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Photonic qubits are made by generating a photon, preparing one of its degrees of freedom to represent a two-state quantum system, and then using optical components to manipulate and measure that state. The photon carries quantum information; it is not, by itself, a complete quantum computer.
What makes a photon a qubit?
A qubit is a quantum system with two logical basis states. In a photonic system, those states are encoded in a chosen feature—or degree of freedom—of a photon. The photon can be prepared in a superposition of the two states, and optical operations can change that state before measurement.
- Polarization: two orthogonal polarization states can represent the logical states.
- Path: two distinct routes through an optical circuit form the basis states.
- Time bin: an early or late arrival can encode the state.
- Frequency: two frequency modes can serve as the basis states.
The encoding affects which components are needed to prepare, transform, transmit, and detect the qubit. It also shapes how the photon fits into a communication link or processing architecture. Reviews of photonic quantum technologies discuss these encoding and system-design choices (Nature Materials, 2025; Nature Reviews Physics, 2023).
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteHow are photons generated for quantum experiments?
Two common approaches are nonlinear optical pair generation and emission from semiconductor quantum dots. They differ in how photons are produced and in the engineering trade-offs involved; neither is a universal winner.
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Nonlinear optics: generate a pair, then herald one photon
A pump laser interacts with a nonlinear material, which can occasionally produce a correlated pair of photons. In spontaneous parametric down-conversion (SPDC), the process uses a second-order, or χ(2), nonlinearity. In spontaneous four-wave mixing (SFWM), it uses a third-order, or χ(3), nonlinearity. Both are probabilistic: a pump pulse does not guarantee that a pair is produced.
When one photon is detected, that detection can herald the presence of its partner, which is then used in the experiment. This is a useful way to prepare a single photon, but probabilistic production makes it harder to scale systems that require many photons to be available together. Reviews of integrated nonlinear sources and photonic systems describe these approaches (Light: Science & Applications, 2017; npj Nanophotonics, 2026).
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Quantum dots: trigger an emitter
A semiconductor quantum dot can act as a solid-state emitter. Laser excitation creates an exciton in the dot; when it decays, it emits a photon. Because the emission can be triggered, this is described as a deterministic or on-demand route in contrast to heralded pair generation.
“Deterministic” describes the operating approach, not a guarantee that every pulse yields a perfect photon. Practical devices must still be engineered for properties such as purity, indistinguishability, efficient extraction, and integration with the rest of the optical system. Reviews of quantum-dot sources and emitter interfaces discuss these requirements (Nature Nanotechnology, 2017; Nature Nanotechnology, 2021).
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How do the two source approaches differ?
| Consideration | Nonlinear pair sources (SPDC and SFWM) | Quantum dots |
|---|---|---|
| Emission mode | Probabilistic pair generation; detecting one photon can herald the other. | Triggered emission, described as deterministic or on demand. |
| Photon quality | Purity and indistinguishability matter when photons must interfere or be processed together. | Purity and indistinguishability remain important engineering goals; triggered emission alone does not guarantee them. |
| Collection and rate | Useful output depends on how efficiently photons are collected and how often usable pairs are generated. | Extraction efficiency and emission rate affect how many useful photons reach the circuit. |
| Integration | Nonlinear materials can be coupled to integrated photonic components. | Solid-state emitters can be coupled to integrated photonic components. |
| Best fit | Depends on the intended circuit, wavelength, and source requirements. | Depends on the intended circuit, wavelength, and source requirements. |
The relevant choice depends on the complete system: required source properties, circuit design, and application. Reviews discuss the trade-offs, but do not establish one source type as the best for every use (Nature Nanotechnology, 2017; npj Nanophotonics, 2026; Nature Reviews Physics, 2023).
What happens between photon generation and measurement?
- Generate: a source emits a photon directly or produces a pair from which one photon is heralded.
- Prepare: optical elements establish the chosen encoding and, where needed, the relative phase between basis states.
- Process and route: a photonic circuit guides modes and can make them interfere, implementing transformations on the encoded state.
- Measure: detectors register the output. The recorded result is used to infer the measurement outcome for the prepared quantum state.
Integrated quantum photonics aims to bring more of generation, processing, routing, and detection onto compact circuits. A 2020 Nature Photonics review described a progression from circuits with a few components operating on two photons to programmable devices approaching 1,000 components in millimetre-scale footprints. That figure refers to a reported device-scale milestone, not to 1,000 qubits or a general-purpose quantum computer. The review describes the field as enabling “the generation, processing and detection of quantum states of light at a steadily increasing scale and level of complexity” (Nature Photonics, published 21 October 2019; volume 14, 2020).
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What can photonic qubits be used for?
Single photons and photonic circuits are studied for quantum communication, computation, simulation, and metrology. Their uses depend on more than the photon alone: source performance, encoding, optical processing, detectors, and the application’s wavelength and system requirements all matter. Progress in these areas should not be confused with universal commercial maturity; the cited reviews describe active research and technology development (Nature Reviews Physics, 2023; Nature Materials, 2025).
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- Emission mode: Is the source heralded and probabilistic, or triggered?
- Purity and indistinguishability: Are photons suitable for the interference or joint processing the system needs?
- Collection and rate: How efficiently do useful photons reach the circuit, and how often are they produced?
- Integration: Can the source work with the circuit and its routing and detection components?
- Application fit: Does the wavelength and encoding suit the communication link or processing architecture?
These are system-level questions, not a simple ranking of source technologies. The requirements differ across applications, and the reviews do not identify a single universally superior approach.
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