In what is likely a major breakthrough for quantum computing, researchers from the University of New South Wales (UNSW) in Australia have managed for the first time to build the fundamental blocks of a quantum computer in silicon. The device was created using standard manufacturing techniques, by modifying current-generation silicon transistors, and the technology could scale up to include thousands, even millions of entangled quantum bits on a single chip. Gizmag spoke to the lead researchers to find out more.
Quantum computers are a peculiar beast. Though the machines we've been building since the 50s have been aiming to be as deterministic and reliable as possible – so a certain input will always result in the same output – in a quantum computer, this dynamic is turned on its head, and predictability is sacrificed for (sometimes) incredible speedups.
The downside to these significant speedups is that due to quantum effects, the results returned by a quantum algorithm are not deterministic. That is, even in the best of cases, a quantum computer is never guaranteed to return the correct result.
This usually means that a quantum algorithm must be run several times in succession to confirm that the solution is correct. So, in practice, classical computers will probably be faster and more practical than quantum computers for day-to-day operations, and quantum computers will only come in useful where massive parallelism is involved. When they are let loose, though, their speed will be spectacular. at UNSW are focusing on the potentially revolutionary approach of building quantum computers out of silicon, a material that is cheap, well-known by the industry, and which could ultimately pave the way for quantum computers with not 300 but thousands, even millions of fully entangled qubits.
Last year, UNSW scientists were able to create single "CMOS type" qubits that leveraged current transistor technology and silicon-28, a very common isotope of silicon, to achieve a very high fidelity of 99.6 percent for quantum operations. Now, the researchers have built on this to create what's known as a CNOT quantum logic gate. Together with a single controllable qubit, this is the basic building block of a quantum computer, and paves the way to quantum chips that can perform just about any operation.
"A CNOT gate is a [...] two-qubit gate [that] flips the state of the target qubit depending on the state of the control qubit," lead author of the paper Menno Veldhorst told Gizmag. "In our case, the target qubit flips its spin if the control qubit is pointing down. If the control qubit is pointing up, the target qubit will remain in the same state.
"This two-qubit gate is most essential for a quantum computer and together with single qubit operations, which we have already demonstrated with very high fidelity, provides what is called a universal gate set. This means that any gate set can be constructed out of [it]."
Although their quantum computers wouldn't work at room temperature, this approach lets the researchers operate their device at approximately 1 Kelvin (-272° C, -458° F). That may not seem like much of an improvement over previous designs, but, the researchers told us, recent advances in cooling technology have resulted in fridges that can easily be operated at these temperatures.
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