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The Nobel Prize in Physics 2022 has been awarded to Alain Aspect, John F. Clauser and Anton Zeilinger “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”. In recognition of this award, Nature Portfolio presents a collection of research, review and opinion articles that celebrates the direct contributions by the awardees and the advances they have inspired.

Quantum optics has come a long way since its birth at the beginning of the 1900s. Nature Photonics spoke to Anton Zeilinger to gain some perspective on its progress.

More than 40 years ago the first Bell tests translated a purely philosophical conundrum to a physical experiment. In doing so, they changed our understanding of quantum mechanics and contributed to the development of quantum technologies.

Quantum teleportation — the transmission and reconstruction over arbitrary distances of the state of a quantum system — is demonstrated experimentally. During teleportation, an initial photon which carries the polarization that is to be transferred and one of a pair of entangled photons are subjected to a measurement such that the second photon of the entangled pair acquires the polarization of the initial photon. This latter photon can be arbitrarily far away from the initial one. Quantum teleportation will be a critical ingredient for quantum computation networks.

In 1997, it was demonstrated that quantum states can be teleported from one location to a distant one. The discovery had huge consequences for the development of quantum communication and computing.

The experimental violation of Bell's inequalities confirms that a pair of entangled photons separated by hundreds of metres must be considered a single non-separable object — it is impossible to assign local physical reality to each photon.

It is easy to dismiss research into the foundations of quantum mechanics as irrelevant to physicists in other areas. Adopting this attitude misses opportunities to appreciate the richness of quantum mechanics.

A deterministic violation of the Bell inequality is reported between two superconducting circuits, providing a necessary test for establishing strong enough quantum entanglement to achieve secure quantum communications.

Device-independent randomness expansion is demonstrated in an experiment that is secure against quantum adversaries as established by the entropy accumulation theorem.

Device-independent randomness expansion is demonstrated in an experiment that is secure in the presence of a classical eavesdropper who does not share any entanglement with the setup.

The BIG Bell Test, which used an online video game with 100,000 participants worldwide to provide random bits to 13 quantum physics experiments, contradicts the Einstein–Podolsky–Rosen worldview of local realism.

The study of higher-dimensional quantum states has seen numerous conceptual and technological developments. This review discusses various techniques for the generation and processing of qudits, which are stored in the momentum, path, time-/frequency-bins, or the orbital angular momentum of photons.

A nondestructive and complete Bell-state measurement is demonstrated between two 60-m-distant atomic qubits in different optical cavities. The main building block is a photon-atom gate, which is executed upon reflection of the photon from the cavity.

A violation of bilocal inequality is demonstrated with two truly independent light sources delivering entanglements to three nodes. To this end, the locality, measurement independence and quantum source independence loopholes are closed simultaneously.

The quantum-delayed choice experiment is implemented with multiple entangled photons under Einstein’s locality condition. The wave–particle quantum superposition is realized by controlling the relative phase between the wave and particle states.

A three-photon entangled state with 3 × 3 × 2 dimensions of its orbital angular momentum is created by using two independent entangled photon pairs from two nonlinear crystals, enabling the development of a new layered quantum communication protocol.

New types of nonlocal correlations can arise in quantum networks, but experiments have not been done for more than two independent sources. Here, the authors violate a chained n-locality inequality in a network with five nodes and four independent sources, relying only on single-qubit measurements.

Bell’s theorem has important implications for quantum information processing. Here the authors experimentally investigate the violation of a Bell-like inequality in the case of distant parties whose correlations are mediated by independent sources, a realistic feature in future quantum networks.

The no-signaling principle constrains which multipartite correlations are allowed, but network scenarios considered so far were limited to specific cases. Here, the authors apply inflation technique to the no-input/binary-output triangle network, and show that it admits non-trilocal distributions.

A Bell experiment that is ‘loophole’ free—leaving no room for explanations based on experimental imperfections—reveals a statistically significant conflict with local realism

According to Bell's theorem, any theory that is based on the joint assumption of realism and locality is at variance with certain quantum predictions. Here, theory and experiment agree that a class of such non-local realistic theories is incompatible with experimentally observable quantum correlations, suggesting that giving up the concept of locality is not sufficient to be consistent with quantum experiments, unless certain intuitive features of realism are abandoned.

The fair-sampling loophole is closed in a Bell inequality violation experiment with entangled photons, making the photon the first physical system for which all the main loopholes have been closed.

In 2000, Asher Peres put forward the paradoxical idea that entanglement could be produced after the entangled particles have been measured, even if they no longer exist. Researchers now experimentally demonstrate this idea using four photons.

Quantum discord is the total non-classical correlation between two systems. This includes, but is not limited to, entanglement. Photonic experiments now demonstrate that separable states with non-zero quantum discord are a useful resource for quantum information processing and can even outperform entangled states.

An experiment distributing entangled photons over 144 km significantly raises the bar on distance, channel loss and transmission time—encouraging news with regard to future long-distance quantum-communication networks.

Four single-photon states are generated and entangled on a single micrometre-scale silicon chip, and provide the basis for the demonstration of chip-to-chip quantum teleportation.

A quantum network formed by three optically connected nodes comprising solid-state qubits demonstrates the teleportation of quantum information between two non-neighbouring nodes, negating the need for a direct connection between them.

Entanglement is often considered the defining feature separating classical physics from quantum physics and provides the basis for many quantum technologies. This Review discusses recent progress in the challenging task of conclusively proving that a physical system features entanglement, surveying detection and certification methods.

Einstein–Podolsky–Rosen entangled beams are sent to a 0.5-m-long optical resonator. To reduce quantum noise in a frequency-dependent manner in the gravitational detector, two-mode frequency-dependent squeezed vacuum states are generated.

Einstein–Podolsky–Rosen entangled beams are sent to a 2.5-m-long cavity mimicking the signal recycling cavity of a gravitational-wave detector. By controlling the wavelength detuning, frequency-dependent squeezed vacuum states were generated.

A three-dimensionally entangled Greenberger–Horne–Zeilinger state, where all three photons reside in a qutrit space, is generated by developing a new multi-port in combination with a novel four-photon source entangled in orbital angular momentum.

Recent advances in quantum information theory reveal the deep connections between entanglement and thermodynamics, many-body theory, quantum computing and its link to macroscopicity.

The benchmark for a global quantum internet — quantum teleportation of independent qubits using active feed-forward over a free-space link whose attenuation corresponds to the path between a satellite and a ground station — has now been successfully achieved over a distance of 143 km, between the Canary Islands of La Palma and Tenerife.

Quantum teleportation of single-photon qubits from a ground observatory to a satellite in low-Earth orbit via an uplink channel is achieved with a fidelity that is well above the classical limit.

This review covers state-of-the-art quantum teleportation technologies, from photonic qubits and optical modes to atomic ensembles, trapped atoms and solid-state systems. Open issues and potential future implementations are also discussed.

Interactive protocols can verify that a quantum computer exhibits a computational speedup using only classical analysis of its output. Exploiting a connection to Bell’s theorem gives a simpler protocol that is much less demanding for experiments.

Decoy-state quantum key distribution from a satellite to a ground station is achieved with much greater efficiency than is possible over the same distance using optical fibres.

This study demonstrates the experimental realization of a complete protocol for quantum key distribution using entangled trapped strontium ions with device-independent quantum security guarantees.

A system based on trapped rubidium atoms for generating quantum secure keys between distant users is presented, which could operate in a device-independent fashion.

A field test of twin-field quantum key distribution was implemented through a 511 km optical fibre. To this end, precise wavelength control of remote independent laser sources and fast time- and phase-compensation systems are developed.

The orbital angular momentum of light is a promising degree of freedom for long-distance information transportation. To create high-dimensional entanglement for pairs of photons, Fickler et al.use an optical mode sorter in reverse to transfer entanglement between the path into the orbital angular momentum.

Unless you are nearby, it is difficult to verify where someone is. Access to a single qubit and classical computation and communication makes it possible to securely check someone’s position as long as adversaries’ quantum resources are limited.

In 2000, Asher Peres put forward the paradoxical idea that entanglement could be produced after the entangled particles have been measured, even if they no longer exist. Researchers now experimentally demonstrate this idea using four photons.

A proof-of-principle experiment on twin-field quantum key distribution is demonstrated. The key rate overcomes the repeaterless secret key capacity bound limit at channel losses of 85 dB, corresponding to 530 km of ultralow-loss optical fibre.

An all-photonic quantum repeater is demonstrated by manipulating state-of-the-art 12-photon interferometry. The enhancement of entanglement-generation rate compared with parallel entanglement swapping proves the feasibility of the concept.

A new quantum imaging experiment demonstrates images made with light that does not encounter the object; one of a pair of photons created at two crystals illuminates the object but is never detected, and the other photon, which is in a joint quantum state with the first and does not interact with the object, forms an image of the object on a camera.

A quantum network that combines 700 fibre and two ground-to-satellite links achieves quantum key distribution between more than 150 users over a combined distance of 4,600 kilometres.

High-performance quantum light sources based on semiconductor quantum dots coupled to microcavities are showing their promise in long-distance solid-state quantum networks.