Credit: Natasha Tomm et al.
To create light-based quantum technologies, scientists and engineers must be able to generate and manipulate photons individually or in groups of a few. To create such quantum photonic logic gates that can be used in an optical quantum computer, a special medium is required that allows strong and controlled interactions of just a few photons.
One way to do this, albeit a difficult one, is to use a “one-dimensional atom,” a device that emits photons based on the input of particular photon states: single photons, photon pairs, photon triplets, and so on. In short, a photon sifter.
Creating a one-dimensional atom (this dimension being the line on which a photon arrives) is not easy. It must interact with the incoming photon almost 100% of the time, be free of noise and decoherence, the one-dimensional atom being perturbed by the photon and its local environment.
Ensembles of atoms have been used, which act together as a superatom or emitter in a waveguide. Another possibility is offered by cavity quantum electrodynamics: a single emitter embedded in a microcavity. The embedded emitter can be atoms, ions, molecules or quantum dots that behave like atoms.
A Swiss and German research team, led by Richard Warburton of the University of Basel, has built such a photon gate using a quantum dot, with their research published in the journal Physical Exam Letters.
Composed of semiconductor nanocrystals, quantum dots are nanometer-sized objects whose optical and electronic properties are governed by the rules of quantum mechanics. This particular quantum dot was 20 nanometers wide and was embedded between two reflective walls of an optical cavity, creating a one-dimensional atom, and was positioned on a device that allows the cavity length to be controlled and modified.
A weak laser light composed of photon states that are one or more photons enters the cavity from above and strikes the quantum dot. The quantum dot absorbs it if the dot has a difference in energy levels that matches the energy of the photon. In this case, the dot then emits a photon of that energy that is reflected upwards.
Schematic of a photon filtering device, showing a quantum dot surrounded by an optical cavity with a polarizing beam splitter above. Credit: Natasha Tomm et al, University of Basel and Ruhr University Bochum via CC-BY (4.0 International) license
But if the photon state entering from the top is composed of two or more photons, the interaction of this state with the quantum dot is modified and the polarization (direction of its electric field) of the outgoing state changes. With a polarization filter (“beam splitter”) placed at the top of the dot, the reflected single emitted photons pass in one direction (port 1) and the reflected multiphoton states are reflected in another direction (port 2).
The incoming beam is thus separated into single-photon and multi-photon states. A source composed of several different photon states produces a single-photon beam that can be used for quantum technologies, optical computing circuits or other applications. The device acts as a mirror for single photons.
In their experiment, the group found that 99.2% of the incoming beam was split into multiphoton states, leaving pure single photons, showing the high efficiency of the interaction between the quantum dots and the optical cavity. Measuring the so-called second-order correlation function (a measure of photon clustering, which is a measure of nonlinearity) gave a value of 587.
The researchers write: “To our knowledge, this is the largest clustering of photons due to nonlinearity observed to date.” The best value previously obtained from other experimental setups was 20.
The cavity configuration allows the transmitted light to be tuned and manipulated by moving the quantum dot relative to the optical cavity without external modification of the configuration. This changes the coupling between the dot and the cavity; strong bunching of the transmitted photons can actually be transformed into anti-bunching.
“The quantum dot behaves radically differently depending on the number of photons,” they write. “This leads to massive clustering, as only multiphoton states are transmitted.”
Discrimination between the observed photon numbers allows for interactions at the single-photon level. These results could lead to the creation of bound photon states, with two or more photons held tightly together. Photons do not typically interact with each other, a useful property for fiber optic communications. But interactions between photons are desired for some applications, such as classical and quantum information processing, but a highly nonlinear medium is required, such as the one developed here.
Such nonlinear photonic processes are already used in applications such as photonic frequency conversion, light amplification, and light detection. Other exotic photonic states generated by this device could prove useful for understanding many body phenomena in a controlled environment.
More information:
Natasha Tomm et al, Realization of a coherent and efficient one-dimensional atom, Physical Exam Letters (2024). DOI: 10.1103/PhysRevLett.133.083602. On arXiv: DOI: 10.48550/arxiv.2402.12568
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