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Quantum Physics and Quantum Technologies

The ability to control individual quantum systems in customised setups and materials, is paving the way for a quantum second revolution where the fundamental notions of quantum superposition and quantum entanglement will be an integral part of the inner workings of operational quantum devices. These will include quantum computers and simulators, quantum sensors and quantum communication systems.

In our group we are working in all aspects of quantum technologies, from the basic science behind them all the way to co-developing operating devices. Our special interest is in quantum simulation and computation In this direction, we study and develop novel approaches in a variety of quantum platforms including superconducting quantum circuits, slow light, and integrated photonic chips.

## Main research threads

Classical computers require enormous computing power and memory to simulate even the most modest quantum systems. That makes it difficult to model, for example, why certain materials are insulators and others are conductors or even superconductors. R. Feynman had grasped this since the 1980s and suggested to use instead another more controllable and perhaps artificial quantum system as a “quantum computer” or specifically in this case a “quantum simulator”.

**Recent projects:**

Spectroscopic signatures of many-body localization with interacting photons in superconducting qubits

(theory and experiment, Science 2017)

Here in collaboration with the Google Quantum Hardware group, using a chain of nine superconducting qubits, we implement a technique for resolving the energy levels of interacting photons. We used this to probe a complex quantum phase of matter known as many-body localized phase, currently beyond the reach of classical approached. We benchmark our method by first capturing the main features of the intricate energy spectrum predicted for two-dimensional electrons in a magnetic field—the Hofstadter butterfly. We then introduced disorder to study the statistics of the energy levels of the system as it undergoes the transition from a thermalized to a localized phase.

Forbidden physics has been seen in an experiment – sort of. Here in collaboration with the Szameit group in Germany, we simulated with light the behaviour of an impossible particle of the Majorana equation, we name the “Majoranon…” Unphysical solutions are ruled out in physical equations, as they lead to behavior that violates fundamental physical laws. One of the celebrated equations that allows unphysical solutions is the relativistic Majorana equation, thought to describe neutrinos and other exotic particles predicted in theories beyond the standard model. The neutrally charged Majorana fermion is the equation’s physical solution, whereas the charged version is, due to charge nonconservation, unphysical and cannot exist. Here, we present an experimental scheme simulating the dynamics of a charged Majorana particle by light propagation in a tailored waveguide chip. Specifically, we simulate the free-particle evolution as well as the unphysical operation of charge conjugation. We do this by exploiting the fact that the wave function is not a directly observable physical quantity and by decomposing the unphysical solution to observable entities. Our results illustrate the potential of investigating theories beyond the standard model in a compact laboratory setting.

Topological physics can greatly assist the design of advanced quantum systems and devices. Topological states are naturally protected against disorder and dissipation and can be used for a variety of tasks in quantum technologies.

Thouless pumping of quantum states of interacting bosons(theory)

We have recently put forward a scheme that can reliably transport quantum states of a few photons along a line of miniature quantum circuits. More specifically we show how to implement topological or Thouless pumping of interacting photons in one-dimensional nonlinear resonator arrays by simply modulating the frequency of the resonators periodically in space and time. The interplay between the interactions and the adiabatic modulations enables robust transport of Fock states with few photons per site. We analyze the transport mechanism via an effective analytic model and study its topological properties and its protection to noise. We conclude by a detailed study of an implementation with existing circuit-QED architectures.

Probing topological relativistic models with slow light in room temperature (theory and experiment)

Here we use a slow light quantum light-matter interface at room temperature to implement an analog simulator of complex relativistic and topological physics. In collaboration with the SUNY group of Eden Figureoa, we have realized the famous Jackiw-Rebbi model (JR), the celebrated first example where relativity meets topology. Our system is based upon interacting dark state polaritons (DSP’s) created by storing light in a rubidium vapor using a dual-tripod atomic system and is based in our earlier theory proposal in 2o14. The DSP’s temporal evolution emulates the physics of Dirac spinors and is engineered to follow the JR regime by using a linear magnetic field gradient. We also probe the obtained topologically protected zero-energy mode by analyzing the time correlations between the spinor components. Our implementation paves the way towards quantum simulation of more complex phenomena involving many quantum relativistic particles.

Driven open quantum systems and floquet stroboscopic dynamics

” Pity poor Schrodinger’s cat”. As if it weren’t enough to wish a cat into a state of being simultaneously dead and alive, physicists now have an idea for how to keep it that way – and the answer is to shake it. ….”

Read more from the CQT highlight for non specialists “Shaking Schrodinger’s cat may protect it from the environment”

Abstract: We provide an analytic solution to the problem of system-bath dynamics under the effect of high-frequency driving that has applications in a large class of settings, such as driven-dissipative many-body systems. Our method relies on discrete symmetries of the system-bath Hamiltonian and provides the time evolution operator of the full system, including bath degrees of freedom, without weak-coupling or Markovian assumptions. An interpretation of the solution in terms of the stroboscopic evolution of a family of observables under the influence of an effective static Hamiltonian is proposed, which constitutes a flexible simulation procedure of nontrivial Hamiltonians. We instantiate the result with the study of the spin-boson model with time-dependent tunneling amplitude. We analyze the class of Hamiltonians that may be stroboscopically accessed for this example and illustrate the dynamics of system and bath degrees of freedom.

### Colloquium Talk (July 2017)

10th year anniversary of CQT

## Collaborations

- Prof. J. Martinis/Dr Roushan (Google/UCSB)
- Prof. Alex Szameit (Rostock, Germany)
- Dr. Robert Keil (Inssbruck, Austria)
- Prof Rozario Fazio (Piza, Italy)
- Prof. Dieter Jacksh group (Oxford, UK)
- Prof Tobias Brandes group (Berlin, Germany)
- Prof. Eden Figueroa group (SUNY, USA)
- Dr Stephen Clark (University of Bath)