[Paper Review] Single-shot measurement of a Rydberg superatom via collective photon burst

Chao-Wei Yang, Jun Li+4 more|arXiv (Cornell University)|Jun 21, 2021

Cold Atom Physics and Bose-Einstein Condensates34 references4 citations

TL;DR

This paper demonstrates deterministic, single-shot measurement of a Rydberg superatom qubit using collective photon burst in a low-finesse cavity, achieving 93.2% fidelity. By leveraging Rydberg blockade and microwave Raman pulses, the system generates sequential photons when in the |r₁⟩ state and suppresses emission in |r₂⟩, enabling high-fidelity state discrimination via photon counting in 4.8 μs with 44% in-fiber retrieval efficiency.

ABSTRACT

With Rydberg dipole interactions, a mesoscopic atomic ensemble may behave like a two-level single atom, resulting in the so-called picture of superatom. It is in potential a strong candidate as a qubit in quantum information science, especially for efficient coupling with single photons via collective enhancement that is essential for building quantum internet to connect remote quantum computers. Previously, preliminary studies have been carried out in demonstrating basic concept of Rydberg superatom, a single-photon source, and entanglement with a single photon, etc. While a crucial element of single-shot qubit measurement is still missing. Here we realize the deterministic measurement of a superatom qubit via photon burst in a single shot. We make use of a low-finesse ring cavity to enhance the atom-photon interaction and obtain an in-fiber retrieval efficiency of 44%. Harnessing dipole interaction between two Rydberg levels, we may either create a sequence of multiple single photons or nothing, conditioned on the initial qubit state. We achieve a single-shot measurement fidelity of 93.2% in 4.8 us. Our work complements the experimental toolbox of harnessing Rydberg superatom for quantum information applications.

Motivation & Objective

• To close the critical gap in experimental quantum control by demonstrating deterministic single-shot measurement of a Rydberg superatom qubit.
• To overcome the limitations of inefficient, probabilistic measurement in atomic ensembles by enabling lossless, high-fidelity readout via collective photon emission.
• To integrate cavity enhancement with Rydberg blockade and microwave control for scalable, high-fidelity qubit measurement in a single shot.
• To enable projective measurements in arbitrary bases for quantum state tomography using engineered photon bursts.

Proposed method

• A low-finesse ring cavity enhances atom-photon interaction, achieving 44% in-fiber single-photon retrieval efficiency with an optical depth of 1.9.
• Two-photon excitation via 795 nm and 474 nm lasers prepares a Rydberg superatom in a collective |r₁⟩ state within the Rydberg blockade radius.
• Microwave Raman pulses coherently transfer the superatom between |r₁⟩ and |r₂⟩ states, enabling state-dependent photon emission.
• Photon burst is generated by repeated excitation and retrieval cycles (12 times), with detection via single-mode fiber-coupled superconducting nanowire single-photon detectors.
• Dynamical noise filtering via a Pockels cell suppresses background noise, improving signal-to-noise ratio for single-shot discrimination.
• Quantum state tomography is performed by applying arbitrary unitary transformations using variable microwave pulses and measuring in different bases.

Experimental results

Research questions

• RQ1 Can a Rydberg superatom qubit be measured in a single shot with high fidelity using collective photon emission?
• RQ2 How does cavity enhancement affect the retrieval efficiency and measurement fidelity of collective atomic excitations?
• RQ3 Can microwave Raman pulses enable state-dependent photon burst generation for deterministic measurement?
• RQ4 What is the achievable fidelity of single-shot measurement when distinguishing between |r₁⟩ and |r₂⟩ superatom states?
• RQ5 Can arbitrary basis measurements be realized for quantum state tomography using engineered photon bursts?

Key findings

• The single-shot measurement fidelity reaches 93.2% after correcting for state preparation infidelity, with 94.6% for |r₂⟩ and 91.8% for |r₁⟩.
• The in-fiber single-photon retrieval efficiency is enhanced to 44% using a low-finesse cavity, significantly outperforming free-space retrieval (21% maximum).
• For the |r₁⟩ state, the system emits 2.63 photons on average in 4.8 μs, while the |r₂⟩ state suppresses emission to 0.19 photons, enabling clear discrimination.
• The photon emission statistics show a Poisson-like distribution with 8.2% zero-photon events for |r₁⟩, and 1.2% dark count noise contributes to residual emission in |r₂⟩.
• Quantum state tomography of superposition states (|r₁⟩+|r₂⟩)/√2 and 0.88|r₁⟩+0.47|r₂⟩ achieves fidelities of 89.3% and 88.6%, respectively.
• The measured raw fidelity is 91.3%, with the primary fidelity degradation attributed to microwave pulse infidelity and mechanical phase instability.