Abstract Short pulses from mode-locked lasers can produce background-free atomic fluorescence by allowing temporal separation of the prompt incidental scatter from the subsequent atomic emission. We use this to improve our quantum state detection of optical-frequency and electron-shelved trapped ion qubits by more than two orders of magnitude. For direct detection of qubits defined on atomic hyperfine structure, however, the large bandwidth of short pulses is greater than the hyperfine splitting, and repeated excitation is not qubit state selective. Here, we show that the state resolution needed for projective quantum measurement of hyperfine qubits can be recovered by applying techniques from coherent control to the orbiting valence electron of the queried ion. We demonstrate electron wavepacket interference to allow readout of the original qubit state using broadband pulses, even in the presence of large amounts of background laser scatter.
We present two novel approaches to state detection of qubits defined with trapped ions. The first uses simple pulse sequences from a mode-locked laser to induce state-dependent excitations in less than 1 ns. The resulting atomic fluorescence occurs in the dark, allowing the placement of non-imaging detectors right next to the atom to improve the qubit state detection efficiency and speed.The second employs the long lived F state in Yb+ which has been used in quantum information science almost exclusively for clocks and optical-frequency qubits. We describe how this resource can be used in conjunction with the ground state S1/2 manifold to aid in the scaling of trapped ion quantum information science. Narrow-band optical pumping into the 2F7/2 from one of the conventional 2S1/2 qubit states is projected to achieve a higher state preparation and measurement (SPAM) fidelity than any other demonstrated technique.
Abstract In order to faithfully detect the state of an individual two-state quantum system (qubit) realized using, for example, a trapped ion or atom, state selective scattering of resonance fluorescence is well established. The simplest way to read out this measurement and assign a state is the threshold method. The detection error can be decreased by using more advanced detection methods like the time-resolved method (Myerson et al 2008 Phys. Rev. Lett. 100 200502 ) or the π -pulse detection method (Hemmerling et al 2012 New. J. Phys. 14 023043 ). These methods were introduced to qubits with a single possible state change during the measurement process. However, there exist many qubits like the hyperfine qubit of where several state change are possible. To decrease the detection error for such qubits, we develop generalizations of the time-resolved method and the π -pulse detection method for such qubits. We show the advantages of these generalized detection methods in numerical simulations and experiments using the hyperfine qubit of . The generalized detection methods developed here can be implemented in an efficient way such that experimental real time state discrimination with improved fidelity is possible.
We propose and demonstrate a protocol for high-fidelity indirect readout of trapped ion hyperfine qubits, where the state of a
9Be+ qubit ion is mapped to a
25Mg+ readout ion using laser-driven Raman transitions. By partitioning the
9Be+ ground-state hyperfine manifold into two subspaces representing the two qubit states and choosing appropriate laser parameters, the protocol can be made robust to spontaneous photon scattering errors on the Raman transitions, enabling repetition for increased readout fidelity. We demonstrate combined readout and back-action errors for the two subspaces of ${1.2}_{\ensuremath{-}0.6}^{+1.1}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ and ${0}_{\ensuremath{-}0}^{+1.9}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ with 68% confidence while avoiding decoherence of spectator qubits due to stray resonant light that is inherent to direct fluorescence detection.
In order to faithfully detect the state of an individual two-state quantum system (qubit) realized using, for example, a trapped ion or atom, state selective scattering of resonance fluorescence is well established.The simplest way to read out this measurement and assign a state is the threshold method.The detection error can be decreased by using more advanced detection methods like the time-resolved method [1] or the π-pulse detection method [2].These methods were introduced to qubits with a single possible state change during the measurement process.However, there exist many qubits like the hyperfine qubit of 171 Y b + where several state change are possible.To decrease the detection error for such qubits, we develope generalizations of the time-resolved method and the π-pulse detection method for such qubits.We show the advantages of these generalized detection methods in numerical simulations and experiments using the hyperfine qubit of 171 Y b + .The generalized detection methods developed here can be implemented in an efficient way such that experimental real time state discrimination with improved fidelity is possible.
The highly popular ytterbium-171 (
171mathrmYb+) ion is commonly employed in quantum information research as a qubit whose excellent coherence time and fast, simple state preparation has allowed cutting edge work in quantum computation and simulation. Despite these large benefits, the demonstrated measurement fidelity of this ion has lagged the state preparation and gate fidelity achieved to date.In this thesis we investigate and realize methods of increasing the measurement fidelity of
171mathrmYb+ in a scaleable way for large quantum systems. Using methods of coherent control, we implement a pulsed state detection scheme using a mode-locked laser to perform background-free spectroscopy of the ``bright'' state of the qubit. The small hyperfine splitting of the ion necessitates the use of multiple (two) pulses to manipulate time dynamics of the ion to excite a single transition. A Mach-Zehnder interferometer is constructed to control these pulse separations both coarsely (
sim 237 ps) and on a fine sub-femtosecond scale. These pulses cause destructive/constructive interference of the electron wave packet of a single ion levitated in vacuum and are engineered to state-selectively excite the qubit. This allows measurement of the qubit whose transition frequency is much smaller than the bandwidth of the interrogation laser.During this spectroscopy, mechanical forces from the mode-locked laser frequency comb can drive the ion into large coherent states of motion. This motion has been dubbed ``phonon lasing''. We investigate the phonon lasing affect and how the ion interacts with multiple comb teeth. The large number of teeth leads to a protection mechanism from runaway energy gain by near-by blue detuned teeth, allowing ions to be trapped and cooled by the mode-locked laser, regardless of its detuning. We further explore these discrete amplitude coherent states by injecting energy into the ion's motion and exciting higher-order oscillations.We, for the first time, implement an ``electron shelving'' of the hyperfine qubit, and incoherently transfer the bright state population in the extremely long-lived ($\\approx$5 yr)
2F
7/2 state of
171Yb
+, functionally disconnected state. This is accomplished via narrow-band optical pumping on the
2S
1/2 to the
2D
5/2 quadrupole which has a leaky dipole channel into the
2F
7/2. Narrow-band optical pumping is again used to rescue the ion at the end of the experiment with the aid of a 760 nm E2 transition back into the cooling cycle. Measurement with this scheme is no longer limited by off-resonant effects from the main cycling transition. Limits of this novel technique, as well as further directions using the F state as a utility for quantum information are explored. Finally, we combine the pulsed background-free spectroscopy with shelving and demonstrate high-fidelity, background free detection of a single trapped
171mathrmYb+ qubit.
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.