Distance Quantum Communication Research Articles

Distribution of telecom entangled photons through a 7.7 km antiresonant hollow-core fiber

State of the art classical and quantum communications rely on standard optical fibers with solid cores to transmit light over long distances. However, recent advances have led to the emergence of antiresonant hollow-core optical fibers (AR-HCFs), which, due to the novel fiber geometry, show remarkable optical guiding properties, which are not as limited by the material properties as solid-core fibers. In this paper, we explore the transmission of entangled photons through a novel 7.7 km AR-HCF in a laboratory environment at 1550 nm, presenting the first successful demonstration of entanglement distribution via a long AR-HCF. In addition to showing these novel fibers are compatible with long distance quantum communication, we highlight the low latency and low chromatic dispersion intrinsic to AR-HCF, which can increase the secure key rate in time-bin-based quantum key distribution protocols.

Detection of network and genuine network quantum steering

The quantum network correlations play significant roles in long distance quantum communication, quantum cryptography and distributed quantum computing. Generally it is very difficult to characterize the multipartite quantum network correlations such as nonlocality, entanglement and steering. In this paper, we propose the network and the genuine network quantum steering models from the aspect of probabilities in the star network configurations. Linear and nonlinear inequalities are derived to detect the network and genuine network quantum steering when the central party performs one fixed measurement. We show that our criteria can detect more quantum network steering than that from the violation of the n-locality quantum networks. Moreover, it is shown that biseparable assemblages can demonstrate genuine network steering in the star network configurations.

Classical clock synchronization for quantum communications using the quantum channel.

We present a time synchronization method for correlation of photon arrival times for quantum communication. The method employs a low-power, low-frequency synchronization beacon that shares the quantum channel and is detected using the same single-photon detectors already present in the quantum communication system. We show experimentally that the system jitter approaches the system resolution limit with negligible impact on the quantum channel in realistic use cases. The presented method is particularly suitable for long distance quantum communication with large clock drifts, such as quantum links involving satellites.

Experimental Quantum Communication Overcomes the Rate-Loss Limit without Global Phase Tracking.

Secure key rate (SKR) of point-point quantum key distribution (QKD) is fundamentally bounded by the rate-loss limit. Recent breakthrough of twin-field (TF) QKD can overcome this limit and enables long distance quantum communication, but its implementation necessitates complex global phase tracking and requires strong phase references that not only add to noise but also reduce the duty cycle for quantum transmission. Here, we resolve these shortcomings, and importantly achieve even higher SKRs than TF-QKD, via implementing an innovative but simpler measurement-device-independent QKD that realizes repeaterlike communication through asynchronous coincidence pairing. Over 413 and 508km optical fibers, we achieve finite-size SKRs of 590.61 and 42.64 bit/s, which are respectively 1.80 and 4.08 times of their corresponding absolute rate limits. Significantly, the SKR at 306km exceeds 5 kbit/s and meets the bitrate requirement for live one-time-pad encryption of voice communication. Our work will bring forward economical and efficient intercity quantum-secure networks.

Quantum Zeno repeaters

Quantum repeaters pave the way for long-distance quantum communications and quantum Internet, and the idea of quantum repeaters is based on entanglement swapping which requires the implementation of controlled quantum gates. Frequently measuring a quantum system affects its dynamics which is known as the quantum Zeno effect (QZE). Beyond slowing down its evolution, QZE can be used to control the dynamics of a quantum system by introducing a carefully designed set of operations between measurements. Here, we propose an entanglement swapping protocol based on QZE, which achieves almost unit fidelity. Implementation of our protocol requires only simple frequent threshold measurements and single particle rotations. We extend the proposed entanglement swapping protocol to a series of repeater stations for constructing quantum Zeno repeaters which also achieve almost unit fidelity regardless of the number of repeaters. Requiring no controlled gates, our proposal reduces the quantum circuit complexity of quantum repeaters. Our work has potential to contribute to long distance quantum communications and quantum computing via quantum Zeno effect.

40 km fiber transmission of squeezed light measured with a real local oscillator

We demonstrate the generation, 40 km fiber transmission, and homodyne detection of single-mode squeezed states of light at 1550 nm using real-time phase control of a locally generated local oscillator (LO), often called a ‘real LO’ or ‘local LO’. The system was able to stably measure up to around 3.7 dB of noise suppression with a phase noise uncertainty of around 2.5°, using only standard telecom-compatible components and a field-programmable gate array. The compactness, low degree of complexity and efficacy of the implemented scheme makes it a relevant candidate for long distance quantum communication in future photonic quantum networks.

Nonclassical correlated deterministic single-photon pairs for a trapped atom in bimodal cavities

Single photons and single-photon pairs, inherently nonclassical in their nature, are fundamental elements of quantum sciences and technologies. Here, we propose to realize the nonclassical correlated deterministic photon pairs at the single-photon level for a single atom trapped in bimodal cavities. It is shown that the photon emissions for bimodal cavities exhibit a high single photon purity and relatively large intracavity photon numbers, which are ascribed to the combination of optical Stark shift enhanced energy-spectrum anharmonicity and quantum interference suppressed two-photon excitation. Our scheme generates photon-photon pairs with strong photon blockade for cavity modes via constructing a highly tunable cavity-enhanced deterministic parametric down-conversion process. Furthermore, we show that the cross-correlation function between the two cavity modes vastly violates the Cauchy-Schwarz inequality of the classical boundary, which unambiguously demonstrates the nonclassicality of the single-photon pairs. Our result reveals a prominent strategy to generate the high-quality two-mode single-photon sources with strong nonclassical correlation, which could provide versatile applications in distribution of quantum networks, long distance quantum communication, and fundamental tests of quantum physics.

Implementation of discrete positive operator valued measures on linear optical systems using cosine-sine decomposition

Positive operator valued measurements (POVMs) play an important role in efficient quantum communication and computation. While optical systems are one of the strongest candidates for long distance quantum communication and information processing, efficient methods to implement POVMs in these systems are scarce. Here we propose an all-optical scheme to implement an arbitrary POVM using linear optical components on m-dimensional Hilbert space of internal degrees of freedom. Linear optical nature of the proposed scheme makes it efficient and robust. We show how the scheme can be applied for state tomography and for preparing arbitrary mixed states.

Optomechanical quantum teleportation

Quantum teleportation, the faithful transfer of an unknown input state onto a remote quantum system, is a key component in long distance quantum communication protocols and distributed quantum computing. At the same time, high frequency nano-optomechanical systems hold great promise as nodes in a future quantum network, operating on-chip at low-loss optical telecom wavelengths with long mechanical lifetimes. Recent demonstrations include entanglement between two resonators, a quantum memory and microwave to optics transduction. Despite these successes, quantum teleportation of an optical input state onto a long-lived optomechanical memory is an outstanding challenge. Here we demonstrate quantum teleportation of a polarization-encoded optical input state onto the joint state of a pair of nanomechanical resonators. Our protocol also allows for the first time to store and retrieve an arbitrary qubit state onto a dual-rail encoded optomechanical quantum memory. This work demonstrates the full functionality of a single quantum repeater node, and presents a key milestone towards applications of optomechanical systems as quantum network nodes.

How efficient is transport of quantum cargo through multiple highways?

Quantum states can be efficiently transferred over a long distance if the entire quantum channel can be divided into several small blocks. We consider a scenario in which each block consists of two copies of a multiparty state — one is used for distributing an arbitrary quantum state to multiple parties while the other channel is required to concentrate it back to a single party. Both in noiseless and local noisy scenarios, we find one-shot quantum capacities of these channels in terms of fidelity, when the initial shared states in each block are the generalized Greenberger–Horne–Zeilinger and the generalized W states. We also consider a situation where optimal local measurements transform multipartite states to bipartite ones which can then be used as single-path channels for quantum state transmission in each segment. We show that in some parameter ranges, the former protocol provides strictly better fidelities than that of the latter, thereby establishing the importance of distributing and concentrating arbitrary quantum states via multipartite entangled states in long distance quantum communication, over the local measurement based protocol. Moreover, we show that in presence of bit flip or bit-phase flip noise, shared generalized Greenberger–Horne–Zeilinger states possess an inherent noise detection and correction mechanism, leading to the same fidelity as in the noiseless case. We consider further noise models also, which do not enjoy the same mechanism. In addition to the fidelity based advantages, the multipath scheme is shown to be useful when one considers a situation in which the completion of the teleportation needs to be delayed. We also find the efficiencies of a quantum channel when a quantum state is transferred over long distances and the entire channel is divided into several small blocks.

OAM tomography with Heisenberg–Weyl observables

Photons carrying orbital angular momentum (OAM) are excellent qudits and are widely used in several applications, such as long distance quantum communication, d-dimensional teleportation and high-resolution imaging and metrology. All these protocols rely on quantum tomography to characterise the OAM state, which currently requires complex measurements involving spatial light modulators and mode filters. To simplify the measurement and characterisation of OAM states, here we apply a recent tomography protocol [Asadian et al, Phys. Rev. A 94 010301 (2016)]. Our scheme for OAM tomography in d dimensions requires only a set of measurements on a path qubit, i.e., a two-dimensional system. This replaces the current complexity of OAM measurements by the ability to perform generalized Pauli operators Xd, Zd on OAM states. Our scheme can be adapted in principle to other degrees of freedom, thus opening the way for more complex qudit tomography.

Polarization design for ground-to-satellite quantum entanglement distribution.

High-fidelity transmission of polarization encoded qubits plays a key role in long distance quantum communication. By establishing the channel between ground and satellite, the communication distance can even exceed thousands of kilometers. Aimed to achieve the efficient uplink quantum communication, here we describe a high-fidelity polarization design of a transmitting antenna with an average polarization extinction ratio of 887:1 by a local test. We also implement a feasible polarization-compensation scheme for satellite motions with a fidelity exceeding 0.995 ± 0.001. Based on these works, we demonstrate the ground-to-satellite entanglment distribution with a violation of Bell inequality by 2.312±0.096, which is well above the classic limit 2.

Coherent Conversion Between Microwave and Optical Photons—An Overview of Physical Implementations

AbstractQuantum information technology based on solid state qubits has created much interest in converting quantum states from the microwave to the optical domain. Optical photons, unlike microwave photons, can be transmitted by fiber, making them suitable for long distance quantum communication. Moreover, the optical domain offers access to a large set of very well‐developed quantum optical tools, such as highly efficient single‐photon detectors and long‐lived quantum memories. For a high fidelity microwave to optical transducer, efficient conversion at single photon level and low added noise is needed. Currently, the most promising approaches to build such systems are based on second‐order nonlinear phenomena such as optomechanical and electro‐optic interactions. Alternative approaches, although not yet as efficient, include magneto‐optical coupling and schemes based on isolated quantum systems like atoms, ions, or quantum dots. Herein, the necessary theoretical foundations for the most important microwave‐to‐optical conversion experiments are provided, their implementations are described, and the current limitations and future prospects are discussed.

Optimized designs for telecom-wavelength quantum light sources based on hybrid circular Bragg gratings.

We present a design study of quantum light sources based on hybrid circular Bragg gratings (CBGs) for emission wavelengths in the telecom O-band. The evaluated CBG designs show photon extraction efficiencies > 95% and Purcell factors close to 30. Using simulations based on the finite element method, and considering the influence of possible fabrication imperfections, we identify optimized high-performance CBG designs which are robust against structural aberrations. In particular, full 3D simulations reveal that the designs show robustness regarding lateral deviations of the emitter position in the device well within reported positioning accuracies of deterministic fabrication technologies. Furthermore, we investigate the coupling of the evaluated hybrid CBG designs to single-mode optical fibers, which is particularly interesting for the development of practical quantum light sources. We obtain coupling efficiencies of up to 77% for off-the-shelf fibers, and again proof robustness against fabrication imperfections. Our results show prospects for the fabrication of close-to-ideal fiber-coupled quantum light sources for long distance quantum communication.

Implementation of quantum repeater scheme based on non-identical quantum memories

Quantum key distribution aims to distribute a secret key among distant parties linked by an optical fiber or free space. Unfortunately, the main problem which limits the long distance direct transmission of qubits is the exponential decay of the signal. This limit can be overcome by introducing quantum repeaters (QRs) between the two distant points. Quantum repeaters suggest a technique for establishing long distance quantum communication by the construction of entangled qubits among the end point of the channel, by dividing the channel into elementary links edged with quantum memory (QM) called nodes. Many schemes use identical QMs (Mastromattei in Assessing the practicality of a simple multi-node quantum repeater. M.Sc. thesis, University of Waterloo, ON, Canada, 2017). These schemes need QMs of long life times and high efficiencies which is not the practical case for QMs. To optimize these requirements, a multi-node sequential quantum repeater of a non-identical QMs is proposed here to interplay between the total channel efficiency and decoherence time T2 and the quantum memories life times and their efficiencies to enhance the execution of the quantum repeater (QR). The outcomes show that the interplay between the decoherence time and efficiency in non-identical QR improves the key rate as compared to that of the identical QR, the distance of the channel can be extended by using non-identical QMs repeater scheme since quantum bit error rate is decreased, and the cost of fabricating non-identical QMs repeater is reduced since we use most QMs with low life time.

Effect of magnetic field on a multi window ladder type electromagnetically induced transparency with 87Rb atoms in vapour cell

In this work, we experimentally study the effect of externally applied magnetic field on a ladder type EIT in a vapour cell consisting of 87Rb atoms. The introduction of magnetic field causes the Zeeman splitting of the hyperfine levels of 87Rb atoms and hence the number of available windows of transparency increases. We report the observation of nine such windows. Such multi window EIT systems are capable of storing pulses at the different frequencies, corresponding to these windows hence paving the way for realization of multi frequency quantum memories. Also, the total bandwidth of storage is 218.4 MHz which is two orders of magnitude higher than that typically obtained in single window EIT based storage systems. These systems have tremendous applications in the field of speedy transmission of data over a long distance quantum communication channel.

Photonic quantum memory using an intra-atomic frequency comb

Photonic quantum memory, such as an atomic frequency comb (AFC), is essential to make photonic quantum computation and long distance quantum communication scalable and feasible. In standard AFC the frequency of different atoms must be stable relative to each other which presents difficulties in realizing the quantum memory. Here we propose a quantum memory using an intra-atomic frequency comb which does not require frequency stabilization. We show that the transitions between two degenerate energy levels of a single atom can be used to construct the frequency comb. The spacing between the teeth of the comb is controlled by applying an external magnetic field. Since the frequency comb is constructed from individual atoms, these atoms can be used alone or in ensembles to realize the quantum memory. Furthermore, the ensemble based quantum memory with intra-AFC is robust against Doppler broadening which makes it useful for high-temperature quantum memory. As an example, we numerically show the intra-AFC in cesium atoms and demonstrate a photon echo which is essential for quantum memory.

Engineering of heralded narrowband color-entangled states

Efficient heralded generation of entanglement together with its manipulation is of great importance for quantum communications. In addition, states generated with bandwidths naturally compatible with atomic transitions allow a more efficient mapping of light into matter which is an essential requirement for long distance quantum communications. Here we propose a scheme where the indistinguishability between two spontaneous four-wave mixing processes is engineered to herald generation of single-photon frequency-bin entangled states, i.e., single-photons shared by two distinct frequency modes. We show that entanglement can be optimised together with the generation probability, while maintaining absorption negligible. Besides, the scheme illustrated for cold rubidium atoms is versatile and can be implemented in several other physical systems.

Intrinsic retrieval efficiency for quantum memories: A three-dimensional theory of light interaction with an atomic ensemble

Duan-Lukin-Cirac-Zoller (DLCZ) quantum repeater protocol, which was proposed to realize long distance quantum communication, requires usage of quantum memories. Atomic ensembles interacting with optical beams based on off-resonant Raman scattering serve as convenient on-demand quantum memories. Here, a complete free space, three-dimensional theory of the associated read and write process for this quantum memory is worked out with the aim of understanding intrinsic retrieval efficiency. We develop a formalism to calculate the transverse mode structure for the signal and the idler photons and use the formalism to study the intrinsic retrieval efficiency under various configurations. The effects of atomic density fluctuations and atomic motion are incorporated by numerically simulating this system for a range of realistic experimental parameters. We obtain results that describe the variation in the intrinsic retrieval efficiency as a function of the memory storage time for skewed beam configuration at a finite temperature, which provides valuable information for optimization of the retrieval efficiency in experiments.

A quantum light-emitting diode for the standard telecom window around 1,550\u2009nm

Single photons and entangled photon pairs are a key resource of many quantum secure communication and quantum computation protocols, and non-Poissonian sources emitting in the low-loss wavelength region around 1,550 nm are essential for the development of fibre-based quantum network infrastructure. However, reaching this wavelength window has been challenging for semiconductor-based quantum light sources. Here we show that quantum dot devices based on indium phosphide are capable of electrically injected single photon emission in this wavelength region. Using the biexciton cascade mechanism, they also produce entangled photons with a fidelity of 87 ± 4%, sufficient for the application of one-way error correction protocols. The material system further allows for entangled photon generation up to an operating temperature of 93 K. Our quantum photon source can be directly integrated with existing long distance quantum communication and cryptography systems, and provides a promising material platform for developing future quantum network hardware.