Photonic quantum memories are required in many applications in quantum information science with varying performance requirements depending on specific applications. Although classical light storage has been demonstrated in time scales of minutes (Dudin et al., 2013; Heinze et al., 2013) to hours (Ma et al., 2021) in different systems, storing true single photons and single photon level coherent pulses are still limited to around a few seconds at most (Wang et al., 2021; Ortu et al., 2022; Hain et al., 2022; Stas et al., 2022). In this question, we would like to explore what the challenges for quantum memory storage for the purposes of quantum communication and the distribution of entanglement are, e.g. in quantum repeaters. Furthermore, recent work has proposed using quantum memories with hour-long storage times for quantum computation (Gouzien and Sangouard, 2021) and physically transporting single photons for astronomical interferometry (Bland-Hawthorn et al., 2021) and global quantum communications (Wittig et al., 2017; Gündoğan et al., 2023).
Photonic quantum memories are required in many applications in quantum information science with varying performance requirements depending on specific applications. Although classical light storage has been demonstrated in time scales of minutes (Dudin et al., 2013; Heinze et al., 2013) to hours (Ma et al., 2021) in different systems, storing true single photons and single photon level coherent pulses are still limited to around a few seconds at most (Wang et al., 2021; Ortu et al., 2022; Hain et al., 2022; Stas et al., 2022). In this question, we would like to explore what the challenges for quantum memory storage for the purposes of quantum communication and the distribution of entanglement are, e.g. in quantum repeaters. Furthermore, recent work has proposed using quantum memories with hour-long storage times for quantum computation (Gouzien and Sangouard, 2021) and physically transporting single photons for astronomical interferometry (Bland-Hawthorn et al., 2021) and global quantum communications (Wittig et al., 2017; Gündoğan et al., 2023).
The ability of storing and retrieving quantum states of light is an important experimental challenge in quantum information science. A powerful quantum memory for light is required in a quantum repeater, which would allow long distance (>500km) quantum communications. To be implemented in such application, the quantum memory must allow on-demand readout, with high fidelity and efficiency, and a long storage time. Additionally a multimode capacity (for temporal or spatial modes) would allow multiplexing. Our approaches focus on rare-earth doped crystals, i.e. solid state quantum memory. I present, in this work, our contributions for a solid-state quantum storage, with good performances in every criteria. In particular, I present the preservation of quantum entanglement during the storage, which paves the way for the implementation of quantum memories in quantum repeaters.
Long-lived storage of single photons is a fundamental requirement for enabling quantum communication and foundational tests of quantum physics over extended distances. While the implementation of a global-scale quantum network requires quantum storage times on the order of seconds to minutes, existing photonic quantum memories have so far been limited to subsecond lifetimes. Although 151Eu3+:Y2SiO5 crystals exhibit substantially extended spin coherence times at the `magic' magnetic field, the concomitant weak optical absorption has until now prevented single-photon storage. Here, we overcome this challenge by integrating a noiseless photon echo protocol--which makes full use of the crystal's natural absorption for photonic storage--with a universally robust dynamical decoupling sequence incorporating adiabatic pulses to efficiently protect delocalized spin-wave excitation, enabling long-lived quantum storage at the `magic' magnetic field. At a storage time of 5.6 s, we achieve a time-bin qubit storage fidelity of 88.0 ± 2.1%, surpassing the maximum fidelity attainable via classical strategies. Our device reaches a 1/e storage lifetime of 27.6 ± 0.5 s, enabling single-photon-level storage for 42 s with a signal-to-noise ratio greater than unity. This work establishes photonic quantum memory in the minute-scale regime, laying a solid foundation for global-scale quantum network and deep-space quantum experiments.
Quantum memory is a key component of quantum networks. A successful development in enabling the distribution of quantum information requires the storage of single-photon light. Usually quantum information is encoded using a 2D space, spanned, for example, by the orthogonal polarization states of a photon. In this case, each photon carries a quantum bit of information. If the photon could be encoded in a high-dimensional space instead, the information carried by each photon could be increased from a bit to |log2d| bits, where d is the dimension of the Hilbert space in which the photon is described. The channel capacity of the network and the transmission efficiency would then be greatly enlarged [1]. Moreover, storing high-dimensional states in quantum memory can reduce its sensitivity to memory coherence time [2], leading to significant improvements in storage capacity [3]. It is thus no surprise that many research groups are actively working on different physical systems for storing light encoded using a high-dimensional space. The degrees of freedom for position, linear momentum, time-bin or orbital angular momentum (OAM) of a photon can all be considered as a high-dimensional space for encoding. The photon's OAM has the most potential for encoding [4]. Electromagnetic waves have both spin and OAM. Spin angular momentum produces polarization. Photons with OAM can be regarded as helices with left and right handedness twisted to varying degrees (Fig. 1). Information is encoded using this handedness and the amount of twist. Because of the inherent infinite degrees of freedom for OAM, a photon can in principle carry an arbitrarily large amount of information, and hence the eagerness to store light encoded in OAM space. Although some work on this has been reported for different physical systems, what is lacking is a way of storing and releasing at some later time a true single photon, including its shape and structure [5–7]. Constructing this basic element of quantum memory has been a major challenge. The wavefront of an OAM light is a helical structure (left and center), with an optical vortex at its center in the intensity distribution of light (right). The vortex of light can carry non-zero OAM of any order m. OAM states of a different order are orthogonal. A group at the Key Laboratory of Quantum Information, Chinese Academy of Science, at the University of Science and Technology of China finally resolved this problem [8]. They reported on the first experimental realization of a true single-photon-carrying OAM stored via electromagnetically induced transparency (EIT) in a cold atomic ensemble (Fig. 2). With such a suitable candidate, this work indicates a means of constructing high-dimensional quantum memory. After this, similar results were reported by Laurat's group at Laboratoire Kastler Brossel, at Université Pierre et Marie Curie in France [9], where an extremely weak laser at single-photon level carrying OAM was also stored via EIT in a cold atomic ensemble. Ding et al. [8] first prepared two cold 85Rb atomic clouds in two magnetic-optical traps (MOT) using laser cooling and trapping techniques. They generated non-classical correlated photon pairs using spontaneous four-wave mixing in an atomic cloud. One photon of each pair was used as a trigger; the other mapped and stored in a second atomic cloud via EIT. Each photon to be stored carried one OAM unit. After a programed storage time, the photon was retrieved. What they achieved during storage included (1) proof that the non-classical correlation between the trigger photon and the retrieved photon is retained by demonstrating a strong violation of the Cauchy–Schwarz inequality, (2) a demonstration that the single-photon property of the signal photon is maintained by performing the Hanbury–Brown and Twiss experiment on the trigger photon, (3) proof that the spatial structure of the photon is also very well preserved by showing a high similarity between input and output, and (4) evidence, gathered by Sagnac interferometry, that the coherence of a single photon is also preserved. A heralded single photon is generated through spontaneous four-wave mixing in MOT 1 (a); this single photon carrying OAM is then stored through EIT in MOT 2 (b); (c) shows an image of the photon carrying OAM. Experimental measurements in the transverse position of the image carried by the photon before and after storage are presented in (d) and (e), respectively; (f) shows the simplified experimental setup for storage of a superposition of OAM. Interference patterns taken at different angle settings of the half-wave plate are shown in (g). The left column represents the experimental angle settings of the half-wave and the quarter-wave plates, the middle column gives the experimental results, and the right column gives results from theoretical simulations; (h) and (i) present the interference patterns of the input signal photon and the retrieved signal with plate angle, respectively. This is a significant breakthrough towards the realization of a quantum internet and, potentially, quantum computing. This work addresses the ability to generate, store, retrieve on demand an arbitrary single-photon-level photonic state encoded in OAM space. It signifies an important step in the development of practical quantum-enabled technologies, opens up the possibility for realizing high-dimensional quantum memories based on this degree of freedom of light, and will have a large impact in a broad physical community, especially in the quantum information and quantum optics communities. The Technology Review website of MIT comments on their work very positively with an article entitled ‘First Quantum Memory That Records the Shape of a Single Photon Unveiled in China’. Further, Nature Photonics selected this work as a research highlight. Clearly, memory that can preserve the spatial structure of photons will make the internet far more flexible and capable. Of course, there are still many basic problems remaining that need to be solved to build the future quantum internet, one of which is the construction of quantum memory exploiting entanglement. This derives from the fact that a quantum network needs the distribution of quantum-entangled photons over channels between different nodes [10]. A high-dimensional quantum internet needs to store OAM-entangled states and perform entanglement swapping to efficiently extend the achievable communication distances. While Ding et al. [8] reported the storage of light with OAM at the single-photon level, the photon was, however, still encoded in a 2D space. Therefore, whether it is possible to construct a high-dimensional quantum memory remains an open issue that needs to be investigated further.
Optical photons are powerful carriers of quantum information, which can be delivered in free space by satellites or in fibers on the ground over long distances. Entanglement of quantum states over long distances can empower quantum computing, quantum communications, and quantum sensing. Quantum optical memories are devices designed to store quantum information in the form of stationary excitations, such as atomic coherence, and are capable of coherently mapping these excitations to flying qubits. Quantum memories can effectively store and manipulate quantum states, making them indispensable elements in future long-distance quantum networks. Over the past two decades, quantum optical memories with high fidelities, high efficiencies, long storage times, and promising multiplexing capabilities have been developed, especially at the single-photon level. In this review, we introduce the working principles of commonly used quantum memory protocols and summarize the recent advances in quantum memory demonstrations. We also offer a vision for future quantum optical memory devices that may enable entanglement distribution over long distances.
Quantum communication founds on the possibility to encode quantum states into photons and allows, for instance, the provable secure distribution of encryption keys. Despite a lot of progress during the past two decades, quantum communication still faces a distance barrier, which can be overcome by quantum repeaters whose currently most challenging ingredient is a memory for quantum states of light. In addition to robustness and ease of use, the requirements for such a quantum memory include high fidelity quantum state storage and the preservation of entanglement, sufficiently long storage time with on-demand recall, high efficiency, large bandwidth, and high multimode capacity. Six years ago, some of these properties had already been demonstrated using atomic vapor. However, practical solid-state memories were virtually unexplored in view of quantum state storage. The main goal of this thesis was to realize a solid-state quantum memory for light with emphasis on meeting requirements for long distance quantum communication. First, we identified a potential material candidate, a thulium doped lithium niobate waveguide, and conducted spectroscopic investigations to assess its suitability for quantum state storage using a photon-echo type approach. Next, we demonstrated that our storage device is suitable for high fidelity quantum state storage and for the storage of photonic entanglement, and that its large spectral multimode capacity allows for on-demand selective recall in conjunction with standard spectral filters and frequency shifters. In addition, by performing two-photon interference experiments and Bell-State measurements, we found that our memory preserves not only quantum information, but, more generally, the entire photonic wave function, which further confirms its suitability for quantum repeaters as well as for linear optics quantum computers. Finally, we showed that our integrated device also allows for general temporal and spectral manipulation of individual quantum optical pulses, which paves the way towards on-chip quantum optical processors. While more work remains to be done, in particular to improve memory efficiency and storage time, the large number of achievements, together with known ways to overcome the remaining obstacles, makes us confident that a quantum memory suitable for quantum repeaters will soon be built.
Since the first protocol of quantum algorithm was put forth in 1994 (Shor, 1994), quantum information processing has been intensively studied (Nielsen & Chuang, 2000). The quantum approach has benefits over the classical optical information processing in areas such as prime number factorization (Shor, 1994), data searching (Grover, 1996), and highresolution lithography (Boto et al., 2000; Yablonovitch, 1999). Compared to conventional cryptography based on public key cryptosystem (RSA cryptosystem) using conventional computers, prime number factorization using quantum computers has demonstrated a potential for a formidable attack on existing cryptographic systems. Like conventional memory which serves in the information processing unit, such as a processing unit together with logic gates, quantum memory is also essential to quantum information and communications networks. For quantum communications via a classical optical channel, the longest communication distance a quantum light can be transmitted is determined by the sensitivity of optical detectors and a lossy classical channel such as an optical fibre. Based on current technologies, the longest distance a single photon can propagate through an optical fibre is about 100 km (Zbinden et al., 1998). This distance should limit applications of quantum information especially for long-distance quantum communications. To solve the limited photon transmission, a quantum repeater has been introduced for virtually unlimited transmission distance (Duan et al., 2001; Jiang et al., 2007; Simon et al., 2007; Waks et al., 2002). Quantum memory is an essential element for the entangled photon swapping in the quantum repeaters. Because quantum repeaters swap entangled photons shared by neighboring remote quantum nodes in a quantum network, and the quantum information must be kept coherently through the quantum network, the minimum storage time of quantum memory is determined by the longest transmission distance of the lossy optical channel. For tranceatlantic quantum communications, roughly a one-second or more storage time is required. So far, such a long photon storage has not been demonstrated, where conventional quantum memory protocols limit the storage time to spin phase decay time at most (≤10-3 second). Unlike classical memories, quantum memory must satisfy a coherent process. Since the first observation of coherent retrieval of a stored optical pulse in a Bose Einstein condensate using slow light (Liu et al., 2001), interest in quantum memories has increased in the last decade (Alexander et al., 2006; Afzelius et al., 2010; Chaneliere et al., 2005. Choi et al., 2008;