Research

Quantum State Manipulation of Superconducting Qubits

Quantum mechanics is the fundamental underpinning of our physical reality. However, various quantum effects are almost never observed in our daily, macroscopic lives. For instance, a conventional electrical circuit does not require a quantum mechanical description, even though the complete dynamics of an electron inside a transistor cannot be explained without the language of quantum mechanics.

When an electronic circuit is made from a superconducting material and brought below its critical temperature, due to its minimal energy dispersion, the effects of quantum mechanics start to become visible. On micrometre (and even millimeter) scales, a superconducting circuit which has been well-designed and is operated at an ultra-low temperature can become a macroscopic quantum device.

A superconducting circuit equipped with Josephson junctions can allow for the realization of quantum two-level systems; qubits. It is possible to coherently manipulate the quantum states of these artificial atoms and to observe their dynamics using microwave frequency techniques, in what is known as the field of cavity quantum electrodynamics (cQED).

The superconducting qubit as an artificial atom can have an enormous electric- or magnetic-dipole moment when compared to a true atomic system. Simply due to a qubit’s large physical size, it couples strongly with electromagnetic waves. By incorporating superconducting resonators or transmission lines into our circuits, we may make use of the strong spatial confinement of electromagnetic modes which further enhances the coupling.

With these carefully designed circuits, the coupling becomes so strong that even the effect of zero-point fluctuations of the electromagnetic field on the superconducting qubit can become observable.

Over the past few decades, research into superconducting qubits has rapidly shown them to be among the most viable platforms for building quantum computers. As groups around the world continue to develop this technology and overcome challenges associated with implementing quantum error correction and scalability, we hope that superconducting qubits will be established as the premier quantum information processing platform.

In our group, we are conducting deep and vast investigations into these devices, as we study quantum optical effects on superconducting qubits in the microwave regime and aim at designing scalable architectures, calibration protocols, and measurement techniques.

Hybrid Quantum Systems

As the technology and methods used to manipulate and measure superconducting qubits continue to become increasingly sophisticated, more complex quantum systems become possible to engineer. By expanding our circuits to systems with multiple qubits and resonators, we are able to unlock and explore previously unobservable physical phenomena.

Currently, we are expanding the utility of our quantum circuits such that they may be used to control and mediate interactions between other quantum systems. Coherent state transfer between different physical systems enriches the possible resources available for quantum information processing and communication. By engineering the means by which our superconducting circuits can interface with other platforms, we unlock new energy regimes and a wider range of applications.

In recent years, research on potential quantum transducers for transferring quantum information between superconducting qubits and optical light has been conducted. To that end, the coupling between qubits and mediating systems such as spin-wave modes in ferromagnetic yttrium iron garnet (YIG) spheres and surface acoustic waves (SAW) in nanomechanical structures have been investigated.

By coupling superconducting qubits to other physical systems, we are able to expand the cavity quantum electrodynamics toolbox to incorporate unique dynamics and mechanisms inherently unobtainable with superconducting circuits alone. Consequently, the attraction of quantum transduction and quantum sensing as fields of study for those working with superconducting circuits greatly increases as new systems enter the coupling ecosystem.

Additionally, there exist several proposals for scalable quantum computation which depend on concepts from communication and networking in which the connection of dissimilar platforms and energy regimes is necessary, which make investigations into hybrid quantum systems valuable to researchers across a wide spectrum of the quantum information landscape.

As we advance toward the development of quantum memories with long lifetimes and the establishment of interfaces between superconducting and optical systems, disparate worlds of physics can be connected and new scientific frontiers can be explored.