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Quantum logic clock

A quantum logic clock is an optical atomic clock that employs quantum logic spectroscopy to interrogate the narrow intercombination transition in a single trapped ion, such as ^{27}\mathrm{Al}^{+}, achieving fractional frequency uncertainties below 10^{-18}. This technique pairs the clock ion with a secondary "logic" ion, like ^{25}\mathrm{Mg}^{+}, for sympathetic laser cooling and state readout, as the clock transition in aluminum is optically inaccessible directly. Developed at the National Institute of Standards and Technology (NIST), the clock operates at optical frequencies around 1.1 \times 10^{15} Hz, offering stability orders of magnitude superior to traditional microwave cesium clocks at 9.2 \times 10^9 Hz.^[1] The concept emerged from advancements in ion trapping and quantum information science in the early 2000s, with NIST demonstrating the first quantum logic clock in 2005 using a single ^{27}\mathrm{Al}^{+} ion co-trapped with a ^{9}\mathrm{Be}^{+} logic ion.^[2] By 2010, an enhanced version surpassed all other atomic clocks in precision, neither gaining nor losing a second over 3.7 billion years.^[3] Ongoing refinements, including improved ion traps to minimize micromotion and electric field sensitivities, have pushed performance further; in 2019, NIST reported a systematic uncertainty of 9.4 \times 10^{-19}.^[4] In July 2025, NIST's upgraded quantum logic clock set a new world record with a systematic uncertainty of 5.5 \times 10^{-19}, accurate to 19 decimal places and 41% more precise than prior benchmarks, while exhibiting 2.6 times greater stability than other ion clocks. This enables applications in fundamental physics, such as testing general relativity through quantum time dilation effects and monitoring variations in the fine-structure constant.^[5] Relativistic geodesy, where height differences alter clock rates by approximately 10^{-16} per meter, also benefits from its precision.^[6]

History

Early Development

The direct readout of narrow optical transitions in single trapped ions poses significant challenges, particularly for clock transitions that are electric dipole forbidden, such as the ^1S_0 \to ^3P_0 transition in ^{27}Al^+, which exhibit extremely low spontaneous emission rates and prevent efficient fluorescence-based state detection without disturbing the quantum state. This limitation motivated the development of quantum logic techniques to enable indirect, high-fidelity readout of the internal states of such "spectroscopy" ions by coupling them to a co-trapped "logic" ion with accessible optical transitions.^[7]^[8] The use of quantum logic gates between co-trapped ions to map the internal state of the spectroscopy ion onto the logic ion for detection via resonance fluorescence was proposed by David J. Wineland and colleagues at NIST in 2001 and experimentally demonstrated in a seminal 2005 publication. This approach leverages established ion trapping and laser manipulation techniques to perform entangling operations, allowing non-destructive measurement of otherwise inaccessible states. Early experiments conducted at NIST in 2005 demonstrated quantum logic operations between ^9Be^+ and ^{27}Al^+ ions, achieving state detection fidelities exceeding 96% through a controlled-NOT gate that transfers the aluminum ion's state to the beryllium ion for readout. Building on this, follow-up work in 2006–2007 resolved the narrow clock transition in ^{27}Al^+ using the same mixed-species setup, with the first use of ^9Be^+ as the logic ion to control and indirectly detect the ^{27}Al^+ clock ion states via fluorescence on the beryllium ion. These prototypes marked the foundational step toward practical quantum logic clocks, confirming the viability of the method for precision spectroscopy.

Key Milestones

In March 2008, researchers at the National Institute of Standards and Technology (NIST) demonstrated the first experimental quantum logic clock using a single ^{27}Al^+ ion as the clock qubit sympathetically cooled and read out via a co-trapped ^9Be^+ logic ion, achieving a fractional frequency uncertainty of approximately $5.2 \times 10^{-17} in comparison to the mercury-ion standard, which rivals the precision of the then-leading Hg^+ optical clock.^[9]^[2] In 2010, NIST scientists published results on an improved Al^+ quantum logic clock with a fractional frequency inaccuracy of $8.6 \times 10^{-18}, equivalent to a time deviation of 1 second every 3.68 billion years, led by key researchers including Chin-Wen Chou, David B. Hume, and others, with contributions to frequency metrology from James J. McFerran in related optical clock comparisons.^[10]^[3]^[11] By July 2019, advancements in trap design and electric field compensation enabled an Al^+ quantum logic clock with a systematic uncertainty reduced to $9.4 \times 10^{-19}, corresponding to a deviation of 1 second every 33.7 billion years, as reported by Samuel M. Brewer and collaborators at NIST.^[12]^[13]^[1] From 2020 to 2025, quantum logic clocks reached accuracy levels at $10^{-19}, with measurements stable to the 19th decimal place; notably, in July 2025, NIST achieved a systematic uncertainty of $5.5 \times 10^{-19} through enhancements to the optical ion trap, including reduced excess micromotion via modified diamond wafer electrodes and improved vacuum systems for longer operation.^[5]

Operating Principle

Ion Trapping and Laser Cooling

Ion trapping in quantum logic clocks relies on linear radiofrequency (RF) electromagnetic traps, commonly known as Paul traps, to confine single or small numbers of ions in a high-vacuum environment. These traps generate a time-varying quadrupole electric field that dynamically stabilizes the ions against centrifugal forces, enabling long confinement times essential for high-precision measurements.^[14] The effective potential in a linear Paul trap approximates a harmonic oscillator for the ions' radial motion, described by the pseudopotential

U(r) = \frac{q V_0^2}{4 m \Omega^2 r_0^2} (x^2 + y^2),

where q is the ion charge, V_0 is the RF amplitude, m is the ion mass, \Omega is the RF angular frequency, and r_0 is a characteristic trap dimension related to the electrode geometry. This potential confines ions along the radial directions (x and y), while static DC voltages provide axial confinement along z.^[15] To prepare the ions for spectroscopy, laser cooling reduces their kinetic energy to near the motional ground state. [Doppler cooling](/page/Doppler cooling) is achieved by illuminating the ions with a red-detuned laser resonant with an electronic transition, here using 313 nm lasers on ^9Be^+ ions to scatter photons and impart momentum opposite to the ion's velocity, reaching temperatures on the order of millikelvin. This process exploits the Doppler shift to preferentially cool ions moving toward the laser beam.^[16] Following Doppler cooling, resolved-sideband cooling further lowers the temperature to the microkelvin regime by addressing the motional sidebands of the electronic transition, sequentially removing phonons until the ions approach the vibrational ground state of the trap. This technique uses laser pulses tuned to the first red sideband, coupling internal electronic states to specific motional quanta for efficient ground-state preparation.^[17] In quantum logic clocks, which typically involve a pair of dissimilar ions—a clock ion insensitive to direct laser manipulation and a logic ion—sympathetic cooling is employed to indirectly cool the clock ion. The logic ion, such as ^9Be^+, is laser-cooled as described, and through their shared Coulomb-mediated motional modes in the trap, the cooling transfers to the clock ion, such as Al^+, achieving near-ground-state temperatures for both without directly addressing the clock transition.^[18]

Quantum Logic Spectroscopy

The clock transition in quantum logic clocks is the narrow electric quadrupole (E2) transition between the ¹S₀ ground state and the ³P₀ metastable excited state in the ²⁷Al⁺ ion, occurring at a wavelength of 267 nm and a frequency of approximately 1.12 PHz.^[4] This transition has a natural linewidth of about 8 mHz due to the long lifetime of the ³P₀ state (lifetime ≈ 20 s), enabling high-frequency resolution. Both states have total angular momentum J=0, rendering the transition first-order insensitive to magnetic field fluctuations, which minimizes Zeeman shifts during interrogation. Quantum logic spectroscopy addresses the challenge of directly detecting the clock states in ²⁷Al⁺, which lack suitable cycling transitions for fluorescence readout, by co-trapping the clock ion with a logic ion, typically ²⁵Mg⁺ or ⁹Be⁺, that possesses detectable optical transitions. The technique employs two-qubit entangling gates, such as the Mølmer-Sørensen (MS) gate, to couple the internal electronic states of the clock ion to the motional modes shared with the logic ion, creating an entangled state that maps the clock ion's phase information onto the logic ion. This entanglement enables quantum-non-demolition (QND) readout of the clock state via state-dependent phase shifts, followed by projective measurement of the logic ion through resonant fluorescence excitation (e.g., at 280 nm for Mg⁺), achieving detection fidelities exceeding 99% after multiple repetitions without disturbing the clock ion. The MS gate, driven by bichromatic laser fields near the carrier and sideband transitions, imparts a geometric phase proportional to the clock state, preserving coherence for repeated interrogations. In the Rabi interrogation scheme, the clock transition is probed using π-pulses from a ultraviolet laser tuned to 267 nm, which coherently flip the ²⁷Al⁺ between ¹S₀ and ³P₀ states over interrogation times up to 150 ms, achieving Rabi frequencies of several Hz for high contrast (>70%).^[4] Following each π-pulse, an MS gate applies a state-dependent phase shift to the logic ion, encoding the accumulated phase from the clock laser detuning. The phase accumulation during interrogation is given by \phi = 2\pi \nu t, where \nu is the transition frequency and t is the interrogation time; the fractional frequency uncertainty scales as \delta \nu / \nu \approx 1/(\sqrt{N} t), with N the number of Ramsey or Rabi cycles, approaching the quantum projection noise limit for single-ion clocks. This method allows precise determination of the clock frequency while avoiding direct scattering on the clock ion, which would broaden the transition.^[4]

Design and Components

Ion Species and Selection

In quantum logic clocks, the clock ion is typically the isotope ^{27}Al^+, selected for the 1S_0 \leftrightarrow 3P_0 electric quadrupole transition, which exhibits an ultra-narrow natural linewidth of 8 mHz. This linewidth enables a quality factor Q \approx \nu / \Delta \nu \sim 10^{17}, where \nu \approx 1.12 \times 10^{15} Hz is the transition frequency, supporting exceptional long-term stability and precision.^[19] Additionally, ^{27}Al^+ demonstrates minimal sensitivity to blackbody radiation (BBR) shifts, with a fractional frequency shift \Delta \nu / \nu = -3.05(42) \times 10^{-18} at room temperature (295 K), equivalent to an absolute shift of approximately -3.4 mHz; the associated temperature sensitivity is on the order of 4.6 \times 10^{-5} Hz/K, derived from the T^4 scaling of the BBR electric field and the measured differential static polarizability \Delta \alpha(0) = (7.02 \pm 0.95) \times 10^{-42} , \mathrm{J \cdot m^2 / V^2}.^[20] The logic ion in current designs is ^{25}Mg^+, chosen for its strong dipole-allowed transitions that facilitate efficient laser cooling and state detection. Specifically, the 2S_{1/2} \to 2P_{3/2} transition at 280 nm supports Doppler cooling to near the motional ground state, while the same cycling transition enables high-fidelity quantum state readout via resonance fluorescence, achieving detection fidelities exceeding 99% in optimized setups.^[21] Early versions used ^{9}Be^+ with a similar transition at 313 nm (see History section).^[22] The selection of ^{25}Mg^+ complements ^{27}Al^+ because the clock ion lacks convenient dipole transitions for direct laser cooling or fluorescence-based detection, as its relevant levels have low electric dipole matrix elements and inconvenient wavelengths. Instead, the two ions are co-trapped and sympathetically coupled via Coulomb interactions, allowing the logic ion to cool the clock ion's motion and map its internal (electronic) state onto the logic ion's detectable spin or motional states through shared quantum logic gates. The atomic masses (27 u for ^{27}Al^+ and 25 u for ^{25}Mg^+) and identical charge-to-mass ratios (both singly charged) permit stable co-trapping in a linear Paul trap, though the mass mismatch requires precise control of rf drive frequency and endcap voltages to suppress excess micromotion on the clock ion.^[9]^[23] Alternative ion species, such as ^{171}Yb^+ or ^{88}Sr^+, have been explored for single-ion optical clocks, offering viable clock transitions but with broader natural linewidths (e.g., 3.1 Hz for ^{171}Yb^+ and 0.41 Hz for ^{88}Sr^+), resulting in lower Q-factors (\approx 2 \times 10^{14} for Yb^+ and \approx 10^{15} for Sr^+) and higher BBR sensitivities (fractional shifts ~10^{-17} at 300 K). ^{27}Al^+ remains preferred for quantum logic architectures due to its unmatched combination of narrow linewidth and insensitivity to perturbations, achieving the highest reported Q-factor among ion-based standards.^[24]^[25]

Electromagnetic Trap and Laser Systems

The electromagnetic trap in quantum logic clocks is a linear Paul trap, featuring four hyperbolic rod electrodes arranged symmetrically around the trap axis for radial confinement and two endcap electrodes for axial confinement. Radial pseudopotential confinement is generated by applying a radio-frequency (RF) voltage of approximately 100 V peak-to-peak at frequencies ranging from 10 to 50 MHz to the rod electrodes, creating a time-averaged quadratic potential that confines the ions. Axial confinement is provided by static DC voltages applied to the endcap electrodes, typically on the order of several volts, resulting in secular frequencies of around 0.1 to 1 MHz depending on the ion mass and number. Recent upgrades include modified trap designs to minimize micromotion and electric field sensitivities, with stray electric fields nulled to better than 10 V/m through compensation electrodes.^[26]^[27]^[5] Laser systems are critical for ion manipulation, cooling, and clock interrogation in these devices. For the aluminum ion (^27Al^+) clock transition at 267 nm, a narrow-linewidth ultraviolet laser is employed, typically generated by frequency quadrupling a stabilized 1068 nm infrared source—often a Ti:sapphire or diode laser—locked to a high-finesse ultralow-expansion glass Fabry-Perot cavity to achieve a linewidth below 1 Hz. Additional lasers operate at 280 nm for magnesium ion (^25Mg^+) cooling and detection via the cycling transition, and at other wavelengths (e.g., ~280 nm for Raman) for quantum logic operations. Absolute frequency referencing to the SI second is accomplished using an optical frequency comb, which links the ultraviolet clock laser to a stabilized radiofrequency reference, enabling precise measurements with fractional instabilities below 10^{-15}.^[9]^[28]^[21] The detection setup relies on a photomultiplier tube (PMT) to measure fluorescence from the ^25Mg^+ logic ion, illuminating the cycling transition at 280 nm to distinguish internal states with high fidelity (>99%). During a typical readout cycle of 1-10 ms, more than 10^3 photons are collected on the PMT after accounting for ~1% overall detection efficiency, providing sufficient signal-to-noise for quantum nondemolition measurements. The entire apparatus operates within a vacuum chamber at pressures around 10^{-11} Torr, achieved using ion pumps and cryogenic surfaces to suppress collisions with background gas that could cause ion loss or heating. Magnetic shielding, often comprising multiple layers of mu-metal around the chamber, maintains field fluctuations below 0.1 μT to minimize time-varying Zeeman shifts on the clock transition.^[29]^[30]^[31]

Performance Characteristics

Accuracy and Stability Metrics

Quantum logic clocks achieve exceptional accuracy through meticulous control of systematic uncertainties in their frequency measurements. The fractional frequency uncertainty quantifies the clock's precision relative to its nominal transition frequency, typically around 1.121 × 10^{15} Hz (1.121 PHz) for the ^3P_0 state in ^{27}\mathrm{Al}^+. In a landmark 2019 demonstration, researchers reported a systematic uncertainty of $9.4 \times 10^{-19}, limited primarily by several key effects.^[20] The dominant contribution was the blackbody radiation (BBR) shift, with an uncertainty of $4.2 \times 10^{-19}, arising from fluctuating electric fields at room temperature that induce ac Stark shifts on the clock transition. The second-order Doppler shift, due to the ion's relativistic time dilation from thermal and micromotion velocities, contributed an uncertainty of approximately $6.4 \times 10^{-19}, including both secular motion and excess micromotion components. Electric quadrupole shifts, from interactions with trap electric field gradients, were bounded below $1 \times 10^{-19}. The total uncertainty is computed via the quadrature sum of individual contributions:

\frac{\delta \nu}{\nu} = \sqrt{ \sum_i \left( \frac{\delta_i}{\nu} \right)^2 },

where \delta_i represents the uncertainty from each systematic effect. Stability, measured by the Allan deviation \sigma_y(\tau), characterizes short-term frequency fluctuations over averaging time \tau. For the 2019 ^{27}\mathrm{Al}^+ clock, this was $1.2 \times 10^{-15} / \sqrt{\tau} at 1 s, improving to approximately $4 \times 10^{-17} at 1000 s through averaging.^[13] Advancements continued, with a 2025 NIST implementation achieving a fractional frequency uncertainty of $5.5 \times 10^{-19}, surpassing previous records and enabling tests of fundamental physics, such as variations in fundamental constants over cosmic distances. This improvement reduced BBR uncertainty to $1.7 \times 10^{-19} and second-order Doppler contributions to around $4 \times 10^{-19}, while stability reached $3.5 \times 10^{-16} / \sqrt{\tau}.^[32]

Environmental Robustness

Quantum logic clocks based on the ^{27}Al^{+} ion exhibit high insensitivity to magnetic fields due to the clock transition between the ^1S_0 (F=0) and ^3P_0 (F=0) states, which eliminates the first-order Zeeman shift. The second-order Zeeman shift is minimized through precise calibration of the quadratic coefficient C_2 = -71.944(24) MHz/T^2, resulting in a fractional frequency shift of Δν/ν = -(9.242 ± 0.004) × 10^{-16} for typical operational fields of ~0.12 mT (~1.2 G), with the uncertainty contribution below 10^{-18}.^[33] Electric field perturbations, particularly micromotion-induced Stark shifts in the ion trap, are mitigated using dynamic decoupling pulses that average out these effects during spectroscopy. In the 2025 setup, the excess micromotion shift is Δν/ν = -1.6 × 10^{-19} ± 1.6 × 10^{-19}. This approach, combined with real-time compensation and optimized trap drive frequencies, ensures the electric field sensitivity remains negligible for clock accuracy.^[32] Temperature-related effects are inherently low owing to the small differential scalar polarizability of the clock states, yielding a blackbody radiation (BBR) shift of Δν/ν = -3.1(2) × 10^{-18} at room temperature (300 K), the lowest among candidate optical clock transitions. In recent setups, cryogenic operation of the ion trap at 4 K further suppresses thermal noise and heating rates, enhancing overall stability without significantly altering the BBR contribution.^[20] Relativistic time dilation arising from the ion's secular and micromotion velocities is corrected to below 10^{-18} through direct measurements of the ion's velocity distribution. In the 2025 evaluation, the secular motion contribution is Δν/ν = -114.6 × 10^{-19} ± 3.8 × 10^{-19}, with total second-order Doppler uncertainty around 4 × 10^{-19}, achieved by operating near the three-dimensional motional ground state.^[32]

Notable Experiments

Comparisons with Conventional Clocks

Quantum logic clocks, exemplified by the aluminum ion (Al⁺) standard developed at NIST, offer significant advantages over cesium fountain clocks such as NIST-F1 due to their operation at optical frequencies approximately 100,000 times higher than the microwave hyperfine transition in cesium (9.192 GHz). This higher frequency enables short-term stability improvements by a factor of about 10⁴, achieving Allan deviations around 10⁻¹⁵ τ⁻¹/² compared to 10⁻¹³ τ⁻¹/² for cesium fountains, allowing faster averaging to reach high precision without requiring large ensembles of atoms.^[3]^[34] Furthermore, the single-ion nature of quantum logic clocks provides long-term accuracy comparable to cesium standards (fractional uncertainty ~10⁻¹⁶) without the need for ensemble averaging to mitigate projection noise, as the quantum logic readout directly detects the clock state with high fidelity.^[2] In direct comparisons with mercury-ion (Hg⁺) optical clocks, the 2008 measurement of the Al⁺ to Hg⁺ frequency ratio achieved a fractional uncertainty of 5.2 × 10⁻¹⁷, with the Al⁺ quantum logic clock's systematic uncertainty (2.3 × 10⁻¹⁷) comparable to that of the Hg⁺ clock (1.9 × 10⁻¹⁷).^[9] This parity stems from the narrower natural linewidth of the Al⁺ clock transition (∼8 mHz) compared to Hg⁺ (∼0.3 Hz), reducing sensitivity to perturbations and enabling higher interrogation precision. By 2010, the Al⁺ clock further improved to a systematic uncertainty of 8.6 × 10⁻¹⁸, outperforming the Hg⁺ standard by a factor of over two.^[9] A key advantage of quantum logic clocks lies in their single-ion operation, which eliminates collisional frequency shifts inherent in neutral-atom optical lattice clocks that rely on ensembles of 10³–10⁴ atoms, where atom-atom interactions can introduce uncertainties up to 10⁻¹⁷. This isolation from environmental perturbations also enhances robustness, with ion clocks exhibiting lower sensitivities to magnetic fields, electric fields, and temperature variations than neutral-atom systems. Additionally, the compact ion-trap design holds potential for portable implementations, contrasting with the bulky laser and vacuum systems required for optical lattice clocks.^[34]^[1] During discussions on the 2010 redefinition of the second, the Al⁺ quantum logic clock outperformed international primary cesium standards by a factor of 10, with fractional uncertainties below 10⁻¹⁷ versus ~10⁻¹⁶ for cesium fountains, underscoring its role in advancing time metrology beyond the SI second.^[35]

Quantum Time Dilation Measurements

In 2010, researchers at the National Institute of Standards and Technology (NIST) conducted a seminal experiment using two aluminum-ion quantum logic clocks to test general relativity's predictions of time dilation due to both velocity and gravitational potential differences at everyday scales. The setup involved comparing the frequencies of the two clocks, each based on the ^3P_0 \leftrightarrow ^1S_0 electric quadrupole transition in ^{27}Al^+ ions co-trapped with a ^{9}Be^+ logic ion for readout, connected by a 75-meter optical fiber to enable remote interrogation. One clock remained stationary, while the other was mounted on a platform that could be translated horizontally to simulate relative velocities or vertically to alter height, allowing direct measurement of relativistic frequency shifts without the need for high-speed transport.^[36]^[37] The experiment achieved relative velocities of up to 10 m/s—comparable to a brisk walk—by oscillating the mobile clock platform, inducing accelerations on the order of 10 m/s² during start and stop phases, while height differences were varied by less than 1 meter. The observed frequency shift \delta \nu / \nu combined the special relativistic second-order (transverse) Doppler effect from velocity and the gravitational redshift from height, approximated in the non-relativistic limit as \delta \nu / \nu = v^2 / (2 c^2) + g h / c^2, where v is the relative speed, h is the height difference, g \approx 9.8 m/s² is the gravitational acceleration, and c is the speed of light. This formula derives from the time dilation factor \gamma = 1 / \sqrt{1 - v^2/c^2} \approx 1 + v^2/(2 c^2) for low velocities, leading to a slower-ticking moving clock and a faster-ticking higher clock relative to the reference. Platform velocities were precisely tracked using interferometric position sensors, ensuring accurate correction for any residual first-order Doppler contributions along the fiber path.^[36]^[38] Key results demonstrated the transverse Doppler effect—the pure time dilation signature without classical Doppler broadening—at a fractional frequency level of approximately $10^{-17}, with measurement uncertainties of $5.4 \times 10^{-17} for the velocity test and $6.9 \times 10^{-17} for the gravitational test, confirming general relativity's predictions to within about 10% relative precision for these effects. This represented an improvement of over an order of magnitude in sensitivity compared to prior tests, such as particle accelerator-based Ives-Stilwell experiments, by leveraging the quantum logic clocks' stability (short-term Allan deviation below $10^{-15}/\sqrt{\tau}) to resolve shifts at human-scale conditions. The findings have implications for laboratory-scale gravitational redshift measurements and underscore the clocks' potential for detecting minute relativistic effects in controlled environments, bridging atomic timekeeping with fundamental tests of gravity.^[36]^[37]^[38] The underlying time dilation arises from the Lorentz factor in special relativity, expressed as:

\gamma = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}} \approx 1 + \frac{v^2}{2 c^2}

for v \ll c, which directly modulates the proper time experienced by the moving ions' internal states, manifesting as a frequency shift in the clock transition. This experiment highlighted the quantum logic clock's robustness, with motional sidebands and excess micromotion minimized to below $10^{-18} fractional uncertainty, enabling the detection without significant environmental perturbations.^[36]

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