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Photonics North (PN)

Superradiance from spectrally tailored absorption lines in a cavity

Sourabh Kumar, Erhan Sağlamyürek, Daniel Oblak

Published May 24, 2022• 0 citations
Publisher

Abstract

Superradiance is a coherent emission phenomenon from an ensemble of emitters that share a collective excitation, and has potential applications in quantum metrology, quantum communication, and quantum computing. We demonstrate superradiant emission in a thulium-doped yttrium aluminium garnet (Tm:YAG) crystal, whose input and output faces are reflection coated to form a cavity that satisfies the impedance matching condition for light on resonance with the Tm transition line. This implementation allows us to observe superradiant emission in the low optical depth regime (OD approx. 1). We engineer the characteristic decay of the superradiant emission by tailoring the inhomogeneously broadened spectrum of Tm:YAG into absorption lines with different shapes, depths, and widths, and perform simulations over a wide range of parameters. We extend this investigation and discuss the possibility of a broadband quantum memory based on superradiance for long-distance quantum networks.

DOI
10.1109/pn56061.2022.9908376

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Superradiance is a coherent emission phenomenon from an ensemble of emitters that share a collective excitation, and has potential applications in quantum metrology, quantum communication, and quantum computing. We demonstrate superradiant emission in a thulium-doped yttrium aluminium garnet (Tm:YAG) crystal, whose input and output faces are reflection coated to form a cavity that satisfies the impedance matching condition for light on resonance with the Tm transition line. This implementation allows us to observe superradiant emission in the low optical depth regime (OD approx. 1). We engineer the characteristic decay of the superradiant emission by tailoring the inhomogeneously broadened spectrum of Tm:YAG into absorption lines with different shapes, depths, and widths, and perform simulations over a wide range of parameters. We extend this investigation and discuss the possibility of a broadband quantum memory based on superradiance for long-distance quantum networks.
We design and implement an atomic frequency comb quantum memory for 793-nm wavelength photons using a monolithic cavity based on a thulium- (Tm-) doped Y3Al5O12{\mathrm{Y}}_{3}{\mathrm{Al}}_{5}{\mathrm{O}}_{12}Y3​Al5​O12​ crystal. Approximate impedance matching results in the absorption of 90% of input photons and a memory efficiency of $(27.5\ifmmode\pm\else\textpm\fi{}2.7)%$ over a 500-MHz bandwidth. The cavity enhancement leads to a significant improvement over the previous efficiency in Tm-doped crystals using a quantum memory protocol. In turn, this allows us to store and recall quantum states of light in such a memory. Our results demonstrate progress toward efficient and faithful storage of single-photon qubits with a large time-bandwidth product and multimode capacity for quantum networking.
Superradiance, characterized by the collective, coherent emission of light from an excited ensemble of emitters, generates photonic signals on timescales faster than the natural lifetime of an individual atom. The rapid exchange of coherence between atomic emitters and photonic fields in the superradiant regime enables a fast, broadband quantum memory. We demonstrate this superradiance memory mechanism in an ensemble of cold rubidium atoms and verify that this protocol is suitable for pulses on timescales shorter than the atoms' natural lifetime. Our simulations show that the superradiance memory protocol yields the highest bandwidth storage among protocols in the same system. These high-bandwidth quantum memories provide unique opportunities for fast processing of optical and microwave photonic signals, with applications in large-scale quantum communication and quantum computing technologies.
The effects of high optical depth phenomena, such as superradiance, are investigated in potential quantum memory materials. The results may have relevance for several schemes, including controlled reversible inhomogeneous broadening, atomic frequency combs, and quantum memories based on electromagnetically induced transparency, which are based on using ensembles as storage media. It is shown that strong superradiant effects manifested as decay rates larger than $1/{T}_{2}^{\ensuremath{\ast}}$ are present even for moderate values of $\ensuremath{\alpha}L\ensuremath{\le}5$ and increases as a function of $\ensuremath{\alpha}L$. For even higher $\ensuremath{\alpha}L$, effects such as off-resonant slow light is demonstrated and discussed and, finally, the efficiency of time-reversed optimized input pulses is tested. A maximum retrieval efficiency of $\ensuremath{\sim}20%$ is reached and agreement with the theoretically expected result is discussed.
In contrast to free space, in waveguides the dispersive and dissipative dipole–dipole interactions among quantum emitters exhibit a periodic behavior over remarkably long distances. We propose a novel setup, to our knowledge, exploiting this long-range periodicity in order to create highly excited subradiant states and facilitate fast controlled collective energy transport among far-apart ensembles coupled to a waveguide. For sufficiently large ensembles, collective superradiant emission into the fiber modes dominates over its free space counterpart. We show that, for a large number of emitters, a fast transverse coherent pulse can create almost perfect subradiant states with up to 50% excitation. On the other hand, for a coherent excitation of one sub-ensemble above an overall excitation fraction of 50% we find a nearly lossless and fast energy transfer to the ground state sub-ensemble. This transport can be enhanced or suppressed by controlling the positions of the ensembles relative to each other, while it can also be realized with a random position distribution. In the optimally enhanced case this fast transfer appears as superradiant emission with subsequent superabsorption, yet, without a superradiant decay after the absorption. The highly excited subradiant states, as well as the superradiant excitation transfer, appear as suitable building blocks in applications such as active atomic clocks, quantum batteries, quantum information protocols, and quantum metrology procedures such as fiber-based Ramsey schemes.
Abstract Rare-earth ion-doped crystals are of great interest for quantum memories, a central component in future quantum repeaters. To assess the promise of 1 % Tm3+-doped yttrium gallium garnet (Tm:YGG), we report measurements of optical coherence and energy-level lifetimes of its 3H 6 ↔ 3 H4 transition at a temperature of around 500 mK and various magnetic fields. Using spectral hole burning (SHB), we find hyperfine ground-level (Zeeman level) lifetimes of several minutes at magnetic fields of less than 1000 G. We also measure coherence time exceeding one millisecond using two-pulse photon echoes. Three-pulse photon echo and SHB measurements reveal that due to spectral diffusion, the effective coherence time reduces to a few µs over a timescale of around two hundred seconds. Finally, temporal and frequency-multiplexed storage of optical pulses using the atomic frequency comb protocol is demonstrated. Our results suggest Tm:YGG to be promising for multiplexed photonic quantum memory for quantum repeaters.