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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

High-power ultrafast laser source with 300 MHz repetition rate for trapped-ion quantum logic

Andreas Jechow, M.J. Petrasiunas, Michael G. Pullen, John Canning, M. Stevenson, Paul S. Westbrook, K. S. Feder, D. Kielpinski

Published May 1, 2011• 0 citations

Abstract

Trapped ions are a major candidate technology for scalable quantum computation. However, current methods for performing quantum logic gates with trapped ions are limited to gate times of about 10 μs. This drawback was solved in a recently proposed scheme that uses pairs of counter-propagating π-pulses, resonant with an allowed ion transition, where the time needed for a gate operation is inversely proportional to the laser repetition rate. The MOPA architecture allows scaling to high repetition rate at constant pulse energy. In this paper, the authors designed a laser system to provide high-power picosecond pulses with a high repetition rate resonant with the 370 nm S-P transition in Yb+. The laser source is almost completely implemented in fibre, lending increased stability to the system. As ultrafast laser sources are not readily available at 370 nm, with sufficient optical power, pulses were prepared and amplified over a number of stages. A harmonically mode-locked Er-doped fibre (EDF) seed laser, with a repetition rate of 300 MHz and a wavelength of 1564 nm is amplified and compressed to form shorter femtosecond pulses at high power.

DOI: 10.1109/cleoe.2011.5943009

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Trapped ions are a major candidate technology for scalable quantum computation. However, current methods for performing quantum logic gates with trapped ions are limited to gate times of about 10 μs. This drawback was solved in a recently proposed scheme that uses pairs of counter-propagating π-pulses, resonant with an allowed ion transition, where the time needed for a gate operation is inversely proportional to the laser repetition rate. The MOPA architecture allows scaling to high repetition rate at constant pulse energy. In this paper, the authors designed a laser system to provide high-power picosecond pulses with a high repetition rate resonant with the 370 nm S-P transition in Yb+. The laser source is almost completely implemented in fibre, lending increased stability to the system. As ultrafast laser sources are not readily available at 370 nm, with sufficient optical power, pulses were prepared and amplified over a number of stages. A harmonically mode-locked Er-doped fibre (EDF) seed laser, with a repetition rate of 300 MHz and a wavelength of 1564 nm is amplified and compressed to form shorter femtosecond pulses at high power.

The conventional approach to perform two-qubit gate operations in trapped ions relies on exciting the ions on motional sidebands with laser light, which is an inherently slow process. One way to implement a fast entangling-gate protocol requires a suitable pulsed laser to increase the gate speed by orders of magnitude. However, the realization of such a fast entangling-gate operation presents a big technical challenge, as such the required laser source is not available off the shelf. For this, we engineer an ultrafast entangling-gate source based on a frequency comb. The source generates bursts of several hundred mode-locked pulses with pulse energy approximately 800 pJ at 5-GHz repetition rate at 393.3 nm and complies with all requirements for implementing a fast two-qubit gate operation. Using a single, chirped ultraviolet pulse, we demonstrate a rapid adiabatic passage in a

{\mathrm{Ca}}^{+}

ion. To verify the applicability and projected performance of the laser system for inducing entangling gates we run simulations based on our source parameters. The gate time can be faster than a trap period with an error approaching ${10}^{\ensuremath{-}4}$.

Fast entangling gates for trapped ion pairs offer vastly improved gate operation times relative to implemented gates, as well as approaches to trap scaling. Gates on a neighbouring ion pair only involve local ions when performed sufficiently fast, and we find that even a fast gate between a pair of distant ions with few degrees of freedom restores all the motional modes given more stringent gate speed conditions. We compare pulsed fast gate schemes, defined by a timescale faster than the trap period, and find that our proposed scheme has less stringent requirements on laser repetition rate for achieving arbitrary gate time targets and infidelities well below 10 −4 . By extending gate schemes to ion crystals, we explore the effect of ion number on gate fidelity for coupling two neighbouring ions in large crystals. Inter-ion distance determines the gate time, and a factor of five increase in repetition rate, or correspondingly the laser power, reduces the infidelity by almost two orders of magnitude. We also apply our fast gate scheme to entangle the first and last ions in a crystal. As the number of ions in the crystal increases, significant increases in the laser power are required to provide the short gate times corresponding to fidelity above 0.99.

Experimental realizations of two qubit entangling gates with trapped ions typically rely on addressing spectroscopically resolved motional sidebands, limiting gate speed to the secular frequency. Fast entangling gates using ultrafast pulsed lasers overcome this speed limit. This approach is based on state-dependent photon recoil kicks from a sequence of counter-propagating, resonant, ultrafast pulse pairs, which can allow sub-microsecond gate speeds. Here, we demonstrate a key component of the ultrafast gate protocol, the coherent excitation of a 171Yb+ ion across the 2S1∕2-2P1∕2 transition with a single near-resonant short optical pulse at 369.53 nm. We achieve a maximum population transfer of 94(1)% using a picosecond pulsed laser that can be tuned across the 2S1∕2-2P1∕2 transition and 42.5(9)% with 190(7) GHz detuning.

We present a model for implementing fast entangling gates (∼1 μs) with ultrafast pulses in arbitrarily long ion chains, that requires low numbers of pulses and can be implemented with laser repetition rates well within experimental capability.We demonstrate that we are able to optimize pulse sequences that have theoretical fidelities above 99.99% in arbitrarily long ion chains, for laser repetition rates on the order of 100-300 MHz.Notably, we find higher repetition rates are not required for gates in longer ion chains, which is in contrast to scaling analyses with other gate schemes.When pulse imperfections are considered in our calculations, we find that achievable gate fidelity is independent of the number of ions in the chain.We also show that pulse control requirements do not scale up with the number of ions.We find that population transfer efficiencies of above 99.9% from individual ultrafast pulses is the threshold for realizing high-fidelity gates, which may be achievable in near-future experiments.