Mar 1 2019
For the first time, researchers from the University of Sydney have been able to improve quantum computers by using codes developed to detect and eliminate errors in the logic gates of such machines.
Lead author Dr Robin Harper from the School of Physics and Sydney Nano. (Image credit: University of Sydney)
“This is really the first time that the promised benefit for quantum logic gates from theory has been realised in an actual quantum machine,” stated Dr Robin Harper, lead author of a new study published this week in Physical Review Letters, a prestigious journal.
Entangled networks of a few quantum bits (or qubits) form quantum logic gates. They are the switches that enable quantum computers to run algorithms, or formulae, to process information and carry out calculations.
Dr Harper and his coworker Professor Steven Flammia, from the School of Physics and University of Sydney Nano Institute, tested the error detection codes using IBM’s quantum computer. They exhibited improvement by an order of magnitude in minimizing infidelity (i.e. error rates) in quantum logic gates, which are the switches that will be the basis of fully functioning quantum computers.
This paper is a great example of how scientists can use our publicly available cloud systems to probe fundamental problems. Here Harper and Flammia show that ideas of fault tolerance can be explored on real devices we are building and already deploying, today.
Dr Jay Gambetta, IBM Fellow, Principal Theoretical Scientist, IBM Q.
Although quantum technologies are only in the inception stage, they have the potential to transform computing in the 21st century by carrying out calculations considered to be out of the ability of the fastest and largest supercomputers.
They will perform this by tapping the abnormal properties of matter at the quantum level, which enable them to process information with the help of qubits. These are computing elements that put to use the fact that quantum objects can occur in an indeterminate state, called superposition, and can become “entangled”—a phenomenon that explains the behavior not observed in traditional computers.
However, these states are easily disrupted by electronic “noise,” rapidly creating errors in quantum computations, thereby rendering the development of useful machines highly challenging.
Current devices tend to be too small, with limited interconnectivity between qubits and are too ‘noisy’ to allow meaningful computations. However, they are sufficient to act as test beds for proof of principle concepts, such as detecting and potentially correcting errors using quantum codes.
Dr Robin Harper, School of Physics, The University of Sydney Nano Institute.
In contrast to classical switches in a mobile phone or laptop, which have the ability to run for many years without error, at this stage, quantum switches start failing after only fractions of a second.
One way to look at this is through the concept of entropy. All systems tend to disorder. In conventional computers, systems are refreshed easily and reset using DRAM and other methods, effectively dumping the entropy out of the system, allowing ordered computation. In quantum systems, effective reset methods to combat entropy are much harder to engineer. The codes we use are one way to dump this entropy from the system.
Steven Flammia, Professor, School of Physics, The University of Sydney Nano Institute.
Flammia was awarded the prestigious Pawsey Medal by the Australian Academy of Science on February 28th, 2019.
Dr Harper and Professor Flammia used codes for the detection and elimination of errors on IBM’s quantum device and demonstrated that the error rates dropped from 5.8% to 0.60%. Therefore, in contrast to 1 in 20 quantum gates failing, only one in 200 would fail, improvement of an order of magnitude.
“This is an important step forward to develop fault tolerance in quantum systems to allow them to scale up to meaningful devices,” stated Dr Harper.
The physicists, who are both scientists at the ARC Centre of Excellence for Engineered Quantum Systems, reiterated that this was a display of fault-tolerant gates on qubit pairs.
“There is still a long way to go before the quantum community can demonstrate fault tolerant computing,” stated Dr Harper. He stated that other teams have demonstrated improvements in other aspects of quantum devices using codes. The subsequent step is to develop and test these strategies on devices of larger scale with a few dozen qubits that allow the reuse and reinitialization of qubits.
Firms such as IBM, Google, Rigetti, and IonQ have already allowed or are about to allow quantum researchers to test their theoretical methods on these small, noisy machines.
These experiments are the first confirmation that the theoretical ability to detect errors in the operation of logical gates using quantum codes is advantageous in present-day devices, a significant step towards the goal of building large-scale quantum computers.
Dr Robin Harper, School of Physics, The University of Sydney Nano Institute.