A New Form Of Light That Could Make Quantum Computing Possible With Photons

  • Physicists have developed a new form of photonics matter by sticking three photons together under certain conditions. 
  • The bound-photons have a fraction of mass of electron and move about 100,000 times slower than usual non-interacting photons.  

We all know that photons do not interact with each other, they simply pass each other by. However, researchers at Harvard University and MIT have demonstrated a method in which photons can be made to interact under certain conditions.

They have developed a new form of photonic matter by grouping and sticking three photons together. This could open possibilities of using photons in quantum computing, if not in light sabers.


Researchers carried out some experiments, in which they shone a weak laser beam through a dense  cloud of rubidium atoms. In order to slow down the motion of these atoms (near standstill), they cooled them to a millionth of a degree above absolute zero.

They observed that photons was sticking together in triplets or pairs, instead of exiting the cloud. This was a clear indication of some kind of attraction among photons.

Photons don’t have any mass and they travel at 299,792,458 meters per second in a vacuum. However, bound photons seem to have a tiny fraction of mass of electron. Also, they move nearly 100,000 times slower than usual non-interacting photons.

Image credit: Christine Daniloff / MIT

Researchers examined the photons coming out of the atom cloud. They tracked number and rate of photons, and calculated the phase of photon, both before and after traveling through the cloud.

The phase tells about the frequency of oscillation of a photon. It tells how strongly they are interacting with each other. The greater the phase, the stronger is the interaction. The phase shift of three-photon particle was 3 times greater than the phase shift of two-photon particle, which means they are all together interacting strongly.

The experiment makes it clear that photons can be attracted or entangled by each other. If they can be forced to interact in different manners, they may be used to perform complex quantum computations.

How Photon-Interaction Happens?

When a single photon passes through the rubidium cloud, it lands on a neighbor rubidium atom for a very short time before skipping to another atom. Similarly, if another photon is passing through the cloud at the same time, it can also land on atom for short duration, creating a polariton, a hybrid part of photon, part atom.

2 polaritons can interact with each other. The atoms remain exactly where they’re at the cloud’s edge, while the exiting photons still attached together. The same can happen for 3 photons, creating an even stronger bond than the 2-particle photon.

Within cloud, this complete interaction takes place for a millionth of a second, which makes photons attached together after they have left the cloud. This attachment can be considered as strongly entangled – the most important property of any quantum computing bit.

Reference: Sciencemag | doi:10.1126/science.aao7293 | MIT

So far, we have been using photons to transfer data, like in optical fibers. This study tells that we can use them to distribute quantum data as well.

What’s Next?

Credit: Timothy Yeo / Centre for Quantum Technologies / National University of Singapore

The three-photon bound state can be seen as photonic solitons in the quantum regime, which can be extended along various directions. For instance, increasing the length of medium while keeping the atomic density constant would eliminate the scattering effect and retain only solitonic bound-state component.

Also, using larger or elliptical probe beam and configuring the mass along different directions, the system could be extended to three dimensions. For now, the medium only supports one, two and three-photon bound state. If the atomic density is increased by 3 times, it could result in resonant photon-photon scattering and a configurable scattering length.

Read: Transparent Materials Can Absorb Light | An Unusual Optical Effect

Researchers plan to test several methods to coerce other interactions, like repulsion (photons scattering each other). Photon-repulsion could produce a regular pattern or may be something else.

Large N-body forces in the system opens interesting possibilities to study exotic several body phases of matter and light, including self-organization in quantum systems and quantum material that can’t be realized with traditional systems.

Written by
Varun Kumar

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