2-bit quantum computer works in diamond

Research at the TU has brought a superfast working quantum computer one important step closer. For the first time, researchers built a working 2-bit quantum computer on a diamond chip. Their spectacular result was published in the science journal Nature.

Artist's impression of two q-bits in their protective bubble - Image: Wolfgang Pfaff, Kavli institute of Nanoscience
Artist's impression of two q-bits in their protective bubble - Image: Wolfgang Pfaff, Kavli institute of Nanoscience

In order to build their quantum computer, TU Delft researchers had to overcome a well-known but major problem, known as ‘quantum decoherence’, which is the irreversible loss of quantum information stored in a qubit, caused by disrupting interactions from its environment.

Researchers at the TU Delft however were able to get rid of the decoherence and make two quantum bits interact. “That is an important milestone, as this is the first time a quantum calculation has been performed with spins on a chip. It demonstrates the superior efficiency of a quantum computer and provides proof that it could actually work,” says Toeno van der Sar, who received his PhD for his research on a diamond-based quantum computer and is now a postdoc at the Kavli Institute of Nanoscience (Applied Sciences).

The key role in the research conducted at TU Delft is played by a tiny magnetic moment of electrons and nuclei – the so-called ‘spins’. In a standard computer, a bit is either 0 or 1. But thanks to electrons, which can have two different spin directions at once, a bit in a quantum computer can be 0 and 1 at the same time. “Spins are used to make this possible, thus creating the opportunity to perform incredibly fast calculations. However, spins are also very sensitive for disruptions,” Van der Sar explains.

The current article publication is a follow-up to research published two years ago in Science. The same team demonstrated how to protect the spin state of a single electron, by continually flipping the spin direction of the electron with nanosecond pulses. This protected the spin from disruptions, but it also made it impossible for the spin of the electron to interact with other spins that van be used as quantum bits, which is necessary to perform calculations. The electron therefore could only store information.

The researchers have now found a way to retain the interaction between the electron spin and the spin of an atomic nucleus, while still using the pulses to protect the sensitive electron spin.

“We used to apply the pulses with a random time delay between them, but now we are able to precisely synchronise the reversal of the electron with the dynamics of the spin of an atomic nucleus,” Van der Sar says. “If we reverse the electron with a pulse at the exact moment the spin of the atomic nucleus turns, the two bits are in sync and can interact with one another. We have to be very precise, as the pulses have an accuracy of one nanosecond. If we’re more than 50 nanoseconds off, there’s no interaction.”

Thanks to the synchronisation the two bits could perform calculations, without disruptions.

The research team performed a search algorithm. Van der Sar: “Our small-scale quantum computer finds an element in an unsorted database of four elements in a single try. A standard computer would need about two attempts.”

The quantum calculation was done in a synthetic diamond chip at room temperature, which is also extraordinary. Most research on chip-based quantum computer systems is done close to the absolute zero.

The diamond chips the team uses are 4 x 4 millimetres. The diamond is placed on a printed circuit board. Thanks to wires fabricated onto the diamond using lithography, an electric current goes through the diamond and creates a magnetic field. “We use this to control the bits,” Van der Sar explains. “By measuring how much light comes off the bits, we can see in what position they are exactly.”

The research was conducted in collaboration with the Ames Laboratory at Iowa State University, the University of California Santa Barbara and the University of Southern California.

→ T. van der Sar, Z.H. Wang et.al, Decoherence-protected quantum gates for a hybrid solid-state spin register, Nature, 5 April 2011, doi: 10.1038/nature10900

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