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Integrated optical multi-ion quantum logic
Karan K. Mehta, Chi Zhang, Maciej Malinowski, Thanh-Long Nguyen, Martin Stadler, Jonathan P. Home
TL;DR
The paper addresses optical-scaling barriers in trapped-ion quantum processors by integrating planar-fabricated optics with a surface-electrode trap. Direct fibre coupling across multiple channels supports cryogenic operation and multi-ion gates, achieving two-ion entangled-state fidelities above 99.3(2)%.
Problem
Optics for precise trapped-ion control remain a barrier to improving quantum-information-processing error rates and scale.
Method
The authors co-fabricate scalable planar optics with a surface-electrode ion trap and deliver light through multiple directly coupled fibres in a cryogenic environment.
Results
Two-ion entangled states were generated with fidelities >99.3(2)%.
Takeaways & Limitations
The integrated hardware reduces alignment requirements and offers a route to robust, parallelizable high-fidelity quantum processors.
Takeaways & Limitations
Motional-frequency drifts remained an experimental limitation, with an average 200 Hz drift between recalibrations and an estimated infidelity of 1 × 10^-3.
Abstract
from arXiv · showhide
Practical and useful quantum information processing (QIP) requires significant improvements with respect to current systems, both in error rates of basic operations and in scale. Individual trapped-ion qubits' fundamental qualities are promising for long-term systems, but the optics involved in their precise control are a barrier to scaling. Planar-fabricated optics integrated within ion trap devices can make such systems simultaneously more robust and parallelizable, as suggested by previous work with single ions. Here we use scalable optics co-fabricated with a surface-electrode ion trap to achieve high-fidelity multi-ion quantum logic gates, often the limiting elements in building up the precise, large-scale entanglement essential to quantum computation. Light is efficiently delivered to a trap chip in a cryogenic environment via direct fibre coupling on multiple channels, eliminating the need for beam alignment into vacuum systems and cryostats and lending robustness to vibrations and beam pointing drifts. This allows us to perform ground-state laser cooling of ion motion, and to implement gates generating two-ion entangled states with fidelities $>99.3(2)\%$. This work demonstrates hardware that reduces noise and drifts in sensitive quantum logic, and simultaneously offers a route to practical parallelization for high-fidelity quantum processors. Similar devices may also find applications in neutral atom and ion-based quantum-sensing and timekeeping.
1. Device design and fabrication
The device integrates SiN waveguides and gratings with a surface-electrode ion trap, using multilayer fabrication and wafer-scale reticle layouts for optical delivery to multiple trap zones.
- Device design: 50% designed radiation efficiency uses substrate reflection to strengthen gratings while selecting a single grating order.The grating wavenumber is chosen relative to the grating period to ensure single-order emission.
- Device design: 1 dB simulated coupling loss connects a 5.4 µm mode-field-diameter optical mode to the 25 nm SiN waveguide mode.The thin SiN waveguides are strongly polarizing, and polarization is maintained in the high-confinement routing waveguides.
- Design verification: 3D FDTD and COMSOL simulations verify grating emission and ion-trap potential designs, with layouts drawn in Cadence Virtuoso.
- Fabrication layout: 2.2 × 2.2 cm2 reticles are repeated across a 4-inch wafer and include trap designs plus independent optical test structures.The fabrication process provides approximately ±2 µm layer alignment and about 300 nm alignment of e-beam grating lines to photolithographic waveguides.
- Optical interfaces: Eight waveguides interface with the fibre array, while inputs 1 and 8 form a loop used to align the fibre V-groove array.
2. Assembly and fibre attachment
The chip uses an eight-channel fibre V-groove array attached at a polished coupling edge, with an assembly designed to preserve alignment through cryogenic operation.
- Fibre interface: Eight standard non-polarization-maintaining fibres are spaced at 127 µm pitch, with in-line polarizers controlling input polarization.
- Attachment process: Static friction passively maintains fibre-chip coupling during attachment before flexible epoxy is applied at the interface edges.The die curvature keeps epoxy away from the optical mode while allowing mechanically suitable, non-transparent epoxy.
- Characterization: The coupling-loss measurement uses transmission from fibre 1 to fibre 8 through an on-chip loop-back structure, subtracting measured waveguide loss.
- Cryogenic performance: 1.4 dB room-temperature coupling loss increases to 2.4 dB at 729 nm after cooling to 7 K, with no further changes after two additional temperature cycles.
3. Waveguide/grating characterization and optical losses
Characterization measures propagation loss, grating emission, and ion-control speed, showing that the integrated optical path delivers the designed beam and supports coherent control.
- Waveguide losses: 2 dB/cm waveguide loss at 729 nm is measured in high-confinement routing structures and corroborated with ring-resonator quality factors.Loss decreases with increasing wavelength, consistent with sidewall-roughness scattering and motivating improved lithography.
- Grating characterization: Grating emission is profiled by imaging beam intensity at multiple heights above the chip with a vertically translated high-NA microscope and CCD camera.
- Ion control: 2.6 µs observed π-time with 1.5 mW input is within 25% of the 2.0 µs first-principles prediction using the measured beam profile and 6.4 dB total loss.The loss budget is 2.4 dB fibre-chip coupling, 1 dB waveguide loss, and 3 dB grating emission loss.
- Power handling: 300 mW at 729 nm is coupled into the single-mode waveguides without observed waveguide damage.Future measurements will examine power limits and self-phase modulation at visible wavelengths.
4. Laser/cryogenic apparatus, and trap operation
The experiments combine stabilized 729 nm laser control, cryogenic trapping, and multi-directional micromotion compensation, while exposed dielectric charging remains a source of motional-frequency drift.
- Laser apparatus: 100 Hz-order linewidth 729 nm light is stabilized to a high-finesse reference cavity for coherent qubit control.
- Cryogenic apparatus: Cryogenic operation provides cryopumping, approximately 2-day trap installation or replacement, no in-vacuum baking, and potentially reduced surface electric-field noise.
- Trap operation: 0.01 micromotion-sideband-to-carrier Rabi-frequency ratio is achieved by minimizing sidebands along two separate 729 nm beam paths for three-dimensional compensation.The integrated grating beam provides sensitivity along x and z, while a second free-space beam provides sensitivity along y.
- Dielectric effects: Charging associated with exposed silicon near grating windows produces effects related to carrier dynamics and can perturb stable trap operation.The device includes a platinum ground plane intended to shield the substrate from trap RF fields and mobile charge carriers.
- Frequency stability: 200 Hz average motional-frequency drift between recalibrations is reduced to a few 100 Hz by pulsing the 729 nm beam during Doppler cooling and stabilizing optical flux.The experiment recalibrates motional frequency every 15 s during Mølmer–Sørensen gate experiments.
5. Contributions to Bell-state infidelity and routes to improvement
The Bell-state infidelity is distributed across motional heating and frequency drifts, laser phase noise, radial-mode effects, and state-detection errors. The authors identify technical improvements that could substantially reduce these contributions.
- Measured error sources: 2(1) × 10^-3 gate error arises from heating of the two-ion stretch mode, whose measured heating rate is 60(30) quanta/s.The stretch-mode heating contribution is estimated using the single-loop gate duration.
- Measured error sources: 1 × 10^-3 infidelity is attributed to carrier-frequency noise inferred from Ramsey measurements, while motional and carrier drifts together contribute about 2 × 10^-3.The carrier-noise estimate is a lower bound because only the dominant noise component is modeled.
- Measured error sources: 4 × 10^-4 infidelity is estimated from warm radial modes, whose occupancy fluctuations shift the stretch-mode frequency through a Kerr-type interaction.Radial modes remain above the ground state for technical reasons related to EIT-cooling beam directions.
- Measured error sources: 0.9 × 10^-4 of Bell-state infidelity comes from misclassifying 1- and 2-bright-ion detection events, while state-preparation infidelity is below 10^-4.Detection uses 250 µs PMT-count acquisition and thresholding among 0-, 1-, and 2-bright-ion outcomes.
- Routes to improvement: About 0.6% total infidelity is accounted for by the summarized error sources, with ground-state cooling of radial modes projected to suppress some errors by more than 100×.Additional proposed improvements include reducing motional frequency drifts, laser noise, and heating rates.
6. Crosstalk between trap zones
The integrated device supports parallel operation across trap zones with very low optical crosstalk. Measurements attribute the residual signal to both on-chip waveguide coupling and grating-emission sidelobes.
- Measured crosstalk: −60 dB relative intensity is measured in a non-addressed zone when adjacent-zone light is applied, corresponding to a 1000× lower Rabi frequency.The test compares direct addressing of zone 3 through input 3 with zone-2 light applied through input 5.
- Measurement method: The crosstalk measurement uses Rabi oscillations at zone 3 to probe light delivered through either its direct input or adjacent-zone inputs.This provides an operational test of crosstalk relevant to parallel multi-zone addressing.
- Crosstalk mechanism: Crosstalk is attributed to on-chip coupling between waveguides and weak sidelobes from the neighboring zone’s grating.Similar −60 dB crosstalk is observed when light is sent through an input that propagates around the device without feeding a grating.
7. Hybrid Zeeman/optical qubit encoding
A hybrid encoding stores qubits in long-lived Zeeman sublevels and activates an optical level only for two-qubit interactions. Global mapping pulses remove sensitivity of the composite operation to a constant laser phase offset.
- Motivation: Hybrid storage addresses the optical transition’s phase-stability and 1.1 s spontaneous-emission constraints by limiting optical excitation to interaction operations.The proposed arrangement combines Zeeman-state storage with optical two-qubit interactions and RF single-qubit gates.
- Encoding scheme: The |0⟩ and |1⟩ Zeeman sublevels serve as a long-term memory qubit, while |1⟩ is mapped to an optical level for multi-qubit gates.The mapping uses π rotations between |1⟩ and |2⟩ before and after the optical interaction.
- Phase insensitivity: The composite unitary R12(φ) U_MS02(φ) R12(φ) is independent of the constant laser phase offset φ.The phase cancellation applies to the described MS gate and, as stated, to any optically implemented unitary.