Scientific Series

The past decade has witnessed tremendous advancements in the size, coherence and control accuracy of qubits across various material platforms. In the case of superconducting qubits fabricated from Josephson junctions, quantum systems with over 50 qubits and >99% gate fidelities can now be reliably fabricated. In this talk, I will introduce the basics of superconducting qubits, with a particular focus on the implementation and calibration of two-qubit gates. I will then describe an experiment demonstrating quantum computational advantage on a specific task, namely sampling from random quantum circuits comprising 53 qubits [1]. The success of this experiment hinges on the chaotic dynamics underlying the random circuits, which generates sufficiently large entanglement to challenge classical simulation within the coherence times of the qubits. To systematically study the physics of quantum chaos, we perform a second experiment which investigates the “fingerprints” of chaos on local quantum observables [2]. We demonstrate how the two fundamental mechanisms of quantum chaos, operator spreading and operator entanglement, may be diagnosed by reversing the flow of time and measuring the so-called out-of-time-order correlators (OTOCs). Additionally, we find that with proper error-mitigation strategies, even local quantum observables such as OTOCs may be measured up to accuracies requiring non-trivial classical computational resources to simulate. These works pave a critical path toward using quantum computers for scientific discovery in the NISQ era.

[1] Google Quantum AI, Nature 574, 505 (2019).

[2] X. Mi et al., Science 374, 1479 (2021).

Zoom Link: https://pitp.zoom.us/j/93446337538?pwd=UXpsUFZ4M0tDSDd6bnA0VFBsazNPQT09

 It is unknown how the Einstein Equivalence Principle (EEP) should be modified to account for quantum features. A possibility introduced in arXiv:2012.13754 is that the EEP holds in a generalized form for particles having an arbitrary quantum state. The core of this proposal is the ability to transform to a Quantum Reference Frame (QRF) associated to an arbitrary quantum state of a physical system, in which the metric is locally inertial. I will show that this extended EEP, initially formulated in terms of the local expression of the metric field in a QRF, can be verified in an interferometric setup via tests on the proper time of entangled clocks (arXiv:2112.03303). Moreover, the same setup can be analyzed with quantum sensing techniques (arXiv:2204.03006): I will talk about how gravitational time dilation may be used as a resource in quantum information theory, showing that it may enhance the precision in estimating the gravitational acceleration for long interferometric times.

Three dimensional conformal field theories(CFTs) describe important critical physics in the real world. In the past few years much progress has been made in 3D CFTs using the conformal bootstrap. In particular, using numerical bootstrap, the critical exponents of the 3D Ising, Super-Ising, O(2), O(3) CFTs have been determined precisely with rigorous error bars, while using analytic bootstrap, the information of the leading twist operators at large spins can be computed analytically. The two bootstrap approaches are sensitive to different regions of the spectrum and complement each other. In this talk, I will first give a general review of the numerical bootstrap and analytic bootstrap. Then I will discuss how to combine the numerical bootstrap with the analytic lightcone bootstrap, i.e. a hybrid bootstrap method. As a result, the bootstrap prediction for the actual CFT can be significantly improved.

Zoom Link: https://pitp.zoom.us/j/94186668943?pwd=ejRRb0M4VXNjeVVCWGdnTHQyaWRyQT09

The current theories of quantum physics and general relativity on their own do not allow us to study situations in which spacetime is in a quantum superposition. In this talk, I propose a general strategy to determine the dynamics of objects on an indefinite spacetime metric, using an extended notion of quantum reference frame transformations. First, we study the situation of the gravitational source mass being in a spatial superposition state and, using a generalized principle of covariance, show how to transform to a frame in which the standard theories of GR and QFT allow to determine the dynamics. In the second part, we consider superpositions of conformally equivalent metrics inhabited by a massive quantized Klein-Gordon field. By requiring invariance of the KG equation under quantum conformal transformations, we find that the superposition is transferred to the quantum field in the form of an effective, spacetime dependent mass term. Overall, the proposed strategy allows to construct the respective explicit quantum frame change operators, and to study physical phenomena such as time dilation and cosmological particle production in different quantum frames.

Zoom Link: https://pitp.zoom.us/j/96903859307?pwd=aEtLUy9tME5GL25nTjBVNXVmb2N3Zz09

There are two different standard ways of endowing a physical theory with a symplectic structure: the canonical and the covariant. The former is derived from the well-known symplectic structure of a certain cotangent bundle. The latter is based on the variational calculus. Including a boundary in the canonical formalism poses no problem, however, in the covariant formalism things break apart. In this talk, I will briefly introduce both formalisms without boundary and explain in detail a new framework that allows us to include boundaries in a straightforward way. To show it in action, time permitting, I will apply it to several theories of gravity. Finally, I will briefly show a new result where we proved that, actually, the canonical and covariant formalisms are equivalent in full generality.

Zoom Link: https://pitp.zoom.us/j/97641588140?pwd=dFl0SFdHOG5BYko2djNVWk11UkhSUT09

Inspired by the corpuscular model of gravity, I will discuss a simplified quantum description of neutral and electrically charged spherically symmetric black holes given by coherent states of toy gravitons. From such states I will derive effective geometries devoid of curvature singularities (replaced by integrable ones) and Cauchy horizons. Lastly, I will present a similar analysis for the Schwarzschild-de Sitter geometry, used as a model for the late Universe, showing that the “reaction” of the de Sitter background to the presence of localised matter sources induces a modified Newtonian dynamics at galactic scales and different values measured for the present Hubble parameter.

Zoom Link: https://pitp.zoom.us/j/93087108672?pwd=ekQrLysrcjRwVWFSZ2J5bndteXBtdz09

Mirror symmetry and time-invariance violating processes were crucial at establishing the Standard Model of fundamental interactions as we know it, their tiny effects eventually measured after enormous experimental effort. Macroscopic material on the other hand can maximally break these symmetries spontaneously. Inside these materials, Dark Matter can exhibit phenomena otherwise impossible in the vacuum. In this talk, I will discuss such a phenomenon that we call the piezoaxionic effect: In crystals whose structure breaks mirror symmetry, which are broadly known as piezoelectric, dark matter candidates called axions will produce a change in the crystals size, i.e. strain. Such a strain can be detected with well-established methods leading to a new observable for this well-motivated class of Dark Matter candidates.

Zoom Link: https://pitp.zoom.us/j/96153495346?pwd=QU1MTENUTzdNOU5sZVc3VzY4NHI1UT09

While quantum computers are naturally well-suited to implementing linear operations, it is less clear how to implement nonlinear operations on quantum computers. However, nonlinear subroutines may prove key to a range of applications of quantum computing from solving nonlinear equations to data processing and quantum machine learning. Here we develop algorithms for implementing nonlinear transformations of input quantum states. Our algorithms are framed around the concept of a weighted state, a mathematical entity describing the output of an operational procedure involving both quantum circuits and classical post-processing.

Zoom Link: https://pitp.zoom.us/j/92831825506?pwd=T2VUQ2M2QlZERmRmUHZ0T1VOelkzZz09

Holographic complexity proposals are interesting because, on one hand, they express universal properties of black hole interiors and, on the other, they go beyond the area-centric view on quantum gravity. Our best take on complexity in quantum field theory is based on assigning a cost to circuits generated by time-dependent Hamiltonians and its minimization. I will discuss recent progress on understanding how the relevant circuits are realized in holography and what are possible gravity duals to good costs. Based on 2112.12158, 2203.08842 and work in progress.

Zoom Link: https://pitp.zoom.us/j/96487059058?pwd=ZHk1ZHpXMEZoT0UycUZqTEdIb1ZpQT09

In this talk, I will present MicroBooNE's first results probing the nature of the excess of low-energy interactions observed by the MiniBooNE collaboration. The talk will cover all four electron neutrino analyses that comprise MicroBooNE's first results, with a particular focus on the exclusive search for CCQE-like electron neutrino interactions containing one electron and one proton in the final state (1e1p). The result of the 1e1p analysis, along with results from the other electron analyses and the single-photon analysis, will be discussed in terms of their implications for the MiniBooNE excess. Specifically, I will examine the viability of the single sterile neutrino explanation of the MiniBooNE excess in light of the MicroBooNE electron neutrino results. Additionally, I will introduce the Coherent CAPTAIN-Mills (CCM) experiment at Los Alamos National Laboratory. CCM has unique sensitivity to a number of exotic BSM models, including leptophobic vector-portal dark matter and axion-like particles, some of which may be able to explain the MiniBooNE anomaly.

Zoom Link: https://pitp.zoom.us/j/93290406752?pwd=ajIvZkhvVU5YMW90WjVpU3IySUphdz09

Considering the scale dependent effective spacetimes implied by the functional renormalization group in d-dimensional Quantum Einstein Gravity, we discuss the representation of entire evolution histories by means of a single, (d+1)-dimensional manifold furnished with a fixed (pseudo-) Riemannian structure.

We propose a universal form of the higher dimensional metric and discuss its properties. We show that, under precise conditions, this metric is always Ricci flat; if the evolving spacetimes are maximally symmetric, their (d+1)-dimensional representative has a vanishing Riemann tensor even. The non-degeneracy of the higher dimensional metric is linked to a monotonicity requirement for the running of the cosmological constant, which we test in the case of Asymptotic Safety.

Furthermore, we allow the higher dimensional manifold to be an arbitrary Einstein space, admitting the possibility that the spacetimes to be embedded have a Lorentzian signature, a prime example being a stack of de Sitter spaces. We “derive” the (A)dS/CFT correspondence by applying the gravitational Effective Average Action approach, by solving the corresponding functional RG and the effective Einstein equations, and finally embedding the 4D metrics into the one single 5-dimensional one. It is an intriguing possibility that in this way one might find a specific solution to the general equations which coincides with the 5D kinematic setting which forms the basis of the conjectured (A)dS/CFT correspondence.

Zoom Link: https://pitp.zoom.us/j/92268060878?pwd=M2E1S1RxcHBFbzBiblhpSUJCMWIzUT09