The demonstration shows the viability of solid-state quantum computers, which – unlike earlier gas- and liquid-state systems – may represent the future of quantum computing because they can be easily scaled up in size. Current quantum computers are typically very small and – though impressive – cannot yet compete with the speed of larger, traditional computers.

Nature - Decoherence-protected quantum gates for a hybrid solid-state spin register

*Quantum gate operation in the presence of decoherence.*

The protocol for controlling quantum information pioneered by researchers at UC Santa Barbara, the Kavli Institute of Nanoscience in Delft, the Netherlands, and the Ames Laboratory at Iowa State University could open the door to larger-scale, more accurate quantum computations.

The researchers demonstrated the high-fidelity execution of a quantum search algorithm using this two-qubit system. Quantum search algorithms, if executed on a larger number of qubits, could provide search results of certain databases considerably faster than search algorithms performed on a classical computer.

The results of this study point to greater possibilities for quantum computers that overcome, according to Awschalom, the perception that spin qubits in semiconductors, such as those used in this work, suffer from too strong of environmental interactions to be useful qubits. These solid state spin systems also offer the added benefit of operating at room temperature, in contrast to other candidate qubit systems which operate at only at a fraction of a degree above absolute zero.

*The quantum circuit used in the demonstration is a 3mm x 3mm chip with a 1mm x 1mm diamond in the middle. Credit: Delft University of Technology/UC Santa Barbara.*

The team's diamond quantum computer system featured two quantum bits (called "qubits"), made of subatomic particles.

As opposed to traditional computer bits, which can encode distinctly either a one or a zero, qubits can encode a one and a zero at the same time. This property, called superposition, along with the ability of quantum states to "tunnel" through energy barriers, will some day allow quantum computers to perform optimization calculations much faster than traditional computers.

Like all diamonds, the diamond used by the researchers has impurities – things other than carbon. The more impurities in a diamond, the less attractive it is as a piece of jewelry, because it makes the crystal appear cloudy.

The team, however, utilized the impurities themselves.

A rogue nitrogen nucleus became the first qubit. In a second flaw sat an electron, which became the second qubit. (Though put more accurately, the "spin" of each of these subatomic particles was used as the qubit.)

Electrons are smaller than nuclei and perform computations much more quickly, but also fall victim more quickly to "decoherence." A qubit based on a nucleus, which is large, is much more stable but slower.

"A nucleus has a long decoherence time – in the milliseconds. You can think of it as very sluggish," said Lidar, who holds a joint appointment with the USC Viterbi School of Engineering and the USC Dornsife College of Letters, Arts and Sciences.

Though solid-state computing systems have existed before, this was the first to incorporate decoherence protection – using microwave pulses to continually switch the direction of the electron spin rotation.

"It's a little like time travel," Lidar said, because switching the direction of rotation time-reverses the inconsistencies in motion as the qubits move back to their original position.

The team was able to demonstrate that their diamond-encased system does indeed operate in a quantum fashion by seeing how closely it matched "Grover's algorithm."

The algorithm is not new – Lov Grover of Bell Labs invented it in 1996 – but it shows the promise of quantum computing.

The test is a search of an unsorted database, akin to being told to search for a name in a phone book when you've only been given the phone number.

Sometimes you'd miraculously find it on the first try, other times you might have to search through the entire book to find it. If you did the search countless times, on average, you'd find the name you were looking for after searching through half of the phone book.

Mathematically, this can be expressed by saying you'd find the correct choice in X/2 tries – if X is the number of total choices you have to search through. So, with four choices total, you'll find the correct one after two tries on average.

A quantum computer, using the properties of superposition, can find the correct choice much more quickly. The mathematics behind it are complicated, but in practical terms, a quantum computer searching through an unsorted list of four choices will find the correct choice on the first try, every time.

Though not perfect, the new computer picked the correct choice on the first try about 95 percent of the time – enough to demonstrate that it operates in a quantum fashion.

Protecting the dynamics of coupled quantum systems from decoherence by the environment is a key challenge for solid-state quantum information processing. An idle quantum bit (qubit) can be efficiently insulated from the outside world by dynamical decoupling as has recently been demonstrated for individual solid-state qubits. However, protecting qubit coherence during a multi-qubit gate is a non-trivial problem: in general, the decoupling disrupts the interqubit dynamics and hence conflicts with gate operation. This problem is particularly salient for hybrid systems in which different types of qubit evolve and decohere at very different rates. Here we present the integration of dynamical decoupling into quantum gates for a standard hybrid system, the electron–nuclear spin register. Our design harnesses the internal resonance in the coupled-spin system to resolve the conflict between gate operation and decoupling. We experimentally demonstrate these gates using a two-qubit register in diamond operating at room temperature. Quantum tomography reveals that the qubits involved in the gate operation are protected as accurately as idle qubits. We also perform Grover’s quantum search algorithm1, and achieve fidelities of more than 90% even though the algorithm run-time exceeds the electron spin dephasing time by two orders of magnitude. Our results directly allow decoherence-protected interface gates between different types of solid-state qubit. Ultimately, quantum gates with integrated decoupling may reach the accuracy threshold for fault-tolerant quantum information processing with solid-state devices

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