TOKYO: Scientists say they have solved a key hurdle to quantum computing by creating a new hybrid device that can be quickly initialised and read out.
Spin-based quantum computers have the potential to tackle difficult mathematical problems that cannot be solved using ordinary computers, but many problems remain in making these machines scalable, according to the study published in the journal Nature Communications.
Researchers led by the RIKEN Center for Emergent Matter Science in Japan have constructed a hybrid device made from two different types of qubit –the fundamental computing element of quantum computers.
As conventional computers appear to be reaching a limit, quantum computers have been touted as potential replacements, and they can tackle problems in a very different and potentially much more rapid way.
However, it has proven difficult to scale them up to the size required for performing real-world calculations.
In 1998, Daniel Loss, one of the authors of the current study, came up with a proposal, along with David DiVincenzo of IBM, to build a quantum computer by using the spins of electrons embedded in a quantum dot — a small particle that behaves like an atom, but that can be manipulated, so that they are sometimes called “artificial atoms.”
In the time since then, Loss and his team have endeavoured to build practical devices.
There are a number of barriers to developing practical devices in terms of speed.
First, the device must be able to be initialised quickly. Initialisation is the process of putting a qubit into a certain state, and if that cannot be done rapidly it slows down the device.
Second, it must maintain coherence for a time long enough to make a measurement.
Coherence refers to the entanglement between two quantum states, and ultimately this is used to make the measurement, so if qubits become decoherent due to environmental noise, for example, the device becomes worthless.
Finally, the ultimate state of the qubit must be able to be quickly read out.
While a number of methods have been proposed for building a quantum computer, the one proposed by Loss and DiVincenzo remains one of the most practically feasible, as it is based on semiconductors, for which a large industry already exists, researchers said.
For the current study, the team combined two types of qubits on a single device.
The first, a type of single-spin qubit called a Loss-DiVincenzo qubit, has very high control fidelity — meaning that it is in a clear state, making it ideal for calculations, and has a long decoherence time, so that it will stay in a given state for a relatively long time before losing its signal to the environment.
The downside to these qubits is that they cannot be quickly initialised into a state or read out.
The second type, called a singlet-triplet qubit, is quickly initialised and read out, but it quickly becomes decoherent.
For the study, the scientists combined the two types with a type of quantum gate known as a controlled phase gate, allowing spin states to be entangled between the qubits in a time fast enough to maintain the coherence.
This allowed the state of the single-spin qubit to be read out by the fast singlet-triplet qubit measurement. (AGENCIES)
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