Quantum Non-Demolition Readout and Coupling Studies of Superconducting Qubits

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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.