FAST LASERS AND COLD TRAPPED IONS

The coherent interaction between the motion of cold atoms and fast (picosecond or femtosecond) lasers is largely unexplored.  Atomic ions, tightly confined in electromagnetic traps, allow clean studies of the effect of pulsed laser on trapped ion motion, and may lead to new opportunities in the design of superfast multi-ion entangling quantum logic gates that may be insensitive to conventional sources of decoherence.

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We concentrate the interaction of trapped cadmium and ytterbium ions with off-resonant nanosecond and resonant picosecond pulsed lasers.  The large fine-structure splittings of the ²P states in these ions (70 THz in Cd⁺ and 100 THz in Yb⁺) permits the use of fsec and psec lasers to be used for spin-dependent impulsive forces for the generation of motional superposition states to study fast decoherence processes and operate fast quantum gates.  Pulsed-laser excitation of excited atomic states is also indispensible for the linking of ion qubits through photonic channels.

Ultrafast coupling of atomic and photonic qubits

• Indirect observation of entanglement between atomic (hyperfine) and photonic (frequency) qubits:

• "Ultrafast Coherent Coupling of Atomic Hyperfine and Photon Frequency Qubits," M.J. Madsen, D.L. Moehring, P. Maunz, R.N. Kohn, Jr., L.-M. Duan, and C. Monroe, Phys. Rev. Lett. 97, 040505 (2006).

• Theoretical description of scalable quantum networks based on this coupling:

• "Probabilistic Quantum Gates between Remote Atoms through Interference of Optical Frequency Qubits," L.-M. Duan, M.J. Madsen, D.L. Moehring, P. Maunz, R.N. Kohn, Jr., and C. Monroe, Phys. Rev. A 73, 062324 (2006).

• Histogram of photon arrival times after exciting the  P_1/2 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-P_1/2)
• 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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• 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-P_1/2 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).

Recent work highlighted at the 2005 Gordon Conference on Atomic Physics:

Future Work

We are currently focusing on the use of resonant picosecond pulsed lasers for two qubit gates proposed by García-Ripoll, Zoller, and Cirac (2003).  This scheme has the advantages that the ions need not be cooled to the ground state of motion and can have arbitrarily fast gate times.  Duan (2004) has shown how this scheme can be scaled to multiple qubits in a large crystal by symmetrizing the pulse sequence and perhaps using pulse-shaping techniques.