Quantum Bit Stored for Record 39 Minutes at Room Temperature

Image: Stef Simmons

A physical state crucial for quantum computing has managed to survive at room temperature for 39 minutes in a record-breaking experiment. The new study gives a huge boost to quantum computing's prospects of storing information under normal conditions for long periods.

The quantum state of superposition allows quantum bits (qubits) of information to exist as both 1s and 0s simultaneously—unlike classical computing bits that exist as either 1 or 0. That makes superposition one of the main keys to unlocking quantum computing's potential of performing calculations much faster than classical computers. But past experiments had only succeeded in maintaining superposition at room temperature for mere seconds, compared to the latest record-breaking run of 39 minutes at 25 degrees C. The longer a quantum state can last, the more quantum computing calculations can be performed with it.

The international team from Canada, the UK, and Germany that created the qubit set another benchmark by maintaining superposition for three hours at a cryogenic temperature of -269 degrees C (four degrees above absolute zero). They also showed how to maintain superposition while cycling from -269 degrees C to 25 degrees C and back again. All the study's achievements were detailed in the 15 November edition of the journal Science.

"These lifetimes are at least ten times longer than those measured in previous experiments," said Stephanie Simmons, a junior research fellow in materials science at Oxford University, in a press release.

Such results seem especially promising because the team used silicon as one of its hardware materials. Future quantum computers based on silicon could leverage the manufacturing processes of the existing semiconductor industry that gave "Silicon Valley" its name. The latest study also raises the tantalizing possibility of a silicon-based quantum computer operating at room temperature, Simmons told CBC News.

The big key to the team's success was the use of ionized phosphorus atoms implanted in the silicon. The nuclear spin states of the phosphorus atoms acted as the bits of information that the team could manipulate with magnetic fields—a spin state can point up to represent a 0 bit, down to represent a 1 bit, or any angle in between when in superposition. 

Other teams, especially in Australia, have tested the combination of phosphorus and silicon for quantum computing experiments before. But Simmons and her colleagues took advantage of recent studies showing how ionized phosphorus atoms—with missing electrons—could maintain their states of superposition for much longer than ordinary neutral phosphorus atoms.

The researchers also stabilized the quantum states of the phosphorus atoms by using isotopically enriched silicon to get rid of possible interference from impurities that arise in natural silicon samples. (Natural silicon is a mix of silicon-28 and silicon-29, but the team used crystals of pure silicon-28). And they applied a method called "dynamic decoupling" that uses electromagnetic pulses to help refocus the stability of a spin state.

Physicists still have a long way to go with quantum computing. The latest study manipulated the nuclear spins of about 10 billion phosphorus atoms so that they existed in the same quantum state—a simple way to run an endurance test.  But Simmons and her colleagues plan to take the next big step of testing different qubits in different quantum states.

Image: Stef Simmons

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