Emergence of irreversible decoherence from unitary dynamics

Abstract

The decoherence of superpositions of classically distinguishable states (cat states) is crucial for understanding quantum-to-classical transitions and quantum measurements. So far, decoherence processes of mesoscopic cat states have been demonstrated in several experiments. However, the issue of how the unitary system-reservoir dynamics can lead to irreversible system decoherence remains largely unexplored in experiments. Here we experimentally explore this fundamental issue with a circuit quantum electrodynamics device, where a bus microwave resonator storing a photonic cat state is connected to many nonlinear electronic oscillators. Each of these oscillators that are effectively coupled to the bus resonator serves as one degree of freedom of the reservoir. By gradually increasing the number of the reservoir's degrees of freedom, we find that the revivable quantum coherence progressively decays, owing to the growth in the amount of inerasable information about the system's state encoded in the reservoir. Our results illustrate that irreversible decoherence of a quantum system is an emergent phenomenon, arising from the unitary dynamics involving the system and many of the reservoir's degrees of freedom, which is crucial for the reconciliation of quantum mechanics and classical physics.

Figures (3)

• Figure 1: (a) Sketch of the device. The circuit quantum electrodynamics architecture contains a frequency-fixed bus resonator ($B$) connected to 10 frequency-tunable Josephson-junction-based nonlinear oscillators, each of which is confined to the lowest two levels and behaves as a qubit. Two of these qubits, denoted as $A_{1}$ and $A_{2}$, are used as ancillas to prepare the photonic cat state of the photonic mode stored in $B$ and to probe its quantum coherence, respectively. The other qubits, labeled $Q_{1}$--$Q_{8}$, serve as the reservoir oscillators that can be selectively coupled to $B$.
• Figure 2: Observation of the reversible decoherence of the cat state induced by a single reservoir qubit ($Q_{1}$). (a) The $\left\vert g\right\rangle$-state population of $Q_{1}$ measured for different interaction times $t$. (b) Wigner functions of the mesoscopic field for different $B$-$Q_{1}$ interaction times. (c) Evolution of the von Neumann entropy of $Q_{1}$. Before the interaction, $B$ is prepared in the amplitude cat state of Eq. \ref{['eq3']} with $\alpha =3.3$ and $Q_{1}$ is initialized in its ground state $\left\vert g\right\rangle _{1}$.
• Figure 3: Progressive transition from reversible to irreversible decoherence. (a) Evolutions of distinguishabilities for different values of $N$. A total of $N$ ($N=1$ to $8$) reservoir oscillators are selectively coupled to the system, prepared in the amplitude cat state. The effective coupling strength and detuning associated with each qubit are controlled by the parameters of the modulation pulses applied to mediate the corresponding sideband interactions. (b) Wigner functions of the mesoscopic field for $N=8$, measured for different interaction times.