic radiation, PCs indirectly reduce the generation rate of quasiparticles from this source. Additionally, PCs patterned in the substrate or superconducting films might scatter high-frequency phonons generated by absorption events, reducing their propagation to sensitive Josephson junction areas. This can potentially reduce the density of non-equilibrium quasiparticles near JJs, increasing T1. This mechanism is more complex and depends on the acoustic properties of the materials, the spectrum of phonons involved, and the efficiency of phonon-quasiparticle conversion. Metallic PCs integrated into a superconducting layer might also alter quasiparticle dynamics or create trapping sites. The lattice type, scatterer shape, lattice constant 'a', scatterer size 'd', fill factor, material contrast, and number of periods N are carefully designed and aligned based on electromagnetic simulations to maximize the attenuation of detrimental modes while minimizing scattering and reflection of desired operational signals. For instance, a PC placed around a readout resonator can suppress its higher-order modes, coupling to the package, and photon leakage, thereby increasing the resonator's quality factor (Q) and improving readout fidelity. A PC placed between qubits or along coupling structures can block crosstalk (e.g., via substrate modes, SAWs, or near-field coupling) at the crosstalk frequency. PCs surrounding control lines can filter out-of-band noise before it reaches the qubit. PCs placed near pads or chip edges can block environmental noise ingress. The number of periods N dictates the rejection depth; typically 3-10 periods are sufficient for >20-40 dB attenuation. Increasing N increases the footprint and potentially the passband loss if not optimally designed. The shape and orientation can be optimized for anisotropic bandgaps or to create waveguides (via line defects) for guiding signals through shielded regions. Dielectric contrast is a primary determinant of dielectric bandgap width and depth. Simulations guide the choice of parameters, ensuring the bandgap aligns with the noise frequencies and the passband aligns with operational frequencies. The design balances noise suppression effectiveness, insertion loss, reflection, chip footprint, and fabrication feasibility. Account for different planar modes (TM, TE). **Illustrative Embodiments and Fabrication Flow:** Example 1: Transmon qubits on high-resistivity silicon with integrated silicon nitride PC shielding (Figure 1c, Figure 2/3). The method begins with a cleaned high-resistivity Si wafer (e.g., using RCA clean, HF dip, and H-termination). Aluminum transmon circuits are fabricated: deposit a high-purity, low-stress Al film (100-150 nm) via EBL-assisted sputtering or evaporation. Pattern Al circuit features (qubit islands, pads, resonators, feedlines) using photolithography or EBL and etch the Al film (e.g., using wet PAN etching or dry Cl2/BCl3 RIE). Al/AlOx/Al Josephson junctions are typically fabricated using high-resolution EBL, shadow evaporation (Dolan method) with suspended masks, and controlled *in situ* thermal oxidation (controlled O2 pressure and time) to form the thin AlOx tunnel barrier (1-2 nm). Resonators and feedlines are patterned simultaneously with the qubit circuit. Rigorous post-fabrication cleaning (solvent rinses, gentle O2 plasma ash, potentially a low-power forming gas anneal if compatible with the thermal budget) removes residues and contaminants. A conformal, low-stress SiNx film (100-300 nm) is then deposited via PECVD (~250°C) or ALD (<100°C, offering better conformality and lower thermal impact). SiNx is chosen for its low loss and compatibility with Si processing. Its thickness is optimized for the desired bandgap. EBL is used to define a 2D periodic lattice (hexagonal is preferred for potentially more isotropic bandgaps for planar modes) of circular holes (with diameter 'd') in the SiNx layer, with a lattice constant 'a' (typically 50-200 µm for GHz bandgaps). These patterned areas are strategically positioned around sensitive areas like transmon island pads, coupling capacitors, feedlines, and resonators, or in inter-qubit regions, based on electromagnetic simulations (Figure 2). Anisotropic RIE (using fluorine-based plasmas like SF6/C4F8, SF6/O2, or CHF3/O2) is used to etch the SiNx down to the Si substrate (Figure 3). The etch process is carefully controlled to minimize damage or contamination to the underlying Al circuit, especially near JJs (using low power, low bias, and selective chemistry). Minimal over-etch into the Si substrate is desired unless deep features are specifically needed (e.g., for acoustic isolation). Careful optimization of plasma parameters minimizes damage, charging, and defect creation. Post-etch cleaning (e.g., downstream plasma, wet cleaning, DI water rinses) removes etching residues and polymers and can help repair plasma damage. The parameters 'a', 'd', lattice type, and N are determined by iterative EM simulations (FDTD, PWE, FEM) to produce a bandgap (e.g., 1-4 GHz, 6-8 GHz for crosstalk/modes) while ensuring high transmission (< 0.1 dB/period insertion loss) at the control and readout frequencies (e.g., 5-6 GHz, 7-8 GHz) in the passband. This integrated SiNx PC functions as an on-chip, frequency-selective shield, attenuating stray microwave radiation, spurious modes, and potentially surface acoustic waves. This process flow interleaves PC fabrication *after* primary qubit circuit fabrication but *before* final chip release, ensuring precise alignment, minimal impact on sensitive components, and effective localized shielding. Alternative integration timings (substrate-first, top-layer) are also possible depending on the target noise mechanism and material stack. Example 2: Metallic PC in a superconducting ground plane on sapphire (Figure 1b, intermediate superconducting layer). A superconducting ground plane (100-200 nm thick Nb or TiN film with low stress and high quality) is deposited on a cleaned sapphire substrate via sputtering. The overall ground plane geometry may optionally be patterned. A periodic array of anti-dots (e.g., etched holes or pillars) is then defined within the superconducting ground plane using EBL and dry etching (RIE/ICP etching is used for Nb or TiN films, typically with fluorine or chlorine chemistry). These anti-dots modify the kinetic inductance of the film and disrupt current paths. The pattern (a, d) is designed based on simulations targeting specific ground plane modes or surface current patterns. This metallic PC creates a bandgap or significant attenuation for surface waves (TE-like polarization), ground plane modes, and potentially magnetic field noise coupled via eddy currents. It is effective in isolating qubits from ground plane noise and crosstalk and suppressing ground plane resonances. The etch must be selective to the superconducting film over the sapphire substrate. This type of PC also suppresses magnetic field noise by disrupting current flow and scattering magnetic field components. Kinetic inductance effects are significant at microwave frequencies. If the PC bandgap is above 2Δ/h, it may interact with quasiparticle dynamics. Combining a metallic ground plane PC with a dielectric PC in other layers (e.g., a top dielectric) offers complementary shielding mechanisms. **Characterization and Performance Validation:** Post-fabrication characterization is crucial to validate the effectiveness of the integrated PC structures and their impact on qubit performance. 1. **Material and Structure Characterization:** Electrical characterization of superconducting films (Tc, resistivity, Jc) and Josephson junctions (Ic, Rn from I-V curves, uniformity across the wafer) verifies their quality after the integrated process flow and ensures that PC fabrication steps did not degrade superconducting properties. Imaging techniques (SEM, TEM, AFM) are used to verify PC feature shape, size, depth, sidewall quality, periodicity (Figure 4), identify potential etch damage, and assess interface quality (Figure 3). Focused Ion Beam (FIB) can be used for cross-sections or localized modifications. EDX, XPS, and SIMS can analyze material composition and surface chemistry to check for contamination. Profilometry and interferometry measure feature depth, film thickness, and wafer bow. XRD assesses film crystallinity and stress. Raman spectroscopy can probe material structure and stress. Ellipsometry measures dielectric thickness and refractive index. Contact angle measurements assess surface wettability, indicating cleanliness. 2. **Microwave Characterization of PCs:** Microwave transmission and reflection spectroscopy (S-parameter measurements) are performed on test structures (e.g., transmission lines or waveguides with integrated PC sections) at cryogenic temperatures (<100 mK) using a Vector Network Analyzer (VNA). This directly measures the frequency response (S21 transmission, S11 reflection) and verifies the designed bandgap (range, bandwidth, attenuation depth, roll-off) and passband performance (insertion loss S21 magnitude, return loss S11 magnitude) (Figure 5). Comparing experimental spectra to simulated results validates the EM design. Measuring transmission through PC structures with varying numbers of periods N allows extraction of attenuation per period. Measuring the quality factor (Q) of resonators placed near PC structures indicates the effectiveness of mode suppression. Broadband spectroscopy (1-20 GHz or wider) is useful. Time-domain reflectometry (TDR) assesses impedance matching and reflections. Noise spectrum analysis can directly verify attenuation of specific noise frequencies. Resonator spectroscopy (measuring internal Qi and external Qe quality factors) quantifies loss reduction associated with PC integration. 3. **Qubit Performance Characterization:** The ultimate validation is the impact on superconducting qubit performance. Standard qubit spectroscopy (measuring qubit frequency vs. control parameters or probe power) determines operating frequencies, anharmonicity, and coupling strengths. Coherence time measurements (T1 via inversion recovery or thermalization; T2 via Ramsey decay, spin echo, or CPMG sequences) and single-qubit gate fidelities (measured using randomized benchmarking (RB), interleaved RB, or state tomography) are performed at milliKelvin temperatures (<20 mK). These measurements are compared for qubits fabricated *with* and *without* the integrated PC shielding (either on the same chip with different regions, or on different chips fabricated with and without the PC process steps, ideally with identical underlying qubit designs). Significant increases (e.g., 2x, 5x, 10x, or more) in measured T1 and T2 times (e.g., achieving >50-100 µs, aiming for milliseconds) and single-qubit fidelities (>99.9%, >99.99%) for PC-integrated qubits, particularly in dense multi-qubit environments where crosstalk and on-chip noise are significant, provide direct evidence of the shielding effectiveness. Crosstalk reduction is quantified by measuring the impact of operations or noise on neighboring elements on the target qubit (e.g., frequency shifts, unintended state changes, increased dephasing) and comparing this effect with and without PC isolation barriers in place. Reduced frequency shifts, suppression of unwanted transitions, and reduced dephasing rates indicate effective crosstalk suppression. Analyze frequency stability (drift, 1/f noise components) and sensitivity to controlled noise sources (e.g., flux, voltage, temperature, microwave power). Qubits themselves act as highly sensitive local probes of the electromagnetic environment. Compare performance statistics (e.g., mean and variance of T1, T2, fidelity) across populations of qubits on a chip or wafer. Spectroscopy on qubit transitions or coupled resonators can reveal suppressed spurious modes. The temperature dependence of coherence can help discern different loss mechanisms. Measuring coherence as a function of qubit or resonator frequency can reveal interactions with modes addressable by PCs. Correlated noise spectroscopy techniques can quantify the spatial extent and reduction of correlated noise and crosstalk. Two-qubit gate fidelities (e.g., using cross-entropy benchmarking, two-qubit RB) assess the impact on interactions during entangled or correlated operations. EPR analysis combined with FEM simulations can show how the PC modifies the coupling of the qubit to lossy environments. **Expected Performance Enhancements:** Integration of on-chip photonic crystal shielding structures is expected to yield substantial improvements in superconducting qubit performance. By effectively suppressing lossy modes and resonances, the method can significantly increase T1 and T2 coherence times by factors of 2x to 10x or more, especially for qubits in dense environments or those known to be susceptible to substrate loss, environmental radiation, or crosstalk. This pushes qubit coherence closer to the levels required for fault-tolerant quantum computing (>100 µs, with potential for ms). It effectively reduces unwanted crosstalk between circuit elements by attenuating electromagnetic fields propagating via substrate modes, surface waves, or near-field coupling. The frequency-selective nature of the bandgap ensures that desired control and readout signals (which are outside the bandgap) pass unimpeded with minimal loss or distortion, thereby maintaining or improving the quality of control pulses and enabling high-fidelity gate operations (>99.9%, aiming for >99.99%). The on-chip nature of the shielding provides localized protection against on-chip noise sources and their propagation. The ability to tailor the bandgap allows for targeted mitigation based on identified noise frequencies. Providing localized, frequency-selective control of the electromagnetic environment is critical for scaling to large-scale, high-coherence processors, enabling reduced interference between elements, improved stability and yield, and potentially increased integration density (allowing closer spacing between qubits). Reduced crosstalk simplifies control and calibration procedures. Enhanced coherence and reduced crosstalk lead directly to higher single- and multi-qubit fidelity and enable longer algorithm execution depths. The method helps qubits reach performance closer to their fundamental limits. Improved yield and performance uniformity across the chip and wafer are significant advantages for large-scale integration. Suppressing spurious modes simplifies frequency planning and improves the predictability of circuit behavior. The ability to suppress specific noise channels or pathways directly on-chip is fundamental for scaling fault-tolerant quantum computation. **Variations and Adaptations:** The present invention encompasses its application to various other types of superconducting qubits, including flux qubits, phase qubits, various cQED architectures (e.g., 3D transmons, capacitively or inductively coupled designs), fluxonium qubits, gatemons, and novel qubit designs. The specific design of the PC structures (e.g., material, structure, lattice type, parameters a/d, N), their placement, and the timing of their integration in the fabrication flow can be adapted based on the dominant qubit decoherence mechanisms (e.g., charge noise, flux noise, critical current noise, temperature effects, radiation, phonons), the mode profiles and frequencies of concern, and the specific circuit geometry. For example, flux-sensitive qubits (e.g., flux qubits, fluxoniums) may particularly benefit from metallic PCs that disrupt magnetic fields or current loops, while charge-sensitive qubits (e.g., transmons, gatemons) may benefit more from dielectric or metallic PCs that disrupt electric fields or surface charges. PC design should also consider the qubit's coupling scheme. The method is compatible with alternative low-loss substrates beyond Si and sapphire, such as intrinsic Si, diamond, silicon-on-insulator (SOI), quartz, and other low-loss oxides. It is also compatible with different superconducting materials (e.g., MoRe, various nitrides and silicides, multilayer stacks, potentially even high-temperature superconductors if compatible fabrication processes are developed). A wide range of PC designs beyond simple hole or pillar arrays can be integrated, including complex lattice types, scatterer shapes, or metamaterial-inspired structures. The method can also incorporate controlled defects in the PC lattice, such as line defects to create low-loss waveguides for guiding signals through shielded regions, or point defects to create high-Q cavities for frequency filtering or local modification of the electromagnetic environment (e.g., for Purcell enhancement of readout or Purcell protection of qubit lifetime). The approach is extendable to more complex 3D PC structures (potentially requiring techniques like focused ion beam milling, multi-layer stacking with precise alignment, or wafer bonding) to achieve complete 3D bandgaps and provide more robust, broadband shielding, although these methods involve significantly increased fabrication complexity. The precise placement, spatial extent, orientation, and parameters of the PCs are highly optimized based on the specific qubit design, detailed electromagnetic simulations (including effects from the cryogenic package and bond wires), noise analysis (spectrum, polarization, origin, and its dependence on qubit coherence and temperature), and the targeted mitigation mechanism. This invention provides a flexible framework for engineering the electromagnetic environment of superconducting qubits, leading to enhanced performance and yield, and enabling the scaling of quantum processors. It offers a significant advantage over global shielding approaches by allowing targeted noise suppression without sacrificing chip area or routing density. The on-chip nature minimizes unshielded path lengths and tailors the shielding to the specific on-chip electromagnetic environment. The PC structures can be seamlessly integrated with other on-chip components, such as control and readout electronics or lumped/distributed filters. The principles developed for PC integration can also be applied to engineer other aspects of the circuit, such as designing high-Q cavities, low-loss transmission lines, or on-chip impedance matching structures. The integrated PC shielding can be combined with other on-chip noise mitigation techniques, such as lumped-element or distributed filters, shielding layers, or quasiparticle traps, for compounded performance improvements. PCs can locally modify the density of states (LDOS) near the qubit, influencing spontaneous emission and coupling rates (Purcell effect), which can be beneficial for tailored qubit lifetime or improved readout. The general principles and fabrication methodologies can be adapted to engineering the electromagnetic environment in other quantum computing material systems that are compatible with nanophotonic or metamaterial fabrication techniques.