Ultrafast Laser Pulses and Trapped Ions

Pulsed optical fields from mode-locked lasers have found widespread use as tools for precision control of quantum systems. The short duration of a mode-locked laser pulse gives it a wide bandwidth that can be used to simultaneously address multiple transitions that are spread over a large frequency range, such as molecular spectra. In contrast, trapped atoms typically exhibit very narrow spectral features compared to the bandwidth of a pico- or femtosecond pulse. By allowing a pulse train from a mode-locked laser to interact with the atoms, however, precise spectral control can be regained since the pulse train is a series of narrow spectral lines in the frequency domain (a frequency comb).

Frequency Combs with AOM shifts

We are exploring the ability of ultrafast lasers to provide a new regime for trapped ion quantum information processing. The wide bandwidth of a single pulse allows a single laser to act as thousands of pairs of phase-locked Raman lasers to drive transitions between qubit states (in our case, separated by more than 10 GHz). By tailoring a train of pulses we can coherently accumulate amplitude for a desired manipulation. We have recently used an optical frequency comb to address motional sidebands, cool the ion to the ground state of the harmonic trap, and entangle two atomic qubits:

"Entanglement of Atomic Qubits Using and Optical Frequency Comb" D. Hayes, D. N. Matsukevich, P. Maunz, D. Hucul, Q. Quraishi, S. Olmschenk, W. Campbell, J. Mizrahi, C. Senko, and C. Monroe, Phys. Rev. Lett. 104, 140501 (2010).

It is also possible for a single pulse to drive coherent transitions between qubit states. We have recently reported the creation of ultrafast single qubit gates that can flip the qubit state in less than 50 picoseconds:

"Ultrafast Gates for Single Atomic Qubits," W. C. Campbell, J. Mizrahi, Q. Quraishi, C. Senko, D. Hayes, D. Hucul, D. N. Matsukevich, P. Maunz, and C. Monroe, quant-ph/1005.4144 (2010).

These ultrafast Raman transitions should also be capable of affecting the motion of the ion. By using counterpropagating pulse trains (all generated by a single laser) we are exploring the potential for such pulse trains to produce momentum kicks that depend on the internal qubit state of the ion. Such kicks are much faster than the motional period of the ions in the trap (10 ps compared to 1 μs) and therefore represent an entirely new regime for the interactions that has the potential to circumvent some of the traditional limits of motion-mediated gates.

External references:

"Speed Optimized Two-Qubit Gates with Laser Coherent Control Techniques for Ion Trap Quantum Computing," J. J. Garcıa-Ripoll, P. Zoller, and J. I. Cirac, Phys. Rev. Lett. 91, 157901 (2003).

"Scaling Ion Trap Quantum Computation through Fast Quantum Gates," L.-M. Duan, Phys. Rev. Lett. 93, 100502 (2004).

"Superfast Laser Cooling," S. Machnes, M. B. Plenio, B. Reznik, A. M. Steane and A. Retzker, Phys. Rev. Lett. 104, 183001 (2010).

Recent Experiments:

 

Entanglement of qubits with an optical frequency comb:

 

  • Optical frequency comb used to address motional sideband transitions, cool an ion to the motional ground state using sideband cooling, couple the internal (qubit) state of an ion with its motional state, and entangle two atomic ion qubits.

 

 

 

Ultrafast coupling of atomic and photonic qubits

 

 

 

 
 

Precision Lifetime Measurements
  • Histogram of photon arrival times after exciting the  P1/2 states of Cd+ using a resonant frequency-quadrupled mode-locked Ti:Sapphire laser, producing an 80 MHz train of 1 picosecond pulses near 226.5nm (S-P1/2)
  • Precision Lifetime Measurements of a Single Trapped Ion with Ultrafast Laser Pulses, D.L. Moehring, B.B. Blinov, D.W. Gidley, R.N. Kohn, Jr., M.J. Madsen, T.D. Sanderson, R.S. Vallery and C. Monroe, Phys. Rev. A 73, 023413 (2006).
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Broadband laser cooling and crystallization of trapped Cd+ ions

  • Image of several crystallized Cd+ ions held in quadrupole trap, fluorescing under the excitation from picosecond laser pulses (bandwidth ~ 420 GHz) tuned to the red of the S-P1/2 resonance.
  • "Broadband Laser Cooling of Trapped Atoms with Ultrafast Laser Pulses," B.B. Blinov, R.N. Kohn, Jr., M.J. Madsen, P. Maunz, D.L. Moehring, and C. Monroe, J. Opt. Soc. Am. B 23, 1170 (2006).
  • Partial Rabi flopping of a 111Cd+ qubit with GHz Rabi frequencies.  Stimulated Raman transitions are drivin with an off-resonant Q-switched Nd:YAG laser at 266nm (5 nsec pulse duration, but >15 GHz bandwidth).

  • "Efficient Photoionization-Loading of Trapped Cadmium Ions with Ultrafast Pulses," L. Deslauriers, M. Acton, B. B. Blinov, K.-A. Brickman, P. C. Haljan, W. K. Hensinger, D. Hucul, S. Katnik, R. N. Kohn Jr., P. J. Lee, M. J. Madsen, P. Maunz, S. Olmschenk, D. L. Moehring, D. Stick, J. Sterk, M. Yeo, K. C. Younge, and C. Monroe, Phys. Rev. A (accepted for publication 2006), quant-ph/0608043 (2006).

Future Work

 

We are currently working to implement fast entangling gates between trapped ion qubits along the lines of L.-M. Duan (2004). Fast entangling gates have the advanatage that they may be performed on a timescale much faster than the period of atomic motion in the trap. Furthermore, such gates can be performed on ions embedded in large crystals without needing to address individual lines in the congested collective motional spectrum of large crystals. This simplification may enable scaling of trapped ion quantum information processing to large numbers of ions in the future.