Combing a qubit

Abstract

We demonstrate the use of an optical frequency comb to coherently control and entangle atomic qubits. A train of off-resonant ultrafast laser pulses is used to efficiently and coherently transfer population between electronic and vibrational states of trapped atomic ions and implement an entangling quantum logic gate with high fidelity. This technique can be extended to the high field regime where operations can be performed faster than the trap frequency. This general approach can be applied to more complex quantum systems, such as large collections of interacting atoms or molecules. DOI: 10.1103/PhysRevLett.104.140501 PACS numbers: 03.67.Bg, 32.80.Qk, 37.10.Rs, 37.10.Vz The optical frequency comb generated from an ultrafast laser pulse train has revolutionized optical frequency metrology [1][2][3][4] and is now playing an important role in high resolution spectroscopy [5]. The spectral purity yet large bandwidth of optical frequency combs also provides a means for the precise control of generic quantum systems, with examples such as the quantum control of multilevel atomic systems [6,7], laser cooling of molecules or exotic atomic species [8,9], and quantum state engineering of spins in semiconductors [10,11] or rovibrational states in molecules [12,13]. The optical frequency comb may become a crucial component in the field of quantum information science, where complex multilevel quantum systems must be controlled with great precision [14].In this Letter, we report the use of an optical frequency comb generated from an ultrafast mode-locked laser to efficiently control and faithfully entangle two trapped atomic ion qubits. The optical pulse train drives stimulated Raman transitions between hyperfine levels [15,16], accompanied by qubit state-dependent momentum kicks [17]. The coherent accumulation of these pulses generates particular quantum gate operations that are controlled through the phase relationship between successive pulses. This precise spectral control of the process along with the large optical bandwidth required for bridging the qubit frequency splitting forms a simple method for controlling both the internal electronic and external motional states of trapped ion qubits, and may be extended to most atomic species. This same approach can be applied to control larger trapped ion crystals with more advanced pulseshaping techniques, and can also be extended to a strong pulse regime where only a few high-power pulses are needed for fast quantum gate operations in trapped ions [17][18][19].High fidelity qubit operations through Raman transitions are typically achieved by phase-locking frequency components separated by the energy difference of the qubit states. This is traditionally accomplished in a bottom-up type of approach where either two monochromatic lasers are phase locked or a single cw laser is modulated by an acoustooptic (AO) or an electro-optic (EO) modulator. However, the technical demands of phase-locked lasers and the limited bandwidths of the modulators hinder their application to ex...