Researchers at Princeton University have developed a tiny laser, the size of a grain of rice. This rice size ‘maser’ provides a huge boost to quantum computing. The laser is powered by single electrons being tunnelled through artificial atoms, known as quantum dots. The laser uses around one billionth of the amount of electricity that a traditional energy that a hairdryer uses, while emitting energy in the microwave length, which allows researchers to see the interaction between light and moving electrons. Quantum dots are ideal for use in lasers as, when they are excited, they emit light.
So, let’s take a look at how this new ‘maser’ works a little bit more. Quantum dots are tiny particles of light emitting crystals, which can absorb light from one wavelength and turn it into highly saturated light at other wavelengths. This means it will emit light at longer wavelengths than normal lasers that we can see. In this case, two sets of two quantum dots were placed inside the end of a narrow cavity, at a distance of 500 nanometres apart, they were the joined by tiny, tiny wires. For a bit of perspective, a human hair is about 100,000 nanometres wide. Once this was set up, the dots functioned as a transistor, with one dot providing a source, while the other was a drain, with the wires acting as gate electrodes.
In this experiment, the maser was cooled to a few thousandths of a degree above zero and joined to a battery. When this current and voltage was created, the electrons in the dots ‘tunnelled’ from the source dot to the drain dot. As the electron ‘tunnels’ through, it emits a photon, the light particle, in the microwave range. Physics professor, Jason Petta, gave the example “It’s like a staircase, when the electron runs down the staircase it emits a photon.” The photon’s wavelength directly matches how much energy it lost, or how tired we would be depending on the height of the staircase we had just run down. Finally, the photons were bounced off mirrors at the end of the cavity to produce a coherent beam of microwave light, or, the laser.
Although quantum dots may not be widely known nowadays, they can have an impact on our day to day life. For example quantum dots can significantly improve the appearance of LCD screens, such as in smart phones and televisions. Big electronics manufacturers such as Apple and Samsung have already been experimenting with, and occasionally using, quantum dots in their products. So they are likely to be around a lot more in the future.
In this case, the creation of the tiny ‘maser’ was not the original aim of the project. The researchers were planning to research the usage of two quantum dots joined together to form a quantum bit, or ‘qubit’. Quantum dots communicate through the entanglement of light particles, or the photons that we mentioned earlier, and the researchers had designed quantum dots that could emit photons when electrons crossed from one dot to the other. This would have enabled the researchers to measure the states of the electrons to see if they were entangled, meaning the states were correlated. In this case, the researchers did not conduct full entanglement experiments, but the use of two quantum dots showed that correlation happens over longer distances. Previous experiments have used only a single dot and separation between particles was approximately 50 nanometres.
So, although the project went off subject a little bit, the change in direction of the experiment seems to be an overall good thing. The new device, as mentioned earlier, provides a big development in the development of quantum computing, both in efforts to build quantum computing materials out of semiconductor materials. Also, the ability to create correlated states over long distances is important in the functioning of such machines and any further research into this is valuable.
Another advantage of the little maser is that the frequency within it is tuneable, whereas the frequency of traditional lasers is fixed. This essentially means that is it possible to adjust the amount of energy the electrodes need to tunnel through, basically put, the dots can be fine-tuned to produce light at other frequencies.
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