The quantum internet just got one step closer to reality thanks to new resonator breakthrough
A new kind of resonator has the ability to transmit quantum information using single photons from a silicon device tipped with a few dozen erbium atoms.
| Jun 21, 2023 10:57 PM EST
Created: Jun 21, 2023 10:57 PM EST
A technical paper titled “All-silicon quantum light source by embedding an atomic emissive center in a nanophotonic cavity” was published by researchers at University of California Berkeley and Lawrence Berkeley National Laboratory.
“Silicon is the most scalable optoelectronic material but has suffered from its inability to generate directly and efficiently classical or quantum light on-chip. Scaling and integration are the most fundamental challenges facing quantum science and technology. We report an all-silicon quantum light source based on a single atomic emissive center embedded in a silicon-based nanophotonic cavity. We observe a more than 30-fold enhancement of luminescence, a near-unity atom-cavity coupling efficiency, and an 8-fold acceleration of the emission from the all-silicon quantum emissive center. Our work opens immediate avenues for large-scale integrated cavity quantum electrodynamics and quantum light-matter interfaces with applications in quantum communication and networking, sensing, imaging, and computing.”
Find the technical paper here. Published: June 2023. Read this related news article from Berkeley Engineering.
Redjem, W., Zhiyenbayev, Y., Qarony, W. et al. All-silicon quantum light source by embedding an atomic emissive center in a nanophotonic cavity. Nat Commun 14, 3321 (2023). https://doi.org/10.1038/s41467-023-38559-6.
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21 Jun 2023
Max Planck and TU Munich researchers say development looks ideal for quantum-encrypted optical networking.
Researchers in southern Germany made a novel optical resonator that they say looks ideal for the construction of quantum networks.
The collaboration featuring the Max Planck Institute of Quantum Optics (MPQ) in Garching and Technical University of Munich (TUM) fabricated an erbium-doped silicon crystal, and found that it could emit single photons at a wavelength of 1536 nm.
Single-photon emitters are seen as a key component for quantum-encrypted optical networking, because the physical properties of a photon will always be altered if the link is intercepted – so any hack of the link will be seen.
They could also be used for future quantum networks, enabling calculations between multiple quantum computers.
The development follows earlier work by the same team to embed individual erbium atoms in crystalline silicon, using a relatively low temperature of 500°C to ensure that large numbers of erbium atoms do not cluster together in the silicon lattice.
Detailing the latest experimental results in the journal Optica, lead author Andreas Reiserer and colleagues wrote that although the promise of individual erbium dopants in silicon has been recognized for quantum networking previously, their integration into optical resonators had not been demonstrated before.
“We have [now] demonstrated that single erbium dopants in silicon can be resolved, and that their emission can be enhanced using a nanophotonic resonator,” stated the team in its summary. “This offers great promise for the implementation of quantum networks over long distances.”
Together with known methods for achieving quantum entanglement, they believe that the latest advance would establish erbium dopants in silicon as a prime candidate for large-scale quantum computing and communication networks.
In a release from MPQ, Reiserer and colleagues explained that their erbium-doped resonator was unlike conventional designs, in that it did not feature any mirrors.
Instead of mirrors, a regular pattern of nanometer-scale holes in the crystalline silicon are used. It means that the entire resonator measures only a few microns in length, and contains only a few dozen erbium atoms.
That nanophotonic structure was then coupled to an optical fiber to allow laser excitation of the ebrium atoms. Andreas Gritsch from the team explained: “In this way, we were able to accomplish the emission of individual photons with the desired characteristics.”
Reiserer commented: “The fact that this is possible in crystalline silicon offers an additional opportunity for the realization of quantum networks, because this material has been used for decades to produce classic semiconductor elements.
“This means that for quantum technology applications, such as the construction of quantum networks, silicon crystals can also be produced in high quality and purity.”
A further advantage of the new design is that it operates not just at absolute zero, but at the relatively high temperature – at least in the quantum world – of 8 K.
“And these few degrees make a big difference in practice,” Reiserer says. “Because such temperatures are technologically easy to achieve by cooling in a cryostat with liquid helium.”
That characteristic is expected to help pave the way towards real applications of the nanophotonic system. The team expects it to be of interest to financial institutions, medical facilities, or government agencies, where sensitive personal data or classified information is handled.
They explain: “While today even the best encryption cannot guarantee complete security, a quantum network would offer perfect data protection: as soon as an eavesdropper tried to intercept the information transmitted by prepared photons, their quantum properties would be lost and the data would become unusable.”
© 2023 SPIE Europe
Download ImageImage Name: Quantum-Light-Source-Concept-Art-777x583.jpg
MIT researchers have discovered that novel photovoltaic nanoparticles can emit streams of identical photons, potentially paving the way for new quantum computing technologies and quantum teleportation devices.
The device emits a stream of single photons and could provide a basis for optical quantum computers.
Using novel materials that have been widely studied as potential new solar photovoltaics, researchers at MIT have shown that nanoparticles of these materials can emit a stream of single, identical photons.
While the work is currently a fundamental discovery of these materials’ capabilities, it might ultimately pave the way to new optically based quantum computers, as well as possible quantum teleportation devices for communication, the researchers say. The results were published on June 22 in the journal Nature Photonics, in a paper by graduate student Alexander Kaplan, professor of chemistry Moungi Bawendi, and six others at MIT.Download ImageImage Name: Perovskite-Nanocrystals-Size-Uniformity-777x518.jpg
Microscopic imaging shows the size uniformity of the perovskite nanocrystals. Credit: Courtesy of the researchers
Most concepts for quantum computing use ultracold atoms or the spins of individual electrons to act as the quantum bits, or qubits, that form the basis of such devices. But about two decades ago some researchers proposed the idea of using light instead of physical objects as the basic qubit units. Among other advantages, this would eliminate the need for complex and expensive equipment to control the qubits and enter and extract data from them. Instead, ordinary mirrors and optical detectors would be all that was needed.
“With these qubit-like photons,” Kaplan explains, “with just ‘household’ linear optics, you can build a quantum computer, provided you have appropriately prepared photons.”
The preparation of those photons is the key thing. Each photon has to precisely match the quantum characteristics of the one before, and so on. Once that perfect matching is achieved, “the really big paradigm shift then is changing from the need for very fancy optics, very fancy equipment, to needing just simple equipment. The thing that needs to be special is the light itself.”
Then, Bawendi explains, they take these single photons that are identical and indistinguishable from each other, and they interact them with each other. That indistinguishability is crucial: If you have two photons, and “everything is the same about them, and you can’t say number one and number two, you can’t keep track of them that way. That’s what allows them to interact in certain ways that are nonclassical.”
Kaplan says that “if we want the photon to have this very specific property, of being very well-defined in energy, polarization, spatial mode, time, all of the things that we can encode quantum mechanically, we need the source to be very well-defined quantum mechanically as well.”
The source they ended up using is a form of lead-halite perovskite nanoparticles. Thin films of lead-halide perovskites are being widely pursued as potential next-generation photovoltaics, among other things, because they could be much more lightweight and easier to process than today’s standard silicon-based photovoltaics. In nanoparticle form, lead-halide perovskites are notable for their blindingly fast cryogenic radiative rate, which sets them apart from other colloidal semiconductor nanoparticles. The faster the light is emitted, the more likely the output will have a well-defined wavefunction. The fast radiative rates thus uniquely position lead-halide perovskite nanoparticles to emit quantum light.
To test that the photons they generate really do have this indistinguishable property, a standard test is to detect a specific kind of interference between two photons, known as Hong-Ou-Mandel interference. This phenomenon is central to a lot of quantum-based technologies, Kaplan says, and therefore demonstrating its presence “has been a hallmark for confirming that a photon source can be used for these purposes.”
Very few materials can emit light that meets this test, he says. “They pretty much can be listed on one hand.” While their new source is not yet perfect, producing the HOM interference only about half the time, the other sources have significant issues with achieving scalability. “The reason other sources are coherent is they’re made with the purest materials, and they’re made individually one by one, atom by atom. So, there’s very poor scalability and very poor reproducibility,” Kaplan says.
By contrast, the perovskite nanoparticles are made in a solution and simply deposited on a substrate material. “We’re basically just spinning them onto a surface, in this case just a regular glass surface,” Kaplan says. “And we’re seeing them undergo this behavior that previously was seen only under the most stringent of preparation conditions.”
So, even though these materials may not yet be perfect, “They’re very scalable, we can make a lot of them. and they’re currently very unoptimized. We can integrate them into devices, and we can further improve them,” Kaplan says.
At this stage, he says, this work is “a very interesting fundamental discovery,” showing the capabilities of these materials. “The importance of the work is that hopefully, it can encourage people to look into how to further enhance these in various device architectures.”
And, Bawendi adds, by integrating these emitters into reflective systems called optical cavities, as has already been done with the other sources, “we have full confidence that integrating them into an optical cavity will bring their properties up to the level of the competition.”
Reference: “Hong–Ou–Mandel interference in colloidal CsPbBr3 perovskite nanocrystals” by Alexander E. K. Kaplan, Chantalle J. Krajewska, Andrew H. Proppe, Weiwei Sun, Tara Sverko, David B. Berkinsky, Hendrik Utzat and Moungi G. Bawendi, 22 June 2023, Nature Photonics.
The research team included Chantalle Krajewska, Andrew Proppe, Weiwei Sun, Tara Sverko, David Berkinsky, and Hendrik Utzat. The work was supported by the U.S. Department of Energy and the Natural Sciences and Engineering Research Council of Canada.
One of the two big news items these days from the realm of computing is quantum computers (the other is artificial intelligence). Recently, IBM published a paper in which it claimed to have demonstrated that a quantum computer could solve a useful problem that today’s conventional computers can’t, a result merited by concerns that their computations might become too unreliable when they also become complicated.
What are qubits?
Quantum computers use qubits as their basic units of information. A qubit can be a particle – like an electron; a collection of particles; or a quantum system engineered to behave like a particle. Particles can do funky things that large objects – like the semiconductors of classical computers – can’t because they are guided by the rules of quantum physics. These rules allow each qubit to have the values ‘on’ and ‘off’ at the same time, for example.
The premise of quantum computing is that information can be ‘encoded’ in some property of the particle, like an electron’s spin, and then processed using these peculiar abilities. As a result, quantum computers are expected to perform complicated calculations that are out of reach of the best supercomputers today.
Other forms of quantum computing use other units of information. For example, linear optical quantum computing (LOQC) uses photons, the particles of light, as qubits. Just like different pieces of information can be combined and processed by encoding them on electrons and then having the electrons interact in different ways, LOQC offers to use optical equipment – like mirrors, lenses, splitters, waveplates, etc. – with photons to process information.
In fact, any particle that can be controlled and manipulated using quantum-mechanical phenomena should, on paper, be usable as an information unit in a quantum computer.
What are phonons?
This is why physicists are wondering if they can use phonons as well. Photons are packets of light energy; similarly, phonons are packets of vibrational energy. So the question is: can we build a quantum computer whose information unit is, colloquially speaking, sound?
According to a paper published in Science this month, it should be possible.
The problem is that researchers can manipulate electrons using electric currents, magnetic fields, etc., and they can manipulate photons with mirrors, lenses, etc. – but what can they manipulate phonons with? To this end, in the new study, researchers from the University of Chicago have reported developing an acoustic beam-splitter.
What is a beam-splitter?
Beam-splitters are used widely in optics research. Imagine a torchlight shining light along a straight line. This is basically a stream of photons. When a beam-splitter is placed in the light’s path, it will split the beam into two: i.e. it will reflect 50% of the photons to one side and let the other 50% pass straight through.
While it seems simple, the working of a beam-splitter actually draws on quantum physics. If you shine a million photons at it, it will create two beams, each of 500,000 photons. We can then reflect these two beams to intersect each other, creating an interference pattern (recall Young’s double-slit experiment). But researchers have found that an interference pattern appears even when they shine photons at the beam-splitter one by one. What are the photons interfering with? The answer is themselves.
This is because a) particles can also behave like waves, and b) until an observation is made, a quantum system exists in a superposition of all its possible states (like a qubit being partly ‘on’ and partly ‘off’ at the same time). So when the single wave interacts with the beam-splitter, it enters a superposition of the two possible outcomes – reflected and transmitted. When these states recombine, an interference pattern shows up.
What did the new study do?
In the new study, the researchers developed an acoustic beam-splitter – a tiny device resembling a comb, with 16 metal bars jutting out of it. It was placed in the middle of a 2-mm-long channel of lithium niobate. Each end of the channel had a superconducting qubit – a qubit whose circuit components were superconducting – that could both emit and detect individual phonons. The whole setup was maintained at an ultra-low temperature.
If these phonons were converted to sound, their frequency would be too high for humans to hear. Each phonon in the study represented, according to the paper, the “collective” vibration of around one quadrillion atoms.
The team found that these phonons interacted with the comb just like photons interact with an optical beam-splitter. When a phonon was emitted from the left side of the channel, it was reflected half of the time and transmitted to the right side the other half. When phonons were emitted simultaneously from the left and the right sides, they both ended up on one side (as expected).
A phonon-based computer…?
“The basic science question is whether phonons … actually behave the way quantum mechanics says they should,” Andrew Cleland, a physicist at the Pritzker School of Molecular Engineering and a member of the study team, told Physics magazine. His team’s tests proved that they do.
But it’s still a long way from here to a functional quantum computer that uses phonons as units of information. As University of Nottingham physicist Andrew Armour put it more broadly to Science News: “What you’re doing is extending the [quantum] toolbox… People will build on it, and it will keep going, and there’s no sign of it stopping any time soon.”
- IBM published a paper in which it claimed to have demonstrated that a quantum computer could solve a useful problem that today’s conventional computers can’t, a result merited by concerns that their computations might become too unreliable when they also become complicated.
- The team found that these phonons interacted with the comb just like photons interact with an optical beam-splitter.
- Beam-splitters are used widely in optics research. Imagine a torchlight shining light along a straight line. This is basically a stream of photons. When a beam-splitter is placed in the light’s path, it will split the beam into two: i.e. it will reflect 50% of the photons to one side and let the other 50% pass straight through.
MIT researchers harness nanoparticles for new source of quantum light
These findings could significantly boost optical quantum computers by making them scalable and affordable without the need for complex equipment.
| Jun 26, 2023 06:20 AM EST
Created: Jun 26, 2023 06:20 AM EST
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