Experimental Protection of Two-Qubit Quantum Gates against Environmental Noise by Dynamical Decoupling

Abstract

Hybrid systems consisting of different types of qubits are promising for building quantum computers if they combine useful properties of their constituent qubits. However, they also pose additional challenges if one type of qubits is more susceptible to environmental noise than the others. Dynamical decoupling can help to protect such systems by reducing the decoherence due to the environmental noise, but the protection must be designed such that it does not interfere with the control fields driving the logical operations. Here, we test such a protection scheme on a quantum register consisting of the electronic and nuclear spins of a nitrogen-vacancy center in diamond. The results show that processing is compatible with protection: The dephasing time was extended almost to the limit given by the longitudinal relaxation time of the electron spin.PACS numbers: 03.67.Pp,03.67.Lx -Introduction. Choosing the most suitable qubit systems and implementing high-precision quantum gate operations on them may be considered as the main challenges for building quantum computers that will be capable of solving certain problems qualitatively faster than classical computers [1,2]. In every physical realization, environmental noise is present, which degrades the information stored in the quantum systems. Several techniques have been developed for protecting quantum systems against this effect, including passive schemes like decoherence-free subspaces [3] or active techniques like dynamical decoupling (DD) [4][5][6]. So far, these techniques have been applied mostly to the protection of quantum states in quantum memories [7][8][9][10][11][12]. However, similar protection is also required for quantum information processing. Here, the protection must be designed in such a way that it decouples the protected system from environmental noise, but not from the control fields that are applied for driving the gate operations. Experimental implementations of protected single-qubit gates and some special 2-qubit gates, such as a controlled-NOT (CNOT) gate, were recently reported [13][14][15][16], but so far no scheme was demonstrated that can be applied to arbitrary twoqubit gates in different physical qubit systems. Implementing quantum computing in hybrid systems is a promising strategy for building quantum computers, as it increases the range of possible systems and allows one to combine useful properties of different types of qubits [17][18][19][20]. As a typical hybrid system, the nitrogen vacancy (NV) center in diamond consists of an electronic spin S = 1 and a nitrogen nuclear spin I = 1 and combines several useful properties for a quantum register. It also poses some challenges, mostly because the characteristic properties of the two types of spins differ by many orders of magnitude. For example, the natural coherence time at room temperature of the electron spin is of the order of 1 µs, while that of the nuclear spin is 5 ms [15]. The electron spin dephasing time is usually shorter than typical durations of gates applied to the nit...

Polarizing the electronic and nuclear spin of the NV-center in diamond in arbitrary magnetic fields: analysis of the optical pumping process

“…Enhancing the occupation probability of the target input state in a certain computational space can result in a better performance in experiments like quantum gate operations. For instance, in an experiment where dynamical decoupling was applied to implement protected operation of a controlled rotation gate in a definite subspace [15], an enhanced purity of the input state increases the signal-to-noise ratio. 3 6 3 3 3 3 , , , 3 3 6 3 3 3 ,…”

“…Most of these applications use the long coherence times of the NV center [5] and the optical initialization and readout of the system [6][7][8]. The NV center has been used for several interesting demonstrations like robust multipartite entanglement persisting over a timescale of milliseconds at room temperature [4], quantum interference between photons [9,10], an efficient quantum memory [11][12][13], single shot readout of single nuclear spins [14] or quantum gate operations where dephasing is protected with the help of dynamical decoupling [15]. These developments of quantum information processing based on the defect centers in diamond have remarkably boosted solid state quantum technology and pioneered a new way towards reliable implementation of quantum computation [16].…”

Floquet engineering to entanglement protection of distant nitrogen vacancy centers

“…Since the driving field does not commute with these terms, it automatically suppresses most of the detrimental effects of environmental noise and stabilize the entanglement present in the system. If necessary, the environmental noise can be suppressed further by combining the driving withDD pulses, provided these pulses are short compared to the Floquet period [50,51]. Other options include additional driving fields that drive transitions between the dressed states.…”

Reviving the precision of multiple entangled probes in an open system by simple π -pulse sequences

“…It has been well applied in one-qubit and two-qubit quantum information processes [27,28]. Here we combine the traditional quantum metrology strategies, the quantum-classical (QC) and quantum-quantum (QQ) metrology protocol [4,5], with the DD method to construct protected QC (P-QC) and protected QQ (P-QQ) protocols as shown in Fig.…”

“…Omitting the global phase √ i N , we can get (N + 1)-qubit |GHZ+ state and those processes can be denoted by an operations U ent,h . If DD sequences [27,28] are interleaved in the generation processes, the decoherence effects can be neglected. It also holds for detection process.…”