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Optimization of Resonant Λ\LambdaΛ-type Quantum Memory with Gaussian Pulses

Kai Shinbrough, B. D. Hunt, Virginia O. Lorenz

Published Aug 31, 2020• 0 citations
OpenAlex

Abstract

Optical quantum memory--the ability to store photonic quantum states and retrieve them on demand--is an essential resource for emerging quantum technologies and photonic quantum information protocols. Simultaneously achieving high efficiency and high-speed, broadband operation is an important task necessary for enabling these applications. In this work, we investigate the optimization of a large class of optical quantum memory protocols based on resonant interaction with ensembles of $\Lambda$-type level systems with the restriction that the temporal envelope of all optical fields must be Gaussian, which reduces experimental complexity. We show that for overlapping signal and control fields there exists a unique and broadband pulse duration that optimizes the memory efficiency, and that this optimized efficiency can be close to the protocol-independent bound. We further optimize over the control field temporal delay and pulse duration, demonstrating saturation of this efficiency bound over a broad range of pulse durations while clarifying the underlying physics of the quantum memory interaction.

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•Jan 1, 2011•0 citations
Optical quantum memory--the ability to store photonic quantum states and retrieve them on demand--is an essential resource for emerging quantum technologies and photonic quantum information protocols. Simultaneously achieving high efficiency and high-speed, broadband operation is an important task necessary for enabling these applications. In this work, we investigate the optimization of a large class of optical quantum memory protocols based on resonant interaction with ensembles of Λ\LambdaΛ-type level systems with the restriction that the temporal envelope of all optical fields must be Gaussian, which reduces experimental complexity. We show that for overlapping signal and control fields there exists a unique and broadband pulse duration that optimizes the memory efficiency, and that this optimized efficiency can be close to the protocol-independent bound. We further optimize over the control field temporal delay and pulse duration, demonstrating saturation of this efficiency bound over a broad range of pulse durations while clarifying the underlying physics of the quantum memory interaction.
Optical quantum memory, the ability to store photonic quantum states and retrieve them on demand, is an essential resource for emerging quantum technologies and photonic quantum information protocols. Simultaneously achieving high efficiency and high-speed, broadband operation is an important task necessary for enabling these applications. We investigate the optimization of a large class of optical quantum memories based on resonant and near-resonant interaction with ensembles of $\mathrm{\ensuremath{\Lambda}}$-type level systems with the restriction that the temporal envelope of all optical fields must be Gaussian, which reduces experimental complexity. Through this optimization we demonstrate an experimentally simple path to saturation of the protocol-independent storage efficiency bound that is valid for a wide range of memory bandwidths, including those that are broadband and high speed. Examining the resulting optimal Gaussian control field parameters, we find a continuous transformation between three physically distinct resonant quantum memory protocols. We compare this Gaussian optimization scheme with standard shape-based optimization.
Quantum memories, devices that can store and retrieve photonic quantum states on demand, are essential components for scalable quantum technologies. It is desirable to push the memory towards the broadband regime in order to increase the data rate. Here, we present a theoretical and experimental study of broadband optical memory based on the electromagnetically induced transparency (EIT) protocol. We first provide a theoretical analysis of the issues and requirements needed to achieve broadband EIT memory. We then present our experimental efforts for the movement of EIT memory in cold atoms towards the broadband or short-pulse regime. A storage efficiency of $\ensuremath{\sim}80%$ with a pulse duration of 30 ns (corresponding to a bandwidth of 14.7 MHz) is realized. Due to limitations of the available intensity of the control beam, we could not achieve an optimal storage for the even shorter pulses but were still able to obtain an efficiency of greater than 5050%50 with a pulse duration of 14 ns (31.4 MHz). The time-bandwidth product achieved at an efficiency of 5050%50 is 1267(89).
Photonic quantum memory is a crucial elementary operation in photonic quantum information processing. While many physically distinct memory protocols and hardware implementations have been applied to this task, the development of a quantum memory performant in all relevant metrics simultaneously (e.g., efficiency, bandwidth, lifetime, etc.) is still an open challenge. In this work, we focus on inhomogeneously broadened ensembles of ΛΛΛ-type quantum emitters, which have long coherence lifetimes and broad bandwidth compatibility, but tend to exhibit low efficiency, in part due to technical constraints on medium growth and preparation, and in part due to inefficient use of a key resource in these systems: the inhomogeneously broadened excited state lineshape. We investigate the properties of electromagnetically induced transparency (EIT) for a survey of inhomogeneous lineshapes that are straightforward to realize experimentally, and optimize the memory efficiency for each lineshape over a large range of experimental parameters. We compare the optimal EIT efficiency to the well-known atomic frequency comb (AFC) protocol, which also relies on spectral shaping of the inhomogeneous broadening, and observe that with sufficient control field power the optimized lineshapes allow more efficient storage. Finally, we optimize over the inhomogeneous lineshape in a protocol agnostic fashion by numerically constructing the linear integral kernel describing the memory interaction and using a singular value decomposition and interpolation procedure to ensure optimality of the resulting lineshape.
Ideal photonic quantum memories can store arbitrary pulses of light with unit efficiency. This requires operating in the adiabatic regime, where pulses have a duration much longer than the bandwidth of the memory. In the nonadiabatic regime of short pulses, memories are therefore imperfect, and information is always lost. We theoretically investigate the bandwidth limitations for setups based on individual atoms, or ensembles thereof, confined inside optical cavities. We identify an effective strategy for optimizing the efficiencies of the storage and retrieval process regardless of the duration or the shape of the pulses. Our protocol is derived almost completely analytically and attains efficiencies that are comparable to those obtained by numerical optimization. Furthermore, our results provide an improved understanding of the performance of quantum memories in several regimes. When considering pulses defined on an infinite time interval, the shapes can be divided into two categories, depending on their asymptotic behaviors. If the intensity of the pulse increases with time slower than or as an exponential function then the storage efficiency is only limited by the pulse width. For pulses defined on a finite interval, on the other hand, the efficiency is determined by the shape at the beginning of the storage or, correspondingly, at the end of the retrieval process.
This thesis is about the experimental implementation of a high-speed and robust quantum memory for light. A novel far off-resonant Raman approach to ensemble-based quantum memories in a room-temperature environment is developed and demonstrated. Storage and retrieval of sub-nanosecond, weak coherent light pulses at the single-photon-level with total efficiencies exceeding 30% and storage times of up to 4 μs are achieved. The coherence of the memory is shown by directly interfering a copy of the incident signal with the retrieved signal from the memory. The unconditional noise floor of the memory is found to be low enough to operate the memory in the quantum regime at room temperature. Multiple readout of a single stored excitation is demonstrated, suggesting that 100% readout is possible in different temporal modes. Furthermore, first results regarding the storage and retrieval of polarisation encoded qubits are obtained. This and the memory’s ability to operate in the quantum regime at room temperature with a low unconditional noise floor illustrate its potential usefulness for real world applications.