Advanced optical excitation for generating optical quantum bits

Hvad går øvelsen ud på?

We are working on the development of future quantum computers. Quantum computer is predicted to efficiently solve many tasks, for example, the problems within quantum chemistry, that are intractable on classical computers.

In optical quantum computing, the quantum bits (Qubits) are encoded in single photons. Therefore, a bright, stable, and on-demand source of indistinguishable single photons is essential to perform efficient optical quantum information processing.

In order to generate thousands of highly indistinguishable single photons, we need to develop an advanced optical excitation scheme.

Some of the most recent and advanced excitation schemes rely on coherent excitation through a pair of slightly detuned laser pulses. Upon absorption of the pulses, the quantum dot is driven to populate its upper energy level, and, eventually, the emission of the single photons can take place.

To generate the two pulses from a single laser pulse that we have in our laboratory, it is necessary to somehow select and filter out only the wavelength that we want.

How is it possible to do that? Before answering to this question, we should also mention that the duration of a single laser pulse is of the order of 150fs (1000e-15) and, unfortunately, there is no way of manipulating such fast pulses in the temporal domain relying on some electrical devices.

Hvordan foregår øvelsen?

To singularly address the spectral components of the pulses we need to separate them in the spatial domain. Such a result can be achieved thanks to the dispersion of light given by a grating. Dispersion is an optical phenomenon according to which the phase velocity of a wave depends on its frequency.

In the experimental exercise, we will try to understand how a grating works and how can we implement it for the generation of the above-mentioned detuned pulses, which we need to efficiently excite a quantum dot.

To do this, we will first implement a halogen lamp producing a white light featuring a broad spectrum, a diffraction (reflective or refractive) grating, a lens to collect the light in an optical fiber, and ultimately, a spectrometer to check what wavelength we managed to collect.

In this way, we can get an idea of how to proceed into the more complex and delicate task of spectrally selecting very narrow and neighbouring frequencies. What is more, the selected wavelength from the laser which we need is not even visible to the human naked eye!

Believe it or not, the quantum dots will lighten up when we shine them with those two laser pulses even if both of them are at a different frequency than the emitter.

Hvilke faciliteter kommer du til at bruge?

You have the opportunity to enter and work in a state-of-the-art quantum optical lab, equipped with advanced tools such as Femtosecond Laser, Optical Parametric Oscillator, Spectrometer, Single-Photon Detector, Microscope, and many more optics tools.