Metamaterials are artificial materials designed to control the electromagnetic properties of a medium. Metasurfaces are the two-dimensional version of metamaterials: extremely thin surfaces made up of numerous subwavelength optical nanoantennas, each designed to serve a specific function upon the interaction with light. The metasurfaces contain regularly spaced nanoparticles that can modulate electromagnetic waves over sub-micrometer wavelength scales.
Metasurfaces allow or inhibit the propagation of electromagnetic waves in desired directions, can concentrate the waves and guide or can control the scattering of light with exceptionally high precision. These devices enable efficient beam steering, local control of optical polarization, and enhancement of emission and detection of light.
A classical optical device such as beam splitter is a simple device where one can only change its reflectivity and thus does not have much functionality. Metasurfaces have much broader functionality and have great potential to manipulate single photons and produce a wide variety of multiphoton entangled states. For example, the metasurface can entangle the orbital and spin degrees of freedom, whereas the beam splitter cannot.
The main advantages of metasurfaces with respect to the existing conventional technology include their low cost, low level of absorption in comparison with bulky metamaterials, and easy integration due to their thin profile. Planar metamaterials with subwavelength thickness, or metasurfaces, consisting of single-layer or few-layer stacks of planar structures can be readily fabricated using lithography and nanoprinting methods, and the ultrathin thickness in the wave propagation direction can greatly suppress the undesirable losses.
Till recently the experimentation with metamaterials was limited to manipulations using classical light. For example, using classical light, researchers have designed invisibility cloaks that can conceal small things from radar, or create a medium that bends light backwards. Over the past decade or so, these materials have been used to create a variety of technological tools ranging from sensors to lenses and imaging techniques. For example, metasurfaces have been demonstrated to produce unusual scattering properties of incident plane waves or to guide and modulate surface waves to obtain desired radiation properties.
Soon, those metamaterials could become the building blocks for quantum optics, quantum computer and quantum sensors. The quantum optical technologies require sources of single photons, entangled photons, and other types of nonclassical light. These also require newer methods of detection. The quantum states could be based on different degrees of freedom of light polarization, direction, orbital angular momentum. For the realization of each of these, metasurfaces have great potential.
Quantum Metamaterials Have Arrived
In 2018, An interdisciplinary team of scientists led by Distinguished Professor Mordechai Segev of the Technion’s Physics Department and Solid State Institute and Professor Erez Hasman of the Technion’s Faculty of Mechanical Engineering reported to have developed a new and innovative scientific field: quantum metamaterials.
The key to the researchers’ discovery? A dielectric metasurface, which acts differently in response to left- and right-handed polarized light. Dielectric metasurfaces impose polarized light on opposite phase fronts that look like screws, one clockwise and one counterclockwise. If the metasurface wasn’t nano-fabricated from transparent materials, the quantum properties would be destroyed.
The team conducted two sets of experiments to generate entanglement between the spin and orbital angular momentum (OAM) of photons, the particles that make up light. Researchers shone a laser beam through a nonlinear crystal to create single photon pairs. Each is characterized by zero orbital momentum, and each had linear polarization — a superposition of right-handed and left-handed circular polarization, which correspond to positive and negative spin.
In the first experiment, the scientists split the photon pairs — directing one through a unique fabricated metasurface and the other to a detector to signal the arrival of the other photon. They measured the single photon that passed through the metasurface and found that it acquired OAM and the OAM became entangled with the spin. In the second experiment, the photon pairs were passed through the metasurface and measured using two detectors to show they had become entangled.
In the simplest of terms, entanglement means the actions performed on one photon simultaneously affect the other, even when the photons are very far apart. In quantum mechanics, photons are believed to exist in both positive and negative spin states, but once measured adopt only one state. By applying metamaterials to quantum information and computing, the Technion team has paved the way for numerous practical applications, including unbreakable encryptions. Their discovery also opens the door for new possibilities for quantum information on chips.
A quantum metasurface that can simultaneously control multiple properties of light
In 2020, research team led by Mikhail Lukin at Harvard University has recently proposed a new type of metasurface that can control both the spatiotemporal and quantum properties of transmitted and reflected light. In a paper published in Nature Physics, the team showed that realizing a quantum metasurface is possible and could be achieved by entangling the macroscopic response of thin atom arrays to light.
“Quantum metasurfaces are an entirely new type of materials designed atom by atom, which enable applications such as quantum computation with photons,” Rivka Bekenstein, the lead author of the recent paper, told Phys.org. “We combined a state-of-the-art technique for manipulating the state of many atoms by long-range interactions (i.e., Rydberg interactions) with a recent discovery of how a single sheet of atoms can reflect light. We identified an architecture that can be realized in the laboratory, in which a single layer of atoms can act as a switchable quantum mirror.”
As part of their study, Bekenstein and her colleagues reviewed different quantum metasurfaces that can be controlled to have different light scattering properties. One of the most prominent sources for the development of quantum technologies are entangled states, which are unique states that only exist for quantum entities. The quantum metamaterial proposed by the researchers enables the production of specific entangled states of many light particles (i.e., photons), which are particularly valuable for quantum information processing applications.
In certain environmental conditions, atoms can be manipulated to become transparent using external electrical fields. Recent studies have also demonstrated that a single sheet of atoms can reflect light, resembling a regular mirror. By employing Rydberg interactions that naturally occur in atomic systems, Bekenstein and her colleagues were able to identify a scheme in which a single layer of atoms simultaneously reflects and transmits light in a quantum superposition. In other words, the resulting quantum metasurface could both become transparent and reflect light, like a mirror.
“In quantum mechanics, entities can co-exist in different states—this is called a superposition state,” Bekenstein said. “Our quantum metasurface is a new type of material that can make light co-exist in two different directions. This is done by manipulating the atoms’ state and then shining a weak laser to scatter from them.”
The design strategy employed by Bekenstein and her colleagues induces quantum entanglement between different metasurfaces and light, as well as between individual light particles. Notably, the architecture they proposed could also be manipulated to have varying amounts of photons in entangled states, which is a crucial capability for most quantum applications, including quantum computing. Through a series of quantitative calculations, the researchers analyzed how their metasurface enables quantum operations between atoms and photons, allowing for the generation of highly entangled photonic states that are ideal for quantum information processing applications.
“A key advantage of our architecture is that only one atom has to be prepared in a quantum superposition state in the laboratory,” Bekenstein said. “Hundreds of atoms construct the quantum metasurface, but only one has to be manipulated on the quantum mechanical level, which make this proposal practical. This is enabled due to the long-range interaction we utilize in the scheme, which naturally exists for atoms in specific energy levels.”
Remarkably, the recent study by Bekenstein and her colleagues introduces a technique to gain quantum control over the response of macroscopic materials to light. This technique could pave the way for the development of an entirely new type of quantum materials, while also potentially revolutionizing the current understanding of quantum optical materials and their response to light. “We are currently exploring additional experimental systems that can realize the quantum metasurfaces we proposed,” Bekenstein said. “We are also interested in revealing the nonlinear response of these quantum metasurfaces to light, which occur for higher intensity light beams. Finally, we are investigating specific practical applications of the proposed quantum metasurfaces for quantum information processing.”
A way of shrinking the devices used in quantum sensing systems has now been developed.
A way of shrinking the devices used in quantum sensing systems has been developed by researchers at the UK Quantum Technology Hub Sensors and Timing, which is led by the University of Birmingham. Sensing devices have a huge number of industrial uses, from carrying out ground surveys to monitoring volcanoes. Scientists working on ways to improve the capabilities of these sensors are now using quantum technologies, based on cold atoms, to improve their sensitivity.
Machines developed in laboratories using quantum technology, however, are cumbersome and difficult to transport, making current designs unsuitable for most industrial uses. The team of researchers has used a new approach that will enable quantum sensors to shrink to a fraction of their current size. The research was conducted by an international team led by University of Birmingham and SUSTech in China in collaboration with Paderborn University in Germany. Their results are published in Science Advances.
The quantum technology currently used in sensing devices works by finely controlling laser beams to engineer and manipulate atoms at super-cold temperatures. To manage this, the atoms have to be contained within a vacuum-sealed chamber where they can be cooled to the desired temperatures. A key challenge in miniaturising the instruments is in reducing the space required by the laser beams, which typically need to be arranged in three pairs, set at angles. The lasers cool the atoms by firing photons against the moving atom, lowering its momentum and therefore cooling it down.
The new findings show how a new technique can be used to reduce the space needed for the laser delivery system. The method uses devices called optical metasurfaces — manufactured structures that can be used to control light. A metasurface optical chip can be designed to diffract a single beam into five separate, well-balanced and uniform beams that are used to supercool the atoms. This single chip can replace the complex optical devices that currently make up the cooling system.
Metasurface photonic devices have inspired a range of novel research activities in the past few years and this is the first time researchers have been able to demonstrate its potential in cold atom quantum devices. Dr Yu-Hung Lien, lead author of the study, says: “The mission of the UK Quantum Technology Hub is to deliver technologies that can be adopted and used by industry. Designing devices that are small enough to be portable or which can fit into industrial processes and practices is vital. This new approach represents a significant step forward in this approach.”
The team have succeeded in producing an optical chip that measures just 0.5mm across, resulting in a platform for future sensing devices measuring about 30cm cubed. The next step will be to optimise the size and the performance of the platform to produce the maximum sensitivity for each application.
Quantum Metasurfaces Manipulate Free Photons, reported in August 2021
A team at Los Alamos National Laboratory proposed that modulated quantum metasurfaces can control all properties of photonic qubits. According to the team, such a breakthrough would affect the fields of quantum information, communications, sensing, imaging, and energy and momentum harvesting.
“People have studied classical metasurfaces for a long time,” said Diego Dalvit of the Physics of Condensed Matter and Complex Systems group in the laboratory’s Theoretical Division. “But we came up with this new idea, which was to modulate in time and space the optical properties of a quantum metasurface that allow us to manipulate, on demand, all degrees of freedom of a single photon.”
The team described the metasurface it developed as looking like an array of rotated crosses, which it can then manipulate with lasers or electrical pulses. Team members then proposed to shoot a single photon through the metasurface, where the photon splits into a superposition of many colors, paths, and spinning states that are all intertwined, generating so-called quantum entanglement — meaning that the single photon is capable of inheriting all these different properties at once.
“When the metasurface is modulated with laser or electrical pulses, one can control the frequency of the refracted single photon, alter its angle of trajectory, the direction of its electrical field, as well as its twist,” said Abul Azad from the Center for Integrated Nanotechnologies at the laboratory’s Materials Physics and Applications Division.
In manipulating these properties, the technology could be used to encode information in photons traveling within a quantum network. Encoding photons is particularly desirable in cryptography, as hackers are unable to view a photon without changing its fundamental physics, which would alert the sender and receiver that the information has been compromised.
The researchers are also working on how to pull photons from a vacuum by modulating the quantum metasurface. “The quantum vacuum is not empty but full of fleeting virtual photons. With the modulated quantum metasurface, one is able to efficiently extract and convert virtual photons into real photon pairs,” said Wilton Kort-Kamp, in the Theoretical Division at the lab’s Physics of Condensed Matter and Complex Systems group.
Harnessing photons that exist within the vacuum and firing them in one direction should create propulsion in the opposite direction. Similarly, stirring the vacuum should create rotational motion from the twisted photons. Structured quantum light could then one day be used to generate mechanical thrust, using only tiny amounts of energy to drive the metasurface.
The research was published in Physics Review Letters (www.doi.org/10.1103/PhysRevLett.127.043603).
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