Flying Optical Cats for Quantum Communication

An atom is trapped in the resonator between two mirrors. A light pulse, which is reflected off the resonator, gets entangled with the atom and may fly freely as a superimposed cat state. (Source: B. Hacker, MPQ)

In 1935 Erwin Schrödinger formu­lated a thought experiment designed to capture the para­doxical nature of quantum physics. The crucial element of this experiment is a cat that is simul­taneously dead and alive. Since Schrödinger proposed his ‘cat paradox’, physicists have been thinking about ways to create such super­position states experi­mentally. A group of researchers led by Gerhard Rempe, Director of the Division of Quantum Dynamics at the Max Planck Insti­tute of Quantum Optics, has now realized an optical version of Schrö­dinger’s thought experiment in the labora­tory.

In this instance, pulses of laser light play the role of the cat. The insights gained from the project open up new prospects for enhanced control of optical states, that can in the future be used for quantum communi­cations. “Accor­ding to Schrödinger‘s idea, it is possible for a micro­scopic particle, such as a single atom, to exist in two different states at once in a super­position. Moreover, when such a particle interacts with a macro­scopic object, they can become entangled, and the macro­scopic object may end up in super­position state. Schrödinger proposed the example of a cat, which can be both dead and alive, depending on whether or not a radio­active atom has decayed – a notion which is in obvious conflict with our everyday ex­perience,” Rempe explains.

In order to realize this philo­sophical experiment in the labora­tory, physicists have turned to various model systems. The one implemented in this instance follows a scheme proposed by the theore­ticians Wang and Duan in 2005. Here, the super­position of two states of an optical pulse serves as the cat. The experi­mental techniques required to implement this proposal – in parti­cular an optical resonator – have been developed in Rempe’s group over the past few years.

The researchers involved in the project were initially skeptical as to whether it would be possible to generate and reliably detect such quantum mechani­cally entangled cat states with the available technology. The major diffi­culty lay in the need to minimize optical losses in their experiment. Once this was achieved, all measure­ments were found to confirm Schrödinger’s predic­tion. The experiment allows the scientists to explore the scope of appli­cation of quantum mechanics and to develop new techniques for quantum communi­cation.

The labora­tory is equipped with all the tools necessary to perform state-of-the-art experiments in quantum optics. A vacuum chamber and high-precision lasers are used to isolate a single atom and mani­pulate its state. At the core of the set-up is an optical resonator, consisting of two mirrors sepa­rated by a slit only 0.5 mm wide, where an atom can be trapped. A laser pulse is fed into the resonator and reflected, and thereby interacts with the atom. As a result, the reflected light gets entangled with the atom. By performing a suitable measure­ment on the atom, the optical pulse can be prepared in a super­position state, just like that of Schrödinger’s cat. One special feature of the experiment is that the entangled states can be generated deter­ministically. In other words, a cat state is produced in every trial.

“We have succeeded in gene­rating flying optical cat states, and demon­strated that they behave in accor­dance with the predictions of quantum mechanics. These findings prove that our method for creating cat states works, and allowed us to explore the essential para­meters,” says PhD student Stephan Welte. “In our experimental setup, we have succeeded not only in creating one specific cat state, but arbi­trarily many such states with different super­position phases – a whole zoo, so to speak. This capa­bility could in the future be utilized to encode quantum infor­mation,” adds Bastian Hacker, who is also a doctoral student at the Institute.

“Schrödinger‘s cat was originally enclosed in a box to avoid any inter­action with the environ­ment. Our optical cat states are not enclosed in a box. They propa­gate freely in space. Yet they remain isolated from the environ­ment and retain their properties over long distances. In the future we could use this tech­nology to construct quantum networks, in which flying optical cat states transmit infor­mation,” says Gerhard Rempe. (Source: MPQ)

Reference: B. Hacker et al.: Deterministic creation of entangled atom–light Schrödinger-cat states, Nat. Phot., online 14 January 2019; DOI: 10.1038/s41566-018-0339-5

Link: Quantum Dynamics (G. Rempe), Max-Planck-Institute for Quantum Optics, Garching, Germany

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