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Quantum optics

Quantum optics

Light–matter entanglement creates Schrödinger-cat states

17 Jan 2019 Isabelle Dumé
Cat state
Cavity cat state

Researchers at the Max Planck Institute for Quantum Optics in Garching, Germany, have succeeded in creating Schrödinger-cat states using a single rubidium-87 atom in an optical cavity to control a propagating light pulse. The feat could help advance the field of quantum state engineering with possible applications in quantum networks and quantum computing.

In 1935 physicist Erwin Schrödinger devised his famous thought experiment involving a cat that could, surprisingly, be both dead and alive at the same time. In his gedanken, the decay of a radioactive atom triggers a mechanism (the breaking of a vial containing a poisonous gas) that kills the cat. However, since the decay of the radioactive atom is a completely random and quantum phenomenon, we cannot know the moment at which the cat dies. Mathematically, the feline is in an entangled superposition of quantum states – known as the “Schrödinger-cat” state.

Recreating this state is no easy task, but researchers have managed to do this in recent years using the quantum superposition of coherent states of a laser field with different amplitudes, or phases, of the field. They have also created these states using a trapped ion (with the vibrational state of the ion in the trap playing the role of the cat) and coherent microwave fields confined to superconducting boxes combined with Rydberg atoms and superconducting quantum bits (qubits).

Controlled superposition of two states

A team led by Gerhard Rempe has now created entangled light-matter Schrödinger-cat states by reflecting coherent laser pulses from an optical cavity containing a single trapped atom that is in a controlled superposition of two states (“spin up” and “spin down”).

The optical cavity consists of two mirrors with a reflectivity of more than 99.99% facing each other such that they can reflect a light pulse back and forth around 10,000 times. The light pulse thus interacts very strongly with the trapped atom.

“We trap a single 87Rb atom in the resonator, which can then phase-shift an impinging light pulse,” explains study lead author Bastian Hacker. “If the atom is in an equal superposition of spin up and spin down states, the light pulse is brought into a superposition state as well.”

The technique is deterministic, he says, because the light pulse becomes entangled with the atom in every single trial.

“Cat states have been postulated to exist for decades now and the recipe to create them with an optical resonator, as in our study, was first put forward in a paper in 2005. To see this finally work is most exciting and it is one of the countless triumphs of quantum mechanics,” he tells Physics World.

Wigner function measured

To confirm that the light pulses are indeed in a Schrödinger-cat state, the researchers measured their Wigner function, which is an important characteristic of non-classical systems. “The properties of light pulses are described in so-called phase space in which each point represents one amplitude and phase of a light wave,” explains Hacker. “The Wigner function is a quantum mechanical probability distribution in phase space and it contains an unambiguous description of an optical state.”

In classical logic and classic physics, probability distributions are always positive and negative probabilities don’t exist, he says. In contrast, the Wigner function of special optical states like cat states may become negative. “This is a genuine quantum feature and such a state cannot be described by classical physics.”

A quantum-logic gate

The researchers did not stop there. As a first application of their proof-of-concept experiment, they made a quantum-logic gate between an atom and a light pulse, with a photonic qubit encoded in the phase of the light field.

“A quantum logic gate has qubits as input and output and performs an elementary computation on any combination of input states – much like a classical logical gate in any computer,” explains team member Severin Daiß. “In our experiment, one qubit is stored on the atom and a second qubit is encoded in the cat state.”

In this set up, the “0” qubit is the light field at a certain phase (the “living cat”) and the “1” qubit is the light field at the opposite phase (the “dead cat”). The gate works by sending the light pulse onto the optical resonator, which amplifies the light field at the location of the atom. “The atom and light field can thus both change each other’s state to perform a controlled-NOT (CNOT) operation, similar to a classical plus operation,” says Daiß. “But in contrast to a classical gate, certain input states lead to an entangled output between the atom and the light pulses – something that we have seen in our work.”

Cat states for quantum networks

Cat states may not only be good for addressing old philosophical questions on the scope of quantum mechanics, however, he says. They do, in fact, allow to encode qubits in such a way that optical losses can be detected and corrected. “This is in stark contrast to single photons, where such losses irreversibly delete the carried information. This means that cat states are attractive for applications in small-scale quantum networks in which atoms in a resonator act as sender and receiver nodes connected by optical fibres through which the states can propagate.”

The Max Planck researchers, reporting their experiments in Nature Photonics 10.1038/s41566-018-0339-5, are now working on improving their optical resonators to reduce losses. “Doing this would allow us to create larger and more complex states,” says Rempe. “We also want to create more powerful resonator-based quantum network nodes with independent control over more than one atom in the resonator.”

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