Quantum technology (QT) applies quantum mechanical properties such as quantum entanglement, quantum superposition, and No-cloning theorem to quantum systems such as atoms, ions, electrons, photons, or molecules. Quantum bit is the basic unit of quantum information. Whereas in a classical system, a bit is either in one state or the another. However, quantum qubits can exist in large number of states simultaneously, property called Superposition.
Quantum entanglement is a phenomenon where entangled particles can stay connected in the sense that the actions performed on one of the particles affects the other no matter what’s the distance between them. No-cloning theorem tells us that quantum information (qubit) cannot be copied.
Quantum computers shall bring power of massive parallel processing, equivalent of supercomputer to a single chip. They can consider different possible solutions to a problem simultaneously, quickly converge on the correct solution without check each possibility individually. This dramatically speed up certain calculations, such as number factoring.
The power of quantum computers depends on the number of qubits and their quality measured by coherence, and gate fidelity. Qubit is very fragile, can be disrupted by things like tiny changes in temperature or very slight vibrations. Coherence measures the time during which quantum information is preserved. The gate fidelity uses distance from ideal gate to decide how noisy a quantum gate is.
Nanoscale systems possessing long-lived spins and the ability to coherently couple to light are highly demanded for quantum devices implementations. Several approaches, like NV centers in diamond, semiconductor quantum dots are intensively investigated in the field, where an outstanding challenge is to preserve properties, and especially optical and spin coherence lifetimes, at the nanoscale.
The investigation of quantum phenomena in nanophotonics systems may lead to new scales of quantum complexity and constitutes the starting point for developing photonic technologies that deliver quantum-enhanced performances.
There are many types of quantum bits, or qubits, ranging from those using trapped ions, superconducting loops or photons. Quantum dots are particularly promising for optical applications due to their optical properties arising from the quantum confinement of electrons and holes.
Nanophotonics for Quantum Computers
Nanophotonics or nano-optics is the study of the behavior of light on the nanometer scale, and of the interaction of nanometer-scale objects with light. It encompasses the investigation of novel optical interactions, materials, manufacturing techniques, and models, as well as the exploration of organic and inorganic, or chemically manufactured structures such as holey fibers, photonic crystals, sub-wavelength structures, quantum dots, and plasmonics.
In the field of photonic quantum computing, photonics companies are currently developing two types of devices: qubit emitters and quantum processors.
Quantum dots have also been suggested as implementations of qubits for quantum information processing. “When a QD is provided with energy,” explains Prof. Felici, “via luminous radition or electric impluses, for example, the system first shifts to an excited state and then loses energy by emitting light or photons. By correctly exciting a QD, it can be primed to emit a single photon for each excitation impulse. This ability is extremely interesting, especially in view of the use of single photons as “quantum bits” or qubits in computing and quantum information protocols. Electrical engineering professor Supriyo Bandyopadhyay is attempting to make quantum computers with quantum dots.”The process is difficult” said Bandyopadhyay, “but the payoff is tremendous.”
Quandela is developing plug-and-play qubit emitters, designed around quantum dot-based single-photon sources, that emit photonic qubits. The qubits can be inserted into an optical fiber connected to a quantum processor to enable quantum computing. The company has also produced the world’s first fully commercially available quantum random number generator based on PICs with sources that are based on compact semiconductors only a few microns in size. The device emits a lineal stream of single, high-quality photons that can be sequentially entangled to generate cluster states in a matter of hours rather than years, as is the case when using a laser.
An example of a quantum processor developer is QuiX, a spinoff of the University of Twente. The company’s main product is a quantum photonic processor, which is basically a reconfigurable photonic chip in the form of a large-scale tunable interferometer that is very stable and capable of scaling. This processor has three main properties. First, it is low loss, as low as 0.1 dB per centimeter, thanks to the SiN waveguides. Second, it is also plug and play, as it comes with a seamless integration of the dedicated control software. Finally, it is fully reconfigurable, resembling a sort of multipurpose, off-the-wall field-programmable gate array.
The use of the SiN platform also provides a wide transparency window, from 400 nm to 3.7 µm. This means that the processor can be interfaced with all common single-photon sources — including, for example, commercially available quantum dots, nonlinear crystal, and spontaneous parametric downconversion sources. The applications of the photonic processors developed at QuiX are various and include quantum information processing, quantum chemistry, and machine learning.
Diamond has recently emerged as a unique material for quantum information processing. In particular, Nitrogen-Vacancy (NV) centers in diamond exhibit quantum behavior up to room temperature. The diamond has many properties that fairly isolates the qubit from the surrounding environment including rigid structure, excellent heat conduction, and conducting electricity not at all.
Nanophotonics for Quantum Communications
Quantum communication refers to a quantum information exchange that uses photons as quantum information carriers over optical fibre or free-space channels.
Today, quantum data transfer rates remain quite low, and so communicating entire messages is not yet practical. Instead, Quantum Cryptography or Quantum key distribution (QKD) is being used that employs single or entangled photons to generate shared secret key between the parties that is perfectly secure. The security is guaranteed by Heisenberg’s uncertainty principle. This ensures that any attempts to intercept and measure quantum transmissions, will introduce an anomalously high error rate in the transmissions and therefore will be detectable.
QKD technology requires single-photon sources (SPSs), single-photon detectors, modulating schemes, and protocols. Sensitive superconducting detectors also require cryogenic refrigerated devices. Currently point to point fiber optic links are commercially available with limited distance due to photon losses. QKD is also being tested on free space channels from ground to satellites and drones. They are now being expanded to quantum network that contains elements such as a quantum repeater and quantum switch.
Quantum physicists at Harvard University are currently developing synthetic diamond-based quantum computer technology that could enable faster data processing and secure communication. Delft University in the Netherlands have also established that diamond spin qubits are a prime candidate for the realization of quantum networks. An experiment in China using diamonds has put quantum code-breaking a step closer to reality, threatening to one day break the digital encryption technologies that safeguard banks, governments and the military.
As part of a current project, Ronald Hanson’s group at the Delft University of Technology in the Netherlands is using the NV defect in diamond as a “quantum repeater node” in a 100% secure quantum internet. In such a network, the nodes are quantum mechanically entangled to build up a chain from the source to the receiver so that quantum information can be transmitted over large distances. Such a demonstration is a challenging target, but there are also many nearer-term applications using the fragility of the quantum states.
In this context, chemically synthesized Eu3+ doped Y2O3 nanoparticles have demonstrated great potential for quantum technologies based on their narrow optical homogeneous linewidth, down to the 10 kHz level, and millisecond-long spin coherence time.
Praseodymium (Pr3+) ions present remarkable interest as an alternative to Eu3+ for quantum memories, quantum computing and single photon emission due to their larger oscillator strength and long optical and spin coherence lifetimes as demonstrated in Y2SiO5 bulk crystals.
This ambition demands new physical insight as well as cutting-edge engineering, with an interdisciplinary approach and a view towards how such ground-breaking technologies may be implemented and commercialized.
Crystals with rare-earth ions could lead to quantum repeaters that enable secure quantum communications over long distances
Researchers proposed in 2017 a way to store light using the weak but narrow optical transitions of rare-earth ions, such as erbium and praseodymium, doped into solids. Rare-earth ions have the unusual property of their unfilled electronic orbitals lying inside their filled shells, which provide shielding against the strong electric fields of the host atoms of the crystal where they are embedded. The ions’ optical transitions are thus narrow, like those of free atoms in a gas. However, the residual interaction with the solid host shifts the transition frequencies differently for each ion, so that the combined spectrum appears broadened.
In addition, the optical transitions are normally forbidden, but they become allowed due to the host’s perturbation. This means the transitions will be weak, and as a consequence, the incoming light must be tuned close to the resonance frequency to increase the probability of absorption (and therefore storage). However, on-resonance excitation combined with the strong inhomogeneous broadening will cause the resulting atomic excitations to rapidly get out of sync. This dephasing would mean the stored information in the putative repeater would be lost.
The trick for overcoming the dephasing, is to shape the spectrum of the ions into an atomic frequency comb (AFC). In this method, a laser “switches off” those dopant ions whose transition frequencies correspond to the spaces between the teeth in the comb. If the remaining ions, with transitions spaced apart in frequency by δ, are resonantly excited with another light pulse, they will rapidly dephase. But thanks to beating between the different frequencies, the ions will return in sync after a time delay of τ=2π∕δ.
For their system, the Geneva group used europium as a dopant in an yttrium orthosilicate ( Y2SiO5) crystalline host. The ICFO team used the same host, but their dopant was praseodymium. In both cases, after optical pumping to produce an AFC spectrum, the teams excited their ions with a “write” pulse. The excited ion states emitted photons, which in a quantum repeater network would be used to entangle the emitting crystal with another crystal. However, in these simple demonstrations, it was enough to show that the emitted photons were quantum correlated with the excited ions in the crystal. For that, the teams used a “read” pulse to re-excite the ions. The researchers then measured the conditional probability that a single photon was detected following the write pulse and following the read pulse. They found that this two-photon coincidence was much higher than could be explained by any classical process, which implied that the emission was correlated to the quantum state of the ions.
The ICFO team initially generated an entangled pair of photons—one at visible wavelengths and the other in an infrared band used in optical fiber telecommunications. The researchers utilized the visible photon to excite their doped crystal, which meant the infrared photon became correlated with the ion excitation. This demonstrates the feasibility of using the scheme at wavelengths compatible with existing optical fiber networks.
Along the way, the researchers have contributed improvements to optical pumping, polarization encoding, coherent control, and enhanced optical efficiency.
Future work is to do in reducing the noise and optical losses, increasing the storage lifetime, and then demonstrating a prototype quantum repeater node. Still, the groundwork laid by the Geneva and ICFO teams shows crystals doped with rare-earth ions remain a promising architecture for tomorrow’s quantum secure World Wide Web.
Integrate optical switches and single-photon detectors in a single chip, reported in March 2021
The work of researchers from Sweden’s KTH Royal Institute of Technology and Austria’s Johannes Kepler University Linz is making it possible to integrate optical switches and single-photon detectors in a single chip. Supported by the EU-funded S2QUIP project, the research team has helped to further the field of quantum computing by developing a new heat-free method for controlling single photons.
The team’s work and findings are published in the journal ‘Nature Communications’. Current optical switches work by heating light guides inside a semiconductor chip. “This approach does not work for quantum optics,” remarks first author Samuel Gyger of S2QUIP project partner KTH Royal Institute of Technology in a news item posted on the ‘EurekAlert!’ website. “Because we want to detect every single photon, we use quantum detectors that work by measuring the heat a single photon generates when absorbed by a superconducting material. If we use traditional switches, our detectors will be flooded by heat, and thus not work at all,” Gyger goes on to explain. The heat generated by reconfigurable photonic circuits is therefore incompatible with heat-sensitive superconducting single-photon detectors, making the integration of these circuits and detectors on one chip difficult.
To solve this problem, the researchers developed an optical switch that’s reconfigured with microscopic electromechanical motion instead of heat. Single photons can therefore be controlled without the semiconductor chip heating up and incapacitating the single-photon detectors. This makes the switch compatible with the heat-sensitive detectors, therefore enabling their integration on a single chip.
In addition to demonstrating the on-chip compatibility of reconfigurable photonic circuits and superconducting single-photon detectors, the researchers also demonstrated three key functionalities of photonic quantum technologies. These are reconfigurable routing of classical and quantum light, high-dynamic range detection of single photons and power stabilisation of optical excitation using a feedback loop. Their results showed that combining microelectromechanical systems and superconducting nanowire single-photon detectors “enables the on-chip integration of not only the main building blocks of quantum optics, but also devices for adaptive control, monitoring, and stabilization of classical and quantum optics,” the study reports.
“Our technology will help to connect all building blocks required for integrated optical circuits for quantum technologies,” observes co-author Carlos Errando-Herranz of KTH Royal Institute of Technology in the ‘EurekAlert!’ article. “Quantum technologies will enable secure message encryption and methods of computation that solve problems today’s computers cannot. And they will provide simulation tools that enable us to understand fundamental laws of nature, which can lead to new materials and medicines.”
The goal of S2QUIP (Scalable Two-Dimensional Quantum Integrated Photonics) is to bring about a paradigm shift in the development of scalable, cost-effective integrated-chip quantum light sources. The project ends in March 2022.
NanOQTech project (Nanoscale Systems for Optical Quantum Technologies)
NanOQTech is a project funded by the FET Open, a highly competitive sub-program of Horizon 2020. It selects research projects that are forward-thinking, interdisciplinary, combining science and engineering to transform Europe’s scientific excellence into a competitive advantage.
The project involves eight laboratories, including our colleagues at the Observatoire de Paris-PSL SYRTE as well as researchers from Institut Néel in Grenoble, Lund University, Aarhus University, the Karlsruher Institut für Technologie, the Institute of Photonic Sciences in Barcelona and one industrial partner: Keysight Technologies. “I am the coordinator, and Diana is manager. The overall aim of the project is to demonstrate the usefulness of doping nanometric materials with rare-earth elements for quantum technologies, particularly those that use light”, said Philippe Goldner, CNRS researcher at Institut de Recherche de Chimie Paris (Chimie Paris – PSL). He is also coordinator of the NanOQTech European project.
You have to keep in mind that future (and current) improvements to our computers and means of communication rely on quantum technologies. While it has been proven that nanometric systems controlled by light bring key functionalities to quantum technologies for communication, computing, and detection, our primary challenge is now to extend as far as possible the life span of the quantum states at this scale for as long as possible. We believe that rare-earth elements inserted into nanoparticles offer intriguing properties to this end, said Diana Serrano is a researcher at the Chimie Paris research institute within the Crystals and Dynamics of Quantum States group. She is also manager of the NanOQTech European project.
As Philippe said, our initial hypothesis was that rare earth spins in nanoparticles had useful properties, but we were not entirely sure how to measure them. In the end, we were able to develop a novel measurement technique which demonstrated that the quantum properties of millimeter-scale crystals can essentially be applied to nanoparticles. The ability the use these materials on a smaller scale will allow for their inclusion in micro- and nanophotonic devices, and the results are extremely promising to that end.
First of all, there’s the use of nanoparticles doped with rare-earth elements as components of future quantum computers. Our research would also support the development of simulation and computation tools that are beyond the reach of current computers. We are also envisioning applications in the field of quantum communications and, more specifically, secure data transmission. Optical fiber, for example, which is commonly used to transmit large quantities of information, could be paired with nanoparticles doped with rare-earth elements to serve as memory for the quantum information. Secure flows of quantum communication over large distances could then be established. This is a very active field: a Chinese satellite has been launched, for example, to establish quantum communications. The field of research is developing quickly.
With the NanOQTech project, we are also exploring the use of nanostructures doped with rare-earth elements for new force sensors and single-photon sources. And we’re working on couplings between rare earth elements and graphene, which would offer new applications in optoelectronics.