Quantum bits are getting warmer


Entangled qubits (Image: Luca Petit / Qutech)

The future belongs to quantum computers, it is said. So far, however, the quantum bits of existing systems have had to be cooled down to extremely low temperatures. Now two research teams have made an important breakthrough: They have developed qubits integrated in silicon that “only” have to be cooled to a little more than one Kelvin. This opens up the possibility of operating such quantum computers with much cheaper cooling systems. In addition, the qubits could be combined in the near future together with the electronic components of the quantum computers to form an integrated circuit – an important prerequisite for more powerful quantum computers.

In the future, quantum computers could perform arithmetic operations that would take thousands of years for conventional computers. Thanks to quantum physical phenomena such as superimposition and entanglement, the quantum bits consisting of atoms, ions or other smallest particles can process even complex tasks in parallel. This reduces the computing time especially for complex tasks and could therefore predestine quantum computers especially for complex data analysis. However, the development of such computers is only at the beginning, previously existing quantum computers consist of only a few qubits and have to be cooled down to temperatures a few fractions of a degree above near zero. Because the fragile quantum states of these particles collapse at higher temperatures. “The quantum information stored in the qubits is quickly lost if it is not cooled down to almost zero Kelvin,” explains Menno Veldhorst from Delft University of Technology.

The temperature is crucial

However, this extreme cooling brings with it several problems: First, the cooling processes are very expensive because, for example, the liquid helium often used as a coolant is not cold enough. In addition, each ultra-cold qubit has been connected to conventional electronic components that run at room temperature via its own cable. This makes it difficult to effectively scale up the number of qubits. Because future quantum computers will have to consist of millions of qubits in order to perform properly. “The current state of quantum technology is comparable to that of classic computers in the 1950s,” explains Veldhorst. “At that time, every component had to be soldered together, which was simply no longer feasible for larger circuits.”

Heat development is also problematic when scaling today’s quantum technologies: “Every qubit pair that is added to the system also increases the heat generated,” explains Andrew Dzurak from the University of New South Wales in Sydney. A possible solution to these problems could be integrated circuits in which electronics and quantum bits work on a single chip. For this, however, electronics and qubits must also approach each other in terms of their temperature requirements.

Small step for qubits, big leap for quantum systems

An important step in this direction has now been taken by two research teams – one around Veldhorst and the second around Dzurak. Because they have managed to stabilize qubits at temperatures of 1.1 and 1.5 Kelvin. “It is still very cold, but it is a temperature that can be achieved by cooling a few thousand dollars instead of the millions of dollars it takes today to cool the chips down to 0.1 Kelvin,” explains Dzurak. “From our everyday perspective, this is difficult to grasp, but in the quantum world this temperature increase is an extreme step.” The value of 1.5 Kelvin is 15 times higher than the temperatures at which current quantum computers from IBM or Google run, for example.

In both approaches, the qubits consist of so-called quantum dots in doped silicon. These are areas in the semiconductor that are only a few nanometers in size, into which individual electrons or electron gaps are locked in such a way that they can only assume very specific, clearly distinguishable states. These states – for example the type of electron spin – correspond to the zeros and ones of the classic bits in electronic computers. In their experiments, the researchers succeeded in stabilizing these silicon qubits in such a way that they could calculate logical operations such as CNOT gates at 1.1 Kelvin in the Veldhorst team and 1.5 Kelvin in the Dzurak team. “In order to be able to perform quantum calculations at 1.1 Kelvin, we had to reduce all sources of interference extremely and develop measurement methods that are temperature-resistant,” reports Veldhorst’s colleague Luca Petit. “But when it all came together and we were able to perform operations with two quantum bits at this temperature for the first time, it was a fantastic moment.” According to the researchers, their results pave the way to quantum computers with larger qubit numbers and integrated quantum Circuits.

Source: C.H. Yang (University of New South Wales, Sydney) et al., Nature, doi: 10.1038 / s41586-020-2171-6; Luca Petit (TU Delft) et al., Nature, doi: 10.1038 / s41586-020-2170-7

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