Quantum Computing: All-fiber QED system aims for deterministic rather than probabilistic quantum entanglement

May 1st, 2019
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A cavity quantum electrodynamics (QED) system is one in which atoms can interact with photons in a confined cavity. Since photons escaping from a cavity can travel long distances and enter another cavity, two cavity QED systems can be connected via a photonic channel. Ultimately, these coupled-cavity QED systems can be used to entangle distant atoms, leading to useful quantum effects such as quantum computers and networks.

Towards that end, researchers at Waseda University (Tokyo, Japan), the Japan Science and Technology Agency (JST; Saitama, Japan), and the University of Auckland (Auckland, New Zealand) built an all-optical-fiber-based, coupled-cavity QED system and have observed a reversible interaction between distant atoms and delocalized photons separated by up to 2 m—a record for this type of QED system.1 With this system, it will be possible to create deterministic (rather than probabilistic) quantum entanglement, which is the first step towards physical implementation of a QED-based distributed quantum computer.

In the coupled-cavity QED system, cavities 1 and 2 of length L1 and L2 (with coupling rates v1 and v2 as well as atom-cavity coupling rates of g1 and g2, respectively) couple to a length of fiber Lf; measurements are made from left to right using a probe beam. (Image credit: Takao Aoki/Waseda University)

Coupled-cavity implementation

In the experimental setup, two cavities measuring 0.92 and 1.38 m in length are connected by a length of optical fiber (with values of 1.23, 0.83, and 2.27 m) to facilitate quantum interactions between atoms and photons. Each cavity consists of a tapered optical fiber between two fiber Bragg grating (FBG) mirrors, effectively creating an expanded, evanescent mode field wherein photons can interact with several tens of atoms “trapped” in the tapered region (see figure).

To measure different interactions, the atoms are either loaded in the first cavity only, the second cavity only, in both cavities, or in neither cavity. The photon-atom interactions are measured by quantifying the spectral output of a laser probe beam input to the left side of the system as the connecting fiber lengths are varied.

Spectral output from the system with different atom-loading conditions and different lengths of connecting optical fiber are experimentally in good agreement with theoretical predictions for the dressed states of distant atoms with delocalized photons. Two empty cavities produce a triple-peak spectral signature with a strong “fiber-dark” central peak and two “fiber-bright” peaks. Atom-loaded cavities split the central fiber-dark peak into two, which is indicative of reversible interaction between distant atoms and delocalized photons in the fiber-dark mode.

The researchers have constructed a coupled-cavity QED system in which two nanofiber cavity QED systems are coherently connected via a meter-long fiber. One can deterministically create quantum entanglement between two quantum systems with reversible interaction between them, and this can be used to realize deterministic quantum gates in a quantum computer. Therefore, a large-scale coupled-cavity QED system will enable a “distributed” quantum computer, where quantum information is stored and processed in distant atoms connected to each other by photons. The current achievement is the first step towards this goal.

“It has been technically challenging to connect multiple cavity-QED systems based on conventional free-space cavities with sufficiently low losses to realize coherent coupling,” says Takao Aoki, professor at Waseda University. “For our all-optical-fiber-based cavities, we can easily connect them with minimal losses, simply by using a commercial fiber splicer,” he adds.—Gail Overton

REFERENCE

1. S. Kato et al., Nat. Commun., 10, 1160 (Mar. 11, 2019).

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