Detecting quantum transmissions without destroying quantum information

Researchers at the Max Planck Institute of Quantum Optics have developed a method for detecting quantum transmissions without destroying the quantum information.

Quantum information is sent over long distances in the form of light particles (photons), but these are lost quickly.

One step that can be taken to reduce the difficulty of information processing would be finding out whether photons are still on track to reach their destination after they have travelled part of the distance. This would have useful applications such as making the encryption of money transfers much more practicable.

Currently, the exchange of qubits, the smallest unit of quantum information, is far too complex to be a widely used secure method of data transmission and exchange.

Light particles are capable of carrying qubits over long distances but are easily deflected from their route in the air or absorbed in glass fibres, which leads to the quantum information being lost. As the majority of photons are lost in a transmission, at a distance of around 100km, thousands of photons would have to be transmitted to directly transmit a single qubit. The transmission of quantum information can therefore be difficult and time consuming.

Now, a team around Dominik Niemietz and Gerhard Rempe at the Max Planck Institute of Quantum Optics has created a physical protocol that can determine if the qubit has been lost at intermediate stations of the quantum transmission.

“If this is the case, the transmitter can send the qubit again with significantly less delay than if the loss is noticed only at the receiving end”, explains  Niemietz, who developed the detector for photonic qubits.

“It is essential that we do not destroy the qubit. We are thus only detecting the qubit photon and not measuring it.

“This is crucial because the laws of quantum physics rule out copying a qubit 1 to 1 – this is what quantum cryptography is based on.”

Using model calculations, the researchers have shown that quantum communication is made more efficient with the detection of photons transporting qubits.

Pau Farrera, part of the research team, commented: “A detector for photonic qubits can also be useful at shorter distances.”

However, this detection would have to work even more reliably than it did in the current experiment.

“This is not a fundamental problem but rather only a technical one”, explains Farrera. The efficiency of the detector currently suffers as the resonator reflects only about one third of the incoming photons. Only in the case of a reflection does a photon leave a trace in the atom.

“However, we can increase this efficiency to almost 100%  by improving the fabrication of the resonators,” he added.

As well as tracking quantum information during transmission, a detector capable of detecting a photonic qubit could also be used to confirm the arrival of quantum post at its destination. This would have useful applications if the information encoded in the photon was to be processed in a complex manner, such as being transferred to entangle atoms. Entanglement is a quantum mechanical phenomenon that can be used to encrypt and process data which involves two spatially widely separated particles becoming a single quantum entity, with changes in one particle directly leading to changes in the other. “Creating entanglement is complex”, said Rempe, who us Director at the Max Planck Institute of Quantum Optics. “You should use it to process a qubit only if you are sure that this qubit is there”.

Going forward, Rempe’s group may be working towards demonstrating how quantum post tracking could be used in information processing.

“We would like to use the detector for quantum communication between our Institute in Garching and a more distant location. For example, to make the step from our laboratory to practical application,” he said.

“In this way, we are once again getting a little closer to our great long-term goal, the quantum internet”.

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