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February 5, 2021

from the Max Planck Institute for Quantum Optics

Today’s quantum computers contain up to several dozen storage and processing units, the so-called qubits. Severin Daiss, Stefan Langenfeld and colleagues from the Max Planck Institute for Quantum Optics in Garching have successfully connected two such qubits in different laboratories to a distributed quantum computer by connecting the qubits with a 60 meter long optical fiber. Over such a distance they implemented a quantum logic gate – the basic building block of a quantum computer. This makes the system the world’s first prototype of a distributed quantum computer.

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Quantum computers differ considerably from conventional “binary” computers: Future implementations of computers are likely to easily perform specific calculations that conventional computers would need months or even years to complete – for example in the area of ​​data encryption and decryption. While the performance of binary computers results from large memories and fast computation cycles, the success of the quantum computer rests on the fact that a single storage unit – a quantum bit, also called a “qubit” – can contain overlays of various possible values ​​at the same time. A quantum computer therefore not only calculates one result at a time, but many possible results in parallel. The more qubits there are connected to one another in a quantum computer; The more complex the calculations it can perform.

The basic arithmetic operations of a quantum computer are quantum logic gates between two qubits. Such an operation changes – depending on the initial state of the qubits – their quantum mechanical states. For a quantum computer to be superior to a normal computer for various calculations, it would have to reliably connect many dozen or even thousands of qubits together for thousands of quantum operations as well. Despite great successes, all current laboratories still struggle to build such a large and reliable quantum computer, as each additional qubit required makes it much more difficult to build a quantum computer in just one setup. The qubits are implemented, for example, with individual atoms, superconducting elements or light particles, all of which have to be perfectly isolated from each other and from the environment. The more qubits are arranged side by side, the more difficult it is to isolate and control them from the outside at the same time.

One way to overcome the technical difficulties of building quantum computers is shown in a new study in the journal Science by Severin Daiss, Stefan Langenfeld and colleagues from the Gerhard Rempe research group at the Max Planck Institute for Quantum Optics in Garching. In this work, supported by the Institute of Photonic Sciences (Castelldefels, Spain), the team managed to connect two qubit modules over a distance of 60 meters so that they effectively form a basic quantum computer with two qubits. “Over this distance we carry out a quantum computer operation between two independent qubit setups in different laboratories,” emphasizes Daiss. This allows smaller quantum computers to be merged into a common processing unit.

The simple coupling of distant qubits to create entanglement between them has been achieved in the past, but now the connection can also be used for quantum computations. For this purpose, the researchers used modules that consist of a single atom as a qubit and are positioned between two mirrors. Between these modules they send a single light quantum, a photon, which is transported in the optical fiber. This photon then becomes entangled with the quantum states of the qubits in the various modules. The state of one of the qubits is then changed in accordance with the measured state of the “Ancilla photon”, as a result of which a quantum mechanical CNOT operation is realized with a fidelity of 80 percent. A next step would be to combine more than two modules and to host more qubits in the individual modules.

The team leader and institute director Gerhard Rempe believes that the result will enable further development of the technology: “Our program opens a new one Development path for distributed quantum computing. ” It could, for example, make it possible to build a distributed quantum computer consisting of many modules with a few qubits connected with the newly introduced method. This approach could circumvent the limitation of existing quantum computers to integrate more qubits into a single setup and could therefore enable more powerful systems.

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