Injection locking of two frequency-doubled lasers with 32  GHz offset for driving Raman transitions with low photon scattering in Ca43^+

Abstract

We describe the injection locking of two infrared (794 nm) laser diodes which are each part of a frequencydoubled laser system. An acousto-optic modulator (AOM) in the injection path gives an offset of 1.6 GHz between the lasers for driving Raman transitions between states in the hyperfine split (by 3.2 GHz) ground level of 43 Ca + . The offset can be disabled for use in 40 Ca + . We measure the relative linewidth of the frequency-doubled beams to be 42 mHz in an optical heterodyne measurement. The use of both injection locking and frequency doubling combines spectral purity with high optical power. Our scheme is applicable for providing Raman beams across other ion species and neutral atoms where coherent optical manipulation is required. c 2013 Optical Society of America OCIS codes: 000.0000, 999.9999.Ions held in rf traps are a standard system for quantum information processing experiments [1]. One of the key challenges is to realize coherent operations with fidelities high enough to allow the implementation of quantum error correction protocols [2]. The sub-states of the ground level of alkali earth ions are commonly used as the qubit states [3,4]. In 43 Ca + with its nuclear spin (I = 7/2) a pair of Zeeman states from the F = 3 and F = 4 hyperfine manifolds can be chosen [5, 6]. At specific magnetic field strengths pairs of magnetic-field insensitive clockstates are available that have recently been shown to have extremely long coherence times [7] making 43 Ca + a promising species for quantum information. Coherent transitions between the qubit states may either be driven by rf/microwave radiation [8] or by a pair of Raman laser beams near the 4S 1/2 to 4P 1/2 transition at 397 nm [4]. Their strong coupling to the motion makes Raman beams particularly attractive for sideband cooling and motional entangling gates.With Raman lasers the choice of detuning ∆ from the 4S 1/2 to 4P 1/2 transition is a compromise between higher speed at small detuning (scaling ∼ 1/∆) and lower offresonant photon scattering (which introduces qubit errors) at large detuning (scaling ∼ 1/∆ 2 ). Typical detunings are in the range of 10−1000GHz. For a desired gate speed, the fidelity of qubit operations depends on the available Raman beam power. Typically, powers around 1 mW have been used. For common experimental parameters and a desired two-qubit gate error probability of 10 −4 in a gate time of 10 µs, beam powers of around 100 mW are required [9,10].The two-photon Raman process only depends on the difference frequency between the two beams and its detuning from the qubit resonance which is why there are no stringent requirements for absolute frequency stability. Instead, relative interferometric stability between the beams is needed. (TAs) are not yet commercially available at 397 nm. Another alternative is injection locking, where only a low intensity seed beam needs to be frequency shifted. This has recently been reported with violet diode lasers [13]. While this approach is currently limited to ∼ 20 mW of output power, it shows ...

High-Fidelity Quantum Logic Gates Using Trapped-Ion Hyperfine Qubits

“…The beams for the single-qubit gates copropagate (< 10 mrad beam angle) giving negligible coupling to the motion (η < 10 −3 ). The ≈ 397 nm Raman beams are derived from a pair of frequency-doubled diode lasers, the second (slave) of which is injection-locked at 794 nm from the first (master) via a double-pass 800 MHz acousto-optic modulator (AOM), allowing the 3.2 GHz qubit splitting to be bridged for qubit carrier and sideband manipulations [17]. The output beam from the master is split, switched using two AOMs, and brought close to the trap using ≈ 1 m long optical fibres; these are the beams used to drive the two-qubit gate.…”

“…The ion separation (3.5 µm) is set to be 12 1 2 wavelengths of the travelling standing wave which results from the interference of the two-qubit gate beams. All beams are derived from a master/slave pair of frequency-doubled lasers whose frequency difference (≈ f0) is set by optical injection locking (at 794 nm) via an acousto-optic modulator (AOM) [17]. The beams are frequency-shifted and switched by further AOMs and brought close to the trap using optical fibres.…”

Generation of 130 mW of 3975 nm tunable laser via ring-cavity-enhanced frequency doubling

“…Compact high-power tunable violet laser sources are of interest for many scientific and technological applications, such as optical data storage, laser printing and lithography, spectroscopy, quantum optics, and cold-atom physics. The cooling and trapping of Ca ions, which play an important role in quantum manipulation and quantum computing areas, have been realized with a tunable 397 nm light source in several experiments [1][2][3][4]. Particularly, in the field of quantum optics, the major objective of the violet laser is the realization of the squeezed lights by pumping an optical parametric oscillator (OPO).…”

Limits on atomic qubit control from laser noise

“…For an LO of an optical qubit, this can be performed using the transmitted light of a high-finesse cavity, such that the resulting laser frequency noise is low-pass filtered by the cavity linewidth [16]. Offset injection locking can be used to passively lock two diode lasers, where the light of a primary laser is shifted in frequency by the qubit frequency and injected into a secondary laser [46]. The effective bandwidth of the phase locking is that of the cavity bandwidth, which, for the short cavity lengths present in ECDLs, can be of order GHz.…”