We are at the dawn of a new age in computing. After the invention of the modern computer 70 years ago, computational power has been growing rapidly due to the exponential growth in technology. Where computers were once large enough to fill entire rooms, today’s smart phone fits into the palm of our hand while being 350 times faster than the computers that were used to send astronauts to the moon. And now, we are going quantum.
The idea of a quantum computer
Quantum mechanics in general have been researched since the early 20th century, but only since the 1970s have quantum computers been a subject of deliberate research. It took another 20 years until the first algorithms were written that would take advantage of the specific properties of quantum computers, which then sparked the interest of the broader science community and thus shifted research into high gear. Today, 30 years later, quantum computing is still in its infancy, but companies like Intel, IBM and Google spend a lot of money on research to make sure that this new technology arrives rather sooner than later.
From a general perspective, quantum computers are comparable to regular computers. They store information, do lots of calculations and generate output. They have a bus system that relays quantum states and a main processing unit that is called a quantum arithmetic logic unit, which performs quantum gate operations. Those are stored in a quantum memory storage and eventually an input/output interface translates quantum states into classical information. So, the general setup is that of the von Neumann architecture. The difference lies in the way in which information is stored and calculations are done. A regular computer uses bits as its smallest unit of information. A bit is an electronic switch that is either closed or open. It can thus hold one of two different values: 0 and 1. Quantum computers instead use qubits. Qubits are (for the most part) physical objects that exhibit quantum mechanical properties. Examples are an electric current that oscillates resistance-free in a 2-micrometer circuit loop or an individual ion that is held in place by a laser. It is at these scales that quantum mechanical phenomena like superposition and entanglement start to emerge. Superposition allows a qubit to hold not only 0 or 1 as possible information, but both at the same time with varying probabilities, while entanglement allows multiple qubits to yield and fix their state after only one of them was measured instantaneously. The combination of the two allows a computer to do specific calculations e. g. for material properties, photonic systems on much bigger data sets and with much higher speeds than what you would normally get with a classical computer.
This is why China is about to invest $ 10 Billion into researching quantum computers. Now that is a vote of confidence.
Regular computers are slowly approaching their natural limits. Moore’s Law predicted that every two years the number of transistors on a circuit board (and with that the overall power of the computer) doubles. This has been true for the last 50 years because we have been able to make transistors smaller and smaller. But soon this will reach a point where components cannot get any smaller. Once we get to the sub-atomic level there is nowhere else to go. Which is why quantum computers are of such high interest.
The physics behind it all
Because the scientific field is still young, standards have yet to be established and the used materials as well as the actual technologies are still up for debate. In total, there are five different technologies that make up the majority of today’s research.
1. First off: superconducting qubits. They take a resistance-free electrical current that oscillates in a circuit loop with a diameter of 2 micrometers and use pulses of microwaves to put the current in a state of superposition. The advantage of this approach is that it uses existing technologies from the semiconductor industry and can build on a lot of pre-existing technology. The disadvantage: the circuit needs to be cooled down close to absolute zero and even then, the state of superposition could so far only be made to last for about 0.00005 seconds across 9 entangled qubits. This approach is researched by Google, IBM and Intel.
2. The next approach uses ion-trapped qubits. Multiple laser beams are pointed at a couple of ions (or electrically charged atoms) to cool them down to almost absolute zero, while an oscillating electromagnetic field keeps them in place. A laser excitation puts them in a superposition state. The biggest benefit is that this setup is very stable. So far, the University of Maryland has managed to create 14 entangled qubits that remained in superposition for over 1.000 seconds. The downside: the whole process takes a lot of time and energy to set up and is highly complex… even in the world of quantum science, where everything is already highly complex to begin with.
3. The third idea is to add silicon atoms to a substrate of pure silicon to create silicon quantum dots. These are then put into superposition by using microwaves. Intel is following this route because they already have a lot of experience working with silicon and other semiconductors. The current benchmark is at two entangled qubits for about 0.03 seconds. While the setup in general is very stable, it requires temperatures at nearly absolute zero and a magnetic field of over 1 Tesla. Not exactly something that can be done at home.
4. A fourth solution is to utilize the nitrogen-vacancies in diamonds in conjunction with nitrogen atoms to add an electron to the lattice of a diamond. This electron can then be put into superposition with light pulses. The big advantage is that it works at room temperature, which already resulted in six entangled qubits for ten seconds. The overall approach is still in its very early stages, though, and none of the big players are researching it.
5. The fifth and last idea is called topological qubits and is researched by Microsoft and Bell Labs. They are channeling electrons through certain semiconductor materials to observe emerging quasiparticles which can then be used to encode quantum states. This technology is still in the earliest stages of development and not enough is known about it to give any specifics about its feasibility.
These five approaches are using what is called quantum gates. These gates are in principle similar to the gates used in classical computers. A different approach, that will only be mentioned briefly, is that of the quantum annealer. Quantum annealers are most prominently researched by D-Wave. They use qubits in their non-excited states as basis for computational states which means they do not rely on time-dependent signals and are therefore much easier to work with. But for this reason they are also unable to perform general-purpose algorithms and are mainly used for optimization tasks that can be solved using a very specific type of algorithm. The differences in technology have already allowed quantum annealers to do calculation using up to 2.000 qubits, though this number cannot be directly compared to the number of qubits used for quantum gates in general-purpose quantum computers. Going forward, both strains of research will most likely receive significant attention without one triumphing over the other, as they are too different in their application.
The real-world applications
The European Union is planning to funnel over € 1.2 Billion into a quantum program that focusses on communication, sensor technology, simulation, and general-purpose computers. The overarching goal is to set up a quantum network that connects all the different quantum technologies with each other. Declared milestones are quantum repeaters that enable point-to-point links and a continent-wide communication network (communication), new algorithms and simulators for material research, drug design and general simulation processes (simulation), faster and more powerful versions of existing sensors (sensor technology), and, of course, quantum computers.
The estimated times of arrival of these technologies are not as far off as one might think. Repeaters, atomic quantum clocks or quantum sensors are expected to become viable in the next five years. Handheld quantum navigation devices and quantum computers that specifically solve materials and chemistry problems could be achievable in five to ten years. A quantum communication network across all of Europe and a general-purpose quantum computer are expected no later than 2035. While these are only rough estimates, they do illustrate where the technologies are currently at and what might be reasonable to expect. And as mentioned before, quantum annealers built by D-Wave systems are already used to solve optimization problems. While limited in scope, they do outperform classical computers in those fields.
The marketAs of today, the overall market value of everything quantum is estimated at $ 530 Million. It is expected to rise to $ 3.2 Billion over the next decade. While quantum computers only make up $ 33 Million of the current market, this is expected to increase to $ 1.9 Billion by 2030 – the biggest share of the market. The biggest share as of right now is held by quantum sensors, which are at $ 400 Million (or roughly 80% of the market) but are only expected to grow to $ 500 Million until 2030. Another field, that is expected to show strong growth rates is cryptography, going from $ 84 Million to about $ 750 Million over the next ten years.*
Where does all this money come from?Besides privately-owned companies like Google, IBM, Intel and Microsoft there are tremendous investments coming from national and supranational governments. China is expected to invest $ 10 Billion into their own research, followed by France with $ 1.4 Billion, the U.S.A. and the European Union at $ 1.2 Billion and the U.K. with $ 1 Billion. Together with $ 790 Million from Russia, $ 650 Million from Germany and $ 350 Million from Israel this brings the total amount of investments to $ 16 Billion.*
The same patterns emerge when you look at patent filings. In 2018, 492 patents came from China and 248 from the U.S.A. South Korea filed 45, the European Union 31 and Japan 30. While these numbers do not necessarily reflect the level of research, they do show who is pushing the research.
The challenges ahead are well defined. On the technical side, the number of qubits and the stability in terms of longevity of the entangled qubits as well as the error tolerance of the measurements needs to increase. The costs of operation need to go down and the requirements for near-zero temperatures need to relax. Moreover, algorithms that can take advantage of the strengths of quantum computers need to be written.
Here at Fraunhofer IZM, we are focusing our research on two areas. One is related to photonic system integration and miniaturization in the context of quantum computing, and the other is about ultra-low temperature integration technologies for superconductive qubits. The overarching goal is to reduce the complexity and cost of quantum systems. In quantum communication, this means to build the infrastructure, the protocols and the technical solutions for transmitting optical qubits from one place to another. For quantum sensors and computers, our research goes into solving the interaction between information carriers and their storage and emitters. This means to reduce the influence of environmental noise, the maximization of the interaction cross section and the miniaturization of the system as a whole – while still being able to scale the manufacturing.
One promising solution is to use glass as a material. Its transparency supports the usage of near-infra-red wavelengths that are typically used in quantum systems. Optical waveguides in glass have much smaller losses than those in silicon at these wavelengths. Furthermore, unlike some semiconductor materials, glass does not react with alkali atoms and its surface can be polished to a very high degree, which reduces the degree of residual light scattering. Some of the questions we are currently addressing are the hermetic bonding of glass for the scalable production of passive micro UHV chambers, the assembly of micro optics inside UHV chambers, the design and manufacturing of glass-based chips coupled to optical glass fibers, and the metallization of glass and the structuring of interlayer connections to enable electrical functionality.
Another approach looks at quantum processors that work in the Milli-Kelvin range. These controller circuits (including driver and read-out electronics) need very close microwave connections to manipulate the qubits. This requires cryogenic packaging approaches that use 3D interposers with through silicon vias (TSV), superconductive metallization for signal lines and shielding as well as high speed, fine pitch flip chip interconnects that can be used in cold environments (e.g. indium).
Sooner or later, all these challenges will be overcome and then we are in for a truly disruptive technology. While it is expected that first the sciences, military and finance will benefit from this new type of computer, it will only be a matter of time, until all of us will have the powers of the quantum realm at our fingertips.
*Source: Yole Développement (2020). Quantum Technologies.
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