Search Talks

Is it possible to simulate efficiently using classical means the workings of a quantum computer that used mixed states if no non-classical correlations are generated in the mixed state? We discuss this question in the context of the DQC1 model of quantum computation and sketch path for efficient classical simulation of the DQC1 circuit that estimates the trace of an implementable unitary under the zero quantum discord condition is presented and the challenges in doing such a simulation are elucidated. This result reinforces the status of non-classical correlations quantified by quantum discord and related measures as the key resource enabling exponential speedups in mixed state quantum computation.

I will briefly review the operational and algorithmic approach for digital quantum simulation using different forms of quantum walk and present the example for simulating Dirac equations [1], many-body systems dynamics, complex quantum networks and open quantum systems [2]. I will also present the progress made in experimentally realizing and controlling quantum walks which with a promise for performing universal quantum computation[3].

[1] Nature Communications 11, 3720 (2020)
[2] New J. Phys. 22, 123027 (2020) ; New Journal of Physics 23, 113013 (2021)
[3] EPJ Quantum Technology 10, 43 (2023); Physical Review A 110 (3), 032615 (2024)

In the last decade, we have witnessed remarkable progress in quantum computing aided by an ever increasing number of qubits, enhanced error correction methods, and advances in hardware. One of the major obstacles that quantum computing must deal with is environmental effects on quantum dynamics. The obstacle originates from quantum systems being – unavoidably – a part of nature and, thereby, not isolated and noise-free. Thoroughly understanding the dynamics of quantum systems connected to the environment, or open quantum systems remains one of the critical research areas.

The primary focus of our research at Spin Lab is the dynamics of open quantum systems. The research relies on home-grown theoretical tools and experimental work using Nuclear Magnetic Resonance spectroscopy. The theoretical part involves the formulation and applications of a novel form of quantum master equation that takes into account the fluctuations in the local environment. To completely incorporate their effects, a propagator is designed to include finite evolution due to the fluctuations and infinitesimal evolution due to system Hamiltonians. The resulting quantum master equation (named, fluctuation-regularized quantum master equation or FRQME) is characterized by the presence of an exponential kernel in the dissipator and – most importantly – by the inclusion of dissipators from external drives and coupling. The later dissipators have been shown to play a major role in explaining many of the hitherto enigmatic features of spin dynamics, such as the emergence of prethermal plateau in spin-locking experiments, the emergence of superradiance in dipolar systems. The new master equation was used to show optimal behavior in various quantum control experiments. FRQME had been used in quantum optics to show the nonlinear behavior of light shifts and in quantum sensing. FRQME has also been used to explore foundational aspects of quantum mechanics.
In the presentation, the FRQME and some of its applications in wide-ranging areas will be highlighted.

Feedback issues relying on classical controllers (optimizing QND measurement via Markovian feedback, quantum state stabilization via Bayesian feedback) and on quantum controllers (stabilization of Schrödinger cats via autonomous feedback).

We introduce a numerical method to sample the distributions of charge, heat, and entropy production in open quantum systems coupled strongly to macroscopic reservoirs, with both temporal and energy resolution and beyond the linear-response regime. Our method exploits the mesoscopic-leads formulation, where macroscopic reservoirs are modeled by a finite collection of modes that are continuously damped toward thermal equilibrium by an appropriate Gorini-Kossakowski-Sudarshan-Lindblad master equation. Focussing on non-interacting fermionic systems, we access the time-resolved full counting statistics through a trajectory unraveling of the master equation. We show that the integral fluctuation theorems for the total entropy production, as well as the martingale and uncertainty entropy production, hold. Furthermore, we investigate the fluctuations of the dissipated heat in finite-time information erasure. Conceptually, our approach extends the continuous-time trajectory description of quantum stochastic thermodynamics beyond the regime of weak system-environment coupling.

In this work, we develop a panoramic schematic of a quantum thermoelectric circuit theory in the steady state regime. We establish the foundations of the said premise by defining the analogs of Kirchhoff's laws for heat currents and temperature gradients. We further show that our approach encompasses various circuits like thermal diode, transistor, and Wheatstone bridge. Additionally, we have been able to develop a model of a quantum thermal step transformer. We also construct a novel model of a thermal adder circuit, paving the way to develop thermal integrated circuits. This sheds new light on the present architecture of quantum device engineering.

We describe the smallest quantum error correcting (QEC) code to correct for amplitude-damping (AD) noise, namely, a 3-qubit code that corrects up to first order in the damping strength. We generalize this construction to create a family of codes that correct AD noise up to any fixed order. We underpin the fundamental connection between the structure of our codes and the noise structure via a relaxed form of the Knill-Laflamme conditions, that are different from existing formulations of approximate QEC conditions. Although the recovery procedure for this code is non-deterministic, our codes are optimal with respect to overheads and outperform existing codes to tackle AD noise in terms of entanglement fidelity. This alternate formulation of approximate QEC in fact leads us to a new class of quantum codes tailored to AD noise and also gives rise to a noise-adapted quantum Hamming bound for AD noise.

We devise an autonomous quantum thermal machine consisting of three pairwise-interacting qubits, two of which are locally coupled to thermal reservoirs. The machine operates autonomously, as it requires no time-coherent control, external driving or quantum bath engineering, and is instead propelled by a chemical potential bias. Under ideal conditions, we show that this out-of-equilibrium system can deterministically generate a maximally entangled steady-state between two of the qubits, or any desired pure two-qubit entangled state, emerging as a dark state of the system. We study the robustness of entanglement production with respect to several relevant parameters, obtaining nearly-maximally-entangled states well-away from the ideal regime of operation. Furthermore, we show that our machine architecture can be generalised to a configuration with 2n−1 qubits, in which only a potential bias and two-body interactions are sufficient to generate genuine multipartite maximally entangled steady states in the form of a W state of n qubits.

In this talk, we describe recent NMR experiments demonstrating fast quantum state preparation. In particular, we describe counter-diabatic drive, quantum alternating operator ansatz, feedback-assisted quantum control, as well as nonlinear evolutions via ancilla-assisted superposition of unitaries.

Exploring continuous time crystals (CTCs) within the symmetric subspace of spin systems has been a subject of intensive research in recent times. Thus far, the stability of the time-crystal phase outside the symmetric subspace in such spin systems has gone largely unexplored. Here, I present results relating the effect of including the asymmetric subspaces on the dynamics of CTCs in a driven dissipative spin model. This results in multistability, and the dynamics becomes dependent on the initial state. Remarkably, this multistability leads to exotic synchronization regimes such as chimera states and cluster synchronization in an ensemble of coupled identical CTCs. Interestingly, it leads to other nonlinear phenomena such as oscillation death and signature of chaos.

(based on work with coauthors reported in Phys. Rev. Lett. 133, 260403, 2024)