Quantum Computing Seminars take place on Wednesdays at 1:30 pm at CSI (Bldg 725) Training Room, unless otherwise noted. All events are free and open to the public. For questions please contact Layla Hormozi.
DEC
18
Wednesday
CSI Q Seminar
"Probing quantum entanglement at the Electron Ion Collider"
Presented by Dmitri Kharzeev, Stony Brook University and BNL
1:30 pm, Training Room, Bldg 725
Wednesday, December 18, 2019, 1:30 pm
Hosted by: Layla Hormozi
The structure functions measured in deep-inelastic scattering are related to the entropy of entanglement between the region probed by the virtual photon and the rest of the hadron. This opens new possibilities for experimental and theoretical studies using the Electron Ion Collider. The real-time evolution of the final state in deep-inelastic scattering can be addressed with quantum simulations using the duality between high energy QCD and the Heisenberg spin chain.
CSI Q Seminar
"Integrating ballistic graphene with superconducting resonators - A new building block for detectors and quantum circuits"
Presented by Olli Saira, BNL
Wednesday, December 11, 2019, 1:30 pm
Training Room, Bldg 725
Hosted by: Layla Hormozi
I present measurements of a bolometer device based on boron nitride-encapsulated graphene operating at temperatures of 300 mK and below. Our experiment probes the exquisite properties of graphene that make it an appealing material for detector applications. First, the specific heat of electronic excitations in graphene is low, promising excellent sensitivity as a thermal photodetector. Second, at low temperatures, superconductivity can be induced in a localized region within the flake. This enables the integration of graphene with superconducting microwave circuits routinely used in quantum processors and astronomical detector arrays. Our initial results demonstrate the operating principle of a graphene bolometer with resonator-coupled temperature readout. However, we also observed unexpected heat leakage out of the flake in our first-generation device, which prevented it from reaching its full theoretical performance.
CSI Q Seminar
"Quantum supremacy using a programmable superconducting processor"
Presented by Pedram Roushan, Google
Wednesday, December 4, 2019, 1:30 pm
Large Seminar Room, Bldg. 510
Hosted by: Layla Hormozi
The promise of quantum computers is that certain computational tasks might be executed exponentially faster on a quantum processor than on a classical processor1. A fundamental challenge is to build a high-fidelity processor capable of running quantum algorithms in an exponentially large computational space. Here we report the use of a processor with programmable superconducting qubits to create quantum states on 53 qubits, corresponding to a computational state-space of dimension 2^53. Measurements from repeated experiments sample the resulting probability distribution, which we verify using classical simulations. Our Sycamore processor takes about 200 seconds to sample one instance of a quantum circuit a million times—our benchmarks currently indicate that the equivalent task for a state-of-the-art classical supercomputer would take approximately 10,000 years. This dramatic increase in speed compared to all known classical algorithms is an experimental realization of quantum supremacy for this specific computational task, heralding a much-anticipated computing paradigm.
CSI Q Seminar
"Designing Two-Qubit Gates for Exchange-Only Quantum Computation"
Presented by Nick Bonesteel, Florida State University and NHMFL
Wednesday, November 20, 2019, 1:30 pm
Conference room 201, Bldg 734
Hosted by: Layla Hormozi
In exchange-only quantum computation, qubits are encoded using three or more spin-1/2 particles and quantum gates can be performed by switching on and off, or "pulsing", the isotropic exchange interaction between spins. Finding efficient pulse sequences for realizing two-qubit gates in this way is complicated by the large search space in which they live, and has typically involved numerical brute force search. Here I will give a simple analytic derivation of the most efficient known exchange-pulse sequence for carrying out a controlled-NOT gate, originally found numerically by Fong and Wandzura. I will then show how the ideas behind this derivation can be used to analytically find new pulse sequences for two-qubit gates beyond controlled-NOT.
CSI Q Seminar
"Quantum Information: History, Development and Applications"
Presented by Vladimir Korepin, Stony Brook University
Wednesday, November 13, 2019, 11 am
Training Room, Bldg 725
Hosted by: Layla Hormozi
History of quantum information will be mentioned. Followed by comments on modern developments. Current projects of the speaker [spin chains and quantum search] will be briefly described.
CSI Q Seminar
"Many-body physics with atoms and molecules under quantum control"
Presented by Sebastain Will, Columbia University
Thursday, November 7, 2019, 3 pm
Conference room 201, Bldg 734
Hosted by: Layla Hormozi
Over the past decade, quantum simulators based on ultracold atoms have emerged as a powerful tool to address open questions in strongly interacting systems and nonequilibrium quantum dynamics that have relevance in all areas of physics, from strongly correlated materials to cosmology. Today, quantum simulators based on ultracold dipolar molecules are within experimental reach, which exploit long-range dipole-dipole interactions and will give access to new classes of strongly correlated many-body systems. In this talk, I will present our efforts towards quantum simulation with ultracold dipolar molecules. In trailblazing experiments we have demonstrated the creation of ultracold molecules via atom-by-atom assembly, which yields complete control over the molecular degrees of freedom, including electronic, vibrational, rotational, and nuclear spin states. Exploiting this control, we have observed long nuclear spin coherence times in molecular ensembles, which makes ultracold molecules an interesting candidate for the realization of a long-lived quantum memory. In addition, the dipole-dipole interactions between molecules can be flexibly tuned via external electrostatic and microwave fields. This motivates our current work towards two-dimensional systems of strongly interacting molecules, which promises access to novel quantum phases, will enable high-speed simulation of quantum magnetism, and points towards potential quantum computing schemes based on ultracold molecules. In the end, I will briefly present our new project on enhancing quantum coherence by dissipation in programmable atomic arrays. For this effort we will develop a novel nanophotonic platform that will enable trapping of individual atoms in optical tweezer arrays with unprecedented accuracy and high-speed tunability.
CSI Q Seminar
"Characterizing readout in quantum computers: does the reading '0' really mean 0 and '1' really 1?"
Presented by Tzu-Chieh Wei, Stony Brook University
Wednesday, October 30, 2019, 3 pm
Training Room, Bldg 725
Hosted by: Layla Hormozi
Typical quantum computation includes three stages: state initialization, gate operations and readout. There are tomographic tools on quantum state and process tomography, as well as one that is often ignored, i.e. the detector tomography. It is important to characterize the readout in interpreting experiments on quantum computers. We use quantum detector tomography to characterize the qubit readout in terms of measurement POVMs on IBM Quantum Computers (e.g. IBM Q 5 Tenerife and IBM Q 5 Yorktown). Our results suggest that the characterized detector model deviates from the ideal projectors, ranging from 10 to 40 percent. This is mostly dominated by classical errors, evident from the shrinkage of arrows in the corresponding Bloch-vector representations. There are also small deviations that are not `classical', of order 3 percent or less, represented by the tilt of the arrows from the z axis. Further improvement on this characterization can be made by adopting two- or more-qubit detector models instead of independent single-qubit detectors for all the qubits in one device. We also find evidence indicating correlations in the detector behavior, i.e. the detector characterization is slightly altered (to a few percent) when other qubits and their detectors are in operation. Such peculiar behavior is consistent with characterization from the more sophisticated approach of the gate set tomography. Finally, we also discuss how the characterized detectors' POVM, despite deviation from the ideal projectors, can be used to estimate the ideal detection distribution.
CSI Q Seminar
"Universal logical gate sets with constant-depth circuits for topological and hyperbolic quantum codes"
Presented by Guanyu Zhu, IBM T.J. Watson Research Center
Wednesday, October 23, 2019, 3 pm
Conference Room 201, Bldg 734
Hosted by: Layla Hormozi
A fundamental question in the theory of quantum computation is to understand the ultimate space-time resource costs for performing a universal set of logical quantum gates to arbitrary precision. To date, common approaches for implementing a universal logical gate set, such as schemes utilizing magic state distillation, require a substantial space-time overhead. In this work, we show that braids and Dehn twists, which generate the mapping class group of a generic high genus surface and correspond to logical gates on encoded qubits in arbitrary topological codes, can be performed through a constant depth circuit acting on the physical qubits. In particular, the circuit depth is independent of code distance d and system size. The constant depth circuit is composed of a local quantum circuit, which implements a local geometry deformation, and a permutation of qubits. When applied to anyon braiding or Dehn twists in the Fibonacci Turaev-Viro code based on the Levin-Wen model, our results demonstrate that a universal logical gate set can be implemented on encoded qubits in O(1) time through a constant depth unitary quantum circuit, and without increasing the asymptotic scaling of the space overhead. Our results for Dehn twists can be extended to the context of hyperbolic Turaev-Viro codes as well, which have constant space overhead (constant rate encoding). This implies the possibility of achieving a space-time overhead of O(d/log d), which is optimal to date. From a conceptual perspective, our results reveal a deep connection between the geometry of quantum many-body states and the complexity of quantum circuits. References: arXiv:1806.06078,arXiv:1806.02358, Quantum 3, 180 (2019) (arXiv:1901.11029).
CSI Q Seminar
"Quantum simulation of quantum field theory on the light front"
Presented by Peter Love, Tufts University and BNL
Tuesday, October 15, 2019, 12 pm
Training Room, Bldg 725
Hosted by: Layla Hormozi
Quantum simulation proposes to use future quantum computers to calculate properties of quantum systems. The simulation of quantum field theories by any means is a challenge, and quantum algorithms for problems in fundamental physics are a natural target for quantum computation. We will show that the light front formulation of quantum field theory is particularly useful in this regard. We analyze a simple theory in 1 + 1D and show how computation of quantities of interest in this theory is analogous to quantum algorithms for chemistry that we understand in detail.