Trapped-ion Quantum Computing

Quantum computing leverages quantum mechanics, where quantum bits (qubits) can exist in multiple states at once, unlike classical bits. This allows quantum computers to explore many possibilities simultaneously, potentially offering breakthroughs in fields such as cryptography and optimization.

A key feature is entanglement, where qubits are linked, and the state of one can affect another, even at a distance. This property enables quantum computers to solve certain problems exponentially faster than classical ones. Despite its potential, quantum computing faces challenges like qubit coherence and error rates. Various hardware platforms, including trapped ions and superconducting qubits, are being explored to overcome these issues and build scalable quantum systems.

Among them, trapped-ion-based quantum computing stands out as one of the most promising and mature platforms, offering advantages such as long coherence times, high gate fidelity, and all-to-all connectivity between qubits.

Ions are confined within electromagnetic fields, where their internal energy levels serve as qubit states. These states can be precisely manipulated using either microwaves or lasers. The Coulomb repulsion between ions sharing the same potential enables qubit interactions, functioning as a quantum bus to facilitate entanglement.

At QuIQCL, we utilize quadrupole ion traps, known as Paul traps, which employ static and oscillating electric fields to confine ions. Paul traps vary in design depending on electrode geometry, and our research focuses on silicon-chip-based surface traps and blade traps. We are investigating phenomena within ion traps to enhance our understanding of their behavior and improve their performance.

Development of a Full Stack Quantum Computation System

Our goal is to develop a scalable quantum computation system, building up from in-house microfabricated chips and custom control hardware to a programmable software interface, pipelined through layers of abstraction that translate high-level algorithms into low-level pulses.

We have demonstrated the trapping of > 80 ions in our surface trap for more than 2 hours, implemented two-qubit gates using individually addressable control hardware, and are now working on building a fully-connected > 5-qubit system with 171 Yb+ ions.

Together with our fabrication team, we are also focused on suppressing sources of errors in our surface traps, which include photo-induced charging and electric field noise associated with the material of the chip. We have successfully mitigated semiconductor charging in our silicon-based chip [1,2], and are now moving towards reducing the heating rate for high-fidelity operations.

Manipulation of Bosonic Modes

Manipulating bosonic modes using trapped ions, specifically by leveraging the harmonic oscillator motional modes, is one of our key focuses. While qubit-based quantum computing remains a major area of interest, we also explore the rich potential of bosonic modes, particularly in continuous-variable quantum computing (CVQC) and hybrid quantum computing.

The work is being conducted with a macroscopic linear Paul trap composed of four blade-like electrodes, resembling a four-rod trap. Due to the relatively large ion-electrode distance, this trap exhibits low heating rates, on the order of a few quanta per second, which allows us to explore motional states with minimal corruption.

One of our key accomplishments is the implementation of the Mølmer-Sørensen gate, a widely used two-qubit gate in trapped ion systems, applied to a two-ion chain. This gate utilizes the motional mode to acquire geometric phases, which are then transferred to the qubit state to achieve two-qubit entanglement. The spin-motion coupling inherent in trapped ion systems provides us with the ability to manipulate the motional state in various ways.

The spin-dependent force operation, which is a key element of the Mølmer-Sørensen gate, enables us to displace the motional state in different directions based on the spin state. This capability allows us to generate cat states and entangled coherent states, both of which are valuable resources in CVQC. Additionally, we can implement the spin-dependent beam splitter interaction, which also serves as a useful resource in CVQC and quantum metrology. Using the spin-dependent beam splitter, we can perform measurements of multimode joint parity and the Wigner function of bosonic states.

Quantum simulation also represents an exciting area of exploration for us. We believe that spin-motion coupling interactions with trapped ions can be used to implement a wide variety of Hamiltonians, further advancing the field of quantum simulations.

Development of a Cryogenic Ion Trap System

Cryogenic ion trap systems offer several advantages over room-temperature setups. Notably, they allow for rapid turnaround when switching trap modules, which will enable efficient testing of various chip designs produced by our fabrication team. The system will focus on manipulating Barium ions, which have gained interest for their transition levels in the visible wavelength range. In particular, we plan to mount our first generation photonics-integrated ion trap chips in the cryogenic system and investigate the scalability of such modules.

Segmented blade trap with Yb and Ba ions

We are currently building a new trap with segmented blade-like DC electrodes. This design allows us to create more flexible DC potentials, enabling us to move the trapped ions and adjust their spacing, among other modifications. Additionally, the setup is designed to facilitate ablation loading of ions and is capable of trapping dual species: Ytterbium (Yb) and Barium (Ba) ions.

By using this trap, we aim to enhance our understanding and capabilities in manipulating Barium ions. Barium is a prominent ion species for quantum applications, as its optical wavelengths are primarily in the visible spectrum, whereas Ytterbium typically requires ultraviolet (UV) wavelengths. Furthermore, trapping dual species simultaneously will enable us to explore the intriguing properties of an inhomogeneous ion chain, where characteristics such as mass and transition wavelengths differ between the species.

References

[1] W. Lee, D. Chung et al. Phys. Rev. A 109, 043106 (2024)

[2] D. Chung, K. Choi et al. arXiv:2411.13955v1

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