Quantum mechanics has played an ever-increasing role in electronics over the past several decades. At first, materials and devices were introduced that were designed with quantum mechanical principles, but still operated on classical information (for example, the silicon transistor). More recently, devices have been developed to store and manipulate quantum bits of information (qubits) towards quantum computing applications. Until the past few years, however, these qubits have only been addressed with classical light signals. A fully quantum mechanical circuit, in which quantum mechanical microwave signals address quantum bits, enables scalable quantum computing architectures and makes possible a full range of quantum optics experiments, all on a single chip in an integrated circuit.

Our research focuses on these fully quantum mechanical integrated circuits, combining basic quantum mechanics, superconducting electronics, microwave circuits, quantum optics, and low-temperature measurement. The backbone of our work is a system known as circuit quantum electrodynamics (cQED). This system consists of a superconducting qubit coupled to an on-chip microwave resonator; the qubit can absorb and re-emit a single photon into the cavity hundreds of times before the photon is lost. This strong coupling opens the door to a vast array of experiments in quantum computing and non-linear optics. These are the two main thrusts of my research.

First, my group is looking at ways of building a robust scalable quantum architecture. While small qubit systems have been developed and microwave cavities have been shown to make a good quantum bus connecting these qubits, large-scale quantum computers remain a distant goal. Quantum information is quite fragile, and individual qubits are currently plagued by information loss, called decoherence. Are there ways of building individual qubits that are robust to dominant noise sources? Even if perfect qubits could be achieved, new problems arise as circuits get more and more complicated. How can we wire up complex systems without destroying the individual parts? These are the types of quantum computing questions we address experimentally.

Second, we study quantum and non-linear optics. Although people tend to think of lasers when they hear the term “optics,” the oscillating voltages and currents in a microwave circuit are also photons, and all principles of quantum optics apply to these devices as well. In fact, non-linearities can be much stronger in microwave devices, allowing us access to a very interesting regime of quantum optics. The goal of this area of research is to address the central question: What happens when a system is non-linear at powers where quantization is important?