C W Chou

National Institute of Standards and Technology, Gaithersburg, MD, United States

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Publications (13)92.42 Total impact

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    ABSTRACT: We present an optical-electronic approach to generating microwave signals with high spectral purity. By circumventing shot noise and operating near fundamental thermal limits, we demonstrate 10 GHz signals with an absolute timing jitter for a single hybrid oscillator of 420 attoseconds (1Hz - 5 GHz).
    Applied Physics Letters 05/2012; 100(23). · 3.79 Impact Factor
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    ABSTRACT: We demonstrate a 10 GHz hybrid oscillator comprised of a phase stabilized optical frequency comb divider and a room temperature dielectric sapphire oscillator. Characterization of the 10 GHz microwave signal via comparison of two independent hybrid oscillators yields a combined phase noise, L(f) = −100 dBc/Hz at a 1 Hz offset and L(f) 1 MHz. The associated absolute timing jitter is 0.93 fs (1 Hz to Nyquist).
    01/2012;
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    ABSTRACT: We demonstrate a general method for state detection of trapped ions that can be applied to a large class of atomic and molecular species. We couple a spectroscopy ion (27Al+) to a control ion (25Mg+) in the same trap and perform state detection through off-resonant laser excitation of the spectroscopy ion that induces coherent motion. The motional amplitude, dependent on the spectroscopy ion state, is measured either by time-resolved photon counting or by resolved sideband excitations on the control ion. The first method provides a simplified way to distinguish clock states in 27Al+, which avoids ground-state cooling and sideband transitions. The second method reduces spontaneous emission and optical pumping on the spectroscopy ion, which we demonstrate by nondestructively distinguishing Zeeman sublevels in the (1)S0 ground state of 27Al+.
    Physical Review Letters 12/2011; 107(24):243902. · 7.73 Impact Factor
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    ABSTRACT: We place two atoms in quantum superposition states and observe coherent phase evolution for 3.4×10(15) cycles. Correlation signals from the two atoms yield information about their relative phase even after the probe radiation has decohered. This technique allowed a frequency comparison of two (27)Al(+) ions with fractional uncertainty 3.7(-0.8)(+1.0)×10(-16)/√[τ/s]. Two measures of the Q factor are reported: The Q factor derived from quantum coherence is 3.4(-1.1)(+2.4)×10(16), and the spectroscopic Q factor for a Ramsey time of 3 s is 6.7×10(15). We demonstrate a method to detect the individual quantum states of two Al(+) ions in a Mg(+)-Al(+)-Al(+) linear ion chain without spatially resolving the ions.
    Physical Review Letters 04/2011; 106(16):160801. · 7.73 Impact Factor
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    ABSTRACT: We compare the rates of two Al+optical clocks. Despite many differences, their rates agree to 1.8 +/- 0.7 x 10-17, within the accuracy limit of the older clock. The newer clock has an accuracy of 8.6 x 10-18and stability near 10-15(?/s)-1/2.
    Laser Science; 10/2010
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    ABSTRACT: Observers in relative motion or at different gravitational potentials measure disparate clock rates. These predictions of relativity have previously been observed with atomic clocks at high velocities and with large changes in elevation. We observed time dilation from relative speeds of less than 10 meters per second by comparing two optical atomic clocks connected by a 75-meter length of optical fiber. We can now also detect time dilation due to a change in height near Earth's surface of less than 1 meter. This technique may be extended to the field of geodesy, with applications in geophysics and hydrology as well as in space-based tests of fundamental physics.
    Science 09/2010; 329(5999):1630-3. · 31.20 Impact Factor
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    ABSTRACT: Frequency standards based on narrow optical transitions in 27 Al+ and 199Hg+ ions have been developed at NIST. Both standards have absolute reproducibil­ ities of a few parts in 10 17 . This is about an order of magnitude better than the fractional uncertainty of the SI second, which is based on the 133Cs hyper­ fine frequency. Use of femtosecond laser frequency combs makes it possible to compare the optical frequency standards to microwave frequency standards or to each other. The ratio ofthe Al+ and Hg+ frequencies can be measured more accurately than the reproducibility of the primary cesium frequency standards. 'Frequency measurements made over time can be used to set limits on the time variation of fundamental constants, such as the fine structure constant Q or the quark masses.
    02/2010;
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    ABSTRACT: We have constructed an optical clock with a fractional frequency inaccuracy of 8.6x10{-18}, based on quantum logic spectroscopy of an Al+ ion. A simultaneously trapped Mg+ ion serves to sympathetically laser cool the Al+ ion and detect its quantum state. The frequency of the {1}S{0}<-->{3}P{0} clock transition is compared to that of a previously constructed Al+ optical clock with a statistical measurement uncertainty of 7.0x10{-18}. The two clocks exhibit a relative stability of 2.8x10{-15}tau{-1/2}, and a fractional frequency difference of -1.8x10{-17}, consistent with the accuracy limit of the older clock.
    Physical Review Letters 02/2010; 104(7):070802. · 7.73 Impact Factor
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    ABSTRACT: We have investigated theoretically and experimentally a method for preparing Dicke states in trapped atomic ions. We consider a linear chain of $N$ ion qubits that is prepared in a particular Fock state of motion, $|m>$. The $m$ phonons are removed by applying a laser pulse globally to the $N$ qubits, and converting the motional excitation to $m$ flipped spins. The global nature of this pulse ensures that the $m$ flipped spins are shared by all the target ions in a state that is a close approximation to the Dicke state $\D{N}{m}$. We calculate numerically the fidelity limits of the protocol and find small deviations from the ideal state for $m = 1$ and $m = 2$. We have demonstrated the basic features of this protocol by preparing the state $\D{2}{1}$ in two $^{25}$Mg$^+$ target ions trapped simultaneously with an $^{27}$Al$^+$ ancillary ion. Comment: 5 pages, 2 figures
    Physical Review A 08/2009; · 3.04 Impact Factor
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    ABSTRACT: Frequency standards (atomic clocks) based on narrow optical transitions in 27Al+ and 199Hg+ have been developed over the past several years at NIST. Both types of standards are based on single ions confined in Paul traps, but differ in the methods used to prepare and detect the internal atomic states. Al+ lacks a strong, laser-accessible transition for laser-cooling and for state preparation and detection. Coupling with a Be+ ion, trapped simultaneously with the Al+ ion, enables state manipulation, detection, and cooling of the Al+ ion. Both standards have achieved absolute reproducibilities of a few parts in 1017. Development of femtosecond laser frequency combs makes it possible to directly compare optical frequencies. The present determination of fAl/fHg is 1.052 871 833 148 990 438 (55), where the uncertainty is expressed in units of the least significant digit. Measurements of fAl/fHg made over about one year show a drift rate consistent with zero. This result can be used to place limits on time variations of fundamental constants such as the fine structure constant α.
    06/2009;
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    ABSTRACT: Repeated measurements of the frequency ratio of 199Hg+ and 27Al+ single-atom optical clocks over the course of a year yield a constraint on the possible present-era temporal variation of the fine-structure constant alpha. The time variation of the measured ratio corresponds to a time variation in the fine structure constant of &dot; alpha /alpha = (-1.6± 2.3) × 10-17/year, consistent with no change. The frequency ratio of these clocks was measured with a fractional uncertainty of 5.2 × 10-17. Stability simulations for optical clocks whose probe period is limited by 1/f-noise in the laser local oscillator provide an estimate of the optimal probe period, as well as a modified expression for the theoretical clock stability.
    04/2009;
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    ABSTRACT: Time has always had a special status in physics because of its fundamental role in specifying the regularities of nature and because of the extraordinary precision with which it can be measured. This precision enables tests of fundamental physics and cosmology, as well as practical applications such as satellite navigation. Recently, a regime of operation for atomic clocks based on optical transitions has become possible, promising even higher performance. We report the frequency ratio of two optical atomic clocks with a fractional uncertainty of 5.2 x 10(-17). The ratio of aluminum and mercury single-ion optical clock frequencies nuAl+/nuHg+ is 1.052871833148990438(55), where the uncertainty comprises a statistical measurement uncertainty of 4.3 x 10(-17), and systematic uncertainties of 1.9 x 10(-17) and 2.3 x 10(-17) in the mercury and aluminum frequency standards, respectively. Repeated measurements during the past year yield a preliminary constraint on the temporal variation of the fine-structure constant alpha of alpha/alpha = (-1.6+/-2.3) x 10(-17)/year.
    Science 04/2008; 319(5871):1808-12. · 31.20 Impact Factor
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    Phys. Rev. Lett. 104(7):070802.