Concept · physics
Quantum optics
9 studiesEvidence last moved Sep 27, 2026
Quantum optics studies light whose statistics cannot be explained classically, such as single photons and correlated photon pairs, and how to make, shape and use them. The evidence here covers solid-state single-photon emitters, frequency and mode conversion of single photons, quantum key distribution over real fibre, and quantum-enhanced imaging and interferometry.
Students often treat 'single photon' as a label rather than something that must be tested (g2(0), visibility thresholds). These papers show how nonclassicality is verified and how far practical advantages actually go.
Studies
9
Findings
5
8 supporting · 0 challenging · 3 qualifying citations
Open tensions
2
Latest change
Concept page published
Quantum optics
Currently
What we know
- Emitters can be made on demand and tuned electrically.
- Real-world QKD with single-photon sources is working.
- Quantum interfaces can match photons from different sources.
- Visibility thresholds, not fringe patterns alone, prove quantumness.
- Quantum correlations give constant-factor gains limited by detection efficiency.
Largest unresolved question
Demonstration versus system: most studies show a component (source, converter, metasurface) without the full application; the OAM interface was never connected to a memory, the metasurface was not built into a sensor, and the QKD link lacked fast random state choice.
Common misconceptions
Seeing doubled-frequency interference fringes proves photons are entangled.
The metasurface authors note weak coherent light also shows them; the evidence rests on visibility exceeding the 70.7% threshold.
Quantum imaging gives unlimited improvement over classical imaging.
The twin-beam phase imaging gain was up to about 40%, limited by 0.57 heralding efficiency, not Heisenberg scaling.
Any bright spot from a quantum dot or defect is a single-photon source.
It must be checked with g2(0): in hBN, higher pulse energies gave multiple peaks and g2(0) of 0.48, only low-energy sites reached 0.09.
Related
Claim ledger
What the evidence shows
Drawn from 9 studies in this library. Mix labels say which citation roles are present; they are not a strength score. Supports means evidence for a finding; Challenges means evidence against a stated position; Qualifies marks scope.
Emitters can be made on demand and tuned electrically.
Single-photon emitters can be engineered and controlled: laser-written hBN defects near threshold gave single emitters with g2(0) down to 0.09 and 94% of 200 emitters with negligible spectral diffusion, and charging CdSe/CdS dots electrochemically sped emission up to 140-fold while keeping single-photon emission above -1.8 V.
- Can voltage make a single quantum dot emit photons faster?— Only about 30% of dots charged beyond two electrons; done in liquid electrolyte.
- Can a laser write single-atom-scale light sources?— Emission wavelength not controlled.
Study Role Design N Population Outcome Can a laser write single-atom-scale light sources? Supports OtherFemtosecond laser writing of defects in exfoliated hBN flakes near a multi-shot-extrapolated threshold, followed by annealing and single-photon characterisation No single N: 16-site array for single-emitter yield, 150 emitters for g2(0) statistics, 200 emitters for stability, roughly ten thousand centres for the wavelength histogram Laser-written colour centres in mechanically exfoliated hexagonal boron nitride flakes Feature size (TEM), number of emission peaks, single-photon purity g2(0), brightness, and photostability Can voltage make a single quantum dot emit photons faster? Supports OtherLab experiment: individual giant-shell CdSe/CdS quantum dots on an ITO electrode in an electrochemical cell, with time-resolved confocal photoluminescence and photon-correlation measurements while the bias was varied from 0 to -2 V. N=37 · 37 individual quantum dots from two batches were tested; 13 showed charging beyond the doubly negative exciton. Headline numbers come from single representative dots. Giant-shell CdSe/CdS colloidal quantum dots (4 nm core; two shell thicknesses) Fluorescence lifetime (decay rate), emission intensity, blinking and second-order photon correlation g(2)(0) versus applied voltage Real-world QKD with single-photon sources is working.
Quantum-dot single photons already carry secure keys over deployed fibre: a 79 km Hannover-Braunschweig link sustained keys for 35 hours at about 5.35 kbit/s with 0.65% error.
- Can single photons from a quantum dot secure a link between cities?— Polarisation states were set statically, not randomly at speed.
Quantum interfaces can match photons from different sources.
Photons can be converted between colours and modes while keeping quantum properties: a thin-film lithium niobate modulator shifted photons by ±641 GHz, raising two-photon interference from 23.1% to 90.5%, and sum-frequency conversion kept OAM photons nonclassical with process fidelities near 0.95.
- Can a chip change the colour of single photons without loss?
- Can a single photon's twisted light survive a colour change?
Study Role Design N Population Outcome Can a chip change the colour of single photons without loss? Supports OtherLab experiment: heralded single photons from down-conversion were phase-modulated by a double-pass thin-film lithium niobate modulator for spectral shearing and time-lens compression, with spectra and two-photon interference measured. No sample count; a single integrated modulator device characterised in several configurations. Telecom-band (~1560 nm) heralded single-photon pulses and one double-pass thin-film lithium niobate phase modulator Single-photon frequency shift, spectral bandwidth after compression, and Hong-Ou-Mandel interference visibility Can a single photon's twisted light survive a colour change? Supports OtherTabletop nonlinear-optics experiment: cavity-enhanced sum-frequency generation in a PPKTP crystal converting OAM-carrying light from 1560 nm to 525 nm, first with classical and attenuated light, then with heralded single photons. No sample size; results are measured conversion efficiencies, cross-correlations and interference visibilities for OAM values l = 0, 1 and 2. Heralded single photons from spontaneous parametric down-conversion and attenuated laser light carrying orbital angular momentum Conversion efficiency, preservation of spatial mode shape, signal-idler cross-correlation, and superposition interference visibility/fidelity Visibility thresholds, not fringe patterns alone, prove quantumness.
Nonclassicality is shown by beating classical thresholds: a metasurface gave a Hong-Ou-Mandel dip of 86% (classical limit 50%) and two-photon fringes of 86.8% visibility (threshold 70.7%), and fibre N00N states doubled a rotation phase.
- Can a flat nanostructured surface entangle photons?
- Can entangled photons measure the Earth's rotation?
Study Role Design N Population Outcome Can a flat nanostructured surface entangle photons? Supports OtherPhoton-pair (SPDC) experiments with a silicon nanofin Pancharatnam-Berry-phase metasurface, HOM coincidence measurements and a folded metasurface interferometer No sample count; coincidence measurements on photon pairs from one source and one metasurface Orthogonally polarized photon pairs sent through an all-dielectric metasurface HOM dip visibility, photon bunching, and interference fringe visibility in coincidences Can entangled photons measure the Earth's rotation? Supports OtherOptical-fibre Sagnac interferometer with 2 km of fibre on a rotatable frame, fed with single photons or two-photon N00N states, with an optical switch to turn the rotation signal on and off. No sample of units; phases extracted from photon-coincidence fringes (11 settings of 30-minute runs at the main angle) plus five further frame angles. Entangled photon pairs at 1546 nm propagating in a large fibre Sagnac interferometer Earth-rotation-induced Sagnac phase shift and entanglement enhancement factor Quantum correlations give constant-factor gains limited by detection efficiency.
Correlated photon pairs reduce imaging noise: subtracting the idler's noise pattern cut uncertainty in transport-of-intensity phase imaging by up to about 40%.
Debates
Tensions and limits
Some items are genuine disagreements on the same question. Others mark different assays, populations, or outcomes.
Demonstration versus system: most studies show a component (source, converter, metasurface) without the full application; the OAM interface was never connected to a memory, the metasurface was not built into a sensor, and the QKD link lacked fast random state choice.
Demonstration versus system: most studies show a component (source, converter, metasurface) without the full application; the OAM interface was never connected to a memory, the metasurface was not built into a sensor, and the QKD link lacked fast random state choice.
- Can a single photon's twisted light survive a colour change?
- Can a flat nanostructured surface entangle photons?
- Can single photons from a quantum dot secure a link between cities?
Study Role Design N Population Outcome Can a single photon's twisted light survive a colour change? Supports OtherTabletop nonlinear-optics experiment: cavity-enhanced sum-frequency generation in a PPKTP crystal converting OAM-carrying light from 1560 nm to 525 nm, first with classical and attenuated light, then with heralded single photons. No sample size; results are measured conversion efficiencies, cross-correlations and interference visibilities for OAM values l = 0, 1 and 2. Heralded single photons from spontaneous parametric down-conversion and attenuated laser light carrying orbital angular momentum Conversion efficiency, preservation of spatial mode shape, signal-idler cross-correlation, and superposition interference visibility/fidelity Can a flat nanostructured surface entangle photons? Supports OtherPhoton-pair (SPDC) experiments with a silicon nanofin Pancharatnam-Berry-phase metasurface, HOM coincidence measurements and a folded metasurface interferometer No sample count; coincidence measurements on photon pairs from one source and one metasurface Orthogonally polarized photon pairs sent through an all-dielectric metasurface HOM dip visibility, photon bunching, and interference fringe visibility in coincidences Can single photons from a quantum dot secure a link between cities? Supports OtherPhysics experiment: a quantum dot single-photon source sent polarisation-encoded photons through lab fibre spools and a deployed 79 km fibre between Hannover and Braunschweig, with key rates computed from measured photon statistics. No participant sample; the system is one quantum-dot source measured in the lab and over one deployed fibre link. An InAs/InGaAs quantum dot in a circular Bragg grating emitting telecom C-band single photons, tested over lab fibre spools and an intercity fibre Quantum bit error ratio, secret key bits per pulse and secret key rate, single-photon purity g(2)(0), and maximum tolerable loss
PaperFren reads this as a limit on how far one study travels — different assays, populations, or outcomes — not a forced fight between papers.
Reported visibilities differ in processing: some are background-subtracted (lithium niobate shifter) while others are raw (microring Franson visibilities), so numbers are not directly comparable.
Reported visibilities differ in processing: some are background-subtracted (lithium niobate shifter) while others are raw (microring Franson visibilities), so numbers are not directly comparable.
- Can a chip change the colour of single photons without loss?
- Can one wavelength carry entanglement to three partners at once?
Study Role Design N Population Outcome Can a chip change the colour of single photons without loss? Supports OtherLab experiment: heralded single photons from down-conversion were phase-modulated by a double-pass thin-film lithium niobate modulator for spectral shearing and time-lens compression, with spectra and two-photon interference measured. No sample count; a single integrated modulator device characterised in several configurations. Telecom-band (~1560 nm) heralded single-photon pulses and one double-pass thin-film lithium niobate phase modulator Single-photon frequency shift, spectral bandwidth after compression, and Hong-Ou-Mandel interference visibility Can one wavelength carry entanglement to three partners at once? Supports OtherPhotonics experiment: two CW lasers pump a silicon nitride microring to drive degenerate and non-degenerate four-wave mixing; photon pairs characterized by coincidence counting and Franson interference, then used for BBM92 key distribution among four nodes. No sample; one microring source serving four network nodes over six wavelength channels. Energy-time entangled photon pairs from a fiber-pigtailed Si3N4 microring resonator Coincidence-to-accidental ratio, Franson interference visibility, quantum bit error rate and secure key rate
PaperFren reads this as a limit on how far one study travels — different assays, populations, or outcomes — not a forced fight between papers.
Timeline
How understanding moved
Study years are when the paper was published. Evidence edits are dated changes to this page's claims. Explanations are when PaperFren added a Discovery — not a claim that the science happened that day.
2026
Concept page published
Quantum optics
Change log
What changed
Dated edits to this page's evidence: studies added or removed from a claim, claims added or withdrawn, and new explanations tagged here. Rewordings are not listed.
- Concept page published
Papers
9 studies in this library bear on Quantum optics, ordered by citations.
- Can a flat nanostructured surface entangle photons?
A single flat silicon metasurface can entangle pairs of photons into a shared path state and then disentangle them again while keeping their quantum phase intact.
- Can a single photon's twisted light survive a colour change?
Single photons carrying twisted-light orbital angular momentum were shifted from telecom infrared to green light while keeping their spatial shape, quantum correlations and superposition coherence.
- Can a chip change the colour of single photons without loss?
An on-chip lithium niobate modulator shifted single photons' frequency by up to about 641 GHz and squeezed their bandwidth 18-fold, without adding noticeable loss or noise.
- Can a laser write single-atom-scale light sources?
Firing laser pulses just above a carefully measured damage threshold made defects smaller than 5 nm in boron nitride, and nearly every spot became a single, bright, stable source of individual photons.
- Can voltage make a single quantum dot emit photons faster?
Pumping extra electrons into a single quantum dot with a voltage made it emit light up to about 210 times faster while it still emitted photons one at a time.
- Can single photons from a quantum dot secure a link between cities?
A quantum dot that emits one photon at a time in the telecom band produced secure keys between two cities with a very low error rate for more than a day.
- Can entangled-photon twins sharpen see-through phase images?
Using the noise of a twin photon beam to cancel shot noise lets phase images of transparent objects be retrieved with up to about 40% less uncertainty than the best classical equivalent.
- Can entangled photons measure the Earth's rotation?
Pairs of entangled photons sent around a giant fibre loop detected Earth's spin and picked up twice the phase shift that single photons did, as quantum theory predicts.
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- Can one wavelength carry entanglement to three partners at once?
Pumping a chip-scale ring resonator with two lasers let one wavelength channel be entangled with three others, halving the channels needed to fully connect four quantum-network users.
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Questions
What is still open
Demonstration versus system: most studies show a component (source, converter, metasurface) without the full application; the OAM interface was never connected to a memory, the metasurface was not built into a sensor, and the QKD link lacked fast random state choice.
Reported visibilities differ in processing: some are background-subtracted (lithium niobate shifter) while others are raw (microring Franson visibilities), so numbers are not directly comparable.
Ask PaperFren about Quantum optics
Study this conceptflashcards and short-answer questions
How would you show that a metasurface produces a genuinely quantum two-photon effect?
Measure a Hong-Ou-Mandel dip and compare its visibility with the 50% classical limit; the metasurface gave 86%. In an interferometer, check that two-photon fringe visibility exceeds 70.7%; it reached 86.8%. Fringes at twice the frequency alone are not enough because classical light can produce them.
What does the 79 km quantum-dot QKD experiment show, and what remains?
It shows a semiconductor single-photon source can deliver secure finite-key rates of about 5.35 kbit/s at 0.65% error over deployed intercity fibre for 35 hours. However, polarisation states were prepared by slow motorised waveplates, so a real-time system was modelled, not demonstrated. Longer reaches are projections.