Journal Article

Generation of Fock states in a superconducting quantum circuit

Weig, E. M., Bialczak, Radoslaw C., Cleland, A. N., O’Connell, A. D., Martinis, John M., Lucero, Erik, Neeley, M., Wang, H., Ansmann, M., Hofheinz, Max

2008

Overview

Cavity quantum electrodynamics: Fock states represent quantum purity In cavity quantum electrodynamics (QED), light–matter interactions between a single emitter (an atom or an atom-like system with discrete energy levels) and a resonant optical cavity are investigated at a fundamental level. Recent advances in solid-state implementations, which offer great design flexibility, have given this field considerable momentum. An outstanding important question has been which features in such a system show true quantum behaviour and cannot be explained with classical models. Hofheinz et al . study a 'circuit' QED system where a superconducting qubit acts as an atom-like two-energy level system and is embedded in a microwave transmission circuit, acting as the optical cavity. They demonstrate in this system the creation of pure quantum states, known as Fock states, which give specific numbers of energy quanta, in this case photons. Fock states with up to six photons are prepared and analysed. The results are important because cavity QED is expected to play a crucial role in the development of quantum information processing and communication applications. A 'circuit' quantum electrodynamics system where a superconducting qubit acts as an atom-like two-energy level system and is embedded in a microwave transmission circuit (acting as the optical cavity) is studied. In this system, it is demonstrated that the creation of pure quantum states, known as Fock states, which give specific numbers of energy quanta, in this case photons. Fock states with up to six photons are prepared and analysed. Spin systems and harmonic oscillators comprise two archetypes in quantum mechanics 1 . The spin-1/2 system, with two quantum energy levels, is essentially the most nonlinear system found in nature, whereas the harmonic oscillator represents the most linear, with an infinite number of evenly spaced quantum levels. A significant difference between these systems is that a two-level spin can be prepared in an arbitrary quantum state using classical excitations, whereas classical excitations applied to an oscillator generate a coherent state, nearly indistinguishable from a classical state 2 . Quantum behaviour in an oscillator is most obvious in Fock states, which are states with specific numbers of energy quanta, but such states are hard to create 3 , 4 , 5 , 6 , 7 . Here we demonstrate the controlled generation of multi-photon Fock states in a solid-state system. We use a superconducting phase qubit 8 , which is a close approximation to a two-level spin system, coupled to a microwave resonator, which acts as a harmonic oscillator, to prepare and analyse pure Fock states with up to six photons. We contrast the Fock states with coherent states generated using classical pulses applied directly to the resonator.

• Publisher: Nature Publishing Group UK,Nature Publishing,Nature Publishing Group
• Subject: Applied sciences / Atomic properties / Circuit design / Circuit properties / Classical and quantum physics: mechanics and fields / Control / Design and construction / Design. Technologies. Operation analysis. Testing / Electric, optical and optoelectronic circuits / Electronic circuits / Electronics / Energy levels (Quantum mechanics) / Evaluation / Exact sciences and technology / Fourier transforms / Frequencies / Humanities and Social Sciences / Integrated circuits / letter / Microwave circuits, microwave integrated circuits, microwave transmission lines, submillimeter wave circuits / multidisciplinary / Nonlinear systems / Oscillators, resonators, synthetizers / Physics / Poisson distribution / Quantum information / Quantum theory / Science / Semiconductor chips / Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices / Superconductive devices / Superconductivity / Superconductors