Periodic Reporting for period 4 - SEQUNET (Semiconductor-based quantum network)
Periodo di rendicontazione: 2020-11-01 al 2021-10-31
Current information technology relies on representing data as zeros or ones in memory elements called bits that obey the laws of classical physics familiar from every day life. Conceptually, it does not matter if we use electronic circuits or pebbles in boxes to represent bits. Using instead objects that obey the laws of quantum mechanics, which usually only manifest themselves in the microscopic world of atoms and molecules, promises qualtitatively new capabilities. For example, cryptographic procedures to transmit secret messages can be protected against undetected eavesdropping by the laws of physics. In computing, practically important tasks such as optimization problems or the simulation of catalysts for chemical processes could be made drastically faster, in some cases allowing us to tackle currently intractable problems.
While first quantum communication and quantum computing systems are already available, their usefulness is still limited by their moderate size and accuracy. Substantial improvements are needed to unleash their full potential. This project tackled two important aspects for semiconductor-based quantum bits (short qubits), which are an attractive approach because of their similarity with the semiconductor technology used for current computers. First, highly accurate methods to execute elementary operations with pairs of qubits, which are the fundamental steps for composing larger algorithms, were developed. The inaccuracy of such operations is a key limiting factor for the performance of current quantum processors. Our simulation results show that with a careful optimization, semiconductor qubits can reach figures of merit that are sufficient for large scale quantum computing. Furthermore, we have realized and demonstrated a device to couple qubits over larger distances than commonly achieved. The key idea here is to move individual electrons across a chip using electric signals. Based on this concept, we have developed a complete architecture that promises to be much better scalable than other approaches.
A second outcome of the project is an approach to optically interlink qubits that are suitable for realizing quantum circuits and processors. Such quantum links can be seen as the first step towards a quantum internet. Small quantum computers could be connected to tackle problems that are too large for each individual ones, and the range of quantum-secure communication could be extended beyond the limit of about 100 km encountered by current commercial devices, which do not yet dispose of fully operational qubits at each communicating node. A long term vision is that every home will connected to a quantum network to secure communication for online banking, purchases, messaging and other sensitive applications. The concrete step completed in the project is the realization of a challenging device design that can equip a promising type of qubit with an optical interface, i.e. a receiver or emitter of quantum states.
While first quantum communication and quantum computing systems are already available, their usefulness is still limited by their moderate size and accuracy. Substantial improvements are needed to unleash their full potential. This project tackled two important aspects for semiconductor-based quantum bits (short qubits), which are an attractive approach because of their similarity with the semiconductor technology used for current computers. First, highly accurate methods to execute elementary operations with pairs of qubits, which are the fundamental steps for composing larger algorithms, were developed. The inaccuracy of such operations is a key limiting factor for the performance of current quantum processors. Our simulation results show that with a careful optimization, semiconductor qubits can reach figures of merit that are sufficient for large scale quantum computing. Furthermore, we have realized and demonstrated a device to couple qubits over larger distances than commonly achieved. The key idea here is to move individual electrons across a chip using electric signals. Based on this concept, we have developed a complete architecture that promises to be much better scalable than other approaches.
A second outcome of the project is an approach to optically interlink qubits that are suitable for realizing quantum circuits and processors. Such quantum links can be seen as the first step towards a quantum internet. Small quantum computers could be connected to tackle problems that are too large for each individual ones, and the range of quantum-secure communication could be extended beyond the limit of about 100 km encountered by current commercial devices, which do not yet dispose of fully operational qubits at each communicating node. A long term vision is that every home will connected to a quantum network to secure communication for online banking, purchases, messaging and other sensitive applications. The concrete step completed in the project is the realization of a challenging device design that can equip a promising type of qubit with an optical interface, i.e. a receiver or emitter of quantum states.
The project uses a particular type of semiconductor qubit using GaAs as the host material. This material system is very favorable for optical connectivity because of its good optoelectronic properties, but suffers from the drawback that the unavoidable presence of nuclear spins can easily destroy the fragile information stored in the quantum states of the devices. We have approached this problem using detailed, realistic simulation for numerically finding the qubit control signals that most robustly manipulate it in the desired way. We show that operations with the accuracy required to build large-scale quantum computing systems (corresponding roughly to at most one error per 1000 operations) should indeed be possible. We have also develped a way to compesate calibration errors that are unavoidable when transfering simulations to a real device with imperfectly known parameters. Besides publishing the results, we have also put a significant effort in making the associated software tools broadly useable and accessible.
Working towards moving individual qubits across a chip, we have realized a corresponding device design and found experimental evidence that it can indeed transport individual electrons. While the devices realized in this specific project unfortunately did not reach a significant quality for more advanced demonstrations, the concepts and tools developed have already turned out to be very effective for other platforms.
Exploring optical links between our qubit requires several elements. First, even though the host material is well-suited for the task, our qubits so far rely exclusively on electrical manipulation and are normally not optically addressable. Other types of qubits shine optically, but are more difficult to connect to larger quantum circuits. Bringing both advantages together requires new device designs with rather difficult fabrication procedures. We have implemented such a device design and carried out some first proof-of-principle experiments that verify the desired device properties. Specifically, we have shown that by patterning gate electrodes on both sides of a 200 nm thick membrane hosting our qubits, one can locally trap excitonic states serving as phontonic interface.
Second, a special type of experimental setup that provides operating temperatures of a tenth of a degree above absolute zero, optical access as well as high frequency electrical control signals is needed. While most of the components can be purchased, some key elements must be home built, and the complete setup must be assembled from a large number of individual components. Within the project, we have constructed and used such a highly specialized instrument. To guide future experiments and to assess the expected performance of procedures to carry out the transfer of a quantum state between a spin qubit and a photon, we have carried out detailed modelling. The results indicate favorable prospects for achieving a good transfer accuracy.
Working towards moving individual qubits across a chip, we have realized a corresponding device design and found experimental evidence that it can indeed transport individual electrons. While the devices realized in this specific project unfortunately did not reach a significant quality for more advanced demonstrations, the concepts and tools developed have already turned out to be very effective for other platforms.
Exploring optical links between our qubit requires several elements. First, even though the host material is well-suited for the task, our qubits so far rely exclusively on electrical manipulation and are normally not optically addressable. Other types of qubits shine optically, but are more difficult to connect to larger quantum circuits. Bringing both advantages together requires new device designs with rather difficult fabrication procedures. We have implemented such a device design and carried out some first proof-of-principle experiments that verify the desired device properties. Specifically, we have shown that by patterning gate electrodes on both sides of a 200 nm thick membrane hosting our qubits, one can locally trap excitonic states serving as phontonic interface.
Second, a special type of experimental setup that provides operating temperatures of a tenth of a degree above absolute zero, optical access as well as high frequency electrical control signals is needed. While most of the components can be purchased, some key elements must be home built, and the complete setup must be assembled from a large number of individual components. Within the project, we have constructed and used such a highly specialized instrument. To guide future experiments and to assess the expected performance of procedures to carry out the transfer of a quantum state between a spin qubit and a photon, we have carried out detailed modelling. The results indicate favorable prospects for achieving a good transfer accuracy.
The project has expanded the technological state of the art to lay very important foundations for a very ambitious research program towards highly scalable semiconductor based quantum computers. Key results include new devices designs and a verification of their properties, realistic numerical models for high performance qubit operations, and ways to tune the experimental operating parameters of qubits in a systematic way. These contribute to advancing qubit development from insight-driven fundamental science to a goal oriented engineering discipline. Furthermore, we have constructed a unique instrument for experiments on optical connectivity of electrically controlled qubits.
One of our visions is to build on the results of the present and other projects to realize small optically interlinked quantum computing nodes with tens of qubits each. This degree of complexity should be sufficient to enable, for example, continental-scale quantum networking. Recently, this idea has been in competition for our attention with our concept to build highly scalable quantum processors with eventually millions of qubits using the on-chip links explored in the project. Perhaps one day they will both be merged to reaslize a quantum network of large scale devices, to interlink different chips in one system, or to combine long and short distance information transfer on a single chip.
One of our visions is to build on the results of the present and other projects to realize small optically interlinked quantum computing nodes with tens of qubits each. This degree of complexity should be sufficient to enable, for example, continental-scale quantum networking. Recently, this idea has been in competition for our attention with our concept to build highly scalable quantum processors with eventually millions of qubits using the on-chip links explored in the project. Perhaps one day they will both be merged to reaslize a quantum network of large scale devices, to interlink different chips in one system, or to combine long and short distance information transfer on a single chip.