Entanglement of Two Atoms using Rydberg Blockade

TL;DR

This work surveys the experimental realization of entanglement between two neutral rubidium atoms via the Rydberg blockade, detailing the physical principles, trapping/readout methods, state preparation, and coherent Rydberg control required for two-qubit operations. The authors implement a controlled-phase (CZ) gate using blockade, demonstrate a CNOT gate, and verify entanglement through Bell-state measurements and parity oscillations, achieving a fidelity of about 0.72 after accounting for atom loss. The paper also discusses practical routes to scale this approach to larger qubit registers, including deterministic loading, dark trap schemes, and alternative excitation schemes, while addressing fundamental and technical limits to fidelity and coherence. Overall, the results establish a viable pathway toward scalable, Rydberg-mediated quantum information processing with neutral atoms and highlight key technical advances needed for higher-fidelity, multi-qubit architectures.

Abstract

Over the past few years we have built an apparatus to demonstrate the entanglement of neutral Rb atoms at optically resolvable distances using the strong interactions between Rydberg atoms. Here we review the basic physics involved in this process: loading of single atoms into individual traps, state initialization, state readout, single atom rotations, blockade-mediated manipulation of Rydberg atoms, and demonstration of entanglement.

Figures (15)

• Figure 1: (left) Geometry of Rydberg blockade. Two atoms are trapped in separate FORTs. Each atom can be addressed with Raman and Rydberg resonant lasers to perform single-atom and two-atom gates. (right) Energy levels and laser wavelengths. The qubit states $|{0}\rangle$ and $|{1}\rangle$ are two hyperfine levels of the 5s ground state of Rb. Raman and Rydberg Rabi excitations are driven by 2-photon resonant lasers through the virtual $|{5p}\rangle$ excited state. The Rydberg-Rydberg interaction $V_{rr}$, present when both atoms are in the Rydberg state $|{r}\rangle$, enables entanglement by blocking excitation of both atoms to the Rydberg state.
• Figure 2: a) Protocol for controlled-phase gate. b) Experimental demonstration of the $\pi$ phase shift induced in the target atom by changing the state of the control atom. From Isenhower2010.
• Figure 3: Experimental setup for Rydberg gate experiments.
• Figure 4: (left) Histogram of photon counts from trapped atoms from Johnson2008, showing also the ability to clearly distinguish a single atom from the background and to detect the presence of one atom without removing it from the trap. (right) Multiple exposure photograph of the spatial distribution of single atoms in the two FORT sites, from Urban2009.
• Figure 5: Optical pumping transients for preparation of the $|{1}\rangle$ qubit level. Left: with both pumping lasers on, the atoms rapidly accumulate in the $F=2,m=0$ ($|{1}\rangle$) level. Right: blocking the $F=1$ laser allows measurement of the rate at which atoms are removed from the $|{1}\rangle$ state.