Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U4: Invited Session: Quantum Reservoir Engineering and Feedback |
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Sponsoring Units: DCMP GQI Chair: Steven Girvin, Yale University Room: Ballroom IV |
Thursday, March 21, 2013 11:15AM - 11:51AM |
U4.00001: Cavity-assisted quantum bath engineering Invited Speaker: Kater Murch In practice, quantum systems are never completely isolated, but instead interact with degrees of freedom in the surrounding environment, eventually leading to decoherence. Precision measurement techniques such as nuclear magnetic resonance and interferometry, as well as envisioned quantum schemes for computation, simulation, and data encryption, rely on the ability to prepare and preserve delicate quantum superpositions and entanglement. The conventional route to long-lived quantum coherence involves minimizing coupling to a dissipative bath. Paradoxically, it is possible to instead engineer specific couplings to a quantum environment that allow dissipation to actually preserve coherence. I will discuss our recent demonstration of quantum bath engineering for a superconducting qubit coupled to a microwave cavity. By tailoring the spectrum of microwave photon shot noise in the cavity, we create a dissipative environment that autonomously relaxes the qubit to an arbitrarily specified coherent superposition of the ground and excited states. In the presence of background thermal excitations, this mechanism increases the state purity and effectively cools the dressed atom state to a low temperature. We envision that future multi-qubit implementations could enable the preparation of entangled many-body states suitable for quantum simulation and computation. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:27PM |
U4.00002: Quantum measurement in action Invited Speaker: Michael Hatridge A quantum system subject to the infinitely-strong measurement of textbook physics undergoes a discontinuous, random state collapse. However, in practice, measurements often involve a finite-strength, continuous process whose iteration leads to a projective evolution only asymptotically. Moreover, if the observation apparatus is fully efficient informationally, the measured system can remain at all times in a pure state. The stochastic evolution of this pure state is trackable from the measurement record. Thus, an initial superposition of states can be usefully transformed by a partial measurement rather than be entirely destroyed. This striking property has been demonstrated in superconducting qubit experiments in which readout is performed by a microwave signal sent through a cavity dispersively coupled to the qubit, and thereafter processed by an amplifier operating at the quantum limit. Such accurate monitoring of a qubit state is an essential prerequisite for measurement-based feedback control of quantum systems. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 1:03PM |
U4.00003: Quantum feedback control in superconducting qubits: Towards creating and stabilizing entanglement in remote qubits Invited Speaker: Rajamani Vijayaraghavan Recent advances in superconducting parametric amplifiers have enabled quantum limited measurements of superconducting qubits, ushering in a new era of measurement based control using quantum feedback. Quantum entanglement is a key aspect of the measurement process. Measurement creates a pointer state which is entangled with the system being measured. Typically, one analyzes the pointer state which in turn determines the state of the original system. I will discuss experiments where we entangle the state of a 3D transmon qubit with a coherent microwave field (the pointer) using the circuit QED architecture. The use of parametric amplifiers to analyze the microwave field enables us to actually observe this entanglement and the resulting strong correlations between the states of the pointer and the qubit. We reconstruct quantum trajectories of the qubit state as it evolves during measurement and show that the final state of the qubit is consistent with the trajectories. Further, we use quantum feedback to actively steer the state of the qubit and demonstrate Rabi oscillations which persist indefinitely [1]. Finally, I will discuss how we can use the pointer states to generate entanglement between remote qubits and stabilize them using feedback. Applications to quantum computing and quantum error correction will also be discussed.\\[4pt] [1] R.Vijay et al., Nature 490, 77-80 (2012) [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:39PM |
U4.00004: Quantum Feedback for preparing and stabilizing photon number states of a field stored in a cavity Invited Speaker: Michel Brune The stabilization of complex classical systems requires feedback. A sensor performs measurements of the system's state whose result is fed into a controller, which decides on an action bringing the system closer to a target state. Operating feedback for preparing and stabilizing against decoherence a quantum state is a promising tool for quantum control. It is however much more demanding than its classical counterpart, since a quantum measurement by the sensor changes the measured state. We present the first continuous operation of a closed feedback-loop for preparing and stabilizing photon number states of a microwave field stored in a high Q superconducting cavity. The field is probed by non-resonant Rydberg atoms performing a Quantum Non-Demolition photon counting. The feedback action consists either in the injection of a small coherent field pulse with a controlled amplitude and phase or in the emission and absorption of single photons with individual resonant atoms. The atomic measurement results are fed into a real-time controller, which performs an estimation of the field's state before deciding on the actuator action bringing it closer to the target. We stabilize number states up to 7. We discuss perspectives for the stabilization of mesoscopic quantum superpositions. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 2:15PM |
U4.00005: Experimental quantum error correction with trapped ions Invited Speaker: Philipp Schindler The computational potential of a quantum information processor can only be utilized if errors occurring during a quantum computation can be controlled and corrected for. Quantum error correction protocols encode the quantum information of a single qubit in a larger register. Errors are then corrected by a quantum-feedback algorithm that is applied repeatedly. We encode a single logical qubit into three physical qubits and perform multiple rounds of error correction with the aid of high-fidelity gate operations and a reset technique for the auxiliary qubits. Furthermore we demonstrate that the same technique can be used to undo a quantum measurement. Full quantum error correction schemes are able to correct for arbitrary errors and enable universal quantum computation, but they require a significant overhead in the number of qubits. This prevents them to be useful for medium-scale systems used for quantum simulation. Therefore, we develop a quantum feedback scheme to reduce the dominant errors in an open-system quantum simulator. Our scheme requires only a single auxiliary qubit regardless of the system size. [Preview Abstract] |
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