Neutral Atom Qubit

The neutral atom qubit encodes quantum information in internal states (typically hyperfine ground states) of individual neutral atoms trapped in optical tweezers or optical lattices. Entangling gates exploit strong dipole-dipole interactions via transient excitation to Rydberg states.

Figure

Description

Neutral atom platforms trap individual alkali atoms (⁸⁷Rb, ¹³³Cs) or alkaline-earth atoms (⁸⁸Sr, ¹⁷¹Yb) in tightly focused optical tweezer arrays. Qubit states are encoded in two hyperfine ground states |0⟩ and |1⟩, which have long coherence times due to weak coupling to the environment. Single-qubit gates are driven by microwave or two-photon Raman transitions. Two-qubit entangling gates use the Rydberg blockade mechanism: when one atom is excited to a high-lying Rydberg state (n ~ 50–100), the strong van der Waals interaction shifts the doubly-excited state out of resonance, creating a conditional phase.

Key advantages include identical qubits (no fabrication disorder), reconfigurable geometry via tweezer rearrangement, and mid-circuit atom reloading. Arrays of 1000+ atoms have been demonstrated, with programmable connectivity limited only by Rydberg interaction range.

Hamiltonian

Single-qubit Hamiltonian in the rotating frame:

$H_{\text{1Q}}=\frac{\hslash \Omega }{2}{\sigma }_x+\frac{\hslash \Delta }{2}{\sigma }_z$

where $\Omega$ is the Rabi frequency and $\Delta$ the detuning.

Rydberg blockade Hamiltonian for two atoms separated by distance $R$:

$H_{\text{2Q}}=\frac{\hslash {\Omega }_1}{2}|g_1⟩⟨r_1|+\frac{\hslash {\Omega }_2}{2}|g_2⟩⟨r_2|+\frac{C_6}{R^6}|r_1{r}_2⟩⟨r_1{r}_2|+\text{h.c.}$

The blockade condition $C_6/R^6\gg \hslash \Omega$ prevents simultaneous Rydberg excitation, enabling a controlled-Z gate.

Performance Metrics

Metric	Value	Notes	Fidelity reference
1Q gate fidelity	99.97%	¹³³Cs tweezers	Evered et al. 2023
2Q gate fidelity (CZ)	99.5%	¹³³Cs tweezers	Evered et al. 2023
T₂ (Ramsey)	~1 s	⁸⁷Rb hyperfine	Levine et al. 2022
T₂ (spin echo)	~10 s	¹⁷¹Yb clock states	Ma et al. 2022
Readout fidelity	99.8%	¹³³Cs fluorescence	Bluvstein et al. 2024
Array size	1225 atoms	Rb tweezer array	Pause et al. 2024
Atom loss per circuit	~0.5% per layer	¹³³Cs	Bluvstein et al. 2024

Scaling Considerations

• Connectivity: Reconfigurable via atom shuttling; effective all-to-all within Rydberg range (~5–10 μm).
• Parallelism: Global Rydberg pulses enable parallel entangling gates on non-interacting pairs.
• Error budget: Dominated by Rydberg state decay, atomic motion (Doppler shifts), and atom loss.
• Mid-circuit operations: Erasure detection via shelving to auxiliary states demonstrated; enables erasure-error conversion.

Linked Papers

• jaksch-2000-rydberg-gate

Related Entries

• rydberg-neutral-atom-qubit
• alkaline-earth-neutral-atom-clock-qubit
• nuclear-spin-neutral-atom-qubit
• surface-code-logical-qubit