If your quantum computer starts to go awry, just give it a high-energy kick. Perfectly timed energy pulses could beat the information-destroying effects of a quantum computer’s environment, according to studies published by two groups.
Making these machines run algorithms that outperform classical computers will involve using millions of qubits, subatomic structures that maintain delicate quantum superpositions and entanglement states to process information. Unlike a traditional computing bit, which is either in the position of a 0 or 1, qubits can be a mixture of both.
However, this state is highly vulnerable to disturbance by thermal and mechanical vibrations, stray electromagnetic fields or fluctuations in the voltages that hold the qubits in place on the apparatus. Any one of these can knock the qubits out of their superposition, which would stop them from being able to compute anything.
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Some qubits are made of ions – charged particles trapped in an array by oscillating electromagnetic fields – but they’re hard to protect in the large-scale systems that will be necessary if quantum computers are to achieve their full potential. “You can’t control every ion absolutely perfectly,” says Winfried Hensinger of the University of Sussex. He leads one of the groups that have demonstrated that qubits can be protected without extensive physical shielding, or an expensive cooling apparatus. All it takes is a well-crafted pulse of energy.
Quantum gates
The innovation applies to a quantum logic gate, a sequence of operations to process quantum information, that uses two trapped ions as qubits. The Mølmer-Sørensen (MS) gate lets researchers build all the necessary architectures for quantum logic operations without the need to cool the ions to their lowest energy states. This makes it more practical than other types of gate.
Like all implementations of quantum computing, the MS gate is still vulnerable to fidelity loss because of external noise, which results in a phenomenon known as decoherence. This is where some of the quantum information describing the properties of the qubits leaks out into the environment. Other quantum particles in the surrounding area become entangled with the pair of ions, rather than the ions being entangled only with one another. This causes the fidelity loss. “Having gates that are resilient to noise is crucial,” says Roee Ozeri at the Weizmann Institute in Israel, who led one of the studies.
In 2016, Florian Mintert and Farhang Haddadfarshi at Imperial College published a theoretical scheme for mitigating this loss by hitting the qubits with carefully-controlled, precisely-tuned pulses of energy that make the ions’ quantum states become less prone to entangling with external particles, without affecting their entanglement with each other.
Stopping the drift
Now, we have the experimental proof of their theory. Hensinger and colleagues have protected the fidelity of qubits made of ytterbium ions using pulses of microwave energy. “We add multiple frequencies that make the system resilient,” Hensinger says. Unlike error-correcting algorithms, which use extra qubits to reset qubits that have gone adrift, this technique stops the drift from happening in the first place.
Ozeri and his colleagues have also put this theory to the test. They maintained fidelity in a string of strontium ions by hitting them with laser pulses composed of an array of frequencies that are selected to maximise the ion-ion entanglement and reduce the ions’ coupling with their environment.
As a result, they engineered entanglements that can withstand operating errors in the way laser pulses are applied to the ions, changes in the voltages that hold the ions in place, and even heat. “We have reduced the dependence on the system’s temperature,” says Yotam Shapira at the Weizmann Institute.
Whether this particular approach can be scaled beyond two-qubit systems remains an open question. However, Chris Monroe of the University of Maryland says the results should be of interest to everyone working with ion traps nonetheless. “It shows that certain types of noise need not be debilitating,” he says.
Physical Review Letters
Physical Review Letters
Read more: Google’s quantum computing plans threatened by IBM curveball
