
chair: Michael Gullans

15:00  15:30

Nicole Yunger Halpern
(Harvard University)
Whence hybrid circuits? Motivation from quantum cognition
Circuits formed from random unitaries and projective measurements have blossomed into a subfield of quantum information (QI). Whence came these circuits? This talk concerns one of the motivations: According to conventional wisdom, quantum phenomena cannot affection cognition much. Fisher postulated a mechanism by which entanglement might enhance coordinated neural firing: Qubits would manifest as the nuclear spins of phosphorus atoms. These nuclei might resist decoherence for days in Posner molecules. Experimental tests of this proposal have begun. If the proposal holds water, how adroitly could Posner systems process QI, and what QI theory can nature motivate? This work established a framework for answering, translating Fisher’s biochemistry into a quantumcomputational model and circuit diagrams. These circuits, motivated by biochemistry, are amongst the earliest formed from random unitaries and projective measurements.
References
1) Yunger Halpern and Crosson, Ann. Phys. 407, 92147 (2019).
https://www.sciencedirect.com/science/article/pii/S0003491618303014
2) Bene Watts, Yunger Halpern, and Harrow, arXiv:1911.09122 (2019).
https://arxiv.org/abs/1911.09122

15:30  16:00

Crystal Noel
(Joint Quantum Institute)
Error correction and dynamics of monitored systems on a trappedion quantum computer
In a quantum error correcting code, entanglement survives in subsystems of the code despite the fact that the system is monitored via stabilizer measurements. In these monitored systems, a phase transition exists between the survival of longrange entanglement in the system, and unrecoverable entanglement with the environment due to a high rate of measurement. We experimentally study both error correction and the dynamics of measurement induced critically using a trappedion universal quantum processor. Our approach takes advantage of individual optical addressing to achieve operations on a long chain of 171Yb+ ions, resulting in one of the largest academic generalpurpose quantum computers. In this talk, we present recent results of implementing a faulttolerant error correcting code, the BaconShor [[9,1,3]] code. We also describe the protocol to use this computer to realize a purification phase transition in a monitored nonequilibrium manybody quantum system. We present preliminary results showing excellent agreement between simulations and experiments on small system sizes.
* This work is supported by the ARO with funding from the IARPA LogiQ program, the NSF STAQ program, the DOE BES and HEP programs, the AFOSR MURI on Quantum Measurement and Verification, and the AFOSR MURI on Interactive Quantum Computation and Communication Protocols.
Authors: Crystal Noel, Pradeep Niroula, Laird Egan, Daiwei Zhu, Debopriyo Biswas, Andrew Risinger, Michael Gullans, David Huse, Marko Cetina, and Chris Monroe

16:00  16:30

Liang Jiang
(University of Chicago)
Efficient classical simulation of noisy random quantum circuits in one dimension
Understanding the computational power of noisy intermediatescale quantum devices is of both fundamental and practical importance to quantum information science. Here, we address the question of whether erroruncorrected noisy quantum computers can provide computational advantage over classical computers. We simulate the realtime dynamics of 1D noisy random quantum circuits via matrix product operators (MPOs) and characterize the computational power of the 1D noisy quantum system by using a metric we call MPO entanglement entropy. We numerically demonstrate that for the twoqubit gate error rates we considered, there exists a characteristic system size above which adding more qubits does not bring about an exponential growth of the cost of classical MPO simulation of 1D noisy systems. The maximum achievable MPO entanglement entropy is bounded by a constant that depends only on the gate error rate, not on the system size.

16:30  17:00

break / discussion with speakers


chair: Justin Wilson

17:00  17:30

Sagar Vijay
(University of California, Santa Barbara)
Measurementdriven entanglement transition in alltoall quantum dynamics
We identify a phase transition in nonlocal but fewbody, monitored quantum dynamics, in which the separability of the steadystate changes as the rate of local projective measurements is tuned. In one phase, a fraction of the system belongs to a fullyentangled state, one for which no subsystem is in a pure state, while in the second phase, the steadystate resembles a product state over extensively many subsystems. The two phases are sharply distinguished by the extent to which local measurements alone can increase the mutual information of disjoint subsystems. We access this “separability” phase transition in a family of solvable quantum circuit dynamics, from which we find a relation to a meanfield percolation transition, and characterize the differences between these entangled steadystates and a random Page state. We argue that this transition coincides with a change in the computational hardness of classically determining the output probability distribution for the steadystate in a certain basis of product states.

17:30  18:00

Timothy Hsieh
(Perimeter Institute)
Measurement protected quantum phases
I will discuss a class of hybrid quantum circuits, with random unitaries and projective measurements, which host longrange order in the area law entanglement phase. Our primary example is circuits with unitaries respecting a global Ising symmetry and two competing types of measurements. The phase diagram has an area law phase with spin glass order, which undergoes a direct transition to a paramagnetic phase with volume law entanglement, as well as a critical regime. Using mutual information diagnostics, we find that such entanglement transitions preserving a global symmetry are in new universality classes. We analyze generalizations of such hybrid circuits to higher dimensions, which allow for coexistence of order and volume law entanglement, as well as topological order without any symmetry restrictions.

18:00  18:30

break / discussion with speakers

18:30  20:00

poster session I
