Quantum information with Rydberg atoms

Abstract: Rydberg atoms with principal quantum number n >> 1 have exaggerated atomic properties including dipole-dipole interactions that scale as n^4 and radiative lifetimes that scale as n^3. It was proposed a decade ago to take advantage of these properties to implement quantum gates between neutral atom qubits. The availability of a strong, long-range interaction that can be coherently turned on and off is an enabling resource for a wide range of quantum information tasks stretching far beyond the original gate proposal. Rydberg enabled capabilities include long-range two-qubit gates, collective encoding of multi-qubit registers, implementation of robust light-atom quantum interfaces, and the potential for simulating quantum many body physics. We review the advances of the last decade, covering both theoretical and experimental aspects of Rydberg mediated quantum information processing.

• The paper demonstrates the feasibility of quantum computing with Rydberg atoms through blockade mechanisms that enable high-fidelity quantum gates.
• It details how optimized Rabi frequencies and composite pulse schemes mitigate errors from spontaneous emission and imperfect blockade.
• The study highlights ensemble encoding and collective atom interactions as promising routes toward scalable quantum information protocols.

Quantum Information with Rydberg Atoms: A Review

Quantum information processing with Rydberg atoms has emerged as a significant field of research, leveraging the exaggerated atomic properties of these atoms. The foundational idea of using Rydberg atoms for quantum computing is attributed to their strong, long-range dipole-dipole interactions which can be coherently turned on and off, thus enabling the implementation of quantum gates between neutral atom qubits.

Rydberg Atom Properties and Interactions

Rydberg atoms, characterized by a high principal quantum number $n \gg 1$, exhibit enhanced atomic interactions, such as dipole-dipole interactions scaling as $n^4$ and radiative lifetimes as $n^3$. These properties make them suitable for quantum information tasks far beyond the initial proposal of quantum gates. The interactions can be categorized as resonant dipole-dipole at shorter ranges or van der Waals at longer ranges, with the crossover point dependent on $n$. The resonant interactions are particularly advantageous as they scale favorably with larger $n$, allowing gates to be operated on separations of several microns with high fidelity.

Quantum Gates and Blockade

The basic guideline for quantum gates with Rydberg atoms is the Rydberg blockade mechanism, which fundamentally relies on the suppression of simultaneous excitation due to strong interactions between Rydberg states. The Rydberg blockade two-qubit gate, proposed by Jaksch and colleagues, uses this mechanism for implementing entangling gates such as CNOT. A primary source of error is the finite lifetime of Rydberg states, typically remedied by fast gate operation with MHz-scale pulses.

Error Analysis

Quantitative analyses indicate that gate fidelity is sensitive to variables such as spontaneous emission from Rydberg states and blockade error, with the latter stemming from imperfect blockade. Optimizing Rabi frequencies and employing composite pulse schemes can mitigate these errors, potentially reducing gate errors to values below the fault-tolerant threshold of $10^{-3}$. High $n$ values allow for gate operation over distances greater than $40 \mu m$ with errors well below this threshold.

Collective Encoding and Ensembles

In addition to single-qubit operations, Rydberg atoms offer opportunities for ensemble-based quantum computing. The concept of encoding logical qubits in many-atom ensembles exploits the collective properties of Rydberg blockade. This brings forth the potential for inherently scalable quantum information protocols without individually addressing each atom. For instance, collective states enable protocols for deterministic single atom loading, qubit encoding, and error correction.

Experimental Progress and Challenges

Notable experimental progress includes demonstration of Rabi oscillations in Rydberg states, Rydberg blockade between atom pairs, and fundamental quantum gate operations. These achievements mark significant steps toward practical implementations of quantum computing with neutral atoms. However, experimental results still lag behind theoretical predictions, primarily due to technical challenges such as laser stability, atomic localization, and suppression of environmental decoherence.

Future Directions and Applications

The versatility of Rydberg atoms in quantum optics and quantum information processing is underscored by proposed applications in quantum interfaces, hybrid systems, and simulations of complex quantum systems. The long-range nature of Rydberg interactions facilitates entanglement across large distances, making them suitable for quantum networking and communication. Furthermore, employing Rydberg atoms for simulating multi-body quantum systems can provide insights into condensed matter physics problems.

Overall, the research on quantum information processing with Rydberg atoms is poised to make significant contributions to the development of quantum technologies. Expanding the operational capabilities and coherence times through continued research and technological advancement will be crucial for realizing scalable, fault-tolerant quantum computers in the future.