Quantum theory of open systems

Physics 1 Mr. Bigler Conservation of Momentum Unit: Momentum MA Curriculum Frameworks (2016): HS-PS2-2 MA Curriculum Frameworks (2006): 2.5 Mastery Objective(s): (Students will be able to...)  Solve problems involving collisions in which momentum is conserved, with or without an external impulse. Success Criteria:  Masses and velocities are correctly identified for each object, both before and after the collision.  Variables are correctly identified and substituted correctly into the correct part of the equation.  Algebra is correct and rounding to appropriate number of significant figures is reasonable. Tier 2 Vocabulary: momentum, collision Language Objectives:  Explain what happens before, during, and after a collision from the point of view of one of the objects participating in the collision.

Physics I.1

Nanomechanical oscillators are at the heart of ultrasensitive detectors of force, mass and motion. As these detectors progress to even better sensitivity, they will encounter measurement limits imposed by the laws of quantum mechanics. If the imprecision of a measurement of the displacement of an oscillator is pushed below a scale set by the standard quantum limit, the measurement must perturb the motion of the oscillator by an amount larger than that scale. Here we show a displacement measurement with an imprecision below the standard quantum limit scale. We achieve this imprecision by measuring the motion of a nanomechanical oscillator with a nearly shot-noise limited microwave interferometer. As the interferometer is naturally operated at cryogenic temperatures, the thermal motion of the oscillator is minimized, yielding an excellent force detector with a sensitivity of 0.51 aN Hz(-1/2). This measurement is a critical step towards observing quantum behaviour in a mechanical object.

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Nanomechanical motion measured with an imprecision below the standard quantum limit

We demonstrate a decoherence-free quantum memory of one qubit. By encoding the qubit into the decoherence-free subspace (DFS) of a pair of trapped 9Be+ ions, we protect the qubit from environment-induced dephasing that limits the storage time of a qubit composed of a single ion. We measured the storage time under ambient conditions and under interaction with an engineered noisy environment and observed that encoding into the DFS increases the storage time by up to an order of magnitude. The encoding reversibly transfers an arbitrary qubit stored in a single ion to the DFS of two ions.

A Decoherence-Free Quantum Memory Using Trapped Ions

The control of quantum systems is of fundamental scientific interest and promises powerful applications and technologies. Impressive progress has been achieved in isolating quantum systems from the environment and coherently controlling their dynamics, as demonstrated by the creation and manipulation of entanglement in various physical systems. However, for open quantum systems, engineering the dynamics of many particles by a controlled coupling to an environment remains largely unexplored. Here we realize an experimental toolbox for simulating an open quantum system with up to five quantum bits (qubits). Using a quantum computing architecture with trapped ions, we combine multi-qubit gates with optical pumping to implement coherent operations and dissipative processes. We illustrate our ability to engineer the open-system dynamics through the dissipative preparation of entangled states, the simulation of coherent many-body spin interactions, and the quantum non-demolition measurement of multi-qubit observables. By adding controlled dissipation to coherent operations, this work offers novel prospects for open-system quantum simulation and computation.

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An Open-System Quantum Simulator with Trapped Ions

In the emerging field of cavity optomagnonics, photons are coupled coherently to magnons in solid-state systems. These new systems are promising for implementing hybrid quantum technologies. Being able to prepare Fock states in such platforms is an essential step towards the implementation of quantum information schemes. We propose a magnon-heralding protocol to generate a magnon Fock state by detecting an optical cavity photon. Due to the peculiarities of the optomagnonic coupling, the protocol involves two distinct cavity photon modes. Solving the quantum Langevin equations of the coupled system, we show that the temporal scale of the heralding is governed by the magnon-photon cooperativity and derive the requirements for generating high fidelity magnon Fock states. We show that the nonclassical character of the heralded state, which is imprinted in the autocorrelation of an optical ``read'' mode, is only limited by the magnon lifetime for small enough temperatures. We address the detrimental effects of nonvacuum initial states, showing that high fidelity Fock states can be achieved by actively cooling the system prior to the protocol.

Magnon heralding in cavity optomagnonics

A large fraction of quantum science and technology requires low-temperature environments such as those afforded by dilution refrigerators. In these cryogenic environments, accurate thermometry can be difficult to implement, expensive, and often requires calibration to an external reference. Here, we theoretically propose a primary thermometer based on measurement of a hybrid system consisting of phonons coupled via a magnetostrictive interaction to magnons. Thermometry is based on a cross-correlation measurement in which the spectrum of back-action driven motion is used to scale the thermomechanical motion, providing a direct measurement of the phonon temperature independent of experimental parameters. Combined with a simple low-temperature compatible microwave cavity readout, this primary thermometer is expected to become a promising alternative for thermometry below 1 K.

Magnon-Phonon Quantum Correlation Thermometry

Generating nonclassical states in macroscopic systems is a long-standing challenge. A promising platform in the context of this quest are novel hybrid systems based on magnetic dielectrics, where photons can couple strongly and coherently to magnetic excitations, although a nonclassical state therein is yet to be observed. We propose a scheme to generate a magnetization cat state, i.e., a quantum superposition of two distinct magnetization directions, using a conventional setup of a macroscopic ferromagnet in a microwave cavity. Our scheme uses the ground state of an ellipsoid shaped magnet, which displays anisotropic quantum fluctuations akin to a squeezed vacuum. The magnetization collapses to a cat state by either a single photon or a parity measurement of the microwave cavity state. We find that a cat state with two components separated by $\ensuremath{\sim}5\ensuremath{\hbar}$ is feasible and briefly discuss potential experimental setups that can achieve it.

Spin cat states in ferromagnetic insulators

New experiments reveal all the ways that light impacts mechanics in cavity magnomechanics, where photons, phonons, and magnons couple in a versatile platform for exploring proposals quantum applications.

Dynamical Backaction Magnomechanics

This work introduces a multimode cavity optomagnonic system based on antiferromagnetic insulators, where optical photons couple coherently to the system's magnon modes. The resulting dynamics is tunable by an external magnetic field and shows unusual features due to cavity-induced magnon-magnon interactions.

Victor A. S. V. Bittencourt

Papers

Magnon heralding in cavity optomagnonics

Magnon-Phonon Quantum Correlation Thermometry

Spin cat states in ferromagnetic insulators

Dynamical Backaction Magnomechanics

Antiferromagnetic cavity optomagnonics