Quantum Distillery for Light

Experiment to distill photons out of custom-shaped laser pulses that reflect from a single atom strongly coupled to an optical resonator. (Source: S. Daiß)

It is indeed reminiscent of the principle behind the distil­lation of alcohol – even though the device housed in a labora­tory at the Max Planck Institute of Quantum Optics looks completely different from something used for distilling schnapps. Just as distil­lation increases the alcohol content of a beverage relative to the water content, the Garching experiment increases the proportion of indi­vidual photons in relation to the vacuum. While this motivation may sound strange to the general public, it leads directly to the strange world of quantum physics. Ulti­mately weak light sources that can deliver exactly one photon play a central role in quantum infor­mation tech­nology. As a quantum bit, a photon can transport the elementary quantum infor­mation required for quantum networks, quantum encryption, and quantum computers – just as current digital tech­nology processes indi­vidual bits as infor­mation carriers.

The construction of single photon sources is a challenge that has been researched worldwide for many years. This sounds as­tonishing because it takes only a single touch of a light switch to illu­minate a room. However, the light from a lamp corres­ponds to a current of enormous numbers of photons. If a light source is dimmed to such an extent that only indi­vidual photons can escape from it, we are confronted with the roll-of-the-dice nature of the quantum world: sometimes there is no photon and then there are two or three photons and so on. It’s a bit like the drops from a still – you can’t predict when the drops will come or how big they will be.

Instead of developing another single-photon light source, the physicists from Gerhard Rempes’ division at the Max Planck Institute of Quantum Optics had a different idea: Their experiment can extract indi­vidual photons from the light of any very weak light source – like a still – and reliably report this event. Strictly speaking, it reduces the fraction of pure vacuum compared to the event of obtaining a photon. This is what you learn from Severin Daiß, doctoral student at the institute. One of the pecu­liarities of the quantum world is that the vacuum itself represents a quantum state. If you want to cleanly prepare a photon, no vacuum must be admixed.

Two challenges come together in the new research work of Rempes’ team. The first challenge is to obtain exactly one photon; the second is to reliably detect it. A single rubidium atom solves both tasks in one step. This atom is  trapped between two almost perfect mirrors facing each other, in a kind of mirror cabinet. The distance of the mirrors in this resonator corre­sponds precisely to a multiple of half a wave­length of light in which the atom could radiate or absorb its own photon. In this system, the atom can flip between two states like a pointer; this plays an important role here.

“We can use this system of the atom in the resonator as a still for the photon”, says Daiß. The Garching group directs extremely weak laser light – from which they want to obtain a single photon – onto the cavity. There it does something that only works in the quantum world: It “entangles” with the atom-resonator arrange­ment, thereby forming a combined quantum state. This entangled state makes the system a still: With a measure­ment on the atom, physicists can extract an even or an odd number of photons from the incident light. However, this does not work like a switch; the roll-of-the-dice nature of the quantum world prevents a photon from coming through at the push of a button. “What is decisive here is that we can now use the atom as a pointer to report a successful single-photon distil­lation”, explains Daiß. The physicists let the arrange­ment roll photons but get the dice count reliably displayed.

In conjunc­tion with ultra-weak light, the “odd photon number” mode can now produce events with one photon because more photons are rarely available. The distil­lation succeeded with a “purity” of 66%, which means that the vacuum content was reduced to one third. Compared with single photon light sources, this is a good result for a first attempt. This purity can be consi­derably increased with better optical cavities. The photon distil­ling elements can be connected in series in order to further increase the purity of the photon that passes through. The quality of the light from other single photon sources can also be improved. It’s like making 60% (or higher) vodka from 40% percent vodka. (Source: MPQ)

Reference: S. Daiss et al.: Single-Photon Destillation via a Photonic Parity Measurement Using Cavity QED, Phys. Rev. Lett. 122, 133603 (2019); DOI: 10.1103/PhysRevLett. 122. 133603

Link: Quantum Dynamics, Max Planck Institute of Quantum Optics, Garching, Germany

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