Quantum Key Distribution (QKD) is a cutting-edge field that leverages the principles of quantum mechanics to enable secure communication between parties involved. Single-photon quantum emitters offer remarkable on-demand photon emission, near-unitary indistinguishability, and low multiphoton generation, thereby enhancing the performance of QKD protocols. Standard approaches in which the polarization degree-of-freedom is exploited are limited by the precise alignment between the communicating parties. To overcome this obstacle, the Orbital Angular Momentum (OAM) of light represents a suitable candidate for encoding the information, as it allows the implementation of rotational-invariant photonic states that remove the need for a fixed physical reference frame between the communicating parties. Here, we demonstrate an alignment-free BB84 implementation using on-demand single photons from a quantum-dot emitter at 925 nm. Time-to-spatial demultiplexing and Q -plates encode rotationally invariant spin–orbit states that we transmit over a 2 m free-space link while rotating the receiver between 0 ∘ and 90 ∘ . The source exhibits g (2) (0)≈0.03; the measured QBER remains (4.04±0.02)% and is essentially independent of the relative angle. A simple security model tailored to sub-Poissonian emission, which explicitly incorporates g (2) (0), yields a positive asymptotic secret fraction and an effective QBER threshold of ∼9.6% under our conditions. These results validate rotationally invariant QKD with an on-demand single-photon source and outline its scalability to long-distance scenarios.
Quantum Key Distribution (QKD) is a cutting-edge field that leverages the principles of quantum mechanics to enable secure communication between parties involved. Single-photon quantum emitters offer remarkable on-demand photon emission, near-unitary indistinguishability, and low multiphoton generation, thereby enhancing the performance of QKD protocols. Standard approaches in which the polarization degree-of-freedom is exploited are limited by the precise alignment between the communicating parties. To overcome this obstacle, the Orbital Angular Momentum (OAM) of light represents a suitable candidate for encoding the information, as it allows the implementation of rotational-invariant photonic states that remove the need for a fixed physical reference frame between the communicating parties. Here, we demonstrate an alignment-free BB84 implementation using on-demand single photons from a quantum-dot emitter at 925 nm. Time-to-spatial demultiplexing and Q -plates encode rotationally invariant spin–orbit states that we transmit over a 2 m free-space link while rotating the receiver between 0 ∘ and 90 ∘ . The source exhibits g (2) (0)≈0.03; the measured QBER remains (4.04±0.02)% and is essentially independent of the relative angle. A simple security model tailored to sub-Poissonian emission, which explicitly incorporates g (2) (0), yields a positive asymptotic secret fraction and an effective QBER threshold of ∼9.6% under our conditions. These results validate rotationally invariant QKD with an on-demand single-photon source and outline its scalability to long-distance scenarios.
"Twisted photons" are photons carrying a well-defined nonzero value of orbital angular momentum (OAM). The associated optical wave exhibits a helical shape of the wavefront (hence the name) and an optical vortex at the beam axis. The OAM of light is attracting a growing interest for its potential in photonic applications ranging from particle manipulation, microscopy, and nanotechnologies to fundamental tests of quantum mechanics, classical data multiplexing, and quantum communication. Hitherto, however, all results obtained with optical OAM were limited to laboratory scale. Here, we report the experimental demonstration of a link for free-space quantum communication with OAM operating over a distance of 210 m. Our method exploits OAM in combination with optical polarization to encode the information in rotation-invariant photonic states, so as to guarantee full independence of the communication from the local reference frames of the transmitting and receiving units. In particular, we implement quantum key distribution, a protocol exploiting the features of quantum mechanics to guarantee unconditional security in cryptographic communication, demonstrating error-rate performances that are fully compatible with real-world application requirements. Our results extend previous achievements of OAM-based quantum communication by over 2 orders of magnitude in the link scale, providing an important step forward in achieving the vision of a worldwide quantum network.
Polarization-entangled photon-based quantum key distribution (QKD) has seen notable advancements, requiring users to share a common frame of reference that might lead to a significant technical overhead of constant monitoring of their frame of reference. In this work, we characterize and demonstrate hybrid entangled photon pairs for their utilization in rotational alignment-free QKD applications. By employing vortex half-wave retarders that have an azimuthally varying fast axis with a topological charge of q=12, we generate high brightness rotationally invariant hybrid entangled photons and show their resilience against the misalignment of the frame of reference. We demonstrate high fidelity (>92%) entanglement and depict a low quantum bit error rate (QBER) of ∼5−6% (lower than the threshold value of 11%) at a significantly high angle of misalignment θ>45°), in contrast to the polarization-entangled photon pairs that suffer from the degradation of entanglement quality due to a rotational misalignment between the reference frame of communicating entities. The rotationally invariant hybrid photon pairs with robust entanglement properties offer a significant advantage for secure and reliable quantum communication, making them lucrative candidates for applications such as satellite-to-satellite and ground-to-satellite QKD networks. We also show the reliability and stability of the entangled photon source over a long period (measured over 6 h) by demonstrating the BBM92 protocol with a raw key rate of ∼2.8 kbps and a QBER of ∼5% in a single-pass configuration. The performance metrics of our quantum source, measured with conventional avalanche photodiodes (SPADs, quantum efficiency ∼60%, dark counts ∼40 cps), are among the best values reported so far with these detectors.
Engineering single-photon states endowed with orbital angular momentum (OAM) is a powerful tool for quantum information photonic implementations. Indeed, due to its unbounded nature, OAM is suitable for encoding qudits, allowing a single carrier to transport a large amount of information. Most of the experimental platforms employ spontaneous parametric down-conversion processes to generate single photons, even if this approach is intrinsically probabilistic, leading to scalability issues for an increasing number of qudits. Semiconductor quantum dots (QDs) have been used to get over these limitations by producing on-demand pure and indistinguishable single-photon states, although only recently they have been exploited to create OAM modes. Our work employs a bright QD single-photon source to generate a complete set of quantum states for information processing with OAM-endowed photons. We first study hybrid intraparticle entanglement between OAM and polarization degrees of freedom of a single photon whose preparation was certified by means of Hong–Ou–Mandel visibility. Then, we investigate hybrid interparticle OAM-based entanglement by exploiting a probabilistic entangling gate. The performance of our approach is assessed by performing quantum state tomography and violating Bell inequalities. Our results pave the way for the use of deterministic sources for the on-demand generation of photonic high-dimensional quantum states.
It has been widely recognized that the underlying principles of quantum mechanics could be used to enable secure communications, without any additional conventional cryptographic systems, while utilizing the properties of the photons. Considerable research efforts have been invested to develop an efficient quantum key distribution (QKD) scheme over either free-space optical or fiber-optics channels. Most of these research efforts have been focused on a two-dimensional QKD, commonly realized by the use of the polarization state of photons. However, the data rates for the quantum key exchange in two-dimensional QKD are still low, while transmission distance is limited. On the other hand, it is well known that photons can carry both the spin angular momentum (SAM) and the orbital angular momentum (OAM), associated with the polarization and azimuthal phase of the complex electric field, respectively. Accordingly, we can define the combined-OAM-SAM state of a photon as |l, σ〉, where l and σ correspond to OAM and SAM indexes, respectively. Since the OAM eigenstates are orthogonal, an arbitrary number of bits per single photon can be transmitted, which could considerably increase the total secure bit rate. To improve secure data rates, we propose two types of protocols, namely, nonentangled-based (such as weak coherent state) and entanglement-assisted protocols, both employing the photon combined-OAM-SAM state, which can be used for secure key distribution over free-space optical and few-mode fiber channels. Two types of entanglement-assisted protocols are described, namely, two-basis and (D + 1)-basis protocols ( D is dimensionality of corresponding Hilbert space). We further describe how to implement the qudit gates required for implementation of these protocols. Finally, we discuss the security issues of the proposed protocols and determine both infinite and finite secret key fraction rates.
Deterministic solid-state quantum light sources are key building blocks in photonic quantum technologies. While several proof-of-principle experiments of quantum communication using such sources have been realized, all of them required bulky setups. Here, we evaluate for the first time the performance of a compact and stand-alone fiber-coupled single-photon source emitting in the telecom O-band (1321nm) for its application in quantum key distribution (QKD). For this purpose, we developed a compact 19'' rack module including a deterministically fiber-coupled quantum dot single-photon source integrated into a Stirling cryocooler, a pulsed diode laser for driving the quantum dot, and a fiber-based spectral filter. We further employed this compact quantum light source in a QKD testbed designed for polarization coding via the BB84 protocol resulting in g(2)(0)=0.10±0.01 and a raw key rate of up to (4.72±0.13)kHz using an external laser for excitation. In this setting we investigate the achievable performance expected in full implementations of QKD. Using 2D temporal filtering on receiver side, we evaluate optimal parameter settings for different QKD transmission scenarios taking also finite key size effects into account. Using optimized parameter sets for the temporal acceptance time window, we predict a maximal tolerable loss of 23.19dB. Finally, we compare our results to previous QKD systems using quantum dot single-photon sources. Our study represents an important step forward in the development of fiber-based quantum-secured communication networks exploiting sub-Poissonian quantum light sources.
Abstract Quantum key distribution (QKD) enables the transmission of information that is secure against general attacks by eavesdroppers. The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss. Semiconductor quantum dots (QDs) are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution. Here we report on the first intercity QKD experiment using a bright deterministic single photon source. A BB84 protocol based on polarisation encoding is realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure. Utilising the 79 km long link with 25.49 dB loss (equivalent to 130 km for the direct-connected optical fibre) between the German cities of Hannover and Braunschweig, a record-high secret key bits per pulse of 4.8 × 10−5 with an average quantum bit error ratio of ~ 0.65% are demonstrated. An asymptotic maximum tolerable loss of 28.11 dB is found, corresponding to a length of 144 km of standard telecommunication fibre. Deterministic semiconductor sources therefore challenge state-of-the-art QKD protocols and have the potential to excel in measurement device independent protocols and quantum repeater applications.