Entangled Photon Source
A crystal that splits one photon into two — twins that know each other's mind across any distance.
Provides the entangled photon pairs that make quantum cryptography, quantum teleportation, and Bell-test experiments possible. The two output photons share a quantum state that cannot be described independently — measuring one instantly determines the outcome of measuring the other.
Spontaneous parametric down-conversion (SPDC). A pump photon (typically 405 nm) enters a nonlinear crystal — usually beta-barium borate (BBO) or periodically-poled lithium niobate. With low probability (~10⁻⁶ per pump photon) the crystal splits it into two daughter photons whose energies sum to the pump's. By cutting the crystal at a precise angle, the two daughters can be made to emerge with their polarizations entangled — neither photon has a defined polarization until the other is measured.
In plain terms
Imagine a magic prism. You shoot one purple flashlight beam in, and out come two red flashlights — and those two flashlights are linked: whenever you tilt one's beam, the other tilts the same way, even if you carry it to the other side of the world.
Also called: SPDC source, BBO crystal.
Why it matters
- The standard workhorse of every quantum optics lab worldwide.
- Powers quantum-key-distribution networks (e.g., China's Micius satellite).
- Enabled the loophole-free Bell tests of 2015 that closed Einstein's last classical objection to quantum mechanics.
- Single device, table-top — anyone can buy a BBO crystal for under $1000.
Where it lives
- Micius quantum-communications satellite
- Vienna and Geneva metropolitan QKD networks
- Loophole-free Bell tests (Delft 2015, NIST 2015, Vienna 2015)
Key numbers
- Pump wavelength: 405 nm (Violet diode laser)
- Output wavelengths: 810 nm + 810 nm (Degenerate SPDC, near-IR)
- Conversion efficiency: ~10⁻⁶ per pump photon
- Crystal size: ~5 × 5 × 1 mm
Surprising facts
- An entangled photon pair has been used to verify the rules of quantum mechanics across 1200 km between a satellite and the ground.
- Each output photon, alone, is completely random — but when you compare results from the two, they correlate more strongly than any classical theory permits. Einstein called this 'spooky action at a distance.'
- Only one in roughly a million pump photons converts. The rest pass through unchanged. SPDC is one of the lowest-yield quantum processes routinely engineered.
- The same crystal that produces these pairs is on the order of a few millimeters across and costs less than a smartphone.
Open questions
- Correlations violating Bell inequalities by 20%: In the 2015 NIST loophole-free Bell test, entangled photon pairs violated Bell's inequality by 20 percent — well above the classical maximum of zero. The probability that this result could have arisen from any local hidden-variable model, including schemes where the devices conspired using all available historical data, was calculated to be less than 4%. No classical explanation has ever been proposed that fits this result.
- Satellite entanglement over 1,200 km: The Micius satellite generated entangled photon pairs and distributed them simultaneously to two ground stations over 1,200 km, violating Bell's inequality with high statistical significance despite atmospheric turbulence and earth's rotation. The photons' correlations remained intact even though the two measurement stations were separated far beyond any plausible classical signaling range, conclusively ruling out local hidden variables at intercontinental distances.
A short history
Spontaneous parametric down-conversion was first observed in 1970 by D. C. Burnham and D. L. Weinberg. Polarization-entangled pair generation was developed by Paul Kwiat and Anton Zeilinger in 1995 and remains the most widely-used technique. The 2022 Nobel Prize in Physics went to Aspect, Clauser and Zeilinger for experimental work on entangled photons that this source class made possible.
Sources
- Kwiat et al., Phys. Rev. Lett. (1995) — entangled-pair source (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.75.4337)
- Nobel Prize in Physics 2022 (https://www.nobelprize.org/prizes/physics/2022/summary/)
- Yin et al., Science (2017) — Micius satellite 1200 km Bell test (https://www.science.org/doi/10.1126/science.aan3211)