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测试。即便如此，关于我们对一些结果的理解和解释，仍然有一些基本的问题没有得到回答。尽管如此，将我们已经知道的量子力学知识用于实际应用是非常重要的。从这个意义上说，量子物理既是一门基础科学，也是一门新的工程。似乎可以肯定的是，在即将到来的世纪里，我们对量子效应的理解和技术掌握将像上个世纪对电的理解和掌握一样快。我们的研究建议是基于最令人兴奋的最新结果之一。1999年，日本研究人员在其他工作的基础上，证明了制造符合量子物理定律的电路是可能的。通常情况下，遵守量子力学的物体都是电子和光子等自然的单一粒子，我们以前从未有机会研究或开发人造量子电路。目前，这种电路是由铝制成的，它们在非常低的温度下工作，在100mk以下，铝是超导的，并且在非常高的频率下工作，通常是10 GHz。现在可以观察能量的离散(量化)变化，可以随意操纵电路进入不同的量子状态，并可以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，世界上已有五个研究小组使用不同的电路设计复制和改进了早期的结果，并取得了不同程度的成功。然而，现在很明显，这五个实验都不是完美的。事实证明，由于尚未完全了解的原因，很难可靠地测量电路的量子态，这就是众所周知的读出问题。此外，这些电路并不是完全稳定的，因为它们周围环境的微观变化干扰了它们的运行，这种效应被称为环境消相干。我们的研究致力于解决这些问题。我们计划采用目前最好的读出技术--美国耶鲁大学开发的量子光子腔谐振器，并将其用于最好的量子电路--法国CEA-Saclay开发的量子电路。在英国建立严肃的独立研究努力的最快方式是与当前最好的研究小组之一合作。考虑到这一点，这项研究的提出者在过去的一年里一直在与CEA-Saclay小组合作。现在，我们将在伦敦大学皇家霍洛威大学启动一项新的研究工作，该大学已经以其对量子计算的贡献而闻名。与CEA-SACLAY的合作将继续下去，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，并建造更好的电路。量子电路为建立量子计算机提供了一条非常有前途的途径，而超导量子比特是目前可用的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的下一步。这样的装置还将使我们能够对量子纠缠进行系统研究，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是如何过渡到经典力学的。人们认为这是通过环境退相干的过程进行的，而环境退相干正是量子电路最容易受到的影响，因此为以非常直接的方式研究这个问题提供了一个独特的机会。

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.