RESEARCH
Research themes pursued by our group include
open quantum systems engineering,
nonreciprocal parametric systems,
superconducting quantum circuits,
quantum-limited measurements and readout,
quantum-information inspired cosmology and thermodynamics.
The wordle on the right, created with titles of our papers, gives a quick overview of the research in our group.
DISSIPATION ENGINEERING
All quantum systems interact with the environment they inhabit, often through processes like decay on qubits or photon loss from resonators. These processes act as undesired error channels and corrupt the encoded information. Traditional quantum information processing techniques based on unitary control thus have a Sisyphean task at hand, where the experimenter must constantly fight against the pervasive phenomenon of decoherence. The usual way to preserve fidelity of information is to employ complex error correction algorithms. There is, however, an alternative approach which embraces and exploits dissipation to flip this issue on its head; here instead of controlling the system and trying to isolate the environment as much as possible, the system is instead strongly coupled to a carefully engineered environment that steers the desired system evolution. In addition to sidestepping many of the issues which hamper unitary control methods, dissipative control also opens the door to a much richer class of quantum dynamics. Our research in this area is primarily focused on production and stabilization of specific states using these non-unitary processes, and on direct and indirect control of the dissipatively stabilized subsystem.
One of the enduring goals of our research program is to realize and unravel breaking of usual reciprocal symmetries associated with photon propagation and frequency conversion in active parametric systems. Such symmetry breaking can lead to directional propagation of light -- see the artistic(!) representation of a 'nonreciprocal window', which sends light through only when illuminated from the right but blocks it when illuminated from the left instead. The key challenge is to do this without introducing any dissipation in the system. This makes realizing dissipationless nonreciprocity a neat theoretical problem especially when one studies such effects on quantum scales, and opens doors to qualitatively new physics. In addition, it offers immediate applications as active nonreciprocity does not rely on any external magnetic fields -- the usual candidates to implement such symmetry breaking in passive systems. Magnets do not get along well with superconductivity and are kind of a bummer if you want to make them really small for on-chip applications (basically one cannot!). The active nonreciprocal schemes, therefore, are essential for realizing integrated quantum logic for future quantum mechanical processors.
QUANTUM-LIMITED NONRECIPROCITY
SUPERCONDUCTING QUANTUM CIRCUITS
Our research in this area focuses on novel nonlinear dynamics in open quantum system, with the primary application platform being superconducting quantum circuits. The high throne of nonlinearity in these systems is occupied by Josephson tunnel junctions (JJ), which wields a strong nonlinearity at the level of few quanta (any one who works with nonlinear devices will tell you why this is such a godsend!). Plus superconductivity ensures (ideally) no loss, a condition crucial for implementing any coherent quantum operations. The only caveat is that JJ circuits need to be opeated below the superconducting transition temperatures of the relevant material (~1-10 K), which commercially available dilution refrigerators really help out with.
In addition, working with JJ-based superconducting circuits offers a lot of flexibility in terms of engineering and design of all sorts of systems, making them open to various applications -- be it the well known such as parametric amplifiers, mixers, qubits, detectors, digital logic circuits; or the exotic such as quantum metrology, superinductances, topological junctions and lattices of artificial atoms. The image shows a scanning electron micrograph of a superinductance where each of the skinny lines is a Josephson junction.
Platforms such as circuit-QED enable measurement and control of qubits, by strongly coupling them with photons confined in a resonator and recording the qubit activity by looking at the footprint it leaves on outgoing photons. This methodology was originally developed for real atoms and was recently recognized with a Nobel prize. and extended to artificial superconducting atoms by groups at Yale. The image shows a typical cQED chip with a long meander CPW resonator coupled to two qubits. One needs to exercise all this care while measuring quantum information systems are not known for their tolerance to probing and poking; they decohere (i.e. shattering into a classical tantrum!) if you try.
We work on different superconducting qubit flavors such as transmon, fluxonium and flux qubits compatible with cQED architectures. The primary research focus in this direction has centered on, (i) mitigating decoherence by qubit design engineering and (ii) developing new protocols for multiplexed qubit readout and control.
The images show a typical circuit-QED chip design and SEM image of a C-shunt flux qubit.
SCALABLE QIP ARCHITECTURE
OUT-OF-EQUILIBRIUM QUANTUM MATTER
Under construction.