Interview What are the remaining challenges for scalable quantum computing after the electric field is connected to the ion trap module
Last week, scientists took a new step toward large-scale quantum computers: the University of Sussex and the Universal Quantum team demonstrated for the first time, with record speed and precision, that quantum bits can be transferred directly between quantum computer microchips; they connected the quantum microchips together like a puzzle to make powerful of quantum computers.
This latest research demonstrates the feasibility of connecting independent ion trap quantum computer modules via an electric field, and recently, Outside correspondents spoke with Dr. Foni Raphaël Lebrun-Gallagher about this groundbreaking research by his team at the University of Sussex. In the interview, Dr. Lebrun-Gallagher further describes the breakthrough of this experiment: the uncertainty of whether it is physically possible to scale up the trapped ion quantum computer is removed; in the future, the team will collaborate with engineering teams such as the German Aerospace Center (DLR) to deploy multi-module quantum computers.
Full Interview
Q: Can you tell us a little bit about your background and how you got involved in this research?
Lebrun-Gallagher: I grew up in France and Luxembourg and received my undergraduate degree in physical sciences at the University of Rennes; while doing a research internship in astrophysics at the University of Queensland in Australia, I inadvertently discovered the fascinating world of quantum physics, and my passion has never waned since. in 2014, I joined the Masters program in Quantum Technologies at the University of Sussex, and I found it incredible to discover that our current level of scientific development not only allows us to use quantum mechanics to describe the microscopic world around us, but also allows us to design inner workings that exploit the useful tools of this theory.
While at Sussex, I met Professor Winfried Hensinger and his research group, who inspired me to join the quest to build a practical quantum computer using trapped ion arrays. Under his guidance, I have been working for over five years now during my PhD to develop solutions to overcome the prominent physics and engineering barriers that prevent the scaling up of quantum computers.
In order for these machines to solve meaningful, wide-ranging problems, quantum computers need to scale to millions of quantum bits in number. However, today's state-of-the-art quantum computers are capable of accommodating only a small number of quantum bits, with the number of quantum bits typically below 100.
From day one, the goal of our research group has been very clear: we need to develop practical technologies and hardware that will allow quantum computers to no longer be constrained by the number of bits.
Q: Could you please describe quantum entanglement?
Lebrun-Gallagher: Quantum entanglement is a phenomenon in which the properties of particles or groups of particles are so tightly intertwined that they can affect each other even when they are far apart. It arises as a direct consequence of the abstract mathematical formalism used in quantum mechanics; however, trying to translate and understand this abstract formalism in the physical world around us yields some very counterintuitive results.
Surprisingly, this unusual connection between particles is predicted to cause instantaneous interactions between them, a peculiarity best known for being pointed out by Einstein (in a 1935 paper on the EPR paradox): he called it "ghostly motion at a distance". In fact, this phenomenon is so strange that it seems to defy some basic principles of our physical reality and can transmit information faster than the speed of light.
Despite early doubts, this remarkable phenomenon proved to be true. In the 1980s, core experiments focused on generating and studying the properties of entangled quantum states, a phenomenon that also won the Nobel Prize in Physics last year. Today, quantum entanglement is seen as one of the central features of quantum mechanics.
We can compare what would happen between two hypothetical quantum coins in place of our particles. Like our particles, coins have a set of properties: for example, a coin that is tossed can land on "heads" or "tails". If our quantum coins are independent of each other, and if we toss them at the same time, each coin will land with a result that is completely unrelated to the other coins. We have no way to know which side they will land on, and we know that if the first coin lands on "heads", this will have no effect on which side the second coin lands on.
Now, in the case of entangled quantum coins, the results of each coin toss are always perfectly correlated. For example, they can be entangled so that they always land on opposite sides of each other, i.e., if the first coin lands on "tails", the other coin will land on "heads". Even when flipped at the same time! Again, there is no way to know which side they will land on before the flip, but we can be sure that by looking at the outcome of one coin, we will know the outcome of the other. In this sense, we cannot think of our coins as two separate systems.
Q: What is the role of quantum entanglement in the operation of quantum computers?
Lebrun-Gallagher: "Quantum entanglement" is one of the key phenomena, alongside "quantum superposition" and "quantum interference", that can significantly improve the computational performance of quantum computers when solving certain problems. It can significantly increase the computational speed of quantum computers.
A characteristic of entangled particles is that the number of representable quantum states increases exponentially, rather than linearly, with each additional particle. In a quantum computer, each quantum state can be used to encode information. Thus, the resources required to store the same amount of information on a quantum computer would be greatly reduced compared to a classical computer.
Quantum computing is then performed by carefully controlling the way the entangled quantum states interfere with each other and with themselves. This leads to an increase or decrease in the probability associated with each quantum state: quantum computing succeeds when the probability that the state associated with the correct output finds its way into the computational process is amplified and enhanced.
Q: What have been the obstacles to creating efficient quantum computers in the past?
Lebrun-Gallagher: First, while quantum computing is still in its infancy, it has made significant progress since it was first conceptualized in the early 1980s. Today, many researchers in academia and industry around the world have the ability to isolate, exploit and manipulate quantum systems with remarkable precision.
Quantum information can be encoded as quantum bits on a variety of physical platforms, of which trapped ions are a prime candidate. In fact, quantum computers using trapped ions today offer some of the most promising properties that quantum computers can seek: quantum states of individual ion quantum bits can be manipulated with unparalleled precision, and entanglement operations for quantum logic can be performed with unparalleled reliability.
Q: What are the revolutionary implications of your latest research on quantum computing?
Lebrun-Gallagher: What our team from the University of Sussex and Universal Quantum has demonstrated is that the concept of connecting independent ion trap quantum computer modules via an electric field is a viable approach. Not only is the method feasible, but it can be used to transfer quantum information at high speed with minimal loss, thus removing a key obstacle to scaling up to a practical scale quantum computer.
However, as discussed earlier, existing quantum computers are currently limited to hosting a small number of quantum bits, which greatly hinders their ability to solve problems that have the potential to have a significant impact on people's lives.
In fact, most disruptive and anticipated quantum computing applications will require millions of quantum bits. This is because, despite the ability to encode large amounts of information with few resources, quantum information is still very fragile and can interact adversely with the noisy environment around it. To further protect quantum computing from errors, a larger number of quantum bits is needed to perform "quantum error correction" so that small errors can be tolerated.
Several physical architectures have been proposed and developed to demonstrate the key steps to achieve such fault-tolerant machines. Such architectures use ion trap microchips that can perform quantum algorithms by applying a simple voltage to an array of electrodes. But even so, the number of ion quantum bits that such devices can carry is still limited by the size of the microchip.
Therefore, to overcome this obstacle, a common-scale quantum computer must be modular and must provide a way to connect the modules to each other in a way that ensures fast and reliable transmission of quantum information.
As a solution to this challenge, our research group theoretically proposed several years ago to connect ion trap quantum computer modules directly via electric fields and to transfer ion quantum bits directly between the modules. However, this concept holds promise for fast and high-quality inter-module connections, which, however, have not been proven until now.
Q: What is revolutionary about your latest research on quantum computing?
Lebrun-Gallagher: What our team from the University of Sussex and Universal Quantum has demonstrated is that the concept of connecting independent ion trap quantum computer modules via an electric field is a viable method. Not only is the method feasible, but it can be used to transfer quantum information at high speed with minimal loss, thus removing a key obstacle to scaling up to a practical scale quantum computer.
More specifically, in experiments co-led by Falk Bonus, Mariam Akhtar and me, we demonstrated that trapped ions can be transferred at a record speed of up to 2424 links per second. The probability of losing ions during the entire operation is only 1 in 15 million, which is quite low! In addition, we show that the quantum information encoded within the captured ions remains protected during the transfer operation.
These are key features for scaling up quantum computers.
Q: How were you able to achieve such impressive reliability at record speeds?
Lebrun-Gallagher: To achieve these results, we constructed a dedicated ion-trapping experiment that features two ion-trapping microchip modules placed in a vacuum device.
These microchips consist of embedded microstructured electrodes that extend all the way to the edge of the module. By applying a voltage to these electrodes, we were able to generate a customized electric field that confined individual atomic ions to the top of each module. In effect, we trapped individual ions, suspended above the surface of the device, at a distance of about the thickness of a human hair (here 125 microns).
The vacuum around the atoms is very high. In fact, the pressure in our vacuum device is 100 trillion times less than the pressure exerted by the air around us. This ensures that the ions are not disturbed and do not have any collisions with the intruding background gas.
Each ion is then cooled by the Doppler cooling effect, using a laser, to bring the ion temperature down to the micro-Kelvin range.
With the ion trap modules closely aligned to within 10 microns, the electric fields generated on each module can be interconnected and form a continuous "confinement" region above and between the two modules. Then, by applying a specific sequence of voltages to the module's electrodes, individual ions capable of carrying quantum information are transferred from one module to the next.
Q: What else needs to be discovered or improved before quantum computing is fully realized in the future?
Lebrun-Gallagher: Right now, there are indeed many challenges still on the way. It will take time before we see a general-purpose quantum computing machine; but most of the remaining challenges are engineering in nature.
Now that the vast majority of the technologies needed for large-scale quantum computing have been demonstrated, the remaining problem is integrating them into a single architecture. This work will be carried out with numerous conflicting requirements and subject to extreme design and operational constraints. There are many difficulties, challenges that need to be addressed. But they are not physically impossible and can be solved with the right resources, time and expertise.
Q: In what areas can this technology be applied in the future?
Lebrun-Gallagher: I should emphasize that quantum computers are a scientific tool that is most likely to enable applications in R&D development and will be used to solve a very specific set of problems. However, quantum computers are not a panacea that will cure all diseases! This is because a quantum computer is a very complex system that is very well suited to solve certain classes of problems. This is not to perform the same classical computations faster, but to find computational problems that, when using the rules of quantum mechanics, can be solved efficiently in fewer steps overall.
We hope that by using the techniques we have demonstrated, quantum computers will scale to a sufficient number of quantum bits to allow them to start solving meaningful problems. I'm referring to advancing various areas that have a real impact on people's lives, such as helping to create new materials and develop new drugs, and also assisting in making society more sustainable.
Q: What is the most exciting aspect of this research for you?
Lebrun-Gallagher: While this does remove one challenge, there are still many challenges standing in the way, and the one that excites me most personally is that we have removed the uncertainty about whether it is physically possible to scale up a quantum computer that captures ions. This change in the nature of the problem alone is a complete paradigm shift in the way we approach the development of quantum computers.
This is fantastic because at both the University of Sussex and Universal Quantum we have a clear engineering roadmap for building a practical quantum computer machine using this technology.
Q: What are the next steps?
Lebrun-Gallagher: We are already working on the upgrade!
The system is currently being installed with improved, high-performance electronic hardware so that we can drive these connection rates faster. In addition, we are already working closely with the rest of the engineering team at Universal Quantum to deploy this technology as part of a multi-module quantum computer that is being built for the German Aerospace Center (DLR).
Reference link：
https://www.azoquantum.com/Article.aspx?ArticleID=406
