̽��ֱ�� of Cambridge - photons

Ultra-thin quantum LEDs could accelerate development of quantum networks

sc604 — Fri, 07 Oct 2016 15:26:04 +0000

Ultra-thin quantum light emitting diodes (LEDs) – made of layered materials just a few atoms thick – have been developed by researchers at the ̽��ֱ�� of Cambridge. Constructed of layers of different ultra-thin materials, the devices could be used in the development of new computing and sensing technologies. ̽��ֱ��ability to produce single photons using only electrical current is an important step towards building quantum networks on compact chips.

̽��ֱ��devices are constructed of thin layers of different materials stacked together: graphene, boron nitride and transition metal dichalcogenides (TMDs). ̽��ֱ��TMD layer contains regions where electrons and electron vacancies, or holes, are tightly confined. When an electron fills an electron vacancy that sits at a lower energy than the electron, the energy difference is released as a photon, a particle of light. In the LED devices, a voltage pushes electrons through the device, where they fill the holes and emit single photons.

A computer built on the principles of quantum mechanics would be both far more powerful and more secure than current technologies, and would be capable of performing calculations that cannot be performed otherwise. However, in order to make such a device possible, researchers need to develop reliable methods of electrically generating single, indistinguishable photons as carriers of information across quantum networks.

̽��ֱ��ultra-thin platform developed by the Cambridge researchers offers high levels of tunability, design freedom, and integration capabilities. Typically, single photon generation requires large-scale optical set-ups with several lasers and precise alignment of optical components. This new research brings on-chip single photon emission for quantum communication a step closer. ̽��ֱ��results are reported in the journal Nature Communications.

“Ultimately, we need fully integrated devices that we can control by electrical impulses, instead of a laser that focuses on different segments of an integrated circuit,” said Professor Mete Atatüre of Cambridge’s Cavendish Laboratory, one of the paper’s senior authors. “For quantum communication with single photons, and quantum networks between different nodes, we want to be able to just drive current and get light out. There are many emitters that are optically excitable, but only a handful are electrically driven.”

̽��ֱ��layered nature of TMDs makes them ideal for use in ultra-thin structures on chips. They also offer an advantage over some other single-photon emitters for feasible and effective integration into nanophotonic circuits.

With this research, quantum emitters are now seen in another TMD material, namely tungsten disulphide (WS₂). “We chose WS₂ because we wanted to see if different materials offered different parts of the spectra for single photon emission,” said Atatüre, who is a Fellow of St John's College. “With this, we have shown that the quantum emission is not a unique feature of WS₂, which suggests that many other layered materials might be able to host quantum dot-like features as well.”

“We are just scratching the surface of the many possible applications of devices prepared by combining graphene with other materials,” said senior co-author Professor Andrea Ferrari, Director of the Cambridge Graphene. “In this case, not only have we demonstrated controllable photon sources, but we have also shown that the field of quantum technologies can greatly benefit from layered materials. Many more exciting results and applications will surely follow.”

Reference:
C. Palacios-Berraquero et al. ‘Atomically thin quantum light emitting diodes.’ Nature Communications (2016). DOI: 10.1038/ncomms12978

Researchers have developed all-electrical ultra-thin quantum LEDs, which have potential as on-chip photon sources in quantum information applications, including quantum networks for quantum computers.

Ultimately, we need fully integrated devices that we can control by electrical impulses.

Mete Atature

Microscope image of a quantum LED device showing bright quantum emitter generating a stream of single photons.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Scientists "squeeze" light one particle at a time

tdk25 — Tue, 01 Sep 2015 04:04:46 +0000

A team of scientists has successfully measured particles of light being “squeezed”, in an experiment that had been written off in physics textbooks as impossible to observe.

Squeezing is a strange phenomenon of quantum physics. It creates a very specific form of light which is “low-noise” and is potentially useful in technology designed to pick up faint signals, such as the detection of gravitational waves.

̽��ֱ��standard approach to squeezing light involves firing an intense laser beam at a material, usually a non-linear crystal, which produces the desired effect.

For more than 30 years, however, a theory has existed about another possible technique. This involves exciting a single atom with just a tiny amount of light. ̽��ֱ��theory states that the light scattered by this atom should, similarly, be squeezed.

Unfortunately, although the mathematical basis for this method – known as squeezing of resonance fluorescence – was drawn up in 1981, the experiment to observe it was so difficult that one established quantum physics textbook despairingly concludes: “It seems hopeless to measure it”.

So it has proven – until now. In the journal Nature, a team of physicists report that they have successfully demonstrated the squeezing of individual light particles, or photons, using an artificially constructed atom, known as a semiconductor quantum dot. Thanks to the enhanced optical properties of this system and the technique used to make the measurements, they were able to observe the light as it was scattered, and proved that it had indeed been squeezed.

Professor Mete Atature, from the Cavendish Laboratory, Department of Physics, and a Fellow of St John’s College at the ̽��ֱ�� of Cambridge, led the research. He said: “It’s one of those cases of a fundamental question that theorists came up with, but which, after years of trying, people basically concluded it is impossible to see for real – if it’s there at all.”

“We managed to do it because we now have artificial atoms with optical properties that are superior to natural atoms. That meant we were able to reach the necessary conditions to observe this fundamental property of photons and prove that this odd phenomenon of squeezing really exists at the level of a single photon. It’s a very bizarre effect that goes completely against our senses and expectations about what photons should do.”

Like a lot of quantum physics, the principles behind squeezing light involve some mind-boggling concepts.

It begins with the fact that wherever there are light particles, there are also associated electromagnetic fluctuations. This is a sort of static which scientists refer to as “noise”. Typically, the more intense light gets, the higher the noise. Dim the light, and the noise goes down.

But strangely, at a very fine quantum level, the picture changes. Even in a situation where there is no light, electromagnetic noise still exists. These are called vacuum fluctuations. While classical physics tells us that in the absence of a light source we will be in perfect darkness, quantum mechanics tells us that there is always some of this ambient fluctuation.

"If you look at a flat surface, it seems smooth and flat, but we know that if you really zoom in to a super-fine level, it probably isn't perfectly smooth at all," Atature said. " ̽��ֱ��same thing is happening with vacuum fluctuations. Once you get into the quantum world, you start to get this fine print. It looks like there are zero photons present, but actually there is just a tiny bit more than nothing."

Importantly, these vacuum fluctuations are always present and provide a base limit to the noise of a light field. Even lasers, the most perfect light source known, carry this level of fluctuating noise.

This is when things get stranger still, however, because, in the right quantum conditions, that base limit of noise can be lowered even further. This lower-than-nothing, or lower-than-vacuum, state is what physicists call squeezing.

In the Cambridge experiment, the researchers achieved this by shining a faint laser beam on to their artificial atom, the quantum dot. This excited the quantum dot and led to the emission of a stream of individual photons. Although normally, the noise associated with this photonic activity is greater than a vacuum state, when the dot was only excited weakly the noise associated with the light field actually dropped, becoming less than the supposed baseline of vacuum fluctuations.

Explaining why this happens involves some highly complex quantum physics. At its core, however, is a rule known as Heisenberg’s uncertainty principle. This states that in any situation in which a particle has two linked properties, only one can be measured and the other must be uncertain.

In the normal world of classical physics, this rule does not apply. If an object is moving, we can measure both its position and momentum, for example, to understand where it is going and how long it is likely to take getting there. ̽��ֱ��pair of properties – position and momentum – are linked.

In the strange world of quantum physics, however, the situation changes. Heisenberg states that only one part of a pair can ever be measured, and the other must remain uncertain.

In the Cambridge experiment, the researchers used that rule to their advantage, creating a tradeoff between what could be measured, and what could not. By scattering faint laser light from the quantum dot, the noise of part of the electromagnetic field was reduced to an extremely precise and low level, below the standard baseline of vacuum fluctuations. This was done at the expense of making other parts of the electromagnetic field less measurable, meaning that it became possible to create a level of noise that was lower-than-nothing, in keeping with Heisenberg’s uncertainty principle, and hence the laws of quantum physics.

Plotting the uncertainty with which fluctuations in the electromagnetic field could be measured on a graph creates a shape where the uncertainty of one part has been reduced, while the other has been extended. This creates a squashed-looking, or “squeezed” shape, hence the term, “squeezing” light.

Atature added that the main point of the study was simply to attempt to see this property of single photons, because it had never been seen before. “It’s just the same as wanting to look at Pluto in more detail or establishing that pentaquarks are out there,” he said. “Neither of those things has an obvious application right now, but the point is knowing more than we did before. We do this because we are curious and want to discover new things. That’s the essence of what science is all about.”

Additional image: ̽��ֱ��left diagram represents electromagnetic activity associated with light at what is technically its lowest possible level. On the right, part of the same field has been reduced to lower than is technically possible, at the expense of making another part of the field less measurable. This effect is called “squeezing” because of the shape it produces.

Reference:
Schulte, CHH, et al. Quadrature squeezed photons from a two-level system. Nature (2015). DOI: 10.1038/nature14868.

A team of scientists have measured a bizarre effect in quantum physics, in which individual particles of light are said to have been “squeezed” – an achievement which at least one textbook had written off as hopeless.

It’s just the same as wanting to look at Pluto in more detail or establishing that pentaquarks are out there. Neither of those things has an obvious application right now, but the point is knowing more than we did before. We do this because we are curious and want to discover new things. That’s the essence of what science is all about.

Mete Atature

An image from an experiment in the quantum optics laboratory in Cambridge. Laser light was used to excite individual tiny, artificially constructed atoms known as quantum dots, to create “squeezed” single photons

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Scientists move closer to “two for one deal” on solar cell efficiency

tdk25 — Mon, 16 Mar 2015 16:00:54 +0000

̽��ֱ��underlying mechanism behind an enigmatic process called “singlet exciton fission”, which could enable the development of significantly more powerful solar cells, has been identified by scientists in a new study.

̽��ֱ��process is only known to happen in certain materials, and occurs when they absorb light. As the light particles come into contact with electrons within the material, the electrons are excited by the light, and the resulting “excited state” splits into two.

If singlet exciton fission can be controlled and incorporated into solar cells, it has the potential to double the amount of electrical current produced from highly energetic blue and green light, capturing a great deal of energy that would normally be wasted as heat and significantly enhancing the efficiency of solar cells as a source of green energy. Until now, however, scientists have not really understood what causes the process, and this has limited their ability to integrate it into solar devices.

Writing in the journal Nature Physics, a team of researchers shows that there is an unexpected link between the splitting process and the vibration of the molecule that occurs when light comes into contact with the electrons. This vibration is thought to drive the production of two excited electrons, revealing for the first time how singlet exciton fission happens.

̽��ֱ��study was carried out by researchers from the Cavendish Laboratory at the ̽��ֱ�� of Cambridge, and the ̽��ֱ�� of Oxford. As well as solving a hitherto mysterious problem of quantum physics, it potentially provides a basis on which new singlet fission materials could be developed for use in solar cells.

Dr Andrew Musser, a post-doctoral research associate and former PhD student at St John’s College, ̽��ֱ�� of Cambridge, who co-authored the research paper, said: “We tend to characterise singlet exciton fission as a sort of two for the price of one deal on electrons, because you get twice as much electrical current. ̽��ֱ��problem is that if we want to implement this in a solar cell, the material needs to be engineered so that it is compatible with all the other components in the device. That means that we need to design a range of materials that could be used, and to do that, we need to understand more about why and how singlet exciton fission occurs in the first place.”

At its most basic, singlet exciton fission is a product of the fact that when light particles, or photons, come into contact with an electron, the electron is excited by the light and moves. In doing so, it leaves a “hole” in the material’s electronic structure. ̽��ֱ��electron and the hole are still connected, however, by a state of mutual attraction, and the two together are referred to by physicists as an “exciton”.

These excitons come in two very different flavours: spin-singlet and spin-triplet, and in rare circumstances, they can convert from one to the other.

In the natural world, spin-singlet excitons are a part of photosynthesis in plants, because the light absorbed by pigments in the plant generates excitons which then carry energy throughout it. Solar cells imitate this process to generate and drive an electrical current. Conventional solar cells are silicon-based, and the absorption of a single photon leads to the formation of a single, excited electron that can be harvested as electrical current.

In a handful of materials, however, singlet exciton fission occurs instead. Rather than producing just one spin-singlet exciton, two spin-triplets appear when a photon is absorbed. This offers the tantalising prospect of a 100% increase in the amount of electrical current generated.

Researchers attempting to solve the puzzle of why the process happens at all, and why only in certain materials, have typically looked at how the electrons behave when they absorb light. In the new study, however, the team instead focused on the fact when the electrons move in response to the light, the molecule of which they are a part vibrates.

̽��ֱ��team used thin samples of TIPS-pentacene, a semiconducting material in which singlet exciton fission is known to occur. They then fired ultra-fast pulses of laser light at the samples, each pulse lasting just 10 “femtoseconds”, or 10 quadrillionths of a second. ̽��ֱ��miniscule timescale was necessary so that large numbers of molecules could be vibrated synchronously, enabling the researchers to measure the response of the molecule and the resulting effect on the electrons as light hit the material. ̽��ֱ��measurements themselves were made using ultra-fast vibronic spectroscopy.

To the researchers’ surprise, they found that the molecules in the pentacene samples not only vibrated as singlet exciton fission occurred, but also continued to do so afterwards. This implies that the formation of two spin-triplet excitons is stimulated by the vibrations themselves, and the resulting tiny, fast changes in the shape of the molecules.

“We are fairly confident that this underlies all ultrafast singlet fission,” Dr Akshay Rao, a Research Associate at St John’s College, Cambridge, who led the Cambridge team, said. “ ̽��ֱ��picture that emerges is that when they are excited by light, the intrinsic vibrations drive the development of a new electronic state.”

By understanding the fundamentals of singlet exciton fission, the study opens up the possibility of designing new singlet fission materials that would enable the process to be effectively integrated into a new generation of highly efficient solar cells. Future research is already being planned in which the group will examine the precise vibrational states that are required for singlet exciton fission to happen, which will further add to this knowledge.

̽��ֱ��work at Cambridge forms part of a broader initiative to harness high tech knowledge in the physical sciences to tackle global challenges such as climate change and renewable energy. This initiative is backed by the UK Engineering and Physical Sciences Research Council (EPSRC) and the Winton Programme for the Physics of Sustainability.

̽��ֱ��causes of a hitherto mysterious process that could enhance the power of solar cells have been explained in a new study.

If we want to implement this in a solar cell, we need to understand more about why and how singlet exciton fission occurs in the first place.

Andrew Musser

Martin Abegglen on Flickr

"Green Power". While conventional solar cells use silicon, it is possible that other materials could eventually be used that would increase their efficiency.

̽��ֱ��text in this work is licensed under a Creative Commons Licence. If you use this content on your site please link back to this page. For image rights, please see the credits associated with each individual image.

Yes

Licence type:

Attribution-ShareAlike