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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - SEQUNET (Semiconductor-based quantum network)

Teaser

Summary

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 new capabilities and substantial performance enhancements. For example, cryptographic procedures to transmit secret messages can be protected against undetected eavesdropping by the fundamental 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 computational 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 tackles 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, are developed. The inaccuracy of such operations is a key limiting factor for the performance of current quantum computing systems. Second, methods to optically interlink qubits suitable for realizing quantum circuits and processors with multiple qubits are explored. 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 building on the project is that every home will connected to a quantum network to secure communication for online banking, purchases, messaging and many other sensitive applications.

Work performed

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. Thus, the realization of accurate qubit operations is particularly difficult. 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. So far, our simulations 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.These simulations will then serve as a starting point for a forthcoming experimental implementation, for which we have fabricated and tested devices.

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 than our approach. Bringing both advantages together requires new device designs with rather difficult fabrication procedures. So far, we have overcome a good part of the associated challenges and carried out some first proof-of-principle experiments that verify the desired device properties.
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. Most of these have been installed and we have completed the design process so that the final assembly can start.

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.

Final results

With the technical prerequisites being largely established, the most interesting results of the project are to be expected during the second half of its duration. A device that we have recently cooled to its near-absolute-zero operating temperature will hopefully enable the realization of a two-qubit operation with unprecedented accuracy, which will be an important step for implementing quantum circuits with several qubits. Our unique new instrument will enable to combine optical connectivity with electrical control of the qubit. We aim to complete the device development and will then use the experimental setup to carry out a detailed characterization of the required properties of our devices and target the demonstration of the transfer of a quantum state between a single photon and our spin qubit. Eventually, we aim to combine multi-qubit operations with optical interfacing to demonstrate the most important primitives for quantum networking. A subsequent step, likely to take longer than this specific project to complete, is to optically connect two different spin-qubit devices, thus forming a minimal quantum network.

Other projects in our group pursue the realization of devices with more than two qubits. A ten-year vision is to bring these together with the results of the present project 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 once the high operating accuracy predicted by simulations can be achieved.