Future quantum photonic networks require coherent optical memories for synchronizing quantum sources and gates of probabilistic nature. Room temperature operation is also desirable for ease of scaling up. Until now, however, room-temperature atomic memories have suffered from an intrinsic read-out noise due to spontaneous four-wave-mixing. Here we demonstrate a new scheme for storing photons at room temperature, the fast ladder memory (FLAME). In this scheme, stimulated two-photon absorption is used instead of the previously used stimulated Raman scattering. As here the competing spontaneous processes would require spontaneous absorption of an optical photon, rather than emission, the noise is greatly suppressed. Furthermore, high external efficiency can be achieved as the control is well separated in frequency from the signal, and could be filtered out using highly efficient interference filters. We run the protocol in rubidium vapour, both on and off single-photon resonance, demonstrating a ratio of 50 between storage time and signal pulse width, an external total efficiency of over 25%, and only 2.3 × 10−4noise photons per extracted signal photon. This paves the way towards the efficient synchronization of probabilistic gates and sources at room temperature, and the controlled production of large quantum states of light.
Future quantum photonic networks require coherent optical memories for synchronizing quantum sources and gates of probabilistic nature. Room temperature operation is also desirable for ease of scaling up. Until now, however, room-temperature atomic memories have suffered from an intrinsic read-out noise due to spontaneous four-wave-mixing. Here we demonstrate a new scheme for storing photons at room temperature, the fast ladder memory (FLAME). In this scheme, stimulated two-photon absorption is used instead of the previously used stimulated Raman scattering. As here the competing spontaneous processes would require spontaneous absorption of an optical photon, rather than emission, the noise is greatly suppressed. Furthermore, high external efficiency can be achieved as the control is well separated in frequency from the signal, and could be filtered out using highly efficient interference filters. We run the protocol in rubidium vapour, both on and off single-photon resonance, demonstrating a ratio of 50 between storage time and signal pulse width, an external total efficiency of over 25%, and only 2.3 × 10−4noise photons per extracted signal photon. This paves the way towards the efficient synchronization of probabilistic gates and sources at room temperature, and the controlled production of large quantum states of light.
Future quantum photonic networks require coherent optical memories for synchronizing quantum sources and gates of probabilistic nature. Room temperature operation is also desirable for ease of scaling up. Until now, however, room-temperature atomic memories have suffered from an intrinsic read-out noise due to spontaneous four-wave-mixing. Here we demonstrate a new scheme for storing photons at room temperature, the fast ladder memory (FLAME). In this scheme, stimulated two-photon absorption is used instead of the previously used stimulated Raman scattering. As here the competing spontaneous processes would require spontaneous absorption of an optical photon, rather than emission, the noise is greatly suppressed. Furthermore, high external efficiency can be achieved as the control is well separated in frequency from the signal, and could be filtered out using highly efficient interference filters. We run the protocol in rubidium vapour, both on and off single-photon resonance, demonstrating a ratio of 50 between storage time and signal pulse width, an external total efficiency of over 25%, and only 2.3 × 10-4noise photons per extracted signal photon. This paves the way towards the efficient synchronization of probabilistic gates and sources at room temperature, and the controlled production of large quantum states of light.
Future quantum photonic networks require coherent optical memories, preferably operating at room temperature, for synchronizing quantum sources and gates of probabilistic nature. Until now, however, room-temperature atomic memories have suffered from an intrinsic read-out noise. Here we demonstrate a fast ladder memory (FLAME) mapping the optical field onto the superposition between electronic orbitals of rubidium vapor. Employing a ladder level-system of orbital transitions with nearly degenerate frequencies simultaneously enables high bandwidth, low noise, and long memory lifetime. We store and retrieve 1.7-ns-long pulses, containing 0.5 photons on average, and observe short-time external efficiency of 25%, memory lifetime (1/e) of 86 ns, and below 10−4 added noise photons. Consequently, coupling this memory to a probabilistic source would enhance the on-demand photon generation probability by a factor of 12, the highest number yet reported for a noise-free, room-temperature memory. This paves the way towards the controlled production of large quantum states of light from probabilistic photon sources.
Summary form only given. A quantum optical memory (QM) is a device that can store and release quantum states of light on demand. Such a device is capable of synchronising probabilistic events, for example, locally synchronising non-deterministic photon sources for the generation of multi-photon states, or successful quantum gate operations within a quantum computational architecture [1], as well as for globally synchronising the generation of entanglement over long distances within the context of a quantum repeater [2]. Desirable attributes for a QM to be useful for these computational and communicational tasks include high end-to-end transmission (including storage and retrieval efficiency), large storage-time-bandwidth product, room temperature operation for scalability and, of utmost importance, noise free performance for true quantum operation.Here we present a new QM protocol that addresses the above issues, the quantum ladder memory (QLAD). This protocol is based on a two-photon `ladder' transition between the 6S1/2 ground state and 6D5/2 excited state of a caesium (Cs) ensemble at room temperature (see Fig. 1a). We characterise the memory with weak coherent states, storing GHz-band pulses with η=22% storage and retrieval efficiency and a characteristic storage time of τ=5.3ns. We measure the noise at the output to be 8×10-6 photons per pulse, giving rise to a noise-to-efficiency ratio of 3.6×10-5, three orders of magnitude better than any memory to date. We generate GHz-band heralded single photons from a spontaneous parametric down conversion source that are matched to the QLAD, coincidence traces seen in Fig. 1b. A heralded second-order autocorrelation function of g(2)=0.02+0.005 is measured for the input photon, and true quantum operation is confirmed by storing and retrieving this photon with g(2)=0.03+0.01 for a 3.5ns storage time. This result is the lowest g(2) on the output of an on-demand memory to date, and represents a significant step toward large-scale quantum technologies.
Quantum optical memories are devices that store quantum states of light, which can allow for the active synchronization of probabilistic events within large-scale quantum networks. Recent work on quantum memories have seen impressive quantum operation, albeit still suffering from noise on the output mode of the device. Here we demonstrate a noise-free quantum memory for light based on the off-resonant cascaded absorption of photons in a warm vapour of caesium atoms. The memory is characterized by measuring a noise floor of 8 × 10−6 photons per pulse. We demonstrate genuine quantum operation by storing and recalling on-demand heralded single photons with a heralded second-order autocorrelation function of g(2) = 0.028 ± 0.009.
Photonic quantum information processing is a key element for scalable quantum technologies, and has applications in secure long-distance quantum communication and connecting nodes of a quantum computation network. However, logical photon-photon gates and state-of-the-art single photon sources rely on probabilistic processes. Quantum memories are devices that enable storage and on-demand recall of quantum states of light, and have been highlighted as a vital component in photonic networks to overcome the scaling problem by synchronising probabilistic processes. The Raman memory has a large storage bandwidth and high synchronising capacity, and is an ideal candidate for local synchronisation. However, previous demonstrations of the Raman memory suffer from four-wave mixing noise, which prohibits quantum level operation. In this thesis I investigate methods to increase the signal to noise ratio in the Raman memory. I investigate increasing the light-matter coupling strength to boost the memory efficiency, and then explore two different methods to suppress four-wave mixing noise. I demonstrate that operating the Raman memory in a cavity is successful in reducing four-wave mixing, but it is technically challenging to maintain a high memory efficiency. I investigate a new method of noise suppression by introducing an absorption feature at the frequency of the unwanted noise field. This technically simple method is successful in reducing the noise by an order of magnitude, and will be applicable to many quantum memory protocols. In the final section of this thesis I explore the temporal mode properties of the Raman memory. I demonstrate that the Raman memory is single mode and can be used to separate and manipulate temporal modes of light. This positions the Raman memory as a key device for enabling high-dimensional photonic quantum information processing, and enhancing light-matter interactions. These results pave the way towards an efficient, low-noise, mode-selective quantum memory.
Quantum memories are essential for large-scale quantum information networks. Along with high efficiency, storage lifetime, and optical bandwidth, it is critical that the memory adds negligible noise to the recalled signal. A common source of noise in optical quantum memories is spontaneous four-wave mixing. We develop and implement a technically simple scheme to suppress this noise mechanism by means of quantum interference. Using this scheme with a Raman memory in warm atomic vapor, we demonstrate over an order of magnitude improvement in noise performance. Furthermore we demonstrate a method to quantify the remaining noise contributions and present a route to enable further noise suppression. Our scheme opens the way to quantum demonstrations using a broadband memory, significantly advancing the search for scalable quantum photonic networks.