Quantum calculations rendered insensitive for disruptions

Scientists from the the Netherlands based Kavli Institute of Nanoscience at Delft University of Technology (TU Delft) and the Foundation for Fundamental Research on Matter (FOM Foundation), together with colleagues from the United States, have found a new way of performing quantum calculations that is insensitive for disruptions from environmental interactions.

This could be an important step on the road towards a future, ultrafast quantum computer. The researchers published their results this week in the prestigious journal Nature.
 
Quantum particles, such as an atomic nucleus or an electron, can be in several states at once. For example, the magnetic moment of an electron, the so-called spin, can have two different directions at once. If the spin direction is used as a quantum bit in the computer then this can be 0 and 1 at the same time. That makes ultrafast calculations possible. The spins are, however, extremely sensitive for disrupting environmental interactions, and this still hinders the development of a large-scale quantum computer.
 
The researchers have now found a way of protecting the quantum bits during the calculation. Using this new method they performed a quantum calculation with an electron spin and a nuclear spin (the spin of an atomic nucleus). This took place in a diamond chip at room temperature. Using this double quantum bit processor the researchers performed a quantum search algorithm.

This search algorithm finds an element in an unsorted database of four elements in a single attempt, whereas a standard computer on average needs more than two attempts to do this. This is a small-scale demonstration of the superior efficiency of a quantum computer. At the same time it is the first time a quantum algorithm has been performed using spin quantum bits on a chip.
 
Two years ago the same research team published an article in Science in which they demonstrated that the spin state of a single electron can be effectively protected from disruptive environmental interactions.

By continually reversing the spin direction of the electron with very brief pulses, the spin behaves as if it is decoupled from its environment. This technique is therefore ideal for the protection of quantum information stored in a spin.

Quantum computers, however, must not only be capable of storing information but also using it to perform calculations. The use of the decoupling technique during a quantum calculation is problematic, as the electron is decoupled from all other quantum bits. That makes calculations between quantum bits impossible.
 
The researchers have now found a solution for this. They have devised a way of very precisely synchronising the reversal of the electron spin with the dynamics of a second quantum bit, the spin of an atomic nucleus. As a result of this the interaction between the two spins is maintained, while the electron spin is still adequately protected from disruptive environmental interactions.

Using this approach the researchers could build a universal set of arithmetic operations for a quantum computer. This set is the basis for each quantum calculation, whereby the electron spin and the nuclear spin now form a fully functioning double quantum bit processor. The researchers now plan to apply the new technique to chips with more quantum bits.
 
The research was carried out together with American colleagues from Ames Laboratory and Iowa State University, the University of California Santa Barbara and the University of Southern California. The research at Delft University of Technology was funded by FOM, the Netherlands Organisation for Scientific Research, the European Commission (SOLID, DIAMANT) and the DARPA QuEST programme.