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arXiv:1703.03316 · Physical Review Letters

Converting Quasiclassical States into Arbitrary Fock State Superpositions in a Superconducting Circuit

W. Wang, Lian Hu, Yuan Xu, K. Liu, Y. Ma, Shi‐Biao Zheng, Rajamani Vijay, Yipu Song, Luming Duan, Luyan Sun

Published Jun 1, 2017• 55 citations
arXivHTMLPDF

Abstract

We propose and experimentally demonstrate a new method to generate arbitrary Fock state superpositions in a superconducting quantum circuit, where a qubit is dispersively coupled to a microwave cavity mode. Here, the qubit is used to conditionally modulate the probability amplitudes of the Fock state components of a coherent state to those of the desired superposition state, instead of pumping photons one by one into the cavity as in previous schemes. Our method does not require the adjustment of the qubit frequency during the cavity state preparation and is more robust to noise and accumulation of experimental errors compared to previous ones. Using the method, we experimentally generate phase eigenstates under various Hilbert-space dimensions and squeezed states, which are useful for the quantum walk and high-precision measurement.

DOI
10.1103/physrevlett.118.223604

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Two-level systems, or qubits, afford complete quantum control using only classical electrical signals. Achieving the same degree of control over harmonic resonators and photons has remained elusive, due to their infinite number of equally spaced energy levels. We exploit our complete qubit control by using a superconducting phase qubit to pump microwave photons into a high-Q coplanar wave guide resonator and, subsequently, use the qubit to read out the resonator state. This scheme has previously allowed us to prepare and detect photon number states (Fock states [1]) in the resonator and to measure their decay. Using a generalization of a scheme developed by Law and Eberly [2], we can now create arbitrary quantum states of the photon field with up to approximately 10 photons [3]. We analyze the prepared states by directly mapping out the corresponding Wigner function, which is the phase-space equivalent of the density matrix, and provides a complete description of the quantum state. Movies of the decay of a variety of non-classical states show a loss of quantum coherence over the energy decay time of the resonator.
We present a method to prepare nonclassical states of the electromagnetic field in a microwave resonator. It is based on a controlled gate that applies a squeezing operation on a SQUID-terminated resonator conditioned on the state of a dispersively coupled qubit. This controlled-squeeze gate, when combined with Gaussian operations on the resonator, is universal. We explore the use of this tool to map an arbitrary qubit state into a superposition of squeezed states. In particular, we target a bosonic code with well-defined superparity which makes photon losses detectable by nondemolition parity measurements. We analyze the possibility of implementing this using state-of-the-art circuit QED tools and conclude that it is within reach of current technologies.
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