From stationary to flying qubits at speeds never reached before…. This feat, achieved by a team from Polytechnique Montréal and France’s Centre national de la recherche scientifique (CNRS), brings us a little closer to the era when information is transmitted via quantum principles.
A paper titled “High-Fidelity and Ultrafast Initialization of a Hole-Spin Bound to a Te Isoelectronic Centre in ZnSe” was recently published in the prestigious journal Physical Review Letters. The creation of a qubit in zinc selenide, a well-known semi-conductor material, made it possible to produce an interface between quantum physics that governs the behaviour of matter on a nanometre scale and the transfer of information at the speed of light, thereby paving the way to producing quantum communications networks.
Classical physics vs. quantum physics
In today’s computers, classical physics rules. Billions of electrons work together to make up an information bit: 0, electrons are absent and 1, electrons are present. In quantum physics, single electrons are instead preferred since they express an amazing attribute: the electron can take the value of 0, 1 or any superposition of these two states. This is the qubit, the quantum equivalent of the classical bit. Qubits provide stunning possibilities for researchers.
An electron revolves around itself, somewhat like a spinning top. That’s the spin. By applying a magnetic field, this spin points up, down, or simultaneously points both up and down to form a qubit. Better still, instead of using an electron, we can use the absence of an electron; this is what physicists call a “hole.” Like its electron cousin, the hole has a spin from which a qubit can be formed. Qubits are intrinsically fragile quantum creature, they therefore need a special environment.
Zinc selenide, tellurium impurities: a world first
Zinc selenide, or ZnSe, is a crystal in which atoms are precisely organized. It is also a semi-conductor into which it is easy to intentionally introduce tellurium impurities, a close relative of selenium in the periodic table, on which holes are trapped, rather like air bubbles in a glass.
This environment protects the hole’s spin – our qubit – and helps maintaining its quantum information accurately for longer periods; it’s the coherence time, the time that physicists the world over are trying to extend by all possible means. The choice of zinc selenide is purposeful, since it may provide the quietest environment of all semiconductor materials.
Polytechnique Montréal and CNRS of France, a team effort
Philippe St-Jean, a doctoral student on Professor Sébastien Francoeur’s team, uses photons generated by a laser to initialize the hole and record quantum information on it. To read it, he excites the hole again with a laser and then collects the emitted photons. The result is a quantum transfer of information between the stationary qubit, encoded in the spin of the hole held captive in the crystal, and the flying qubit – the photon, which of course travels at the speed of light.
This new technique shows that it is possible to create a qubit faster than with all the methods that have been used until now. Indeed, a mere hundred or so picoseconds, or less than a billionth of a second, are sufficient to go from a flying qubit to a static qubit, and vice-versa.
Although this accomplishment bodes well, there remains a lot of work to do before a quantum network can be used to conduct unconditionally secure banking transactions or build a quantum computer able to perform the most complex calculations. That is the daunting task which Sébastien Francoeur’s research team will continue to tackle.
Dmitry Fedyanin from the Moscow Institute of Physics and Technology and Mario Agio from the University of Siegen and LENS have predicted that artificial defects in the crystal lattice of diamond can be turned into ultrabright and extremely efficient electrically-driven quantum emitters. Their work published in New Journal of Physics demonstrates the potential for a number of technological breakthroughs, including the development of quantum computers and secure communication lines, which, in contrast to previously proposed schemes, would be able to operate at room temperature.
The research conducted by Dmitry Fedyanin and Mario Agio is focused on the development of efficient electrically-driven single-photon sources – devices that emit single photons when an electrical current is applied. In other words, using such devices, one can generate a photon “on demand” by simply applying a small voltage across the devices, the probability of an output of zero photons is vanishingly low and generation of two or more photons simultaneously is fundamentally impossible.
Until recently, it was thought that quantum dots (nanoscale semiconductor particles) are the most promising candidates for true single-photon sources. However, they operate only at very low temperatures, which is their main drawback – mass application would not be possible if a device has to be cooled with liquid nitrogen or even colder liquid helium, or using refrigeration units, which are even more expensive and power-hungry. At the same time, it was known that certain point defects in the crystal lattice of diamond, which occur when foreign atoms (such as silicon or nitrogen) enter the diamond accidentally or through targeted implantation, can efficiently emit single photons at room temperature. However, this has only been achieved by optical excitation of these defects using external high-power lasers. This method is ideal for research in scientific laboratories, but it is very inefficient in practical devices. Experiments with electrical excitation, on the other hand, did not yield the best results – in terms of brightness, diamond sources lost out significantly (by several orders of magnitude) to quantum dots. As there were no theories describing the photon emission from colour centres in diamonds under electrical excitation, it was not possible to assess the potential of these single-photon sources to see if they could be used as a basis for the quantum devices of the future.
The new publication gives an affirmative answer – defects in the structure of diamond at the atomic level can be used to design highly efficient single-photon sources that are even more promising than their counterparts based on quantum dots.
Operation at the single‐photon level will not only increase the energy efficiency of the existing data processing and data transmission devices by more than one thousand times, but will also lay the foundations for the development of novel quantum devices. Building quantum computers is still a prospect of the future, but secure communication lines based on quantum cryptography are already starting to be used. However, today they do not use true single-photon sources; instead, they rely on what are known as attenuated lasers. This means that not only is there a high probability of sending zero photons into a channel, which greatly reduces the speed of data transfer, but there is also a high probability of sending two, three, four, or more light quanta simultaneously. One could intercept these “extra” photons and neither the sender nor the recipient would know about it. This makes the communication channel vulnerable to eavesdropping and quantum cryptography loses its main advantage – fundamental security against all types of attacks.
For quantum computing it is also essential to have the ability to manipulate individual photons. The quantum of light can be used to represent a qubit – the fundamental unit of quantum information processing, – which is a superposition of two or more quantum states. For example, a qubit can be encoded in the polarization of a single photon. The advantage of the optical quantum computing paradigm is that one can natively combine quantum computations with quantum communication and design high-performance, large and scalable quantum supercomputers, which is not possible to do using other physical systems, such as superconducting circuits or trapped ions.
Dmitry Fedyanin and Mario Agio are the first to successfully reveal the mechanism of electroluminescence of colour centres in diamond and develop a theoretical framework to quantify it. They found that not all states of colour centres can be excited electrically, despite the fact that they may be “accessible” under optical excitation. This is because under optical pumping defects behave like isolated atoms or molecules (such as hydrogen or helium), with virtually no interaction with the diamond crystal. Electrical excitation, on the other hand, is based on the exchange of electrons between the defect and the diamond crystal. This not only brings limitations, but also opens up new possibilities. For example, according to the researchers, certain defects can emit serially two photons at two different wavelengths from two different charge states in a single act of the electroluminescence process. This feature could lead to the development of a fundamentally new class of quantum devices that had simply been disregarded before because these processes are not possible with optical excitation of colour centers. But the most important result of the study is that the researchers found out why high-intensity single-photon emission from colour centers was not observed under electrical pumping. The reason for this was the technologically complex process of doping of diamond by phosphorus, which cannot provide sufficiently high density of conduction electrons in diamond.
The calculations show that using modern doping technologies it is possible to create a bright single-photon source with an emission rate of more than 100,000 photons per second at room temperature. It is truly remarkable that the emission rate only increases as the device temperature increases achieving more than 100 million photons per second at 200 degrees Celsius. “Our single-photon source is one of few, if not the only optoelectronic device that should be heated in order to improve its performance, and the effect of improvement is as high as three orders of magnitude. Normally, both electronic and optical devices need to be cooled by attaching heat sinks with fans, or by placing them in liquid nitrogen,” says Dmitry Fedyanin from the Laboratory of Nanooptics and Plasmonics at MIPT. According to him, the technological improvement of diamond doping will further increase the brightness 10-100 times.
One hundred million photons is very low compared to household light sources (e.g. a normal light bulb emits more than 10^18 photons per second), but it should be emphasized that the entire flow of photons is created by a tiny (~10^-10 metres in size) defect in the crystal lattice of diamond and, unlike a light bulb, photons follow strictly one after the other. For the quantum computers mentioned above, around ten thousand photons per second would be enough – the possibility of developing a quantum computer is currently limited by entirely different factors. In quantum communication lines, however, the use of electrically-driven diamond single-photon sources will not only guarantee complete security, but will also greatly increase the speed of information transfer compared to the pseudo single-photon sources based on attenuated lasers used today.
With the help of a semiconductor quantum dot, physicists at the University of Basel have developed a new type of light source that emits single photons. For the first time, the researchers have managed to create a stream of identical photons.
They have reported their findings in the scientific journal Nature Communications together with colleagues from the University of Bochum.
A single-photon source never emits two or more photons at the same time. Single photons are important in the field of quantum information technology where, for example, they are used in quantum computers. Alongside the brightness and robustness of the light source, the indistinguishability of the photons is especially crucial. In particular, this means that all photons must be the same color. Creating such a source of identical single photons has proven very difficult in the past.
However, quantum dots made of semiconductor materials are offering new hope. A quantum dot is a collection of a few hundred thousand atoms that can form itself into a semiconductor under certain conditions. Single electrons can be captured in these quantum dots and locked into a very small area. An individual photon is emitted when an engineered quantum state collapses.
Noise in the semiconductor
A team of scientists led by Dr. Andreas Kuhlmann and Prof. Richard J. Warburton from the University of Basel have already shown in past publications that the indistinguishability of the photons is reduced by the fluctuating nuclear spin of the quantum dot atoms. For the first time ever, the scientists have managed to control the nuclear spin to such an extent that even photons sent out at very large intervals are the same color.
Quantum cryptography and quantum communication are two potential areas of application for single-photon sources. These technologies could make it possible to perform calculations that are far beyond the capabilities of today’s computers.