Do you remember your first computer? And your first internet connection?  Sure, they were not as powerful as today’s technology but it was something completely new and opened many possibilities. A quantum computer, ideally connected to quantum internet, must then be even more remarkable. Although it is true that algorithms for quantum computers focus on abstract mathematical tasks such as factoring large numbers, everyday life applications will certainly come as well. After all, classical computers were also originally seen solely as calculators.

We have now pretty good idea what the quantum internet could look like. Because quantum systems are very sensitive to disturbances and quantum features do not survive for long, the ideal medium for transmitting quantum signals is light. It travels fast and almost does not interact with the surrounding environment so quantum effects can survive a long-distance transfer.

Quantum computers, on the other hand, can in principle be built in many different ways. Some scientists trap ions in electric fields and use them as the basic building blocks. Others try to build the whole quantum computer from a single molecule and use different parts of this molecule as quantum bits that store information. Some try to use light to perform quantum computations since such quantum computers are then easily connected via quantum internet. There are also those who use superconducting systems.

In a way, superconducting systems are, in their form, most similar to classical computers. You can build a chip from the right material, similarly to an integrated circuit in a normal computer. Then you cool the chip down to temperature of a few Kelvin (around -270 degrees Celsius) and it becomes superconducting — it starts to transmit current without any resistance. Quantum bits can then be represented by superconducting currents of various strength, similar to normal computers.

There is just one problem with superconducting quantum computers — it is not possible to connect them to optical quantum internet. Energy of superconducting qubits is much smaller than that of an optical photon so they do not interact well. Superconducting systems can interact with microwave fields but those cannot be transmitted as easily as light because they require low temperatures (just like superconducting systems) to overcome noise.

The solution is simple: We let the superconducting qubits interact with microwave photons which can then be converted to light using mechanical oscillators. Or we can even skip the microwave field and couple superconducting qubits directly to mechanical oscillators. That is possible because superconducting qubits are built using capacitors and some other elements. If one of the capacitor plates can vibrate, its position will affect the state of the qubit and the state of the qubit, in turn, determines the position of the vibrating plate.

Because we do not have quantum computers just yet, we can start with a smaller task — we can try to entangle two superconducting qubits that sit on two different chips. That would be a first step towards building quantum internet with superconducting systems.

Measurement of the number of excitations
Number of excitations of two qubits can be measured if the signal from the first qubit (the sphere with arrow) is converted using a transducer (blackbox), transmitted and converted back.

The approach I like is based on measurement feedback and there are two options how to use it. The first one uses entanglement swapping where each of the qubits interacts with a microwave field in a way that generates entanglement between them. The microwave field is then converted to light and travels to a detector where both the fields are measured together. In this way, the entangled state is teleported from a microwave field to a qubit and both qubits become entangled.

Entanglement swapping with two qubits
Entangling qubits with their transducers locally and then performing joint measurement on the light fields, one can entangle the two qubits.

Another option is to engineer the system in such a way that we perform a measurement of the number of excitations of the two qubits. Each qubit has two levels — denoted by 0 and 1 and thus showing the number of excitations in the qubit. If we prepare the qubits in a suitable state and the measurement reveals that one qubit is excited but we do not know which one, they become entangled. That is commonly done with superconducting qubits (without coupling to light, though). With the optical link, this can be done in the following way: we let one qubit interact with a microwave field which then gets converted to light. The light gets transmitted to the second qubit where it is converted back to microwave frequency, interacts with the second qubit, and is measured.

So far it seems that such tasks can be performed with mechanical oscillators that need not be much better that what is available currently. We thus might see the first steps towards quantum networks with superconducting qubits in the near future. But it will still be a long way to go if we want to build quantum computers connected by quantum internet.

This post is loosely based on talk I held at the Spring meeting of the German Physical Society in Heidelberg, March 2015.

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