Hi all,
Tomorrow our visitor Mario Berta will tell us about his work on 'Quantum state preparation without coherent arithmetic’ (https://arxiv.org/abs/2210.14892). See below for the abstract. The talk will take place at 2pm in HIT E 41.1 or on Zoom: https://ethz.zoom.us/j/362994444.
Best,
Ladina
Quantum state preparation without coherent arithmetic
We introduce a versatile method for preparing a quantum state whose amplitudes are given by some known function. Unlike existing approaches, our method does not require handcrafted reversible arithmetic circuits, or quantum memory loads, to encode the function values. Instead, we use a template quantum eigenvalue transformation circuit to convert a low cost block encoding of the sine function into the desired function. Our method uses only 4 ancilla qubits (3 if the approximating polynomial has definite parity), providing order-of-magnitude qubit count reductions compared to state-of-the-art approaches, while using a similar number of Toffoli gates if the function can be well represented by a polynomial or Fourier approximation. Like black-box methods, the complexity of our approach depends on the 'L2-norm filling-fraction' of the function. We demonstrate the efficiency of our method for preparing states commonly used in quantum algorithms, such as Gaussian and Kaiser window states.
Hi all,
Tomorrow Sébastien Garmier will tell us about his semester thesis entitled 'The Black Hole Information Loss Paradox in the Context of Analogue Gravity'. See below for the abstract. The talk will take place at 1.30pm in HIT E 41.1 or on Zoom: https://ethz.zoom.us/j/362994444.
Best,
Ladina
Titel:
The Black Hole Information Loss Paradox in the Context of Analogue Gravity
Abstract:
We investigate the possibility of approaching the black hole information loss paradox from the point of view of analogue gravity models. More generally, we ask whether analogue models can at all make inferences about gravity. To this end we give an introduction to black holes, the black hole information loss paradox, and analogue gravity models, before attempting to formulate the information loss paradox in the context of analogue gravity. We find that crucially, the notion of black hole entropy is missing, placing a discussion of the paradox in that context out of reach. Simultaneously, we argue based on the ubiquity and generality of analogue models, that they are unlikely to possess deep connections with gravity.
Hi all,
Tomorrow Tony Metger will tell us about ‘Concentration bounds for quantum states and limitations on the QAOA from polynomial approximations'. See below for the abstract. The talk will take place at 2pm in HIT E 41.1 or on Zoom: https://ethz.zoom.us/j/362994444.
Best,
Ladina
Hi all,
Tomorrow our visitor Patrik Potts will tell us about his work on 'Optical coherent feedback control of a mechanical oscillator'. See below for the abstract. The talk will take place at 2pm in HIT E 41.1 or on Zoom: https://ethz.zoom.us/j/362994444.
Best,
Ladina
Optical coherent feedback control of a mechanical oscillator
We present the theoretical description and experimental realization of an optical coherent feedback platform to control the motional state of a nanomechanical membrane in an optical cavity. The coherent feedback loop consists of a light field interacting twice with a mechanical oscillator in different cavity modes. Tuning the optical phase and delay of the feedback loop allows us to control the motional state of the mechanical oscillator, its frequency shift and damping rate, which we use to cool the membrane close to the ground state. In the optimal cooling conditions, we derive an expression for the minimal number of phonons and show that this new technique enables ground state cooling. Experimentally, we show that we can cool the membrane to a state with 4.89 ± 0.14 phonons (480 μK) in a 20 K environment. This lies below the theoretical limit of dynamical backaction cooling in the unresolved sideband regime. The described feedback scheme is very versatile and could be implemented in various optomechanical systems.
Hi all,
Tomorrow Vilasini will tell us about her work on 'Embedding cyclic causal structures in acyclic spacetimes: no-go results for process matrices'. See below for the abstract. The talk will take place at 2pm in HIT E 41.1 or on Zoom: https://ethz.zoom.us/j/362994444.
Best,
Ladina
%%%%%%%%%
Title: Embedding cyclic causal structures in acyclic spacetimes: no-go results for process matrices
V. Vilasini and Renato Renner, based on https://arxiv.org/abs/2203.11245
Abstract: Causality can be defined in terms of a space-time structure or based on information-theoretic structures, which correspond to different notions of causation. The process matrix framework describes quantum indefinite causal structures in the information-theoretic sense, but the physicality of such processes remains an open question. At the same time, there are several experiments in Minkowski spacetime (which gives a definite spacetime notion of causality) that claim to implement indefinite information-theoretic causal structures, suggesting an apparent tension between these notions. To address this, we develop a general framework that disentangles these two notions and characterises their relationship in scenarios where quantum systems are not necessarily localised in spacetime. Formulating (possibly cyclic) quantum causal structures in terms of a composition of quantum maps through feedback loops, we proceed to describe their embedding in acyclic background spacetimes. Then relativistic causality takes the form of a compatibility condition between the information-theoretic and spacetime causal order relations. Connecting the process matrix formalism to cyclic causal structures with a focus on causal relations that can be inferred operationally through agents’ interventions, we derive a number of no-go results for physical realisations of process matrices in a spacetime. In particular, this reveals that it is impossible to physically realise indefinite causal order processes with spacetime localised systems. Further, we show that any realisation of an indefinite causal order process respecting relativistic causality in a background spacetime ultimately admits a fine-grained description in terms of a definite acyclic information-theoretic causal structure that is consistent with the light-cone structure of the spacetime. This resolves the apparent tension between the two causality notions. Finally, we discuss the operational meaning of indefinite causal structures in light of our results.