  Quantum superpositions of distinct coherent states in a single-mode harmonic
oscillator, known as "cat states", have been an elegant demonstration of
Schrodinger's famous cat paradox. Here, we realize a two-mode cat state of
electromagnetic fields in two microwave cavities bridged by a superconducting
artificial atom, which can also be viewed as an entangled pair of single-cavity
cat states. We present full quantum state tomography of this complex cat state
over a Hilbert space exceeding 100 dimensions via quantum non-demolition
measurements of the joint photon number parity. The ability to manipulate such
multi-cavity quantum states paves the way for logical operations between
redundantly encoded qubits for fault-tolerant quantum computation and
communication.
