Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S46: Continuous Measurements and Quantum FoundationsFocus
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Sponsoring Units: GQI Chair: Philippe Campagne-Ibarcq, CEA-Saclay Room: 393 |
Thursday, March 16, 2017 11:15AM - 11:51AM |
S46.00001: Quantum State Smoothing: what did an open quantum system do when you weren't watching? Invited Speaker: Howard Wiseman For a classical system that is subject to unobserved forces, and monitored imperfectly (i.e. yielding a noisy measurement record) the optimal way to estimate its properties at a certain time $t$ is by smoothing. This means making use of the measurement record obtained both before $t$ and after $t$. Generalizing this idea to the quantum domain is not straightforward. One can restrict to estimating an unknown classical process influencing a quantum state [Tsang, Phys. Rev. Lett 102, 250403 (2009)] or to estimating the unknown result of a measurement at time $t$ [Gammelmark \textit{et al}., Phys. Rev. Lett. 111, 160401 (2013)]. We propose instead quantum state smoothing [Guevara and Wiseman, Phys. Rev. Lett. 115, 180407 (2015)]: we consider a quantum system that is coupled to two environments, observed respectively by Alice and Bob. It is the nature of quantum mechanics that such measurements yield a noisy measurement record, and subject the system to back-action forces. The true state of the system given Alice's and Bob's records is, under some assumptions, pure. But say Alice does not have access to Bob's noisy record -- then she cannot know the true state. We show that she can better estimate the true state of the system at time $t$ using her record in the future of $t$ as well as the past. However, the state she should use to estimate the true state depends on what is meant by the `best' estimate. We show that the `standard' smoothed quantum state introduced by Guevara and Wiseman in 2015 is the one that minimizes the expected trace mean square error with the true state. Maximizing the expected fidelity with the true state, on the other hand, yields a different, `lustrated', smoothed state. [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:03PM |
S46.00002: Arrow of time for repeated and continuous quantum measurement Andrew Jordan, Justin Dressel, Areeya Chantasri, Kater Murch, Alexander Korotkov We will present theoretical results on the statistical arrow of time for a quantum system being monitored by a sequence of measurements. For a continuous qubit measurement example, we demonstrate that time-reversed evolution is always physically possible, provided that the measurement record is also negated. Despite this restoration of dynamical reversibility, a statistical arrow of time emerges, and may be quantified by the log-likelihood difference between forward and backward propagation hypotheses. We then show that such reversibility is a universal feature of non-projective measurements, with forward or backward Janus measurement sequences that are time-reversed inverses of each other. [Preview Abstract] |
Thursday, March 16, 2017 12:03PM - 12:15PM |
S46.00003: Probing quantum contextuality with superconducting circuits Arkady Fedorov, Markus Jerger, Yarema Reshitnyk, Markus Oppliger, Anton Potocnik, Mintu Mondal, Andreas Wallraff, Kenneth Goodenough, Stephanie Wehner, Kristinn Juliusson, Nathan K. Langford Quantum physics cannot be reconciled with the classical philosophy of noncontextual realism. Realism demands that system properties exist independently of whether they are measured, while noncontextuality demands that they do not depend on the order in which different measurements are made. The Bell-Kochen-Specker theorem states that noncontextual realism cannot reproduce the measurement statistics of a single three-level quantum system (qutrit). Noncontextual realistic models may thus be tested using a single qutrit without relying on the notion of quantum entanglement which enables the better-known Bell violations. Using a superconducting qutrit with deterministic, binary-outcome readouts, we violate a noncontextuality inequality while addressing the detection, individual-existence and compatibility loopholes. Demonstrating state-dependent contextuality of a solid-state system is also an important conceptual ingredient for universal quantum computation in surface-code architectures. [Preview Abstract] |
Thursday, March 16, 2017 12:15PM - 12:27PM |
S46.00004: Experimental test of macroscopic realism in a superconducting flux qubit Kosuke Kakuyanagi, George Knee, Mao-Chuang Yeh, Yuichiro Matsuzaki, Hiraku Toida, Hiroshi Yamaguchi, Shiro Saito, Anthony Leggett, William Munro A superconducting flux qubit has been considered a macroscopic quantum system because its energy eigenstates correspond to clockwise and anti-clockwise macroscopic current. In order to test macroscopic realism in a superconducting flux qubit, we can measure the violation of the traditional Leggett-Garg inequality (LGI). The LGI is always satisfied if realism is correct, however it can be violated in systems that do not obey realism, for example microscopic systems (atoms, photons) described by quantum mechanics. To show violation of realism in a quantum system, we used a Josephson bifurcation amplifier (JBA) to read out the quantum state of our system in a fast, but low back-action fashion. We tested macroscopic realism with a simplified (but equivalent) LGI and obtained strong and significant evidence for the superposition of states of nontrivial macroscopic objects. [Preview Abstract] |
Thursday, March 16, 2017 12:27PM - 12:39PM |
S46.00005: What does a continuously monitored qubit readout really show? Justin Dressel, Luis Pedro García-Pintos For continuous measurements of a quantum observable it is widely recognized that the measurement output approximates the expectation value of the observable, hidden by additive white noise. Filtering the measurement readout can thus approximately uncover the dynamics of the expectation value, during a single realization. However, using information from the entire output history yields a different, smoothed, observable estimate. We derive the form of this smoothed estimate and show that the observed readout quantitatively tracks it with higher fidelity than the expectation value during a single realization, making it an objectively meaningful quantity. In the weak measurement limit this smoothed estimate approximates a weak value, with no need for additional postselection. [Preview Abstract] |
Thursday, March 16, 2017 12:39PM - 12:51PM |
S46.00006: State dragging using the quantum Zeno effect Shay Hacohen-Gourgy, Leigh Martin, Luis Pedro García-Pintos, Justin Dressel, Irfan Siddiqi The quantum Zeno effect is the suppression of Hamiltonian evolution by continuous measurement. It arises as a consequence of the quantum back-action pushing the state towards an eigenstate of the measurement operator. Rotating the operator at a rate much slower than the measurement rate will effectively drag the state with it. We use our recently developed scheme, which enables dynamic control of the measurement operator, to demonstrate this dragging effect on a superconducting transmon qubit. Since the system is continuously measured, the deterministic trajectory can be monitored, and quantum jumps can be detected in real-time. Furthermore, we perform this with two observables that are set to be either commuting or non-commuting, demonstrating new quantum dynamics. [Preview Abstract] |
Thursday, March 16, 2017 12:51PM - 1:03PM |
S46.00007: A two-fold quantum delayed-choice experiment in a superconducting circuit Yuan Xu, Ke Liu, Weiting Wang, Shi-Biao Zheng, Tanay Roy, Suman Kundu, Madhavi Chand, A. Ranadive, R. Vijay, Y. P. Song, L.-M. Duan, L. Sun Wave-particle duality is among the most fundamental properties of quantum mechanics. We propose and experimentally demonstrate a two-fold quantum delayed-choice experiment where wave or particle nature of a superconducting interfering device can be post-selected twice after the interferometer. The wave-particle complementarity is controlled by a quantum which-path detector (WPD) in a superposition of its on and off states implemented through a superconducting cavity. The WPD projected to its on state records which-path information, which manifests the particle nature and destroys the interference associated with wave nature of the system. In our experiment, we can recover the interference signal through a quantum eraser even if the WPD has selected out the particle nature in the first round of delayed-choice detection, showing that a quantum WPD adds further unprecedented controllability to test of wave-particle complementarity through the peculiar quantum delayed-choice measurements. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:15PM |
S46.00008: Quantum smoothing for classical mixtures Dian Tan, Mahdi Naghiloo, Klaus M\"olmer, Kater Murch We employ a superconducting qubit embedded in a 3D cavity to study quantum smoothing using projective measurements. Whereas the density matrix $\rho(t)$, which depends on the evolution dynamics and measurements performed prior to time $t$ makes predictions about the outcomes of measurements performed at time $t$, further probing of the qubit allows us to refine our prediction in hindsight. We introduce an auxiliary matrix $E(t)$, which is conditioned on the measurement record from $t$ to a final time $T$. The pair of matrices ($\rho(t)$, $E(t)$) exhaust our ability to make a smoothed prediction for the measurement outcome at an earlier time $t$. If the combined dynamics and measurements on a system lead to $\rho(t)$ with only diagonal elements in a given basis $\{|n\rangle\}$, it may be treated as a classical mixture. If continued probing and dynamics of the system lead to $E(t)$ that is also diagonal in the basis $\{|n\rangle\}$, we examine whether the classical mixture description is still valid in determining the smoothed probabilities for the measurement outcome at time $t$. We show experimentally that the smoothed probabilities do not, in the same way as the diagonal elements of $\rho(t)$, permit a classical mixture interpretation of the state of the system at the time $t$. [Preview Abstract] |
Thursday, March 16, 2017 1:15PM - 1:27PM |
S46.00009: Positive operator valued measurements from destructive weak measurements Todd Brun, Yi-Hsiang Chen Earlier work has shown that any generalized measurement can be performed in principle by a sequence of weak measurements. For certain well-defined families of weak measurements, the families of achievable generalized measurements have also been determined. But this earlier work does not include an important experimental case, where the only available measurements are \textit{destructive}. For example, most practical methods of measuring states of light require the absorption of the light and hence the destruction of the measured system. Such destructive measurements are best described by \textit{positive operator valued measurements} (POVMs). In the current work, we make a first approach to this problem using a simple qubit model, where the only measurements allowed are on the output of an amplitude damping channel. Surprisingly, for this model it is possible to perform any two-outcome POVM on the measured system by a sequence of such lossy weak measurements. We discuss how this approach may be generalized to more complicated systems and what limitations may apply. [Preview Abstract] |
Thursday, March 16, 2017 1:27PM - 1:39PM |
S46.00010: Demonstration of entanglement assisted invariance on IBM's Quantum Experience Sebastian Deffner Quantum entanglement is among the most fundamental, yet from classical intuition also most surprising properties of the fully quantum nature of physical reality. We report several experiments performed on IBM's Quantum Experience demonstrating envariance -- entanglement assisted invariance. Envariance is a recently discovered symmetry of composite quantum systems, which is at the foundational origin of physics and a purely quantum phenomenon. These very easily reproducible and freely accessible experiments on Quantum Experience provide simple tools to study the properties of envariance, and we illustrate this for several cases with ``quantum universes'' consisting of up to five qubits. [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 1:51PM |
S46.00011: Tight Bell Inequalities and Nonlocality in Weak Measurement Mordecai Waegell A general class of Bell inequalities is derived based on strict adherence to probabilistic entanglement correlations observed in nature. This derivation gives significantly tighter bounds on local hidden variable theories for the well-known Clauser-Horne-Shimony-Holt (CHSH) inequality, and also leads to new proofs of the Greenberger-Horne-Zeilinger (GHZ) theorem. This method is applied to weak measurements and reveals nonlocal correlations between the weak value and the post-selection, which rules out various classical models of weak measurement. Implications of these results are discussed. [Preview Abstract] |
Thursday, March 16, 2017 1:51PM - 2:03PM |
S46.00012: Abstract Withdrawn
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Thursday, March 16, 2017 2:03PM - 2:15PM |
S46.00013: Probing quantumness with joint continuous measurements of non-commuting qubit observables Luis Pedro Garcia-Pintos, Justin Dressel In this talk we consider continuous weak measurements as a means to probe foundational issues in quantum mechanics. We consider the simultaneous monitoring of two noncommuting observables---as recently implemented by the Siddiqi group at UC Berkeley. Contrary to naive expectation, the output of such experiment can be used to simultaneously track the approximate observable dynamics. Despite this seeming realism, we also show that the readouts violate macrorealistic Leggett-Garg inequalities for arbitrarily short temporal correlations, and that the derived inequalities are manifestly violated even in the absence of Hamiltonian evolution. Such violations should indicate the failure of at least one postulate of macrorealism: either physical quantities do not have well defined values at all times, or the measurement process itself disturbs what is being measured. Despite this macrorealism violation, we construct a realistic, but epistemically restricted, model that perfectly emulates both the qubit evolution and the observed noisy signals, thus also emulating the violations. [Preview Abstract] |
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