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arXiv:1612.02193 · Scientific Reports

A controlled ac Stark echo for quantum memories

Byoung Seung HAM

Published Aug 3, 2017• 11 citations
arXivHTMLPDF

Abstract

A quantum memory protocol of controlled ac Stark echoes (CASE) based on a double rephasing photon echo scheme via controlled Rabi flopping is proposed. The double rephasing scheme of photon echoes inherently satisfies the no-population inversion requirement for quantum memories, but the resultant absorptive echo remains a fundamental problem. Herein, it is reported that the first echo in the double rephasing scheme can be dynamically controlled so that it does not affect the second echo, which is accomplished by using unbalanced ac Stark shifts. Then, the second echo is coherently controlled to be emissive via controlled coherence conversion. Finally a near perfect ultralong CASE is presented using a backward echo scheme. Compared with other methods such as dc Stark echoes, the present protocol is all-optical with advantages of wavelength-selective dynamic control of quantum processing for erasing, buffering, and channel multiplexing.

DOI
10.1038/s41598-017-08051-5

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A quantum memory protocol of controlled ac Stark echoes (CASE) based on a double rephasing photon echo scheme via controlled Rabi flopping is proposed. The double rephasing scheme of photon echoes inherently satisfies the no-population inversion requirement for quantum memories, but the resultant absorptive echo remains a fundamental problem. Herein, it is reported that the first echo in the double rephasing scheme can be dynamically controlled so that it does not affect the second echo, which is accomplished by using unbalanced ac Stark shifts. Then, the second echo is coherently controlled to be emissive via controlled coherence conversion. Finally a near perfect ultralong CASE is presented using a backward echo scheme. Compared with other methods such as dc Stark echoes, the present protocol is all-optical with advantages of wavelength-selective dynamic control of quantum processing for erasing, buffering, and channel multiplexing.
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.
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 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 controlled double rephasing (CDR) photon echo is analyzed for inversion-free photon echoes using controlled coherence conversion (CCC), where an optical Rabi flopping between an excited and an auxiliary ground states is used for atom phase control for emissive photon echoes. Unlike the rephasing mechanism in photon echoes, the CCC induces coherence inversion to the intrinsic absorptive echoes in a double rephasing scheme. Thus, the CDR echo protocol can be vastly applied for modified photon-echo quantum memories.
A near perfect, storage time-extended photon echo-based quantum memory protocol has been analyzed by solving the Maxwell-Bloch equations for a backward scheme in a three-level system. The backward photon echo scheme is combined with a controlled coherence conversion process via control Rabi flopping to a third state, where the control Rabi flopping collectively shifts the phase of the ensemble coherence. The propagation direction of photon echoes is coherently determined by the phase matching condition between the data (quantum) and the control (classical) pulses. Herein we discuss the classical controllability of a quantum state for both phase and propagation direction by manipulating the control pulses in both single and double rephasing photon echo schemes of a three-level system. Compared with their well-understood use for two-level photon echoes, the Maxwell-Bloch equations to a three-level system have a critical limitation regarding the phase change when interacted with an arbitrary control pulse area.