Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session D33: Monitored Quantum Dynamics and Random Circuits |
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Sponsoring Units: DCMP Chair: Oliver Hart, University of Colorado, Boulder Room: Room 225 |
Monday, March 6, 2023 3:00PM - 3:12PM |
D33.00001: Anomalous purification dynamics in measurement-only quantum circuits Kai Klocke, Michael Buchhold Symmetries play a central role in quantum dynamics and critical phenomena. In measurement-only circuits, the subtle interplay of symmetry and measurement frustration can give rise to an anomalous dynamical regime in the purification of mixed states. Moreover, the exponent z* found from scaling-collapse in this regime is different from that found in the stationary state and in pure-state dynamics, indicating a transient critical scaling regime. By considering generalized cluster models and random stabilizer circuits in one and two dimensions, we give a systematic analysis of the role of symmetry, measurement-frustration, and geometric frustration in determining the existence of the transient scaling regime and the magnitude of z*. Finally, we explore the relationship between the exponent z* and the spread of correlations in real-space via the propagation of mutual information through the circuit. |
Monday, March 6, 2023 3:12PM - 3:24PM |
D33.00002: Free Bosons Purify Faster than Free Fermions in 0D Asadullah Bhuiyan, Jasmine Mo, Chao-Ming Jian In quantum many-body systems subject to random unitary gates and local measurements, the purification dynamics |
Monday, March 6, 2023 3:24PM - 3:36PM |
D33.00003: Majorana random walks with braiding Kyle Kawagoe, Brian J Skinner, Ojas Deshpande The field of statistical mechanics has a long history of studying random walks. Usually, the random walkers are endowed with classical properties, such as their diffusion rate or some aggregation or annihilation rules based on the identity of the walker. A natural generalization of such a system is to allow some of these properties to be quantum mechanical in nature. In this talk, we will present a model of Majorana fermions which are allowed to perform classical random walks in 1+1D, but have local rules for interactions which account for the highly non-local nature of Majorana fermions. In particular, we will present exact results which demonstrate that the interplay of braiding and pairwise annihilation processes of Majorana fermions gives rise to a robust universality class of non-equilibrium, semi-classical systems. |
Monday, March 6, 2023 3:36PM - 3:48PM Author not Attending |
D33.00004: Effects of interactions in free-fermion hybrid entanglement transitions Joseph W Merritt, Lukasz Fidkowski Quantum hybrid dynamics, involving unitary gates and projective measurements, have been shown in a wide array of cases to exhibit a measurement-induced entanglement transition. These have been shown numerically to appear in bosonic and fermionic systems. While bosonic systems generally display an area-law to volume-law transition, a system with free fermion dynamics will tend to show an area-law to logarithmic-law transition, where entropy increases as the logarithm of the system size. In this work, we simulate a system where the unitary evolution consists mostly of free fermion unitary gates, but interspersed with interacting unitary gates, in an attempt to investigate a transition between logarithmic-law and volume-law phases. |
Monday, March 6, 2023 3:48PM - 4:00PM |
D33.00005: Universal properties of the measurement-induced phase transition with U(1) symmetry Ahana Chakraborty, Aidan Zabalo, Justin H Wilson, Kun Chen, Jed Pixley Recently measurement-induced phase transitions have been studied in non-unitary quantum circuit evolving with a U(1) conserved charge. With increasing rate of measurements, the circuit exhibits a new type of charge-sharpening phase transition followed by the entanglement transition from a volume law to an area law phase. In this talk, we present a numerical study of the critical behavior of the entanglement transition and find it is described by a new universality class that is distinct from both the percolation transition and the Haar random circuit without a conservation law. We provide convincing numerical evidence based on computing the mutual information between two locally coupled Ancilla qubits in the same global charge sector and we estimate the bulk critical exponent. Further, emergent Lorentz invariance at the transition allows us to probe the properties of the underlying (1+1)d conformal field theory via its effective central charge (ceff) and the leading scaling dimensions of the operators using a numerical transfer matrix method. Our numerical analysis predicts that both the bulk critical exponent and ceff have much larger values than those of the percolation or random Haar circuit and thus uncover distinct signatures of global constraints in the dynamics on the measurement-induced criticality. |
Monday, March 6, 2023 4:00PM - 4:12PM |
D33.00006: Measurement-induced criticality in monitored quantum circuits with U(1) symmetry Hisanori Oshima, Yohei Fuji The entanglement entropy shows a phase transition from a volume-law phase to an area-law phase, called measurement-induced entanglement transition, when a pure quantum state evolved under unitary dynamics is subject to local projective measurements. It is intensively studied due to emergence of conformal invariance at the phase transition, which is rarely found in non-equilibrium systems. In this talk, I will discuss critical phenomena in unitary-projective hybrid quantum circuits with U(1) symmetry (charge conservation). Numerical results show that, in addition to the conventional entanglement transition, a new transition obeying the Tomonaga-Luttinger liquid theory appears. The latter transition is characterized by critical behaviors in a subsystem charge fluctuation, a charge correlator, and the entanglement resolved by conserved charges. I also show that a correlator of subsystem charges can be used to locate this new transition point. |
Monday, March 6, 2023 4:12PM - 4:24PM |
D33.00007: Measurement-Protected Order in Monitored Quantum Circuits with Continuous Symmetry Jacob Hauser, Matthew A Fisher, Yaodong Li Monitored quantum circuits, composed of local unitary operators and projective measurements, have emerged as a rich setting for studying non-equilibrium quantum dynamics. In these systems, measurements can protect various monitored steady state phases with area-law entanglement, including phases which can host measurement-protected Ising spin-glass order. To begin exploring whether this measurement-protected order is a generic phenomenon or is reliant on the discrete Ising symmetry, we introduce a circuit model with continuous symmetry where unitary feedback is used to generate local order. In one dimension, we find that long-range order arises in the steady state of our model but that this order is fragile to symmetry-respecting perturbations. Additionally, we find that the continuous symmetry leads to diffusive dynamics that are otherwise not generally present. |
Monday, March 6, 2023 4:24PM - 4:36PM |
D33.00008: Universality of charge transport: an approach to SEP in random unitary circuits Ewan R McCulloch, Romain Vasseur, Sarang Gopalakrishnan, Jacopo de Nardis We investigate the full counting statistics (FCS) of charge transport in U(1) conserving random unitary circuits. We consider an initial mixed state prepared with different chemical potentials in the left and right half of the system and study the FCS of the charge transferred across the central bond, finding a hydrodynamic approach to the counting statistics of a symmetric simple exclusion process (SEP). This approach is seen in TEBD simulations with an effective statistical mechanics model. To understand this approach analytically, we propose an effective stochastic model for the computation of higher order cumulants. |
Monday, March 6, 2023 4:36PM - 4:48PM |
D33.00009: Observing the effects of measurements in many-body quantum systems without post-selection Samuel J Garratt How do measurements disentangle complex quantum states? If we measure many degrees of freedom there is a fundamental barrier, the so-called 'post-selection problem', to answering this question in experiment: it is extremely unlikely that we will ever observe the same post-measurement state twice. We show how, through a synthesis of experiment and simulation, resource-efficient probes for the effects of measurement can nevertheless be constructed. These probes are cross-correlations between results obtained from experimental quantum systems and from classical simulations. As an application, we discuss how these quantum-classical observables can be used to verify the dramatic effects that measurements have on critical ground states [arXiv:2207.09476]. |
Monday, March 6, 2023 4:48PM - 5:00PM |
D33.00010: Entanglement negativity transition with measurement and feedforward Alireza Seif, Yuxin Wang, Ramis Movassagh, Aashish A Clerk In monitored quantum systems, where the dynamics consist of both measurements and unitary time evolution, the state of the system conditioned on measurement outcomes, also known as a trajectory, can be highly entangled. However, this entanglement is often obscured when we look at the unconditional state averaged over measurement outcomes. In this work, we show that the entanglement in trajectories can be revived in the unconditional state using local operations and classical communications. We uncover a sharp transition in the entanglement negativity of the unconditional state as a function of the number of measurement and feedforward channels acting on the system. The unconditional dynamics can also be viewed as the evolution of the system interacting with an environment, and the entanglement transition can be interpreted as a classical-to-quantum transition of the environment. We use tools from random matrix theory together with numerical simulations to shed light on the mechanism of this transition. Finally, we discuss an experimental protocol for observing this transition in engineered quantum devices. |
Monday, March 6, 2023 5:00PM - 5:12PM |
D33.00011: Crystalline Quantum Circuits Grace M Sommers, David A Huse, Michael J Gullans Random quantum circuits continue to inspire a wide range of applications in quantum information science, while remaining analytically tractable through probabilistic methods. Motivated by the need for deterministic circuits with similar applications, we construct classes of nonrandom unitary Clifford circuits by imposing translation invariance in both time and space. Further imposing dual-unitarity, our circuits effectively become crystalline lattices whose vertices are SWAP or iSWAP cores and whose edges are decorated with single-qubit gates. Working on the square and kagome lattice, one can further impose invariance under (subgroups of) the crystal's point group. We use the formalism of Clifford quantum cellular automata to describe operator spreading, entanglement generation, and recurrence times of these circuits. A full classification on the square lattice reveals, of particular interest, a "non-fractal good scrambling class" with dense operator spreading that generates codes with linear contiguous code distance and high performance under erasure errors at the end of the circuit. We also break unitarity by adding spacetime-translation-invariant measurements and find a class of circuits with fractal dynamics. |
Monday, March 6, 2023 5:12PM - 5:24PM |
D33.00012: Measurement-induced phase transitions on dynamical quantum trees Xiaozhou Feng, Brian J Skinner, Adam Nahum Monitored many-body systems fall broadly into two dynamical phases, ``entangling'' or ``disentangling'', separated by a transition as a function of the rate at which measurements are made on the system. Producing an analytical theory of this measurement-induced transition is an outstanding challenge. So far, however, there are no exact solutions for dynamics of qubits with ``real'' measurements, whose outcome probabilities are sampled according to the Born rule. Here we define dynamical processes for qubits, with real measurements, that have a tree-like spacetime interaction graph. It yields an exactly solvable measurement transition. We explore these processes analytically and numerically, exploiting the recursive structure of the tree. Our model exhibits a transition at a nontrivial value of the measurement strength and an exponential scaling of the entanglement near the transition. On the basis of our results we propose a protocol for realizing a measurement phase transition experimentally via an expansion process. |
Monday, March 6, 2023 5:24PM - 5:36PM |
D33.00013: Entanglement Generation and Information Spreading in Weakly Monitored Quantum Circuits Shengqi Sang, Zhi Li, Timothy Hsieh, Beni Yoshida We study the non-equilibrium dynamics in (1+1)D weakly monitored quantum circuits, focusing on the entanglement generation and information spreading. Due to the non-local nature of projective measurements, entanglement dynamics in monitored circuits is "faster" than the unitary ones in several ways. Specifically, we find that a pair of well-separated regions can build up nontrivial entanglement in a timescale l2/3, sub-linear in their distance l; and initially local information can spread super-ballistically as t. By viewing the dynamics as a dynamical error correcting code, we find the code distance grows sub-linearly as t1/2 until saturation; and the input state's local information contained in size l is lost at a timescale l2. Some notions we developed to quantify information dynamics apply to more general monitored quantum processes and are of their own interest. |
Monday, March 6, 2023 5:36PM - 5:48PM |
D33.00014: Entanglement structure in the volume-law phase of hybrid quantum automaton circuits Yiqiu Han, Xiao Chen We study entanglement fluctuations and quantum error correction in the weakly-monitored volume-law phase of quantum automaton circuits subject to repeated local measurements. We numerically observe that the entanglement entropy exhibits strong fluctuation belonging to the Kardar-Parisi-Zhang (KPZ) universality class, the same as other local random circuits studied previously. We also investigate the dynamically generated quantum error correction code in the purification process and show that this model has different contiguous code distances for two types of errors. We give an interpretation of these results by mapping them to various quantities in a classical particle model. Finally, we show that this classical particle dynamics itself has a type of error correction ability, and can dynamically generate a classical linear code. |
Monday, March 6, 2023 5:48PM - 6:00PM |
D33.00015: Measurement-induced phase transitions in quantum teleportation Alexey Milekhin, Fedor K Popov We demonstrate that some quantum teleportation protocols exhibit measurement-induced phase transitions in Sachdev--Ye--Kitaev model. Namely, Kitaev--Yoshida and Gao--Jafferis--Wall protocols have a phase transition if we apply them at a large projection rate or at a large coupling rate respectively. It is well-known that at small rates they allow teleportation to happen only within a small time-window. We show that for large rates, the teleportation can be performed at any moment. These two regimes are separated by a phase transition. In order to analyze Kitaev--Yoshida case, we argue that certain projections can be approximated by low-energy quantum channels, which do not heat the system but qualitatively behave like a real projection. An unusual feature of this measurement-induced phase transition is that it can be diagnosed by an observable linear in the density matrix. |
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