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Frontiers in Optics

Programmable quantum memory in atomic ensembles

B. C. Buchler, Mahdi Hosseini, Ben M. Sparkes, Jiao Geng, J. Bernu, Geoff Campbell, Quentin C. Glorieux, Olivier Pinel, Nicholas P. Robins, Ping Koy Lam

Published Jan 1, 2013• 0 citations
Publisher

Abstract

Our quantum memory uses a photon echo scheme that works with warm and cold atomic ensembles. We demonstrate light storage that is noiseless, highly efficient (up to 87%), allows spectral manipulation of the light while it is stored, allows storage of higher order spatial modes and allows non-linear phase shifts of stored light. We present results from both warm and cold atomic ensembles.

DOI
10.1364/fio.2013.fm3d.2

Related papers

Storage and manipulation of optical information using gradient echo memory in warm vapours and cold ensembles

Ben M. Sparkes

Quantum memories for light lie at the heart of long-distance provably-secure communication [1], while containing the potential to help break current encryption methods [2], and allow better measurement of quantities than ever before [3]. Demand for a functioning quantum memory is therefore at a premium. Unfortunately, the same properties of light that make it such an effective carrier of quantum information make it difficult to store. Furthermore, by the laws of quantum mechanics, storage must be achieved without measurement to preserve the quantum state. A quantum memory needs to have an efficiency approaching unity without adding noise to the state, and storage times from milliseconds to seconds. Ideally it would also have a high bandwidth and be able to store many pieces of information simultaneously. Many different techniques are currently being developed and much experimental progress has been made over the past few years, with: efficiencies approaching 90% [4]; storage times of over seconds [5]; bandwidths of gigahertz [6, 7]; and over 1000 pieces of information stored at one time [8]. These results were, however, achieved using different memory schemes in different storage media. The challenge now is to reproduce these results with one memory. This thesis focuses on extending the gradient echo memory (GEM) scheme, which shows great promise due to the high efficiencies achieved (87%) [4]. GEM has also been used to demonstrate temporal compression and stretching of pulses, as well as a capacity to arbitrarily resequence stored information [9] and the interference of initially time-separated pulses [10]. Firstly, we demonstrate the noiseless nature of GEM storage in a warm vapour cell to prove that the output from the memory is the best-possible copy of the input allowed by quantum mechanics. We show GEM’s ability to coherently and precisely spectrallymanipulate stored information by having fine control over the memory’s frequency gradient, with potential applications for dynamic conditioning of information inside quantum networks [11]. We demonstrate cross-phase modulation of a stored light pulse with an additional optical field, a process with applications in quantum computing [12]. We also carry out storage of different spatial modes and arbitrary images, demonstrating the potential for orders of magnitude improvement in storage capacity. We then switch from warm vapour cells to cold atomic ensembles to improve the storage time of GEM, seeing a maximum coherence time of 350 μs (seven times that of the warm vapour system) and achieving efficiencies of up to 80%, on a par with the highest efficiency achieved with a cold atomic ensemble [13]. In the process we developed an ultra-dense cold atomic cloud with potential applications in a range of quantum optics experiments. Cold atoms, and the small volumes they occupy, also allowed us to develop an alternative to using magnetic field gradients for our alkali-atom memories in the form of a light-field gradient. This holds promise for extremely fast gradient switching and fine control over the gradient. We also present a digital locking code with application in a range of quantum optics experiments.

•Jan 1, 2013•0 citations

Quantum optical storage and processing using raman gradient echo memory

Mahdi Hosseini

The non-interacting and high-speed nature of light makes it an ideal carrier of information that is essential for transmission of quantum information. Indeed, many proposals and demonstrations of quantum cryptography rely on the use of fibre-optic networks. Construction of a memory that can store light and preserve its quantum properties will be useful in a range of quantum information systems such as secure quantum communication and quantum computation. This is why a quantum memory for light is a remarkable objective. The key to quantum memory is to store the probability amplitude of the possible outcomes of measurement but without measurement. An important criterion for a quantum memory is that the efficiency of the recall must exceed 50%. This is the crucial no-cloning limit for security of information, since it guarantees that nobody can access the information by copying it. This benchmark is important because any kind of deterministic amplification of quantum information is fundamentally impossible. On-demand retrieval of information and ability to controllably manipulate the quantum information are also important for quantum applications. When light is absorbed by atoms, it is actually possible to reverse the absorption process. In our memory system: light is absorbed by an ensemble of atoms and, using careful conditioning and control, we can cause the stored light to be regenerated and released at a later time. This is done by applying a gradient of magnetic field along the atomic ensemble that is the basis for our optical memory. To recall the light we flip the sign of the gradient field. This kind of reversible absorption is called photon echo, hence the name of our scheme: The Gradient Echo Memory (GEM). This simple protocol is used in our experiment and can be applied to a range of different atomic systems. We have extended the GEM protocol and experimentally implemented a memory using three-level atoms. We used an off-the-shelf Rb vapour cell operating above room temperature as the memory medium. In this realisation, we broke the efficiency record with 87% recall of the input light pulse. Moreover, through complete state tomography of coherent states we have demonstrated the ability of our memory to noiselessly store quantum states of light. We have also demonstrated that the memory can store a string of pulses and then recall the pulses ondemand in arbitrary order allowing re-sequencing of the stored information. Furthermore, we have shown that pulses could be time-compressed, time-stretched or split into multiple smaller pulses and selectively recalled in several pieces. This technique enables the construction of an optical random-access memory for quantum information. Moreover, the scheme to manipulate the spectral properties of optical data, stored inside the memory, has been introduced. We have also investigated the possibility of obtaining large nonlinear phase shifts between single photons inside the memory. Such strong interactions can be used for the implementation of universal quantum gates.

•Jan 1, 2012•2 citations

Highly-efficient entanglement storage of light in cold-atom quantum memories

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Highly-efficient entanglement storage in quantum memories is a critical requirement for quantum networks. We present an experiment where we stored single-photon entanglement into two atomic-ensemble based quantum memories with an overall efficiency of 87%.

Conference on Lasers and Electro-Optics•Jan 1, 2021•1 citation

Toward high-dimensional-state quantum memory in a cold atomic ensemble

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Quantum memories have been realized in different physical systems, such as atomic ensembles or solid systems. To date, all quantum memories have realized the storage and retrieval of photons encoded using only a two-dimensional space spanned, for example, by the orthogonal polarization states, and hence can only store a quantum bit. Using electromagnetically induced transparency in a cold atomic ensemble, we report the experimental realization of a quantum memory storing a photon encoded in a three-dimensional space spanned by orbital angular momentum (OAM) states. We experimentally reconstruct the storage process density matrix with a fidelity of $85.3%\ifmmode\pm\else\textpm\fi{}1.8%$ using a 4-f imaging system. We also realize storage of two special photonic qutrit states as examples. Toward storing a higher-dimensional state encoded in OAM space, the efficiency difference between different OAM states should be considered according to the experimental results. The capability to store high-dimensional quantum states with high fidelity is a key step towards building high-dimensional quantum networks.

arXiv:1310.3092•Physical Review A•Oct 1, 2014•32 citations

Quantum memories and monolithic resonators

Geoff Campbell

This thesis presents progress towards two complementary goals. The first part discusses our aim of building high-efficiency quantum memories for quantum optical states and exploring the potential of the memory for performing operations as well as for passive storage. The second part presents our work towards developing a compact source of quantum light by fabricating optical resonators from non-linear optical crystals. Our high-efficiency memories operate by storing the information carried by a weak laser field in the coherence between the hyperfine-split electronic energy levels in a vapour of atomic rubidium. We analyse the performance of a three-level gradient echo memory protocol that is used to store and retrieve optical information from the atomic coherence using an off-resonant Raman transition. Using a warm rubidium vapour, we find that the memory preserves the quantum state of the input light better than any classical means of storage could achieve. We also present results showing that trapped, cold atomic vapours provide an increased storage lifetime and analyse the contribution that four-wave-mixing makes to the retrieved light. Both the warm- and cold-atom implementations of the memory are shown to store and recall light with over 80% efficiency. We also present a number of extensions to the basic memory protocol. We demonstrate that the light-atom coupling can be controlled by the bright coupling field that is used to complete the Raman transition. This control is used to perform a beam-splitting operations between two optical pulses that are separated in time. Using frequency-multiplexed states, we show that beam-splitting operations are also possible between optical modes that are separated in frequency. This operation can be extended to perform arbitrary operations by using a series of memories. Finally, we present a high-fidelity memory for dual-rail q-bits encoded across two frequency modes. The resonators that were fabricated to explore compact sources of quantum light demonstrated many of the properties that are required to obtain the stable operation of a quantum source. We demonstrated that non-linear optical crystals can be machined into monolithic resonators that use total internal reflections to confine light. Two geometries were tested, whispering gallery resonators and square travelling wave cavities. Because both types of resonators use total internal reflection instead of dielectric coatings, they feature very low loss rates. Lithium niobate resonators of both types exhibited cavity decay rates that were dominated by bulk absorption in the crystal. The resonators were shown to be frequency tunable using an applied voltage and could be coupled to free-space modes with a tunable coupling rate. We present preliminary results of non-linear frequency conversion in the resonators. We also investigated calcium fluoride and magnesium fluoride resonators for use as high-finesse reference cavities. We report a finesse of 5000 in a calcium fluoride whispering gallery resonator. A square calcium fluoride monolithic resonator was also produce to have a low insertion loss when coupled to a Gaussian free-space mode. We demonstrate a power coupling efficiency of 94% to that resonator. We also present a proposal to use a square lithium niobate resonator as a tunable coupler for a high-finesse calcium fluoride whispering gallery resonator. The coupled resonator system could be used as a compact high-efficiency optical quantum memory.

•Jan 1, 2015•0 citations

Atom-field quantum state manipulation and storage

Aurélien Dantan, Alberto Bramati, Nicolas Treps +1 more

We present various schemes to manipulate and store quantum optical fields into atomic ensembles and show how to realize quantum memory operations, entangle atomic ensembles or teleport an atomic ensemble quantum state with high efficiency.

European Quantum Electronics Conference (EQEC)•Jan 18, 2006•0 citations
Our quantum memory uses a photon echo scheme that works with warm and cold atomic ensembles. We demonstrate light storage that is noiseless, highly efficient (up to 87%), allows spectral manipulation of the light while it is stored, allows storage of higher order spatial modes and allows non-linear phase shifts of stored light. We present results from both warm and cold atomic ensembles.
Quantum memories for light lie at the heart of long-distance provably-secure communication [1], while containing the potential to help break current encryption methods [2], and allow better measurement of quantities than ever before [3]. Demand for a functioning quantum memory is therefore at a premium. Unfortunately, the same properties of light that make it such an effective carrier of quantum information make it difficult to store. Furthermore, by the laws of quantum mechanics, storage must be achieved without measurement to preserve the quantum state. A quantum memory needs to have an efficiency approaching unity without adding noise to the state, and storage times from milliseconds to seconds. Ideally it would also have a high bandwidth and be able to store many pieces of information simultaneously. Many different techniques are currently being developed and much experimental progress has been made over the past few years, with: efficiencies approaching 90% [4]; storage times of over seconds [5]; bandwidths of gigahertz [6, 7]; and over 1000 pieces of information stored at one time [8]. These results were, however, achieved using different memory schemes in different storage media. The challenge now is to reproduce these results with one memory. This thesis focuses on extending the gradient echo memory (GEM) scheme, which shows great promise due to the high efficiencies achieved (87%) [4]. GEM has also been used to demonstrate temporal compression and stretching of pulses, as well as a capacity to arbitrarily resequence stored information [9] and the interference of initially time-separated pulses [10]. Firstly, we demonstrate the noiseless nature of GEM storage in a warm vapour cell to prove that the output from the memory is the best-possible copy of the input allowed by quantum mechanics. We show GEM’s ability to coherently and precisely spectrallymanipulate stored information by having fine control over the memory’s frequency gradient, with potential applications for dynamic conditioning of information inside quantum networks [11]. We demonstrate cross-phase modulation of a stored light pulse with an additional optical field, a process with applications in quantum computing [12]. We also carry out storage of different spatial modes and arbitrary images, demonstrating the potential for orders of magnitude improvement in storage capacity. We then switch from warm vapour cells to cold atomic ensembles to improve the storage time of GEM, seeing a maximum coherence time of 350 μs (seven times that of the warm vapour system) and achieving efficiencies of up to 80%, on a par with the highest efficiency achieved with a cold atomic ensemble [13]. In the process we developed an ultra-dense cold atomic cloud with potential applications in a range of quantum optics experiments. Cold atoms, and the small volumes they occupy, also allowed us to develop an alternative to using magnetic field gradients for our alkali-atom memories in the form of a light-field gradient. This holds promise for extremely fast gradient switching and fine control over the gradient. We also present a digital locking code with application in a range of quantum optics experiments.
The non-interacting and high-speed nature of light makes it an ideal carrier of information that is essential for transmission of quantum information. Indeed, many proposals and demonstrations of quantum cryptography rely on the use of fibre-optic networks. Construction of a memory that can store light and preserve its quantum properties will be useful in a range of quantum information systems such as secure quantum communication and quantum computation. This is why a quantum memory for light is a remarkable objective. The key to quantum memory is to store the probability amplitude of the possible outcomes of measurement but without measurement. An important criterion for a quantum memory is that the efficiency of the recall must exceed 50%. This is the crucial no-cloning limit for security of information, since it guarantees that nobody can access the information by copying it. This benchmark is important because any kind of deterministic amplification of quantum information is fundamentally impossible. On-demand retrieval of information and ability to controllably manipulate the quantum information are also important for quantum applications. When light is absorbed by atoms, it is actually possible to reverse the absorption process. In our memory system: light is absorbed by an ensemble of atoms and, using careful conditioning and control, we can cause the stored light to be regenerated and released at a later time. This is done by applying a gradient of magnetic field along the atomic ensemble that is the basis for our optical memory. To recall the light we flip the sign of the gradient field. This kind of reversible absorption is called photon echo, hence the name of our scheme: The Gradient Echo Memory (GEM). This simple protocol is used in our experiment and can be applied to a range of different atomic systems. We have extended the GEM protocol and experimentally implemented a memory using three-level atoms. We used an off-the-shelf Rb vapour cell operating above room temperature as the memory medium. In this realisation, we broke the efficiency record with 87% recall of the input light pulse. Moreover, through complete state tomography of coherent states we have demonstrated the ability of our memory to noiselessly store quantum states of light. We have also demonstrated that the memory can store a string of pulses and then recall the pulses ondemand in arbitrary order allowing re-sequencing of the stored information. Furthermore, we have shown that pulses could be time-compressed, time-stretched or split into multiple smaller pulses and selectively recalled in several pieces. This technique enables the construction of an optical random-access memory for quantum information. Moreover, the scheme to manipulate the spectral properties of optical data, stored inside the memory, has been introduced. We have also investigated the possibility of obtaining large nonlinear phase shifts between single photons inside the memory. Such strong interactions can be used for the implementation of universal quantum gates.
Quantum memories have been realized in different physical systems, such as atomic ensembles or solid systems. To date, all quantum memories have realized the storage and retrieval of photons encoded using only a two-dimensional space spanned, for example, by the orthogonal polarization states, and hence can only store a quantum bit. Using electromagnetically induced transparency in a cold atomic ensemble, we report the experimental realization of a quantum memory storing a photon encoded in a three-dimensional space spanned by orbital angular momentum (OAM) states. We experimentally reconstruct the storage process density matrix with a fidelity of 85.385.3%\ifmmode\pm\else\textpm\fi{}1.8%85.3 using a 4-f imaging system. We also realize storage of two special photonic qutrit states as examples. Toward storing a higher-dimensional state encoded in OAM space, the efficiency difference between different OAM states should be considered according to the experimental results. The capability to store high-dimensional quantum states with high fidelity is a key step towards building high-dimensional quantum networks.
This thesis presents progress towards two complementary goals. The first part discusses our aim of building high-efficiency quantum memories for quantum optical states and exploring the potential of the memory for performing operations as well as for passive storage. The second part presents our work towards developing a compact source of quantum light by fabricating optical resonators from non-linear optical crystals. Our high-efficiency memories operate by storing the information carried by a weak laser field in the coherence between the hyperfine-split electronic energy levels in a vapour of atomic rubidium. We analyse the performance of a three-level gradient echo memory protocol that is used to store and retrieve optical information from the atomic coherence using an off-resonant Raman transition. Using a warm rubidium vapour, we find that the memory preserves the quantum state of the input light better than any classical means of storage could achieve. We also present results showing that trapped, cold atomic vapours provide an increased storage lifetime and analyse the contribution that four-wave-mixing makes to the retrieved light. Both the warm- and cold-atom implementations of the memory are shown to store and recall light with over 80% efficiency. We also present a number of extensions to the basic memory protocol. We demonstrate that the light-atom coupling can be controlled by the bright coupling field that is used to complete the Raman transition. This control is used to perform a beam-splitting operations between two optical pulses that are separated in time. Using frequency-multiplexed states, we show that beam-splitting operations are also possible between optical modes that are separated in frequency. This operation can be extended to perform arbitrary operations by using a series of memories. Finally, we present a high-fidelity memory for dual-rail q-bits encoded across two frequency modes. The resonators that were fabricated to explore compact sources of quantum light demonstrated many of the properties that are required to obtain the stable operation of a quantum source. We demonstrated that non-linear optical crystals can be machined into monolithic resonators that use total internal reflections to confine light. Two geometries were tested, whispering gallery resonators and square travelling wave cavities. Because both types of resonators use total internal reflection instead of dielectric coatings, they feature very low loss rates. Lithium niobate resonators of both types exhibited cavity decay rates that were dominated by bulk absorption in the crystal. The resonators were shown to be frequency tunable using an applied voltage and could be coupled to free-space modes with a tunable coupling rate. We present preliminary results of non-linear frequency conversion in the resonators. We also investigated calcium fluoride and magnesium fluoride resonators for use as high-finesse reference cavities. We report a finesse of 5000 in a calcium fluoride whispering gallery resonator. A square calcium fluoride monolithic resonator was also produce to have a low insertion loss when coupled to a Gaussian free-space mode. We demonstrate a power coupling efficiency of 94% to that resonator. We also present a proposal to use a square lithium niobate resonator as a tunable coupler for a high-finesse calcium fluoride whispering gallery resonator. The coupled resonator system could be used as a compact high-efficiency optical quantum memory.