Abstract
Continuous weak measurement allows localizing open quantum systems in state space and tracing out their quantum trajectory as they evolve in time. Efficient quantum measurement schemes have previously enabled recording quantum trajectories of microwave photon and qubit states. We apply these concepts to a macroscopic mechanical resonator, and we follow the quantum trajectory of its motional state conditioned on a continuous optical measurement record. Starting with a thermal mixture, we eventually obtain coherent states of 78% purity-comparable to a displaced thermal state of occupation 0.14. We introduce a retrodictive measurement protocol to directly verify state purity along the trajectory, and we furthermore observe state collapse and decoherence. This opens the door to measurement-based creation of advanced quantum states, as well as potential tests of gravitational decoherence models.
Highlights
Efficient quantum measurement schemes have previously enabled recording quantum trajectories of microwave photon and qubit states. We apply these concepts to a macroscopic mechanical resonator, and we follow the quantum trajectory of its motional state conditioned on a continuous optical measurement record
Within the Copenhagen interpretation of quantum mechanics, the quantum state of an isolated physical system is represented by its wave function
Pure conditional states are obtained through measurements of high efficiency ηmeas 1⁄4 Γmeas=ðγ þ ΓqbaÞ, where Γmeas is the measurement rate; and γ and Γqba are decoherence rates induced by a thermal bath and the measurement quantum backaction, respectively [13,14]
Summary
If information becomes available on how the system has interacted with the environment, it is possible to restore and retain the purity of the quantum state (i.e., the extent to which the mixture is dominated by a single random but known wave function) Measurements can yield such information; over a finite time interval, the obtained information is often incomplete [1,2]. We extend these ideas to measurements of the motion of a macroscopic mechanical resonator [7,8,9,10,11,12] In this setting, pure conditional states are obtained through measurements of high efficiency ηmeas 1⁄4 Γmeas=ðγ þ ΓqbaÞ, where Γmeas is the measurement rate; and γ and Γqba are decoherence rates induced by a thermal bath and the measurement quantum backaction, respectively [13,14]. This allows us to directly observe the collapse of the conditional state, as
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