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Entropy

Analysis of Imperfect Rephasing in Photon Echo-Based Quantum Memories

Byoung Seung HAM

Published Sep 27, 2022• 0 citations
Publisher

Abstract

Over the last two decades, quantum memories have been intensively studied for potential applications of quantum repeaters in quantum networks. Various protocols have also been developed. To satisfy no noise echoes caused by spontaneous emission processes, a conventional two-pulse photon-echo scheme has been modified. The resulting methods include double-rephasing, ac Stark, dc Stark, controlled echo, and atomic frequency comb methods. In these methods, the main purpose of modification is to remove any chance of a population residual on the excited state during the rephasing process. Here, we investigate a typical Gaussian rephasing pulse-based double-rephasing photon-echo scheme. For a complete understanding of the coherence leakage by the Gaussian pulse itself, ensemble atoms are thoroughly investigated for all temporal components of the Gaussian pulse, whose maximum echo efficiency is 26% in amplitude, which is unacceptable for quantum memory applications.

DOI
10.3390/e24101367

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Advances in Applied Science Research•Jan 1, 2018•2 citations
Over the last two decades, quantum memories have been intensively studied for potential applications of quantum repeaters in quantum networks. Various protocols have also been developed. To satisfy no noise echoes caused by spontaneous emission processes, a conventional two-pulse photon-echo scheme has been modified. The resulting methods include double-rephasing, ac Stark, dc Stark, controlled echo, and atomic frequency comb methods. In these methods, the main purpose of modification is to remove any chance of a population residual on the excited state during the rephasing process. Here, we investigate a typical Gaussian rephasing pulse-based double-rephasing photon-echo scheme. For a complete understanding of the coherence leakage by the Gaussian pulse itself, ensemble atoms are thoroughly investigated for all temporal components of the Gaussian pulse, whose maximum echo efficiency is 26% in amplitude, which is unacceptable for quantum memory applications.
A double rephasing photon echo scheme inherently satisfies no-population inversion condition for quantum memory applications, while the resultant absorptive coherence prohibits echo radiation out of the medium. To solve this absorptive echo problem, a controlled coherence conversion (CCC) has been proposed and demonstrated for the collective atom phase control converting an absorptive echo into an emissive one. The recent observations of absorptive photon echoes in double rephasing schemes, however, seemingly contradict the CCC theory. Here the rephasing process of individual ensemble atoms is investigated by using typical Gaussian light pulses to prove that the absorptive echo observations are due to imperfect rephasing-caused coherence leakage. Although the imperfect rephasing-cuased photon echoes has nothing to do with fidelity, the lower retrieval efficiency limits quantum memory applications due to potential eavesdropping caused by device-dependent loopholes or limited quantum error corrections in fault-tolerant quantum computing.
Photon-echo-based quantum memories use inhomogeneously broadened, optically thick ensembles of absorbers to store a weak optical signal and employ various protocols to rephase the atomic coherences for information retrieval. We study the application of two consecutive, frequency-chirped control pulses for coherence rephasing in an ensemble with a ``natural'' inhomogeneous broadening. Although propagation effects distort the two control pulses differently, chirped pulses that drive adiabatic passage can rephase atomic coherences in an optically thick storage medium. Combined with spatial phase-mismatching techniques to prevent primary echo emission, coherences can be rephased around the ground state to achieve secondary echo emission with close to unit efficiency. Potential advantages over similar schemes working with $\ensuremath{\pi}$ pulses include greater potential signal fidelity, reduced noise due to spontaneous emission, and better capability for the storage of multiple memory channels.
Unlike slow-light-based quantum memories, photon echoes offer the benefit of high speed and wide bandwidth. Over the last decade, the rephasing mechanism of photon echoes has been studied for quantum memories to overcome fundamental limitations in photon echoes, such as population inversion and low retrieval efficiency. Although these limitations have been overcome in modified photon echo schemes, photon storage time is still too short to apply it to long-distance quantum communications. For long-distance quantum communications, ultralong photon storage time of the order of seconds is needed to implement quantum repeaters. In this review article, challenging techniques for ultralong photon storage are presented, where ultralong storage is obtained via a coherence conversion process between optical and spin states by using an optical locking technique. To remove population-inversion-caused quantum noise, a double rephasing scheme is addressed, where rephasing-pulse-caused population inversion hinders photon echoes for quantum memory applications.
A double rephasing scheme of a photon echo is analyzed for inversion-free photon echo-based quantum memories using controlled Rabi flopping, where the Rabi flopping is used for phase control of collective atom coherence. Unlike the rephasing-caused π-phase shift in a single rephasing scheme, the control Rabi flopping between the excited state and an auxiliary third state induces coherence inversion. Thus, the absorptive photon echo in a double rephasing scheme can be manipulated to be emissive. Here, we present a quantum coherence control of atom phases in a double rephasing photon echo scheme for emissive photon echoes for quantum memory applications.
A double rephasing photon echo is analyzed for inversion-free photon echo-based quantum memories using controlled Rabi flopping, where the Rabi flopping is used for phase control of atom coherence. Unlike the rephasing-caused pi-phase shift in a single rephasing scheme, the control Rabi flopping between the excited state and an auxiliary third state induces coherence inversion. Thus, the absorptive photon echo in a double rephasing scheme can be manipulated to be emissive via the controlled Rabi flopping. Here we present a quantum coherence control of atom phases in a double rephasing photon echo scheme for emissive photon echoes for quantum memory applications.
Photon echo-based quantum memory protocols presented over the last decade are primarily to solve the population inversion constraint in conventional photon echoes. In addition to population inversion, other major constraints limiting quantum memory applications are the low retrieval efficiency and short storage time, in which the importance of these factors has recently increased because of the challenges of the entangled qubit scalability and quantum repeaters. Here, I present, analyze and discuss the solution model for a Controlled Double Rephasing (CDR) echo protocol to satisfy non-inversion, near-perfect retrieval efficiency, and ultra-long storage time. In the CDR echo, a coherent Rabi pulse pair plays a key role for controlling both the ensemble phase and echo propagation direction, where the followings are the major results. Firstly, a counter propagating control Rabi pulse-pair-induced backward echo is free from echo reabsorption, resulting in a near perfect retrieval efficiency. Secondly, the control Rabi pulse pair induces an optical-spin coherence conversion like in the ultraslow light-based photon storage, resulting in a storage time extension up to spin decay time. Finally, the control Rabi flopping to a third state induces a π phase shift to the ensemble coherence, resulting in an emissive echo in the double rephasing scheme, where the π phase shift is due to optical-spin coherence conversion process. Conclusion: This study reveals that the control Rabi flopping in a three-level system applied to conventional photon echoes for storage time extension induces a π phase shift to the coher ent ensemble.