Bulletin of the American Physical Society
48th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 62, Number 8
Monday–Friday, June 5–9, 2017; Sacramento, California
Session P8: Focus Session: Quantum Error CorrectionFocus
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Chair: Roee Ozeri, Weizmann Institute of Science Room: 314 |
Thursday, June 8, 2017 2:00PM - 2:30PM |
P8.00001: Fault-tolerant encoding of a logical qubit Invited Speaker: Norbert M. Linke The discovery of quantum error correction codes gave credibility to the idea of scaling up quantum computers to large sizes [1]. Showing that all elements of error correction can be realized in a fault-tolerant way is therefore of fundamental interest. Fault tolerance removes the assumption of perfect encoding and decoding operations of logical qubits. We present the implementation of the [[4,2,2]] code, an error detection protocol [2] which uses four physical qubits to encode two logical qubits, one of which can be made fault-tolerant by appropriate construction of the encoding and stabilizer circuits [3]. Remarkably, it works with a bare ancilla qubit. \newline The results demonstrate for the first time the robustness of a fault-tolerant qubit to imperfections in the very operations used to encode it, as errors are suppressed by an order of magnitude below the physical error probability. We present data to show that this advantage over a non-fault-tolerant qubit persists even with large added error rates and experimental calibration errors [4]. \newline The experiment is performed on a programmable quantum computer comprised of five trapped $^{171}$Yb$^+$ ions. It provides a fully connected system of atomic clock qubits with long coherence times and high gate fidelities that can be programmed to execute arbitrary quantum circuits [5]. \newline \newline [1] A. R. Calderbank and Peter W. Shor, PRA 54 (1996). \newline [2] M. Grassl, Th. Beth, and T. Pellizzari, PRA 56 (1997). \newline [3] D. Gottesman, arXiv 1610.03507 (2016). \newline [4] N. M. Linke, M. Gutierrez, K. A. Landsman, C. Figgatt, S. Debnath, K. R. Brown, C. Monroe, arXiv 1611.06946 (2016). \newline [5] S. Debnath, N. M. Linke, C. Figgatt, K. A. Landsman, K. Wright, and C. Monroe, Nature 536 (2016). [Preview Abstract] |
Thursday, June 8, 2017 2:30PM - 3:00PM |
P8.00002: Validating quantum systems in the presence of errors Invited Speaker: J. M. Taylor Experimental groups around the world are developing complex quantum systems for quantum simulation and eventually computation. These systems will hopefully soon exceed our ability to classically simulate or reasonable understand their dynamics. I will discuss efforts to develop testing tools for such systems to confirm various aspects of their performance in the presence of errors, disorder, and noise. The overall formalism focuses on using quantum communication concepts to validate performance, and I will show how it can extend to a variety of quantum systems. [Preview Abstract] |
Thursday, June 8, 2017 3:00PM - 3:12PM |
P8.00003: Autonomous Quantum Error Correction with Application to Quantum Metrology Florentin Reiter, Anders S. Sorensen, Peter Zoller, Christine A. Muschik We present a quantum error correction scheme that stabilizes a qubit by coupling it to an engineered environment which protects it against spin- or phase flips. Our scheme uses always-on couplings that run continuously in time and operates in a fully autonomous fashion without the need to perform measurements or feedback operations on the system. The correction of errors takes place entirely at the microscopic level through a build-in feedback mechanism. Our dissipative error correction scheme can be implemented in a system of trapped ions and can be used for improving high precision sensing. We show that the enhanced coherence time that results from the coupling to the engineered environment translates into a significantly enhanced precision for measuring weak fields. In a broader context, this work constitutes a stepping stone towards the paradigm of self-correcting quantum information processing. [Preview Abstract] |
Thursday, June 8, 2017 3:12PM - 3:24PM |
P8.00004: Delayed Quantum Feedback Hannes Pichler We study the dynamics of photonic quantum circuits consisting of nodes coupled by quantum channels. We are interested in the regime where the time delay in communication between the nodes is significant. This includes the problem of quantum feedback, where a quantum signal is fed back on a system with a time delay. We develop a tensor network state approach to solve the quantum stochastic Schr\"odinger equation with time delays, which accounts in an efficient way for the entanglement of nodes with the stream of emitted photons in the waveguide, and thus the non-Markovian character of the dynamics. [Preview Abstract] |
Thursday, June 8, 2017 3:24PM - 3:36PM |
P8.00005: A Quantum Non-Demolition Parity measurement in a mixed-species trapped-ion quantum processor Matteo Marinelli, Vlad Negnevitsky, Hsiang-Yu Lo, Christa Flühmann, Karan Mehta, Jonathan Home Quantum non-demolition measurements of multi-qubit systems are an important tool in quantum information processing, in particular for syndrome extraction in quantum error correction. We have recently demonstrated a protocol for quantum non-demolition measurement of the parity of two beryllium ions by detection of a co-trapped calcium ion. The measurement requires a sequence of quantum gates between the three ions, using mixed-species gates between beryllium hyperfine qubits and a calcium optical qubit. Our work takes place in a multi-zone segmented trap setup in which we have demonstrated high fidelity control of both species and multi-well ion shuttling. The advantage of using two species of ion is that we can individually manipulate and read out the state of each ion species without disturbing the internal state of the other. The methods demonstrated here can be used for quantum error correcting codes as well as quantum metrology and are key ingredients for realizing a hybrid universal quantum computer based on trapped ions. Mixed-species control may also enable the investigation of new avenues in quantum simulation and quantum state control. [Preview Abstract] |
Thursday, June 8, 2017 3:36PM - 3:48PM |
P8.00006: Theory of real-time feedback on oscillating qubits using weak measurement Hermann Uys, Pieter du Toit, Shaun Burd, Humairah Bassa, Thomas Konrad We review our recent work on state estimation and feedback control of single quantum systems based on weak measurement. We discuss two classes of feedback protocols used to control qubit oscillations. The first relies on standard proportional-integral-differential control while the second comprises unitary operations aimed at reversing the phase kicks due to measurement back-action. Analytical expressions for the convergence of state estimation fidelity are also obtained in the continuous measurement limit, by evaluating the fidelity change in an incremental step of the estimation protocol. [Preview Abstract] |
Thursday, June 8, 2017 3:48PM - 4:00PM |
P8.00007: A state comparison amplifier with feed forward state correction Luca Mazzarella, Ross Donaldson, Robert Collins, Ugo Zanforlin, Gerald Buller, John Jeffers The Quantum State Comparison AMPlifier (SCAMP) is a probabilistic amplifier that works for known sets of coherent states. The input state is mixed with a guess state at a beam splitter and one of the output ports is coupled to a detector. The other output contains the amplified state, which is accepted on the condition that no counts are recorded. The system uses only classical resources and has been shown to achieve high gain and repetition rate. However the output fidelity is not high enough for most quantum communication purposes. Here we show how the success probability and fidelity are enhanced by repeated comparison stages, conditioning later state choices on the outcomes of earlier detections. A detector firing at an early stage means that a guess is wrong. This knowledge allows us to correct the state perfectly. The system requires fast-switching between different input states, but still requires only classical resources. Figures of merit compare favourably with other schemes, most notably the probability-fidelity product is higher than for unambiguous state discrimination. Due to its simplicity, the system is a candidate to counteract quantum signal degradation in a lossy fibre or as a quantum receiver to improve the key rate of continuous variable quantum communication. [Preview Abstract] |
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