Error-free quantum computer: The silicon shooting star

"Entanglement is the secret of the exponential computing power of a quantum computer.

 

Competition for the miracle calculators from Google & Co. A future quantum computer may calculate with quantum bits made of silicon. That would have many advantages. Three research groups have created an important prerequisite.

 

Even if the first commercial quantum computers are already finding their way into data centers - such as the "Advantage" system from D-Wave at Research Center Jülich recently - the development of a universally programmable and fault-tolerant quantum computer is still a long way off. 

The quantum physical information units, the quantum bits, which are mainly realized by stored ions or superconducting microwave resonators, are still quite fragile and prone to failure. Thermal radiation, noise and stray fields can destroy the states of the qbits, which inevitably leads to calculation errors. In addition, scalability poses difficulties: the performance of quantum processors cannot be increased simply by increasing the number of quantum bits. The more Qbits there are, the more difficult it becomes to shield and control each one from disturbing influences.

 

Many research groups have long seen an alternative in quantum computers made of silicon, especially to the superconducting systems favored by IBM, Google and D-Wave. This also includes the developers of the chip manufacturer Intel. Extremely compact and stable quantum bits can be realized in the semiconductor material. By increasing the working temperature, the cooling effort could also be reduced in comparison to the superconducting systems. Another advantage: Quantum circuits based on silicon can be manufactured using standard semiconductor technologies and accommodated on a microchip. This makes it possible to scale silicon quantum systems in a relatively simple manner.

 

Three research groups from Australia, Japan and the Netherlands have now been able to clear another hurdle that has so far stood in the way of practicability. They have developed simple silicon quantum processors that calculate as reliably as the superconducting quantum bits in IBM or Google systems already do.

 

Detect calculation errors early and correct them quickly

 

In the past, it was possible to keep individual qubits stable for 35 seconds – an eternity for quantum systems. For comparison: The superconducting Qbits remain coherent for only 100 milliseconds. However, if you tried to combine two silicon quantum bits into logic gates in order to actually carry out arithmetic operations, you had to expect a high error rate in the results. Using different approaches, the three research groups have succeeded in reducing the error rate to well below one percent. Errors that occur despite all measures can now be detected in good time and corrected quickly, write the scientists in the journal "Nature". This clears the way for the construction of larger, fault-tolerant silicon quantum processors.

 

The Dutch scientists led by Lieven Vandersypen from the TU Delft and the Japanese physicists led by Akito Noiri from the Riken research center near Tokyo used the intrinsic angular momentum (spins) of individual electrons as quantum bits, which were isolated in tiny silicon structures, so-called quantum dots, like in a kind of cage. Spins are ideal quantum bits: They can “point” down and up at the same time or – more precisely – exist as a superposition of both states. The scientists had embedded the quantum dots in thin silicon layers stacked on top of one another for shielding.

 

With the help of electric and magnetic fields, the distances between the quantum bits can be controlled and the alignment of the electron spins can be manipulated in a targeted manner. In this way, it was possible to bring two qbits so close that an entanglement occurred. 

 

The result was fast elementary gates that could be used to perform simple arithmetic operations. The switching times could be reduced to about a hundred nanoseconds, which was reflected in the stability and the low error rate of the gates. Another factor: all groups worked with isotopically pure silicon and used sensitive methods to read the results of the quantum gates without causing any interference. The Japanese and Dutch researchers state the reliability of their processors at around 99.5 percent. A value of 99 percent is considered the threshold for fault-tolerant quantum computers.

 

The gate of the researchers from the University of New South Wales in Sydney calculated almost as reliably (99.4 percent). Instead of electron spins, however, they used the nuclear spins of phosphorus atoms that they had deliberately injected into a thin layer of silicon. Atomic nuclei have the advantage that they are less susceptible to interference and more robust than electron spins. Nuclear spins isolated in silicon can be kept stable for several seconds. Encoded quantum information can be stored with these Qbits for a correspondingly long time.

 

The next challenge awaits

 

However, it is also much more difficult to combine nuclear spins into a gate. The researchers led by Andrea Morello used a neighboring electron as a mediator. Using microwave pulses, it was possible to convert the two atomic nuclei and the electron into an entangled state. The nuclear spins were thus coupled to one another and the prerequisites for a stable logic quantum gate were created.

 

For the quantum physicist Tommaso Calarco from Research Center Jülich, the results of the three research groups are convincing. "In the still open race for the scalability of quantum computers, semiconductor technologies have now caught up closing a key gap." Errors in a two-qbit arithmetic operation are now comparable with the error rates of other platforms based on stored ions and superconductors, for example. However, these quantum computers on stored ions and superconductors are already working with a few dozen quantum bits, which the current silicon processors are still a long way off.

 

The IBM quantum computer “Eagle” currently holds the record with 127 superconducting quantum bits. Efficient correction processes already ensure that the error rate is low. "The next challenge will now be to demonstrate comparable performance capabilities using multiple silicon qbits." It is remarkable for Calarco that three laboratories made the breakthrough at the same time.

 

However, the scientists at Research Center Jülich are not working on silicon processors themselves. They are committed to developing a quantum computer based on superconducting chips. The goal is that the "OpenSuperQ" system should have a good hundred quantum bits in the final configuration. The operating system will be open-source software that, in principle, every user should be able to access the quantum computer."