These entangled photons are stored in a crystal

Quantum applications, such as cryptography and computation, often leverage the benefits of entangled photon particles. 

However, storing photons has always been a hurdle, which creates a particularly interesting dilemma for quantum computing where storage is clearly important.

Fortunately, researchers recently demonstrated a method of storing entangled photons by neatly embedding them in a crystal.

As John Timmer of Ars Technica explains, the scientists used crystals “doped” with rare earth elements to store the photons. 

“The trick to making these crystals hold a photon involves manipulating the energy state of the electrons in the crystal.

“Normally, transitions between different energy levels in the doped atoms happen very quickly, but it’s possible to manipulate them so that they take much longer; the papers use either a laser or a magnetic field to shift the crystals between these two states.”

According to Timmer, with the crystal properly prepared, it’s all just a matter of preparation.

“By matching the photon and crystal, it’s possible to arrange things so that the photon can only be absorbed when the crystal is in its fast transition state.

“Once it’s absorbed, however, the crystal can be shifted to its slow transition state. Once that shift occurs, the photon is trapped. It’ll either be released at the slow rate, or will stick around until the next time the crystal is switched to the fast state.”

Yes, this all may sound very theoretical, but in fact, the above-mentioned methodology could be applied to real life science. 

For example, the altered crystals were found to accept light at much more useful wavelengths than previous demonstrations, making them viable for everyday quantum applications. 

In addition, the second entangled photon was placed on the same wavelength commonly used in fiber optic communications – meaning it could potentially be used in a quantum encryption system for the long range transmission of photons.

Although these theoretical applications do exist, there are still some significant limitations to practical use. 

To be sure, the crystals need to sit within a few Kelvin of absolute zero, meaning they’re not exactly ready for practical computing applications. 

Note:  While storing a photon in altered crystals has been previously demonstrated, the above-mentioned research appears to demonstrate that entangled temporal modes are successfully maintained when the photon emerges from the crystal.

(Via Ars Technica & Nature)