Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session C51: Quantum Measurement and Feedback |
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Sponsoring Units: GQI Chair: Rusko Ruskov, Laboratory for Physical Sciences Room: 398 |
Monday, March 13, 2017 2:30PM - 2:42PM |
C51.00001: Entanglement measurements with propagating two-mode squeezed microwave states S. Pogorzalek, K. G. Fedorov, P. Yard, P. Eder, M. Fischer, J. Goetz, E. Xie, A. Marx, F. Deppe, R. Gross Josephson parametric amplifiers (JPAs) can be employed for the generation of itinerant quantum signals in the form of propagating two-mode squeezed states (TMSSs), which are essential for quantum communication protocols [1]. Further applications of TMSSs include quantum information processing with continuous variables [2], or novel ideas of building quantum annealing networks based on JPAs [3]. All these fields make use of multiple JPAs for entanglement generation and manipulation, and therefore, require detailed knowledge of its physical properties. In our experiments, we employ two flux driven JPAs at the inputs of an entangling hybrid ring in order to generate two mode squeezing between the hybrid ring outputs. We perform tomography of the resulting TMSSs and experimentally investigate robustness of the entanglement to noise and finite-time delays. [1] R. Di Candia \textit{et al.}, EPJ Quan. Tech. \textbf{2}, 25 (2015). [2] J. Yoshikawa \textit{et al.}, Phys. Rev. Lett. \textbf{25}, 250501 (2008). [3] Puri \textit{et al.}, arXiv:1609.07117 (2016). [Preview Abstract] |
Monday, March 13, 2017 2:42PM - 2:54PM |
C51.00002: Nonclassical photon number distribution in a superconducting cavity under a squeezed drive Shingo Kono, Yuta Masuyama, Toyofumi Ishikawa, Yutaka Tabuchi, Rekishu Yamazaki, Koji Usami, Kazuki Koshino, Yasunobu Nakamura A superconducting qubit in the strong dispersive regime of a circuit QED is known to be a powerful probe for photon states in a cavity. In this regime, a qubit spectrum is split into multiple peaks, with each peak corresponding to the individual photon number in a cavity. Here, we measure qubit spectra in the cavity that is driven externally and continuously with various microwave states. We use a thermal state, a coherent state, and a squeezed vacuum state as the cavity drive field and observe that the peak weights in the qubit spectra reflect the characteristics of each photon number distribution. By fitting the qubit spectra with a model which takes into account the finite probe power, the photon number distributions are determined dissimilarly from the apparent peak weights. When a squeezed vacuum state generated by a Josephson parametric amplifier is injected into the cavity, the photon number distribution fulfills Klyshko's criterion for the nonclassicality. [Preview Abstract] |
Monday, March 13, 2017 2:54PM - 3:06PM |
C51.00003: Quantum entanglement and dynamical Lamb effect for two superconducting qubits in a nonstationary cavity Oleg Berman, Roman Kezerashvili, Yurii Lozovik We consider the realistic physical realization to observe the quantum entanglement and the dynamical Lamb effect (DLE) for two artificial atoms, formed by superconducting qubits connected with superconducting line, which plays a role of an optical cavity, with varying boundary conditions. The DLE is a novel effect of nonstationary cavity quantum electrodynamics, which is photonless, parametric excitation of an atom, embedded in a nonstationary cavity, by shaking its photonic coat due to nonadiabatic change of the boundary conditions for virtual photons. The quantum entanglement and the probability of the DLE were evaluated for two qubits, coupled to the superconducting line, caused by nonadiabatic fast change of the boundary conditions. The quantum entanglement under consideration is not caused by interaction between two qubits, but due to change of boundary conditions of the cavity. As a measure of the dynamical quantum entanglement, the conditional concurrence of two qubits for each fixed number of created photons in a nonstationary cavity is derived and analyzed. The DLE and quantum entanglement of two qubits due to the change of cavity boundary conditions offer a new possibility of control of qubits. [Preview Abstract] |
Monday, March 13, 2017 3:06PM - 3:18PM |
C51.00004: Adaptive phase estimation with two-mode squeezed-vacuum and parity measurement Zixin Huang, Keith R. Motes, Petr M. Anisimov, Jonathan P. Dowling, Dominic W. Berry A proposed phase-estimation protocol based on measuring the parity of a two-mode squeezed-vacuum state at the output of a Mach-Zehnder interferometer shows that the Cramer-Rao sensitivity is sub-Heisenberg [Phys.\ Rev.\ Lett.\ 104, 103602 (2010)]. However, these measurements are problematic, making it unclear if this sensitivity can be obtained with a finite number of measurements. This sensitivity is only for phase near zero, and in this region there is a problem with ambiguity because measurements cannot distinguish the sign of the phase. Here, we consider a finite number of parity measurements, and show that an adaptive technique gives a highly accurate phase estimate regardless of the phase. We show that the Heisenberg limit is reachable, where the number of trials needed for mean photon number $\bar{n}=1$ is approximately one hundred. We show that the Cramer-Rao sensitivity can be achieved approximately, and the estimation is unambiguous in the interval ($-\pi/2, \pi/2$). [Preview Abstract] |
Monday, March 13, 2017 3:18PM - 3:30PM |
C51.00005: Efficient continuous-variable state tomography using Padua points Olivier Landon-Cardinal, Luke C.G. Govia, Aashish A. Clerk Further development of quantum technologies calls for efficient characterization methods for quantum systems. While recent work has focused on discrete systems of qubits, much remains to be done for continuous-variable systems such as a microwave mode in a cavity. We introduce a novel technique to reconstruct the full Husimi Q or Wigner function from measurements done at the Padua points in phase space, the optimal sampling points for interpolation in 2D. Our technique not only reduces the number of experimental measurements, but remarkably, also allows for the direct estimation of any density matrix element in the Fock basis, including off-diagonal elements. [Preview Abstract] |
Monday, March 13, 2017 3:30PM - 3:42PM |
C51.00006: Quantum Jumps and Photon Counting Statistics in Waveguide QED Xin Zhang, Harold Baranger Using a novel quantum jump approach, we explore counting statistics for photons in a waveguide scattering from strongly coupled qubits. The system consists of one or two qubits coupled to a one-dimensional waveguide with a coherent state as input. Bunching/antibunching of the photons after interacting with the qubits is typically characterized by the second-order correlation function, neglecting higher-order correlations. Here, we adapt the quantum jump approach to study the full counting statistics of the photons. Our approach takes into account the interference of input photons with photons emitted by the qubits, thereby allowing each jump to be identified as an individual photon (not qubit emission). We present the waiting time distribution of photon arrivals and joint distribution of adjacent waiting times for both systems, noting differences and power dependence. Compared to the second-order correlation function, the waiting time distribution gives a more accurate and clearer view of bunching/antibunching. [Preview Abstract] |
Monday, March 13, 2017 3:42PM - 3:54PM |
C51.00007: Linear feedback stabilization for a continuously monitored qubit Taylor Patti, Areeya Chantasri, Justin Dressel, Andrew Jordan We explore continuous measurement-based quantum state stabilization through linear feedback control for a single quantum bit. We consider a continuous measurement of the $\sigma_z$ observable of the qubit. By applying a time-varying Rabi drive that includes a linear feedback term, we show that the fixed points of the continuous measurement may be relocated. Numerical simulations are used to characterize the stability of the set of possible fixed points, as well as their modified collapse time-scales. We include the effects of realistic experimental non-idealities, such as environmental energy relaxation, dephasing, time-delay, and inefficient measurement. [Preview Abstract] |
Monday, March 13, 2017 3:54PM - 4:06PM |
C51.00008: Unitarity, Feedback, Interactions -- Dynamics Emergent from Repeated Measurements. Paulina Corona Ugalde, Natacha Altamirano, Robert Mann, Magdalena Zych Modern measurement theory dispenses with the description of a measurement as a projection. Rather, the measurement is understood as an operation, whereby the system's final state is determined by an action of a completely positive trace non-increasing map and the outcomes are described by linear operators on the system, distributed according to a positive-operator valued measure (POVM). The POVM approach unifies the theory of measurements with a general description of dynamics, the theory of open quantum systems. Engineering a particular measurement and engineering a particular dynamics for the system are thus two complementary aspects of the same conceptual framework. This correspondence is directly applied in quantum simulations and quantum control theory . With this motivation, we study what types of dynamics can emerge from a model of repeated short interactions of a system with a set of ancillae. We show that contingent on the model parameters the resulting dynamics ranges from exact unitarity to arbitrary fast decoherence. For a series of measurements the effective dynamics includes feedback-control, which for a composite system yields effective interactions between the subsystems. We quantify the amount of decoherence accompanying such induced interactions. The simple framework used in the present study can find applications in devising novel quantum control protocols, or quantum simulations. [Preview Abstract] |
Monday, March 13, 2017 4:06PM - 4:18PM |
C51.00009: Generation of non-classical states of a harmonic oscillator by measurement using a two-level system Mehmet Canturk, Adrian Lupascu |
Monday, March 13, 2017 4:18PM - 4:30PM |
C51.00010: Abstract Withdrawn Coherence and entanglement are the fundamental quantumness features of single and multipartite systems, respectively. Generally, measuring the coherence or entanglement of an unknown state requires accurate state tomography, which is experimentally challenging and impractical for large dimensional states. On the other hand, the purity of a state can be easily obtained independent of its dimension by interfering two copies of the state. Given such purity values, we show that the amount of quantumness, i.e., the relative entropy of coherence and the coherent information of entanglement, can be lower bounded analytically. Furthermore, we simulate our result for randomly chosen states. We show that even only with the purity measurement, the lower bound estimation of quantumness works well especially for states that are mixtures of an arbitrary pure state and white noise. We show numerically that the bound is rather good even in the worst case for systems sizes up to 4 X 4 dimensions. |
Monday, March 13, 2017 4:30PM - 4:42PM |
C51.00011: Optimal feedback scheme for Hamiltonian parameter estimation Haidong Yuan Measurement and estimation of parameters are essential for science and engineering, where the main quest is to find out the highest achievable precision with given resources and design schemes to attain it. Two schemes, the sequential feedback scheme and the parallel scheme, are usually studied in quantum parameter estimation. We show that the sequential feedback scheme has a 3-fold improvement over the parallel scheme for Hamiltonian parameter estimations on 2-dimensional systems, and an order of $O(d+1)$ improvement for Hamiltonian parameter estimation on $d-$dimensional systems. We also show that, contrary to the conventional belief, it is possible to simultaneously achieve the highest precision for estimating all three components of a magnetic field, which sets a benchmark on the local precision limit for the estimation of a magnetic field. [Preview Abstract] |
Monday, March 13, 2017 4:42PM - 4:54PM |
C51.00012: Active measurement-based quantum feedback for preparing and stabilizing superpositions of two cavity photon number states Yves Berube-Lauziere The measurement-based quantum feedback scheme developed and implemented by Haroche and collaborators [Dotsenko et al., Phys. Rev. A 80, 013805 (2009) and Sayrin et al., Nature 477, 73-77 (2011)] to actively prepare and stabilize specific photon number states in cavity quantum electrodynamics (CQED) is a milestone achievement in the active protection of quantum states from decoherence. This feat was achieved by injecting, after each weak dispersive measurement of the cavity state via Rydberg atoms serving as cavity sensors, a low average number classical field (coherent state) to steer the cavity towards the targeted number state. This talk will present the generalization of the theory developed for targeting number states in order to prepare and stabilize desired superpositions of two cavity photon number states. Results from realistic simulations taking into account decoherence and imperfections in a CQED set-up will be presented. These demonstrate the validity of the generalized theory and points to the experimental feasibility of preparing and stabilizing such superpositions. This is a further step towards the active protection of more complex quantum states than number states. This work, cast in the context of CQED, is also almost readily applicable to circuit QED. [Preview Abstract] |
Monday, March 13, 2017 4:54PM - 5:06PM |
C51.00013: Efficient generation of arbitrary Fock-state superpositions in a superconducting cavity Wang Weiting, Hu Ling, Xu Yuan, Liu Ke, Ma Yuwei, Zheng Shibiao, Vijay R, Song Yipu, Duan Luming, Sun Luyan In this talk, I will discuss our experimental demonstration of an efficient method to generate arbitrary Fock-state superpositions in a superconducting quantum circuit, where a qubit is dispersively coupled to a microwave cavity mode without the need of fine-frequency tuning. Compared with the previous multi-step state-synthesis schemes, our method requires only a single step of unitary evolution and measurement-based post-selection, and thus is more robust to noise and accumulation of experimental errors. Using the method, we experimentally generate high-fidelity phase eigenstates under various Hilbert-space dimensions and squeezed states, which are useful for quantum walk and high-precision measurements. [Preview Abstract] |
Monday, March 13, 2017 5:06PM - 5:18PM |
C51.00014: Measuring the phase of a single photon using continuous adaptive measurements Leigh Martin, Shay Hacohen-Gourgy, William Livingston, Emmanuel Flurin, Howard Wiseman, Irfan Siddiqi Although phase is an essential property of quantum systems, it does not correspond to an observable in the sense of a standard projective measurement. Nevertheless, there exists a generally accepted canonical phase measurement using the more general formalism of POVMs. A scheme to implement such a measurement in the context of quantum optics has been known for some time [1], but has yet to be demonstrated experimentally. We present progress toward performing a canonical phase measurement on a microwave field which contains a superposition of the vacuum and one photon state. Quantum mechanics necessitates that measurement disturbs the wave function, so it is imperative that one acquire information only about the phase, and not the conjugate variable, amplitude. By operating a Josephson parametric amplifier in phase sensitive mode, we apply quantum feedback to adapt the amplification phase continuously as the photon impinges upon it. This allows us to choose the measurement axis to only acquire information about the relative phase between the zero- and one-photon states. Our work presents a tool for optical communication and highlights an important capability afforded by continuous measurement and quantum trajectories. [Preview Abstract] |
Monday, March 13, 2017 5:18PM - 5:30PM |
C51.00015: Approximating continuous coupler distributions on devices with limited precision Zheng Zhu, Andrew J. Ochoa, Helmut G. Katzgraber Special purpose computers, such as the D-Wave 2X quantum annealer or the FPGA-based Janus Computer, are typically restricted by memory constraints, limited precision, or analog noise. This means that the study of problems with interactions drawn from continuous distributions can be difficult. Here we extend the approach introduced by Leuzzi {\em et al.}~[Phys.~Rev.~Lett.~103, 267201 (2009)] to approximate a continuous Gaussian distribution by using quadratures. Our approach allows us to approximate any continuous distribution using only a few discrete weights. From a classical point of view, this reduces the simulation's memory footprint of continuous problems drastically, as well as the simulation time, because multiple quantities and expensive operations, such as exponentials, can be precomputed and tabulated. For quantum annealing architectures this means that problems that require continuous distributions can be encoded within the restrictions of finite precision and analog noise on these devices. Advantages and disadvantages of this approach to simulations are discussed. [Preview Abstract] |
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