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  • richardmitnick 10:25 am on February 25, 2019 Permalink | Reply
    Tags: "Quantum dots can spit out clone-like photons", , , It’s a new phenomenon and will require much work to develop to a practical level, , , Perovskite quantum dots still have a long way to go until they become applicable in real applications, Perovskites, , , Such coherent photons could also be used for quantum-encrypted communications applications, The ability to produce individual photons with precisely known and persistent properties including a wavelength or color that does not fluctuate at all could be useful for many kinds of proposed quant, They need to achieve 100 percent indistinguishability in the photons produced   

    From MIT News: “Quantum dots can spit out clone-like photons” 

    MIT News
    MIT Widget

    From MIT News

    February 21, 2019
    David L. Chandler

    1
    Scanning Transmission Electron Microscope image (STEM) of single perovskite quantum dots. New study shows that single perovskite quantum dots could be a fundamental building block for quantum-photonic technologies for computing or communications. Image courtesy of the authors.

    System that generates coherent single particles of light could help pave the way for quantum information processors or communications.

    In the global quest to develop practical computing and communications devices based on the principles of quantum physics, one potentially useful component has proved elusive: a source of individual particles of light with perfectly constant, predictable, and steady characteristics. Now, researchers at MIT and in Switzerland say they have made major steps toward such a single photon source.

    The study, which involves using a family of materials known as perovskites to make light-emitting particles called quantum dots, appears today in the journal Science. The paper is by MIT graduate student in chemistry Hendrik Utzat, professor of chemistry Moungi Bawendi, and nine others at MIT and at ETH Zürich, Switzerland.

    The ability to produce individual photons with precisely known and persistent properties, including a wavelength, or color, that does not fluctuate at all, could be useful for many kinds of proposed quantum devices. Because each photon would be indistinguishable from the others in terms of its quantum-mechanical properties, it could be possible, for example, to delay one of them and then get the pair to interact with each other, in a phenomenon called interference.

    “This quantum interference between different indistinguishable single photons is the basis of many optical quantum information technologies using single photons as information carriers,” Utzat explains. “But it only works if the photons are coherent, meaning they preserve their quantum states for a sufficiently long time.”

    Many researchers have tried to produce sources that could emit such coherent single photons, but all have had limitations. Random fluctuations in the materials surrounding these emitters tend to change the properties of the photons in unpredictable ways, destroying their coherence. Finding emitter materials that maintain coherence and are also bright and stable is “fundamentally challenging,” Utzat says. That’s because not only the surroundings but even the materials themselves “essentially provide a fluctuating bath that randomly interacts with the electronically excited quantum state and washes out the coherence,” he says.

    “Without having a source of coherent single photons, you can’t use any of these quantum effects that are the foundation of optical quantum information manipulation,” says Bawendi, who is the Lester Wolfe Professor of Chemistry. Another important quantum effect that can be harnessed by having coherent photons, he says, is entanglement, in which two photons essentially behave as if they were one, sharing all their properties.

    Previous chemically-made colloidal quantum dot materials had impractically short coherence times, but this team found that making the quantum dots from perovskites, a family of materials defined by their crystal structure, produced coherence levels that were more than a thousand times better than previous versions. The coherence properties of these colloidal perovskite quantum dots are now approaching the levels of established emitters, such as atom-like defects in diamond or quantum dots grown by physicists using gas-phase beam epitaxy.

    One of the big advantages of perovskites, they found, was that they emit photons very quickly after being stimulated by a laser beam. This high speed could be a crucial characteristic for potential quantum computing applications. They also have very little interaction with their surroundings, greatly improving their coherence properties and stability.

    Such coherent photons could also be used for quantum-encrypted communications applications, Bawendi says. A particular kind of entanglement, called polarization entanglement, can be the basis for secure quantum communications that defies attempts at interception.

    Now that the team has found these promising properties, the next step is to work on optimizing and improving their performance in order to make them scalable and practical. For one thing, they need to achieve 100 percent indistinguishability in the photons produced. So far, they have reached 20 percent, “which is already very remarkable,” Utzat says, already comparable to the coherences reached by other materials, such as atom-like fluorescent defects in diamond, that are already established systems and have been worked on much longer.

    “Perovskite quantum dots still have a long way to go until they become applicable in real applications,” he says, “but this is a new materials system available for quantum photonics that can now be optimized and potentially integrated with devices.”

    It’s a new phenomenon and will require much work to develop to a practical level, the researchers say. “Our study is very fundamental,” Bawendi notes. “However, it’s a big step toward developing a new material platform that is promising.”

    The work was supported by the U.S. Department of Energy, the National Science Foundation, and the Swiss Federal Commission for Technology and Innovation.

    See the full article here .


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  • richardmitnick 11:17 am on January 8, 2016 Permalink | Reply
    Tags: , , , Perovskites, Silicon and solar materials   

    From MIT Tech Review: “Promising New Solar Material Boosts Performance of Silicon” 

    MIT Technology Review
    M.I.T Technology Review

    January 7, 2016
    Mike Orcutt

    Silicon probably won’t be replaced as the dominant solar material anytime soon, but it might not be too long before it gets a partner from a promising class of materials called perovskites.

    A group led by Henry Snaith, a physicist at the University of Oxford and leading perovskite researcher, has demonstrated what it says is a viable pathway to a device that combines a conventional silicon cell with a perovskite cell to boost the efficiency of that silicon cell by several percentage points.

    Perovskites, which have captured the interest of solar researchers and energy policy experts because of their rapidly improving performance and low cost, are distinguished by a chemical structure that gives rise to unique electronic properties that make them attractive for solar technology (see “Could a New Solar Material Outperform Silicon?”). Snaith and his colleagues say the new composition they’ve developed overcomes a fundamental obstacle to designing a highly efficient device that combines the light-absorbing characteristics of silicon with those of a perovskite material.

    The researchers say the result suggests it should be possible to make a silicon-perovskite “tandem” device that is more than 25 percent efficient, higher than the performance of today’s commercially available silicon cells, which are about 17 to 20 percent efficient. The measurements they took were in a laboratory environment, but the approach could eventually be used to achieve significantly higher efficiencies than the best silicon panels on the market today.

    High-performance tandem devices made of semiconductors other than perovskite have already achieved efficiencies in the lab of over 40 percent, but they are extremely expensive because they require very technically complex manufacturing processes. Making perovskite solar cells is much simpler and cheaper, and the process could be integrated into existing silicon panel manufacturing lines by adding a few steps. Many experts believe the most realistic near-term commercial application of perovskites will be a tandem device with silicon.

    Several groups have demonstrated working tandem devices made of a silicon cell and a perovskite cell, but the efficiencies have been limited because the range of the solar spectrum the perovskite absorbed did not fully complement the range that silicon absorbs. Attempts to tweak the range of light the perovskite absorbs led to instabilities within the material’s structure that compromised performance. Snaith and his colleagues came up with a method, which relies on substituting certain ions in the material with cesium ions, to achieve the desired photovoltaic properties while maintaining the material’s structural stability.

    The researchers have only demonstrated the new composition at a small scale, and a lot of work would be needed before we might see it in commercially available panels. But a company Snaith cofounded, Oxford PV, is also focused on developing silicon-perovskite tandem devices.

    Chris Case, chief technology officer of Oxford PV, says results like this reflect how quickly researchers are addressing the inherent challenges to making reliable, high-performing tandem cells. Case won’t reveal the specifics of his company’s technology, but says Oxford PV is close to demonstrating full-size devices that are 23 percent efficient and could hit 25 percent shortly thereafter. Case says it’s not unrealistic to think 28 or even 30 percent efficiency is possible within just a few years.

    Perovskite-based technologies still face challenges due to the material’s sensitivity to moisture and air, and questions remain about whether perovskite cells can be made durable enough to survive the long lifetimes required of power systems. Still, Case says Oxford PV is on track to deliver a commercial product—aimed at silicon panel manufacturers who want to “upgrade” the efficiency of their products—in 2017.

    See the full article here .

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