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Coherent control, quantum memories and quantum repeaters

Jiří Minář

Published Jan 1, 2010• 1 citation
Publisher

Abstract

In the domain of quantum communication, one of the main challenges is the distribution of entanglement over long distances. A scheme, the so-called quantum repeater, has been proposed in order to overcome the problem of channel losses. In this thesis we adresse different problems related to quantum repeaters, both theoretically and experimentally. We present an alternative scheme of quantum repeater based on atomic ensembles and linear optics. It is based on the use of a qubit amplifier and two photon interference. The main features of the protocol are that the distributed entangled state does not need post selection and the entanglement distribution rates approach the optimal rates one might expect from quantum repeaters using probabilistic swapping. An important concern of the whole class of quantum repeaters based on one photon interference is the phase stability in the communication channels. We performed an experiment evaluating the realistic phase noise in installed optical fibers in the Geneva urban area. We found fluctuations of order of 0.1 rad on the time scale of order of 100 $mu$s. The feasibility of the phase stabilization is discussed. One of the essential building blocks of quantum repeater is a quantum memory. We focus our attention on quantum memories based on solid state atomic ensembles and photon echo like techniques. In particular, we show, that the atomic coherence in eryso can be controlled to a very high degree using an external electric field. This allowed us to implement the CRIB protocol in this material and realize the first storage of light at the single photon level at telecommunication wavelength. A necessary step for the implementation of CRIB and AFC storage protocols, in order to achieve long storage times and unit efficiencies, is the use of control light pulses. We developped a numerical Maxwell-Bloch simulator allowing us to study a coherent control of atomic excitations. In particular, we investigate the use of adiabatic chirped pulses in the context of AFC protocol. We study the relationship between the pulse characteristics and the echo properties. The comparison with the conventional $pi$ pulses and the impacts on the multimode capacity of the AFC protocol are also discussed.

DOI
10.13097/archive-ouverte/unige:39728

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In the domain of quantum communication, one of the main challenges is the distribution of entanglement over long distances. A scheme, the so-called quantum repeater, has been proposed in order to overcome the problem of channel losses. In this thesis we adresse different problems related to quantum repeaters, both theoretically and experimentally. We present an alternative scheme of quantum repeater based on atomic ensembles and linear optics. It is based on the use of a qubit amplifier and two photon interference. The main features of the protocol are that the distributed entangled state does not need post selection and the entanglement distribution rates approach the optimal rates one might expect from quantum repeaters using probabilistic swapping. An important concern of the whole class of quantum repeaters based on one photon interference is the phase stability in the communication channels. We performed an experiment evaluating the realistic phase noise in installed optical fibers in the Geneva urban area. We found fluctuations of order of 0.1 rad on the time scale of order of 100 mumumus. The feasibility of the phase stabilization is discussed. One of the essential building blocks of quantum repeater is a quantum memory. We focus our attention on quantum memories based on solid state atomic ensembles and photon echo like techniques. In particular, we show, that the atomic coherence in eryso can be controlled to a very high degree using an external electric field. This allowed us to implement the CRIB protocol in this material and realize the first storage of light at the single photon level at telecommunication wavelength. A necessary step for the implementation of CRIB and AFC storage protocols, in order to achieve long storage times and unit efficiencies, is the use of control light pulses. We developped a numerical Maxwell-Bloch simulator allowing us to study a coherent control of atomic excitations. In particular, we investigate the use of adiabatic chirped pulses in the context of AFC protocol. We study the relationship between the pulse characteristics and the echo properties. The comparison with the conventional pipipi pulses and the impacts on the multimode capacity of the AFC protocol are also discussed.
The distribution of quantum states over long distances is limited by photon loss. Straightforward amplification as in classical telecommunications is not an option in quantum communication because of the no-cloning theorem. This problem could be overcome by implementing quantum repeater protocols, which create long-distance entanglement from shorter-distance entanglement via entanglement swapping. Such protocols require the capacity to create entanglement in a heralded fashion, to store it in quantum memories, and to swap it. One attractive general strategy for realizing quantum repeaters is based on the use of atomic ensembles as quantum memories, in combination with linear optical techniques and photon counting to perform all required operations. Here the theoretical and experimental status quo of this very active field are reviewed. The potentials of different approaches are compared quantitatively, with a focus on the most immediate goal of outperforming the direct transmission of photons.
The distribution of quantum states over long distances is limited by photon loss. Straightforward amplification as in classical telecommunications is not an option in quantum communication because of the no-cloning theorem. This problem could be overcome by implementing quantum repeater protocols, which create long-distance entanglement from shorter-distance entanglement via entanglement swapping. Such protocols require the capacity to create entanglement in a heralded fashion, to store it in quantum memories, and to swap it. One attractive general strategy for realizing quantum repeaters is based on the use of atomic ensembles as quantum memories, in combination with linear optical techniques and photon counting to perform all required operations. Here we review the theoretical and experimental status quo of this very active field. We compare the potential of different approaches quantitatively, with a focus on the most immediate goal of outperforming the direct transmission of photons.
Quantum memories are used for storing the quantum mechanical states of photons and release identical photons at a later time. This has many applications within the field of quantum computing and is also essential in order to achieve long range quantum communication and cryptography. In this thesis Atomic frequency comb (AFC) quantum memories were investigated. In particular, three-level memory schemes were investigated in detail and a program for simulating these protocols was developed. These schemes make use of control pulses to transfer the population from the optically excited state to a ground state hyperfine level, as well as radio frequency fields to swap the population in the two ground state hyperfine levels. In this work control and RF pulses, tailored to give good transfer rates, are suggested and effects due to off resonant excitation was investigated. A memory protocol which could give efficiencies up to 60% for Praseodymium doped Yttrium Silicate based AFC memories is proposed. The main bottleneck for reaching higher efficiencies is that the AFC peak spacing needs to be small in order to give enough time to transfer the population to the spin state with the control pulses. With the control pulses suggested in this work and the current technology of tailoring AFC peaks in Pr-based memories, this gives a maximum finesse of 6 of the AFC structure.
(English) Future networks for quantum communication will require a new form of technology in order to operate over long distances. The no-cloning theorem precludes the use of amplification, so quantum repeater schemes have been proposed to achieve long-distance quantum communication through the distribution of entanglement. A key device for this setup is a quantum memory, which is required in order to store entanglement and synchronise its distribution. Among the many physical systems than can be used as a quantum memory, rare-earth ion-doped crystals stand out due to their long coherence times and potential for multiplexing and integration. This thesis reports the use of one such system, Pr3+:Y2SiO5, as a quantum memory using the atomic frequency comb (AFC) protocol. Building on previous work, we improved the performance of the memory to be able to demonstrate light-matter entanglement with a telecommunications wavelength photon and the quantum memory, for both a bulk memory and an integrated version. To measure light-matter entanglement, we use photon pairs generated through cavity-enhanced spontaneous parametric down conversion, where one photon (the signal) is stored in the memory and the other (the idler) is at a telecommunications wavelength. These photon pairs exhibit energy-time entanglement, which we analyse in the time basis using fibre- and AFC-based Mach-Zehnder interferometers. In the first experiment, we demonstrate light-matter entanglement where the bulk quantum memory is used with on-demand retrieval via storage in a spin-wave. We measure interference fringes with average visibilities of 89(2)% for AFC storage, and 70(3)% for spin-wave storage, sufficient to demonstrate entanglement. This is the first such demonstration using an on-demand multimode solid-state memory and telecom photon. We show that entanglement is maintained for up to 47.7 µs of storage. These results were made possible through improvements in the AFC efficiency and the noise filtering. Furthermore, we perform on-demand storage and retrieval of up to 30 temporal modes. We take the next step towards implementing our system in a real world scenario by measuring entanglement over a distance. To do so, we send the idler photons through both optical fibre spools and deployed fibre in the metropolitan area of Barcelona, Spain, while the photon detection still occurs in the same lab. The resulting visibilities (80% or higher) demonstrate that the photonic qubit does not decohere during transmission through the optical fibre. We then decouple the photon creation and detection by measuring non-classical correlations between signal photons detected in the lab, and idler photons detected 16 km away (44 km in fibre) in a different location. The final experiment uses a fibre-integrated memory for light-matter entanglement. We use a Pr3+:Y2SiO5 crystal containing a Type I laser-written waveguide, interfaced directly with optical fibre. The fibre-integration allows for improved transmission and AFC efficiency compared to a free-space coupled waveguide memory. To analyse the performance of the memory, we demonstrate non-classical correlations for storage times up to 28 µs. We perform qubit tomography in the time basis for storage times of 3 and 10 µs, measuring two-qubit fidelities of 86(2)% and 86(4)%, respectively, demonstrating storage of entanglement. These experiments demonstrate the potential of our system for use as part of a quantum repeater, with many opportunities for further improvement to the storage time, efficiency, and multiplexing capability.
Quantum repeaters hold the promise to prevent the photon losses in communication channels. Most recently, the serious efforts have been applied to achieve scalable distribution of entanglement over long distances. However, the probabilistic nature of entanglement generation and realistic quantum memory storage times make the implementation of quantum repeaters an outstanding experimental challenge. We propose a quantum repeater protocol based on the deterministic storage of a single photon in atomic ensembles confined in distant high-finesse cavities and show that this system is capable of distributing the entanglement over long distances with a much higher rate as compared to previous protocols, thereby alleviating the limitations on the quantum memory lifetime by several orders of magnitude. Our scheme is robust with respect to phase fluctuations in the quantum channel, while the fidelity imperfection is fixed and negligibly small at each step of entanglement swapping.
This thesis is an exploration of quantum repeaters from a practical point of view. Quantum repeaters are devices which help improve the quantum communication capacity of a lossy bosonic channel (which include photonic channels such as optical fiber) beyond what is possible using a purely lossy channel. The analyses in this thesis involve modeling the experimental imperfections inherent in the various devices which comprise a quantum repeater, then combining these models to calculate various quantities of interest. \n \nTwo systems are analyzed in this thesis. One is a simple quantum repeater, while the other is a potential building block for more complex quantum repeaters. \n \nThe simple quantum repeater scheme can be implemented with currently available technology. In it, two parties perform quantum key distribution (QKD) by exchanging photons with two quantum memories placed between them. Its secret key rate ideally scales as the square root of the transmittivity of the optical channel, which is superior to QKD schemes based on direct transmission because key rates for the latter scale at best linearly with transmittivity. Taking into account imperfections in the setup, such as detector efficiency and dark counts, we present parameter regimes in which our protocol outperforms protocols based on direct transmission. We find that implementing our scheme with trapped ions is a promising way to reach the necessary parameter regimes, and that the regimes are easier to reach if the optical channels are very lossy. \n \nThe creation of entanglement between two quantum memories is an important building block in some quantum repeater schemes. We consider a specific quantum memory consisting of an atom trapped in a cavity. The system allows a CNOT operation to be performed between an atom and a photon. We study three methods for taking advantage of this to entangle two atoms: (1) interacting a coherent pulse with each atom, then performing an entangling measurement on the pulses; (2) interacting a single coherent pulse with each atom sequentially; (3) emitting an entangled photon from one atom and interacting it with the other atom. The success probability of each method is compared, as well as the quality of the entangled states produced by each one, taking into account imperfections which appear in a specific experimental implementation of such memories. We find that there is a tradeoff between success probability and entangled state quality when coherent states are used, and that method 3 provides higher-quality entangled states than is possible with the other two methods.