03 December 2012

Applying Magnetism and Microwave To Bismuth Atoms As Qubits For Quantum Computers

Researchers are looking at Bismuth atoms and nuclei as a source for qubits used in a quantum computer. Using magnetism and microwaves, they find that this process may be the next step to practical quantum computers.

Moore's law states that the processing power of a computer will continue to double every eighteen months. For the last 50 years, the law seems to be the norm. But technology is fast approaching the limit to the number of transistors that can fit into a silicon chip.

The current record for most number of transistors put on a chip is 2 billion.

By the year 2030, Moore's law would have brought computers to the atomic level. This would be the time of quantum computers. The speed and computing power of a quantum computer far surpasses that of present day computers (also called classical computers).

The secret behind a quantum computer is its use of qubits and not bits to manipulate data. Bits that classical computers use are made up of two states (charged states), either on (1) or off (0). These bits hold and process information depending on its state or in common terms, these information are processed digitally. This is the reason we call our time, the Digital Age.

Qubits are different. Instead of a charged state, the spin state of an atom is used to hold information. Atoms either spin up or spin down which is similar to the two states of a bit. But aside from the two spins, an atom can also enter a superstate where the atom is both in an spin up and spin down state.

This would mean that qubits can hold more information and also exponentially speed up the processing power of the computer. This is a simplified statement though, a thorough knowledge of quantum mechanics and entanglement is needed to approach this kind of computing power.

To have an idea on how fast a quantum computer can calculate, a program to dial one thousand phone numbers for a classical computer will execute it by dialing each number one by one. A quantum computer will dial all one thousand numbers at the same time.

Bismuth Atoms Used For Quantim Qubits

New research has demonstrated a way to make bismuth electrons and nuclei work together as qubits in a quantum computer.

The discovery, published in Nature Materials, takes us a key step further to creating practical quantum computing which could tackle complex programs that would otherwise take the lifetime of the universe to finish.

The collaboration partners are based in the University of Warwick, UCL, ETH Zurich and the USA Sandia National Labs.

Video: Defining Quantum Computing

Information on our normal computers is stored as bits, which are either ones or zeros. Quantum bits work differently in that each quantum bit can try out being a one and a zero at the same time, which makes them much more powerful for solving certain problems.

Researchers have explored influencing the direction of spin in electrons to create those states but this approach has had its challenges.

Dr Gavin Morley from the University of Warwick's Department of Physics said: "Bismuth atoms in silicon crystals are great at working as quantum bits. Each bismuth atom has a spare electron, which has a "spin" that can be influenced by magnets.

"If we put the electron into a magnet, it lines up with the magnetic field, behaving like a compass needle.

"We can control the direction that the electron is pointing in, using microwaves. Microwaves let us flip the direction the electron is pointing in, and these "up and down" directions are what constitute the "one and zero" in our quantum bit.

"Unfortunately, our electron is constantly prone to interference from nearby atoms that are out of our control.

"And the more time we waste, the greater the chance that our poor electron will suffer from interference, making it unusable to us."

"Now, this electron is coupled to the bismuth nucleus, which has its own spin: a smaller compass needle. Using this as an extra quantum bit and flipping it at the same time as our electron, would really help out. We can control this smaller compass needle too, but as it's smaller, it takes longer to control, and we need to use radiowaves instead of microwaves to do this."

Hybridizing electronic and nuclear spin qubits makes quantum control much easier. This equation, spelt out in child's plastic letters on a fridge, describes the pure states of bismuth-doped silicon for which the nuclear and electronic qubits become maximally hybridized at specific magnetic fields.

"The good news is that as it's slow to respond, our bismuth nucleus's smaller compass needle suffers less from interference by nearby rogue atoms than our electron's larger compass needle. Unfortunately in the time we spend controlling our bismuth nucleus, these rogue atoms interfere with our electron."

"However we found that if we reduce the magnetic field just enough, then the electron and the nucleus become hybridized. Our new experiments at ETH Zurich show that through hybridisation, we can flip both compass needles easily using microwaves."

Dr Morley compares it to the magnetic resonance imaging we find in hospitals.

He said: "MRI works by controlling the nuclear spins in your body.

"We have hybridized electron and nuclear spins and found that this makes it easier to control them.

"It's an easy new way to make slow and fast quantum bits work together. There are lots more challenges to face before anyone has a working computer with enough quantum bits to be useful, but with this hybridization as part of a computer's design, we are one step closer."


University of Warwick
Nature Materials
ETH Zurich
Sandia National Laboratories
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