Quantum Non-Demolition Readout and Coupling Studies of Superconducting Qubits

量子力学定律是我们所知道的最基本的物理定律。它们在各种情况下都经过了严格的考验。即便如此，关于我们对一些结果的理解和解释，仍然存在一些基本的未解问题。尽管如此，实际应用我们已经知道的量子力学是非常重要的。从这个意义上说，量子物理学既是一门基础科学，也是一门新的工程。似乎可以肯定的是，在即将到来的世纪，我们在量子效应的理解和技术掌握方面的进步，将像上一个世纪我们在电学方面所做的那样快。我们的研究计划是基于最近最令人兴奋的结果之一。1999年，日本研究人员在其他工作的基础上，表明有可能制造出遵守量子物理定律的电路。通常情况下，服从量子力学的物体是“自然”的单个粒子，如电子和光子，我们以前从未有机会研究或利用人工量子电路。目前，这种电路由铝制成，它们在非常低的温度下工作，低于100 mK，其中铝是超导的，并且在非常高的频率下工作，通常为10 GHz。现在可以观察能量的离散（量子化）变化，随意操纵电路进入不同的量子态，并在这些人造电路上进行所有基本的原子物理实验。到目前为止，世界上有五个研究小组能够使用不同的电路设计复制和改进早期的结果，并取得了不同程度的成功。然而，现在很清楚，这五个实验都没有完美的运作。已经证明，由于尚未完全理解的原因，很难可靠地测量电路的量子态，这被称为读出问题。此外，这些电路并不完全稳定，因为它们周围环境的微观变化会干扰它们的运行，这种效应称为环境退相干。我们的研究致力于解决这些问题。我们计划采用最好的读出技术，即美国耶鲁大学开发的量子光子谐振腔，并将其用于最好的量子电路，即法国CEA-Saclay开发的量子电路。在英国建立一个严肃的独立研究工作的最快方法是与当前最好的研究小组之一合作。考虑到这一点，这项研究的提议者在过去的一年里一直与CEA-Saclay小组合作。现在，我们将在伦敦大学的皇家霍洛威学院发起一项新的研究工作，该学院已经因其对量子计算的贡献而闻名。与CEA-Saclay的合作将继续下去，并将有不同但互补的研究方案，研究方案致力于了解和消除上述问题，并建立更好的电路。量子电路为构建量子计算机提供了一条非常有前途的途径，超导量子比特是目前最好的固态量子比特。我们希望生产一种将两个量子比特耦合在一起的设备，这是生产量子计算机的必要下一步。这样的设备也将使我们能够对量子纠缠进行系统的研究，这可能是量子力学中最不为人所知的领域。我们还计划探索量子力学如何过渡到经典力学。人们认为这是通过环境退相干过程进行的，这正是量子电路最容易受到的影响，因此提供了一个以非常直接的方式研究这个问题的独特机会。

The laws of quantum mechanics are the most fundamental laws of physics that we know of. They have been stringently tested in a variety of situations. Even so, there are still basic unanswered questions concerning our understanding and interpretation of some of the results. Despite this, it is very important to make practical use of what we already know about quantum mechanics. In this sense, quantum physics is both a fundamental science and new engineering. It seems a certainty that in the forthcoming century we will progress in our understanding and technical mastery of quantum effects as quickly as we have done with electricity in the last. Our research proposal is based on one of the most exciting recent results. In 1999 Japanese researchers, building on other work, showed that it is possible to make an electrical circuit that obeys the laws of quantum physics. Normally objects that obey quantum mechanics are 'natural' single particles such as electrons and photons, never before have we had the opportunity to study or exploit an artificial quantum circuit. Presently such circuits are made from Aluminium, they operate at very low temperatures, below 100mK where the Aluminium is superconducting and at very high frequencies, typically 10 GHz. It is now possible to observe the discrete (quantised) changes in energy, to manipulate the circuit at will into its different quantum states, and to perform all the basic atomic physics experiments on these man made electrical circuits. Five research groups in the world have so far been able to reproduce and improve on the early results using different designs of circuits and with varying degrees of success. However, it is now clear that none of these five experiments operate perfectly. It has proven difficult to measure reliably the quantum state of the circuit for reasons that are not yet fully understood, this is known as the readout problem. In addition the circuits are not completely stable in the sense that microscopic changes in the environment around them interfere with their operation, an effect known as environmental decoherence. Our research is dedicated to solving these problems. We plan to take the best available readout technology, a quantised photon cavity resonator developed at Yale University in the USA and use it on the best available quantum circuit, the quantronium circuit developed at the CEA-Saclay, France. The fastest way to establish a serious independent research effort in the UK is to collaborate with one of the best current research groups. With this in mind, the proposer of this research has spent the past year working with the CEA-Saclay group. Now we will initiate a new research effort at Royal Holloway, University of London, already well known for its contributions to quantum computing. The collaboration with the CEA-Saclay will continue and there will be distinct but complementary research programmes.The research programme is dedicated to understanding and eliminating the problems referred to above and to building better circuits. Quantum circuits offer a very promising route to building a quantum computer and superconducting qubits are presently the best available solid state qubits. We wish to produce a device that couples two qubits together, this is the necessary next step in the production of a quantum computer. Such a device would also allow us to make systematic studies of quantum entanglement, perhaps the least well understood area of quantum mechanics. We also plan to explore how it is that quantum mechanics makes the transition to classical mechanics. It is thought that this proceeds through the process of environmental decoherence, which is precisely the effect to which a quantum circuit is most vulnerable, hence presenting a unique opportunity to study this problem in a very direct way.