Hafezi Research Group

The application of topology in optics has led to a new paradigm in developing photonic devices with robust properties against disorder. Although considerable progress on topological phenomena has been achieved in the classical domain, the realization of strong light-matter coupling in the quantum domain remains unexplored. We experimentally demonstrated a strong interface between single quantum emitters and topological photonic states. Our approach created robust counterpropagating edge states at the boundary of two distinct topological photonic crystals.

A team of researchers has devised a simple way to tune a hallmark quantum effect in graphene—the material formed from a single layer of carbon atoms—by bathing it in light. Their theoretical work, which was published recently in Physical Review Letters, suggests a way to realize novel quantum behavior that was previously predicted but has so far remained inaccessible in experiments.

Recently, we theoretically showed how to realize two-component fractional quantum Hall phases in monolayer graphene by optically driving the system. A laser is tuned into resonance between two Landau levels, giving rise to an effective tunneling between these two synthetic layers. Remarkably, because of this coupling, the interlayer interaction at non-zero relative angular momentum can become dominant, resembling a hollow-core pseudo-potential.

Topology plays a central role in the modern condensed matter, quantum information and high-energy physics. Certain Geometric manipulation of the manifold which supports a particular topological, known as the modular transformations, can be used as fault-tolerant logical operations in the context of both topological phases and topological quantum error correction codes. We realized that such transformations can be implemented in a single shot (i.e., with constant circuit depth), using local transversal SWAP operations between patches in a folded system with twist defects (wormholes in the synthetic dimension).

A promising near-term application of a quantum computer consisting of O(100) qubits is quantum simulation of fermionic systems, which exceeds the computational power of the world’s largest classical supercomputer due to the exponential growth of the Hilbert-space size. The target systems range from large molecules in quantum chemistry such as fertilizer made with lower energy cost, to strongly correlated electronic materials such as the notoriously difficult high-Tc superconductors. This killer app happens to coincide with Feynman’s original vision of universal quantum simulator, which uses a quantum system to simulate another and hence “fight fire with fire”.

Entanglement spectrum, the full spectrum of the reduced density matrix of a subsystem, plays a major role in characterising many-body quantum systems. In recent years, it has been widely studied in the fields of condensed matter physics, quantum information, high energy and black-hole physics.  As first pointed out by Haldane and Li in the context of fractional quantum Hall effect, the entanglement spectrum can serve as fingerprint of topological order (TO), which is itself a non-local feature and a pattern of long-range entanglement.

A hallmark feature of topological physics is the presence of one-way propagating chiral modes at the system boundary. The chirality of these edge modes is a consequence of the topological character of the bulk. For example, in an integer quantum Hall system, edge modes manifest as mid-gap states between two topologically distinct bulk bands. The number of these edge modes, called the winding number, is a topological invariant and is related to the bulk topological invariant, the Chern number.

The Kavli Institute of Theoretical Physics holds program on the subject of Many-Body Physics with Light.

JQI Fellow Mohammad Hafezi was announced as a recipient of a 2015 ONR Young Investigator award. ONR's website describes the program as being designed to promote the professional development of early-career academic scientists – called investigators, or YIPs – both as researchers and instructors. For awardees, the funding supports laboratory equipment, graduate student stipends and scholarships, and other expenses critical to ongoing and planned investigational studies.
“These recipients demonstrate the type of visionary, multidisciplinary thought that helps the U.S. Navy anticipate and adapt to a dynamic battlespace,” said Dr. Larry Schuette, ONR’s director of research. “The breadth of their research and combined value of awards underscore the significance the Navy places on ingenuity, wherever it’s harbored, and support the framework for a Naval Innovation Network built on people, ideas and information.”

For more info: Clark school news story and CMNS news story.