SESSION 9b: Error mitigation and correction (Alvarado E)

Chair: (Jim Harrington)
3:45pm - 4:15pmBrandon Ruzic, Sandia National Laboratories
Characterizing errors in entangled-atom interferometry
Abstract. Recent progress in generating entanglement between neutral atoms provides opportunities to advance quantum sensing technology. In particular, entanglement can enhance the performance of accelerometers and gravimeters based on light-pulse atom interferometry. We study the effects of error sources that may limit the sensitivity of such devices, including errors in the preparation of the initial entangled state, spread of the initial atomic wave packet, and imperfections in the laser pulses. Based on the performed analysis, entanglement-enhanced atom interferometry appears to be feasible with existing experimental capabilities.
4:15pm - 4:45pmBibek Pokharel, University of Southern California
Demonstration of fidelity improvement using dynamical decoupling with superconducting qubits
Abstract. Quantum computers must be able to function in the presence of decoherence. The simplest strategy for decoherence reduction is dynamical decoupling (DD), which requires no encoding overhead and works by converting quantum gates into decoupling pulses. Here, using the IBM and Rigetti platforms, we demonstrate that the DD method is suitable for implementation in today’s relatively noisy and small-scale cloud-based quantum computers. Using DD, we achieve substantial fidelity gains relative to unprotected, free evolution of individual superconducting transmon qubits. To a lesser degree, DD is also capable of protecting entangled two-qubit states. We show that dephasing and spontaneous emission errors are dominant in these systems, and that different DD sequences are capable of mitigating both effects. Unlike previous work demonstrating the use of quantum error correcting codes on the same platforms, we make no use of postselection and hence report unconditional fidelity improvements against natural decoherence.
4:45pm - 5:15pmZhang Jiang, Google
Quantum simulation of fermions: geometric locality and error mitigation
Abstract. We consider mappings from fermionic systems to spin systems that preserve geometric locality in more than one spatial dimension. They are useful to simulating lattice fermionic systems on a quantum computer, e.g., the Hubbard model. Locality-preserving mappings avoid the large overhead associated with the nonlocal parity terms in conventional mappings, such as the Jordan-Wigner transformation. As a result, they often provide solutions with much lower circuit depths. Here, we construct locality-preserving mappings that can also detect/correct single-qubit errors without introducing extra physical qubits beyond those required by the original mappings. We discuss error mitigation strategies based on these encodings for quantum algorithms such as the variational quantum eigensolver.
5:15pm - 5:45pmVictor V. Albert, California Institute of Technology
Characterizing and developing bosonic error-correcting codes
Abstract. Continuous-variable or bosonic quantum information processing is a field concerned with using one or more harmonic oscillators to protect, manipulate, and transport quantum information. The large oscillator Hilbert space provides alternative encodings that are currently outperforming encodings into registers of many qubits: break-even error correction has been achieved with the bosonic cat codes but not yet with a many-qubit system. However, an analysis of theoretical capabilities of bosonic codes is missing. We have undertaken a program identifying (1) Which codes are able to protect against dominant noise in realistic bosonic systems? (2) Why those codes perform so well? and (3) How to extend codes to multiple modes advantageously? We provide answers to all these questions. First, we calculate the error-correction conditions of single-mode codes, showing that Gottesman-Kitaev-Preskill (GKP) codes offer the best performance. Second, we prove that GKP codes achieve the quantum capacity (up to a constant offset) of the thermal loss channel. Third, we present a multimode extension of the cat codes that increases both experimental feasibility and theoretically achievable performance.
5:45pm - 6:15pmSepehr Nezami, Stanford University
Continuous symmetries and approximate quantum error correction
Abstract. Quantum error correction and symmetries are relevant to many areas of physics, including many-body systems, holographic quantum gravity, and reference-frame error-correction. Here, we determine that any code is fundamentally limited in its ability to approximately error-correct against erasures at known locations if it is covariant with respect to a continuous local symmetry. Our bound vanishes either in the limit of large individual subsystems, or in the limit of a large number of subsystems. In either case, we provide examples of codes that approximately achieve the scaling of our bound: an infinite-dimensional rotor extension of the three-qutrit secret-sharing code, an infinite-dimensional five-rotor perfect code, and a many-body Dicke-state code. Furthermore, we prove an approximate version of the Eastin-Knill theorem that puts a severe quantitative limit on a code’s ability to correct erasure errors if it admits a universal set of transversal logical gates. This bound goes to zero only inversely in the logarithm of the local physical subsystem dimension. We provide examples of codes circumventing the Eastin-Knill theorem: random unitary covariant codes, many-body generalized W-state code, and families of codes whose transversal gates form a general group G. In the context of the AdS/CFT correspondence, our approach provides insight into how time evolution in the bulk corresponds to time evolution on the boundary without violating the Eastin-Knill theorem.

SQuInT Chief Organizer
Akimasa Miyake, Associate Professor

SQuInT Local Organizers
Rafael Alexander, Postdoctoral Fellow
Chris Jackson, Postdoctoral Fellow

SQuInT Administrator
Gloria Cordova
505 277-1850

SQuInT Assistant
Wendy Jay

SQuInT Founder
Ivan Deutsch, Regents' Professor, CQuIC Director

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