Physics

Identical light particles (photons) are essential for several quantum-physics-based technologies. A team of Basel and Bochum researchers has created identical photons with distinct quantum dots, which is a crucial step toward applications such as tap-proof communications and the quantum internet.

Many quantum-based devices rely on photons that are all precisely the same. However, creating such photons is incredibly challenging. Not only must they have the same wavelength (color), but they must also be the same form and polarity.

In partnership with colleagues at the University of Bochum, a team of researchers led by Richard Warburton at the University of Basel has now succeeded in producing identical photons originating from distinct and widely dispersed sources.

Single photons from quantum dots

The researchers employed quantum dots, which are structures in semiconductors that are only a few nanometers in size, in their tests. Electrons are confined in quantum dots and can only take on highly particular energy levels. When moving from one level to another, light is emitted. Single photons can therefore be manufactured at the push of a button using a laser pulse that causes such a transition.

“Other researchers have already made similar photons with various quantum dots in recent years,” Lian Zhai, a postdoctoral researcher and first author of the work published in Nature Nanotechnology, says. “However, they had to choose and choose the most comparable photons from a large number of photons using optical filters.” As a result, there were just a few useful photons left.

Warburton and his colleagues took a different, more daring approach. First, the Bochum specialists created very pure gallium arsenide from which the quantum dots were created. Natural differences between distinct quantum dots might therefore be minimized. The Basel researchers then used electrodes to subject two quantum dots to carefully adjusted electric fields. These fields changed the quantum dots’ energy levels, and they were adjusted so that the photons released by the quantum dots all had the same wavelength.

93% identical

The researchers used a half-silvered mirror to demonstrate that the photons were indeed indistinguishable. They noticed that the light particles nearly always traveled through the mirror as a pair or were reflected as a pair. They may deduce that the photons were 93 percent similar based on this finding. In other words, even though the photons were “born” entirely independently of one another, they created twins.

Furthermore, the researchers were able to actualize a key component of quantum computers, the controlled NOT gate (or CNOT gate). Quantum algorithms that handle specific problems significantly quicker than conventional computers can be implemented using such gates.

“Right now, our yield of identical photons is only about 1%,” admits Ph.D. student Gian Nguyen. He assisted his colleague Clemens Spindler in carrying out the experiment. “However, we already have a pretty good understanding of how to boost that yield in the future.” This would prepare the twin-photon technique for prospective applications in other quantum technologies.

(Image: University of Basel, Department of Physics)

What is the quantum photonic processor?

A quantum photonic processor is a multimode, low-loss interferometer that can be reconfigured. In the classical or quantum realm, our processor allows the user to execute arbitrary, controlled interference between a number of optical channels.

What does a quantum processor do?

Quantum computers, in contrast to conventional computers, execute calculations based on the probability of an object’s state before it is measured, rather than only 1s or 0s, allowing them to process exponentially more data.

What is quantum computer and how does it work?

Quantum computers, as opposed to conventional computers, execute calculations based on the probability of an object’s condition before it is measured, rather of only 1s or 0s, allowing them to process exponentially more data.

Quantum computers have the potential to push computing far beyond what it is now, but this potential has yet to be fulfilled. Researchers working on the EU-funded PHOQUSING project are constructing a hybrid computer system based on cutting-edge integrated photonics that mixes classical and quantum processes in their quest to show quantum supremacy. The purpose of the project is to create a quantum sampling machine that will propel Europe to the forefront of photonic quantum computing. PHOQUSING project partner QuiX Quantum in the Netherlands has developed the world’s biggest quantum photonic processor compatible with quantum dots with this purpose in mind (nanometer-sized semiconductor crystals that emit light of various colors when illuminated by ultraviolet light). The processor is the heart of the quantum sampling machine, a quantum computing device that can demonstrate a quantum advantage in the near future.

According to a news item on the QuiX Quantum website, “quantum sampling devices based on light are regarded to be particularly promising for exhibiting a quantum advantage.” “By allowing light to propagate [sic] through such quantum sampling devices, the issue of taking samples from a probability distribution, which is theoretically too difficult for a conventional computer, may be readily handled. Large-scale linear optical interferometers, or photonic processors, are at the heart of quantum sampling machines.”

A look at the chip

The processor produced by the research team is a “record-size” 20-mode silicon nitride photonic chip that operates at a wavelength of 925 nanometers and is suited for usage in the near-infrared wavelength range. The 20 input modes, 190 unit cells, and 380 adjustable components, according to a webinar video introducing the processor, make it the most complicated photonic chip accessible today. The quantum photonic processor has a large number of modes, as well as low optical losses (2.9 decibels per mode) and great fidelity (99.5 percent for permutation matrices and 97.4 percent for Haar-random matrices). Quantum interference with excellent visibility is also possible with the turnkey processor (98 percent ).

“The existing high-performance photonic technology supplied by QuiX Quantum is vital for the project’s success as it fulfills the requirement for a science-to-technology transfer needed for establishing usable quantum computers,” says Prof. Fabio Sciarrino. The project brings together seven partners from France, Italy, the Netherlands, and Portugal: five academic and research institutions, as well as two industry partners, all of which are European pioneers in quantum information processing and integrated photonics.

Whenever people look out into the sky, we can perceive magnetic fields embedded in all of the astrophysical objects they view. This is true not just in the vicinity of stars and planets, but also in the vast space between galaxies and galaxy clusters. These fields are weak—much weaker than a refrigerator magnet—but they are dynamically important in the sense that they have a considerable impact on the universe’s dynamics. The origin of these cosmic magnetic fields remains one of the most fundamental mysteries in cosmology, despite decades of intensive attention and inquiry.

Scientists had already discovered how turbulence, the churning motion intrinsic to all fluids, might magnify preexisting magnetic fields via the so-called dynamo process. However, this astonishing discovery just added to the enigma. Where did the “seed” magnetic field come from in the first place, since a chaotic dynamo could only enhance an existing field?

We wouldn’t have a comprehensive and self-consistent solution to the formation of astrophysical magnetic fields until we knew how the seed fields formed. New research by MIT graduate student Muni Zhou, her advisor Nuno Loureiro, an MIT professor of nuclear science and engineering, and colleagues at Princeton University and the University of Colorado at Boulder provides an answer that demonstrates the basic processes that generate a field from a completely unmagnetized state to the point where the dynamo mechanism takes over and amplifies the field to the magnitudes that we observe.

Magnetic fields are everywhere

Magnetic fields are found naturally everywhere in the cosmos. They were initially noticed on Earth thousands of years ago, as a result of their interaction with magnetic minerals such as lodestone, and were used for navigation long before people understood their nature or origin. The effects of magnetism on the sun’s spectrum of light were identified at the beginning of the twentieth century. Since then, more powerful telescopes scanning deep into space have discovered that the fields are everywhere.

While scientists have long known how to create and employ permanent magnets and electromagnets for a variety of applications, the natural sources of magnetic fields in the cosmos remained a mystery. Part of the answer has come from recent research, but many elements of the subject are still up for discussion.

The dynamo effect amplifies magnetic fields

Scientists began to explore this issue by looking at how electric and magnetic fields are generated in the laboratory. Electric fields are formed when conductors, such as copper wire, move in magnetic fields. Electrical currents can then be driven by these fields or voltages. This is how we get the power we use on a daily basis. Large generators, or “dynamos,” turn mechanical energy into electromagnetic energy, which powers our homes and workplaces, via this induction process. One of the most distinguishing characteristics of dynamos is that they require magnetic fields to function.

But, because there are no visible cables or large steel buildings in the cosmos, how can the fields form? Scientists began working on this topic around a century ago when they were trying to figure out where the Earth’s magnetic field came from. Seismic wave propagation research at the time revealed that much of the Earth was liquid under the colder surface layers of the mantle and that there was a core made of molten nickel and iron. Researchers thought that the Earth’s field was generated in some way by the convective motion of this hot, electrically conductive liquid and the rotation of the Earth.

Models eventually appeared that demonstrated how convective motion may magnify a pre-existing field. This is an example of “self-organization,” which occurs when large-scale structures emerge spontaneously from small-scale dynamics in complex dynamical systems. However, much like in a power plant, a magnetic field was required to create a magnetic field.

Throughout the cosmos, a similar mechanism is at work. However, the electrically conducting fluid in stars, galaxies, and the space between them is plasma, which is a state of matter that exists at extremely high temperatures and in which electrons are torn away from their atoms. Plasmas can be seen on Earth in the form of lightning or neon lights. The dynamo effect can magnify an existing magnetic field in such material if it starts at a low level.

Making the first magnetic fields

According to physicists, magnetic fields may be generated spontaneously by general large-scale movements as basic as sheared flows. On powerful supercomputers, Zhou constructed the underlying theory and ran numerical simulations. The plasma that occurs between stars and galaxies is extremely diffuse, which is an essential feature. Mechanical energy was turned into magnetic energy in the same way as it was in terrestrial instances. Although the origin of magnetic fields in the cosmos is unclear, Chinese scientists claim they have developed a self-consistent model for their formation on a cosmic scale.

Their next study will focus on determining if the technique can function on a time scale that is consistent with astronomical findings. “This study represents the first step in the construction of a new paradigm for understanding magnetogenesis,” the researchers write.