9.1 Single-photon emitters (atoms, quantum dots, NV centers)

Single-photon emitters are quantum systems that release one photon at a time. Atoms, quantum dots, and NV centers in diamond are key examples, each with unique properties that make them useful for different quantum applications.

These emitters play a crucial role in quantum optics and information processing. They're essential for creating indistinguishable photons, entangled pairs, and secure communication systems, paving the way for quantum technologies like cryptography and computing.

Single-photon emission mechanisms

Quantum systems and their properties

• Single-photon emission occurs when a quantum system (atom, quantum dot, or NV center) emits one photon at a time upon excitation
• The energy levels and transitions of the quantum system determine the wavelength and properties of the emitted photons
• The single-photon emission process is governed by the principles of quantum mechanics (superposition and entanglement of quantum states)

Emission processes in different systems

• In atoms, single-photon emission occurs due to electronic transitions between discrete energy levels, typically involving the excitation and relaxation of an electron
• Quantum dots are semiconductor nanostructures that confine electrons and holes in a small volume, leading to discrete energy levels and single-photon emission upon optical or electrical excitation
• Nitrogen-vacancy (NV) centers in diamond are point defects consisting of a substitutional nitrogen atom next to a vacancy, which exhibit stable single-photon emission due to their unique electronic structure

Single-photon emitter properties

Physical characteristics

• Atoms, quantum dots, and NV centers differ in their physical structure, size, and composition, which influence their emission properties
• Emission wavelength: Atoms typically emit in the visible to near-infrared range, while quantum dots and NV centers can be engineered to emit at specific wavelengths from the visible to the telecom range
• Emission linewidth: Atoms have narrow emission linewidths due to their well-defined energy levels, while quantum dots and NV centers may have broader linewidths due to inhomogeneous broadening effects

Performance metrics

• Photon indistinguishability: Atoms and NV centers can emit highly indistinguishable photons, which is crucial for quantum interference and entanglement experiments. Quantum dots may have lower indistinguishability due to charge noise and other environmental factors
• Emission rate and efficiency: Quantum dots and NV centers generally have higher emission rates and efficiencies compared to atoms, making them more suitable for high-speed and high-efficiency single-photon sources
• Scalability and integration: Quantum dots and NV centers are more easily integrated into solid-state devices and photonic structures, offering better scalability compared to atomic systems

Single-photon emitters for quantum optics

Advantages and limitations of different emitters

• Atoms:
  • Advantages: High photon indistinguishability, narrow emission linewidths, and long coherence times, making them suitable for quantum interference and entanglement experiments
  • Limitations: Difficulty in trapping and manipulating individual atoms, limited scalability, and the need for complex vacuum systems and laser cooling techniques
• Quantum dots:
  • Advantages: High emission rates, tunable emission wavelengths, and the possibility of integration into solid-state devices and photonic structures for scalable quantum photonic systems
  • Limitations: Lower photon indistinguishability compared to atoms and NV centers, sensitivity to charge noise and other environmental factors, and the need for cryogenic temperatures for optimal performance
• NV centers:
  • Advantages: Room-temperature operation, high photon indistinguishability, long spin coherence times, and the ability to control and manipulate the spin state for quantum information processing
  • Limitations: Lower emission rates compared to quantum dots, the need for careful engineering of the diamond host material, and the presence of phonon sidebands in the emission spectrum

Applications in quantum optics experiments

• Quantum interference and entanglement: Single-photon emitters are essential for generating indistinguishable photons and entangled photon pairs, which are the building blocks for quantum interference and entanglement experiments (Hong-Ou-Mandel effect, Bell state generation)
• Quantum communication: Single-photon emitters are used as sources for quantum key distribution (QKD) protocols, enabling secure communication over long distances (BB84 protocol, decoy-state QKD)
• Quantum metrology: Single-photon emitters, particularly NV centers, are employed in quantum metrology applications, such as high-precision magnetometry, electric field sensing, and nanoscale imaging

Single-photon emitters for quantum information processing

Suitability for specific quantum information tasks

• Quantum key distribution (QKD): All three types of single-photon emitters can be used for QKD, with atoms and NV centers being particularly suitable due to their high photon indistinguishability and the possibility of generating entangled photon pairs
• Quantum repeaters: NV centers are promising candidates for quantum repeaters due to their long spin coherence times and the ability to store and manipulate quantum information in their spin states
• Linear optical quantum computing (LOQC): Quantum dots are well-suited for LOQC due to their high emission rates and the possibility of integration into photonic circuits for scalable quantum information processing
• Quantum networks: Atoms and NV centers are suitable for quantum networks due to their ability to generate entangled photon pairs and their compatibility with quantum memories for long-distance entanglement distribution

Integration with quantum hardware and platforms

• Solid-state devices: Quantum dots and NV centers can be integrated into solid-state devices, such as photonic integrated circuits (PICs) and optoelectronic devices, enabling scalable and compact quantum information processing systems
• Hybrid quantum systems: Single-photon emitters can be interfaced with other quantum systems, such as superconducting qubits, trapped ions, and optomechanical resonators, to create hybrid quantum platforms that combine the advantages of different technologies (quantum transducers, quantum memories)
• Quantum networks: Single-photon emitters are essential components for building quantum networks, where they serve as sources of entangled photons and quantum repeaters for long-distance entanglement distribution and quantum communication (quantum internet, distributed quantum computing)