The coherent control of light with matter, enabling storage and manipulation of optical signals, was revolutionized by electromagnetically induced transparency (EIT), which is a quantum interference effect. For strong electromagnetic fields that induce a wide transparency band, this quantum interference vanishes, giving rise to the well-known phenomenon of Autler-Townes splitting (ATS). To date, it is an open question whether ATS can be directly leveraged for coherent control as more than just a case of "bad" EIT. Here, we establish a protocol showing that dynamically controlled absorption of light in the ATS regime mediates coherent storage and manipulation that is inherently suitable for efficient broadband quantum memory and processing devices. We experimentally demonstrate this protocol by storing and manipulating nanosecondslong optical pulses through a collective spin state of laser-cooled Rb atoms for up to a microsecond. Furthermore, we show that our approach substantially relaxes the technical requirements intrinsic to established memory schemes, rendering it suitable for broad range of platforms with applications to quantum information processing, high-precision spectroscopy, and metrology.When a strong electromagnetic field resonantly drives a transition, that transition can be split into a doublet due to the dynamic, or ac, Stark effect of the field, as first reported by Autler and Townes [1]. This effect can be directly probed using a weak electromagnetic field that couples the split levels to a third level ( Fig. 1a-c). Since its discovery, this splitting, commonly referred to as Autler-Townes splitting (ATS), has been observed in numerous atomic and molecular media [2,3], and extensively studied to describe underlying quantum optical phenomena in laser cooling, cavity quantum electrodynamics [4][5][6] and high resolution spectroscopy [7][8][9].In the context of coherent storage and manipulation of light with matter, remarkable advances have been made possible by EIT, which relies on quantum interference [10,11]. The ATS regime emerges when this quantum interference is washed out due to the strong electromagnetic fields that induce a spectrally broad transparency window via the ac Stark effect. The crossover between EIT and ATS, identified with narrow and wide transparency, respectively, is an active topic of research [12][13][14][15][16][17][18], begging the question of whether it is possible to directly leverage ATS for coherent control of light beyond treatment as an unfavorable EIT regime. Apart from a theoretical proposal for a photon-echo interaction in the ATS regime [19], this possibility remains unexplored.Here, we develop an approach where absorption of light pulses by dynamically controlled ATS lines mediates coherent storage, which is intrinsically suitable for efficient, broadband, and long-lived quantum memories. We demonstrate an experimental implementation of our protocol in a Λ-type three-level system of cold Rb atoms by reversible mapping of coherence from nanoseconds-long optical ...
Electromagnetically induced transparency (EIT) and Autler-Townes splitting (ATS) are similar, but different quantum optical phenomena: EIT results from a Fano interference, whereas ATS is described by the AC-Stark effect. Likewise, despite their close resemblance, light-storage techniques based on the EIT memory protocol and the recently-proposed ATS memory protocol (E. Saglamyurek et al. Nature Photonics 12, 2018) are distinct: the EIT protocol relies on adiabatic elimination of absorption, whereas the ATS protocol is based on absorption. In this article, we elaborate on the distinction between EIT and ATS memory protocols through numerical analysis and experimental demonstrations in a cold rubidium ensemble. We find that their storage characteristics manifest opposite limits of the light-matter interaction due to their inherent adiabatic vs. non-adiabatic nature. Furthermore, we determine optimal memory conditions for each protocol and analyze ambiguous regimes in the case of broadband storage, where non-optimal memory implementations can possess characteristics of both EIT and ATS protocols. We anticipate that this investigation will lead to deeper understanding and improved technical development of quantum memories, while clarifying distinctions between the EIT and ATS protocols.
Broadband spin-photon interfaces for long-lived storage of photonic quantum states are key elements for quantum information technologies. Yet, reliable operation of such memories in the quantum regime is challenging due to photonic noise arising from technical and/or fundamental limitations in the storage-and-recall processes controlled by strong electromagnetic fields. Here, we experimentally implement a single-photon-level spin-wave memory in a laser-cooled Rubidium gas, based on the recently proposed Autler-Townes splitting (ATS) protocol. We demonstrate storage of 20-ns-long laser pulses, each containing an average of 0.1 photons, for 200 ns with an efficiency of 12.5% and signal-to-noise ratio above 30. Notably, the robustness of ATS spin-wave memory against motional dephasing allows for an all-spatial filtering of the control-field noise, yielding an ultra-low unconditional noise probability of 3.3 × 10 −4 , without the complexity of spectral filtering. These results highlight that broadband ATS memory in ultracold atoms is a preeminent option for storing quantum light.For large-scale quantum networks to become practical, storage and on-demand recall of photonic quantum states must be available at timescales of up to several milliseconds [1,2]. Interfacing non-classical light with these memories is necessary, but has proven to be difficult for two reasons: the substantial mismatch between the inherently large bandwidth of quantum light (from most popular single-photon sources) and the narrow acceptance bandwidth of well-studied atomic memories, and the unfaithful storage and recall processes due to photonic noise introduced by memory itself, which may degrade or fully destroy quantum nature of the stored light. This noise is particularly problematic with on-demand memories that require control electromagnetic fields, and is typically much more detrimental for broadband implementations [3][4][5].A promising approach to noise-free broadband memory is a family of photon-echo-based protocols that feature inherently fast (non-adiabatic) memory operation [6]. The Controlled Reversible Inhomogeneous Broadening (CRIB) [7,8] and Gradient Echo Memory (GEM) [9,10] are widely studied protocols that rely upon the absorption of light via artificially broadened spectral features controlled by external electric or magnetic field gradients. However, implementing a broadband CRIB or GEM memory is technically challenging due to the infeasibility of large field gradients with rapid switching times. The atomic frequency comb (AFC) technique offers a solution to this limitation, as this approach relies on tailoring a comb-shaped spectral feature for light absorption without needing controlled broadening [11]. To this end, broadband AFC quantum memories have been successfully demonstrated for high-fidelity storage of entangled photons in the GHz-bandwidth regime using ensemble of two-level rare-earth (RE) ions in solids [12][13][14][15]. But, intrinsically short and pre-programmed storage times in these memories restrict their us...
Large-scale quantum networks require quantum memories featuring long-lived storage of non-classical light together with efficient, high-speed and reliable operation. The concurrent realization of these features is challenging due to inherent limitations of matter platforms and light–matter interaction protocols. Here, we propose an approach to overcome this obstacle, based on the implementation of the Autler–Townes-splitting (ATS) quantum-memory protocol on Bose–Einstein condensate (BEC) platform. We demonstrate a proof-of-principle of this approach by storing short pulses of single-photon-level light as a collective spin-excitation in a rubidium-BEC. For 20 ns long-pulses, we achieve an ultra-low-noise memory with an efficiency of 30% and lifetime of 15 μs. The non-adiabatic character of the ATS protocol (leading to high-speed and low-noise operation) in combination with the intrinsically large atomic densities and ultra-low temperatures of the BEC platform (offering highly efficient and long-lived storage) opens up a new avenue toward high-performance quantum memories.
Using cold and ultracold rubidium atoms controlled with lasers intense enough to put the system into the Autler-Townes regime, fast, efficient, and broadband storage and manipulation of photonic signals is achieved. In demonstrating single-photon-level operation, signals are read out with exceptionally low-noise, which is inherent to this method. Finally, high efficiency and longer storage times are made possible by using Bose-condensed samples as the storage medium.
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