Rydberg Neutral-Atom Qubit

Figure

Description

The Rydberg neutral-atom qubit architecture encodes quantum information in long-lived hyperfine ground states of neutral atoms (typically alkali species such as ${}^{87}\text{Rb}$ or ${}^{133}\text{Cs}$), with $|0⟩$ and $|1⟩$ defined in hyperfine “clock” states that are first-order insensitive to magnetic field fluctuations.

Entangling interactions are turned on transiently by laser excitation to high-lying Rydberg states (principal quantum number $n\sim 50$–$100$). The Rydberg blockade effect is the key mechanism: when one atom is excited to a Rydberg state, nearby atoms within the blockade radius $R_b$ experience a large energy shift from the van der Waals interaction ($V_{ij}=C_6/R_{ij}^6$), preventing simultaneous double excitation. This conditional dynamics enables controlled-phase (CZ) entangling gates.

Atoms are individually trapped and positioned using reconfigurable optical tweezer arrays, providing native all-to-all connectivity at the array level through tweezer rearrangement. Arrays of 100–1000+ atoms have been demonstrated, making this one of the most scalable qubit platforms.

This is the core hardware model for modern optical-tweezer neutral-atom processors being developed by Harvard/MIT (Lukin group), QuEra, Pasqal, Atom Computing, and others.

Hamiltonian

A standard driven Rydberg-array Hamiltonian (rotating frame) is:

$H={\sum }_i\left[\frac{{\Omega }_i(t)}{2}{\sigma }_x^{(i)}-{\Delta }_i(t)n_i\right]+{\sum }_{i<j}{V}_{ij}{n}_i{n}_j$

where $n_i=|r{⟩}_i⟨r|$ projects onto the Rydberg state, ${\Omega }_i$ is the Rabi drive, ${\Delta }_i$ detuning, and $V_{ij}=C_6/R_{ij}^6$ (van der Waals) or $\propto 1/R_{ij}^3$ in resonant dipole regimes.

Blockade condition for two atoms $i,j$:

$V_{ij}\gg \Omega$

suppresses double excitation $|rr⟩$, enabling controlled-phase entangling gates.

Motivation

Rydberg neutral-atom qubits combine three key advantages: (1) native all-to-all reconfigurability via tweezer rearrangement at the array level, (2) strong, switchable interactions for fast entangling gates with no residual always-on coupling, and (3) a direct path from analog quantum simulation to digital gate-based computing on the same hardware. The platform has demonstrated the largest qubit arrays of any technology and is a leading candidate for near-term fault-tolerant quantum computing.

Experimental Status

Foundational Rydberg blockade gate proposal — Jaksch et al. (2000):

• Proposed fast quantum gates for neutral atoms using the Rydberg blockade mechanism
• Established the theoretical basis for the entire Rydberg entangling gate paradigm

Logical quantum processor — Bluvstein et al. (2024):

• Demonstrated a logical quantum processor based on reconfigurable atom arrays
• 48 logical qubits with entangling operations between logical qubits
• Two-qubit CZ gate fidelity of 99.5% with erasure conversion
• Single-qubit gate fidelity of 99.5–99.9% via global and local Raman gates
• Hyperfine clock state coherence $T_2$ of 1–4 s with spin-echo

Key Metrics

Metric	Value	Notes	Fidelity reference
$T_1$ (qubit lifetime)	1–10 s	Hyperfine clock states in optical traps; vacuum-limited	Bluvstein et al. 2024
$T_2$ (coherence)	1–4 s	With spin-echo; limited by trap-induced dephasing	Bluvstein et al. 2024
1Q gate fidelity	99.5–99.90%	Global and local Raman gates	Bluvstein et al. 2024
2Q gate fidelity (CZ)	99.5%	Rydberg blockade CZ; SOTA with erasure conversion	Bluvstein et al. 2024
Gate time (2Q)	200–500 ns	Rydberg pulse sequence	Bluvstein et al. 2024
Architecture scale	100–1000+ physical atoms	Optical tweezer arrays; reconfigurable	—
2Q gate mechanism	Rydberg blockade	$C_6/R^6$ van der Waals interaction	—
Operating temperature	~10–50 μK	Laser-cooled; in ultrahigh vacuum	—

References

Original proposal

• D. Jaksch et al., “Fast Quantum Gates for Neutral Atoms,” Phys. Rev. Lett. 85, 2208 (2000)

Experimental demonstrations

• D. Bluvstein et al., “Logical quantum processor based on reconfigurable atom arrays,” Nature 626, 58 (2024) — arXiv:2312.03982

Linked Papers

• jaksch-2000-rydberg-gate
• henriet-2020-quantum-computing-with-neutral
• wu-2020-a-concise-review-of

Related Entries

• trapped-ion-qubit — competing digital quantum computing platform
• alkaline-earth-neutral-atom-clock-qubit — related neutral-atom qubit using alkaline-earth species
• nuclear-spin-neutral-atom-qubit — related neutral-atom qubit using nuclear spin encoding
• cesium-133-neutral-atom-qubit — cesium Rydberg qubits with larger polarizability than rubidium