Session T8: Focus Session: Quantum Simulation

Using time reversal to detect entanglement and spreading of quantum information

Characterizing and understanding the states of interacting quantum systems and their non-equilibrium dynamics is the goal of quantum simulation. For this it is crucial to find experimentally feasible means for quantifying how entanglement and correlation build up and spread. The ability of analog quantum simulators to reverse the unitary dynamics of quantum many-body systems provides new tools in this quest. One such tool is the multiple-quantum coherence (MQC) spectrum previously used in NMR spectroscopy which can now be studied in so far inaccessible parameter regimes near zero temperature in highly controllable environments. I present recent progress in relating the MQC spectrum to established entanglement witnesses such as quantum Fisher information. Recognizing the MQC as out-of-time-order correlation functions, which quantify the spreading, or scrambling, of quantum information, allows us to establish a connection between these quantities and multi-partite entanglement. I will show recent experimental results obtained with a trapped ion quantum simulator and a spinor BEC illustrating the power of time reversal protocols.   

Towards a photonic Mott insulator in superconducting circuits

Recent developments in circuit QED provide superconducting circuits as a unique platform for exploring quantum many-body phenomena with light. The absence of particle number conservation, however, makes creating and understanding of many-body photonic states challenging. Here we make a one-dimensional lattice of coupled superconducting qubits with an additional pumping site and a lossy site incorporated at the end of the chain, which serves as an effective chemical potential for photons. When driven on the pumping site, the photons can spontaneously thermalize into the ground state of the lattice while the excess energy is dissipated via the lossy site. In the presence of strong photon-photon interaction via the qubit non-linearity, we expect the creation of a Mott insulator state of light, which we can probe with temporal- and spatially-resolved measurements. The performance of such an autonomous stabilizer can be compared to both analytical and numerical results from a simple model. These experiments will give insights to the microscopic investigation of non-equilibrium thermodynamics in strongly-interacting quantum system, including the interplay between external driving and dissipation. The work also provides a new approach to preparation of more exotic quantum photonic states.   

Analog Quantum Simulation of Complex Dynamics

Recent advances in quantum control have made analog quantum simulation (AQS) a promising tool for the study of complex many-body physics. However, as experimental AQS grows in sophistication, questions arise about how much and in which ways we can trust the outcome of a given simulation. Notably, the absence of error correction makes it critical to understand the role of imperfections when the simulated dynamics are chaotic and therefore hypersensitive to errors. The quantum kicked top (QKT) is an ideal model for such studies. We discuss results from recent work using the d = 16 electronic ground state manifold of an individual Cs atom for AQS of a QKT with spin J = 15/2. As a baseline, we see close agreement between simulated and predicted dynamics in a mixed phase space over hundreds of kicks. Earlier studies have hinted at features in the QKT dynamics that reflect classical phase space structures even when the fidelity of microscopic behavior (quantum state) is poor, suggesting the existence of “global” properties that can be reliably simulated in the presence of errors. We present data from experiments and numerical simulations in the presence of deliberately applied errors, showing that the frequency content of the perturbation plays a central role in the robustness of AQS.   

Cryogenic Ion Chains for Large scale Quantum Simulations

Ions confined in RF Paul traps are a useful tool for quantum simulation of long-range spin-spin interaction models. As the system size increases, classical simulation methods become incapable of modeling the exponentially growing Hilbert space, necessitating quantum simulation for precise predictions. Current experiments are limited to less than 30 qubits due to collisions with background gas that regularly destroys the ion crystal. We report results achieved in our cryogenic ion-trap quantum simulator, where we can routinely trap up to 100 ions in a linear chain and hold them for hours, thanks to differential cryo-pumping that reduces residual background pressure. Such a long chain provides a platform to investigate simultaneous cooling of many vibrational modes which will enable quantum simulations that outperform their classical counterpart. Our apparatus serves as a versatile test-bed to investigate a variety of Hamiltonians, including spin 1 and spin 1/2 systems with Ising or XY interactions. This work is supported by the ARO Atomic Physics Program, the AFOSR MURI on Quantum Measurement and Verification, the IC Postdoc Fellowship Program and the NSF Physics Frontier Center at JQI.   

Bang-bang shortcut to adiabaticity in trapped ion quantum simulators

We model the bang-bang optimization protocol as a shortcut to adiabaticity in the ground-state preparation of an ion-trap-based quantum simulator. The bang-bang protocol is a double quench of the field with a hold time in between. We compare the ground-state population after the ``diabatic" preparation protocol for a locally adiabatic ramp of the field, an exponential ramp of the field, and the bang-bang shortcut. We find that for long-range spin-spin couplings, the bang-bang protocol is superior, being overtaken by the locally adiabatic one as the range of the interaction shrinks. It is always better than an exponential ramp. But, unlike the locally adiabatic ramp, which requires detailed knowledge of the energy spectra for all field values, the bang-bang approach can be optimized knowing nothing about the underlying Hamiltonian. Hence, this method may be advantageous in examining properties of complex systems, especially those for which we do not know the low-lying energy spectra a priori. Examples of the bang-bang approach and an explanation for why it works will be given in the presentation as well.