Direct Generation of Light in Superposition

Scheme of the direct generation of light in a state that is simultaneously a single photon, two photons, and no photon at all. (Source: J. C. Loredo et al., C2N)

Physicists at the french Center for Nanoscience and Nano­technology C2N have demonstrated for the first time the direct generation of light in a state that is simul­taneously a single photon, two photons, and no photon at all. They showed that the same kind of light emitters used for decades are also able to generate these quantum states, and expect that this holds true for any kind of atomic system.

Quantum superposition is a property of quantum physics that allows objects to exist simul­taneously in different states. In the quantum world, super­position allows the object to try all different paths simultaneously, therefore finding the exit much faster. For light, super­position has been shown in several of its properties. For instance, in its polarization, where the electro­magnetic field of a single-photon oscil­lates both vertically and horizontally; or in path, taking upon all possible trajectories inside inter­ferometers, the photonic versions of a maze. Superposition has been achieved even in time, with photons existing simul­taneously at earlier and later moments.

However, creating light in a state that is simultaneously a single photon, two photons, or no photon at all, in other words a quantum superposition of photon-numbers, has remained elusive. Some complex experiments had already managed to obtain these super­position states a few times, but it had never been achieved on demand, which means with success at every experi­mental run. Moreover, it was not known whether direct emitters of these states existed. The researchers at the C2N and the Institut Néel (CNRS, Grenoble), have demons­trated for the first time the on-demand generation of light in a quantum super­position of photon-numbers.

The researchers studied the emission of an artificial atom, a semi­conductor quantum dot inserted in an optical micro­cavity, a tech­nology that has recently provided the most efficient single photon sources. By performing a coherent excitation of the quantum dot with optical pulses, they showed that the quantum coherence in the atomic state is preserved through the process of spontaneous emission and imprinted onto the emitted photonic state, generating a quantum super­position of zero, one, and two photons.

Such observations, never seen before in any atomic system, demonstrate that artificial atoms like quantum dots are now controlled to such a point that they behave as the systems described in textbooks. These new quantum states of light based on the coherent super­position of photon-number states open exciting paths for designing and imple­menting new schemes in quantum communi­cation and compu­tation. (Source: C2N)

Reference: J. C. Loredo et al.: Generation of non-classical light in a photon-number superposition, Nat. Phot., online 19 August 2019; DOI: 10.1038/s41566-019-0506-3

Link: CNRS Centre for Nanoscience and Nanotechnology, Université Paris-Sud, Université Paris-Saclay, Palaiseau, France

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