COMPACT ATOMIC CLOCKS EMERGING TECHNOLOGY COMPACT ATOMIC CLOCKS

Our ability to determine time with ever- increasing precision underpins modern life. A BRIEF HISTORY OF TIMEKEEPING Global satellite navigation systems, the internet and utility infrastructures all rely on the accurate timing signals provided by atomic clocks, with transport infrastructures and logistics, safety-of-life critical systems and even global financial transactions increasingly demanding better timing accuracy. When physicist Louis Essen demonstrated the world’s first accurate atomic clock in 1955 at the National Physical Laboratory (NPL) in the UK, it was not possible to foresee the impact that it would have on society. Since 1967, atomic clocks have defined our base unit of time, the second, as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. This was the first SI unit to be defined in terms of a fundamental unchanging quantity, paving the way for more robust definitions of the metre and The first accurate atomic clock, a caesium standard based on a certain transition of the caesium-133 atom, was built by Louis Essen (pictured right) in 1955 at the National Physical Laboratory (NPL) other units. in Middlesex. Calibration of the caesium standard atomic clock was carried out by the use of the The beat of Essen’s atomic clock was astronomical time scale ephemeris time (ET). This led to the internationally agreed definition of a second being based on atomic time © National Physical Labaratory Strontium ion optical clock © National Physical Laboratory provided by microwave radiation whose frequency is adjusted to coincide with an Timekeeping based on mechanical systems such as pendulums started to gain popularity atomic absorption, thereby exciting electrons in the 16th century. It was during this period that horologists first suggested that a precision in the atoms to move between particular clock might hold the key to the longitude problem by allowing mariners to accurately Over the last five decades, the passage of time has been defined by room-sized atomic discrete energy levels. transfer ‘port time’ to sea. Clocks in those days tended to lose or gain time and did not clocks that are now stable to one second in 100 million years. Dr John Christensen and The state of the art in atomic timekeeping operate reliably at sea or in different climatic conditions. The problem was solved by John is being driven forward by institutions such as Harrison’s revolutionary H3 sea clock, which removed the need for a pendulum. By the 20th Professor Patrick Gill of the Time and Frequency Group at the UK’s National Physical the NPL and similar labs such as the National century, more accurate clocks regulated by the steady vibration of quartz crystals began to Laboratory and Sir Peter Knight of Imperial College London and past President of the Institute for Standards and Technology (NIST) replace mechanical devices. Lord Kelvin first introduced the idea of using atomic transitions to measure time in 1879, but it was not until the 1950s that the concept was practically Institute of Physics, describe a new generation of miniature atomic clocks that promise in Colorado, US, with the latest atomic fountains offering stabilities of one second in demonstrated. The first atomic clock based on microwave atomic transitions was accurate to the next revolution in timekeeping. more than 100 million years. These clocks, around one second in 300 years, making it 10 times more accurate than quartz-based clocks. Researchers are now developing a new type of atomic clock based on optical atomic which work by launching a cloud of ultra-cold transitions that will allow time to be measured with a stability corresponding to one second caesium atoms upwards and interrogating in the age of the universe. them using microwave radiation, require a

24 INGENIA INGENIA ISSUE 57 DECEMBER 2013 25 COMPACT ATOMIC CLOCKS EMERGING TECHNOLOGY

vacuum vessel that stands nearly 3m tall in phenomena such as enhanced solar flare addition to extensive electronics racks and activity – can have serious consequences. APPLICATIONS FOR MINIATURE ATOMIC CLOCKS optoelectronic components to cool the Ships crossing busy stretches of water such • Satellite navigation atoms to micro-Kelvin temperatures. as the English Channel rely on GPS to stay Navigating in areas of intermittent GPS coverage would become easier if receivers were in the correct lane, while certain to contain a miniature atomic clock, which would allow local synchronisation to be SMALLERSCALE CLOCKS telecommunications networks would drift maintained for longer periods and reacquisition of the GPS signal to be achieved in a A quiet revolution is also taking place in the and eventually seize up were they to be couple of minutes rather than tens of minutes. area of ‘secondary’ industrial clocks and time denied the GPS reference signals. Although • Financial transactions standards. Lower accuracy commercial additional global satellite navigation High-frequency trading requires a network to be synchronised with an accuracy of caesium and rubidium clocks are routinely constellations such as Europe’s Galileo are around 0.1 microseconds, otherwise it may appear that trades have occurred before used to disseminate time for a wide variety of soon to provide alternatives to the they have actually been executed. Such networks are typically synchronised to GPS time and are hence susceptible to GPS outage. Synchronisation specialist Chronos Technology applications. Typically housed within a US-operated GPS, it is not possible to Ltd has integrated a chip-scale atomic clock 500 x 500 x 130 mm rack-mountable guarantee that satellite-based timing signals • Flexible networks with a GPS receiver to prolong the time during which the clock is synchronized with the satellite enclosure, they use thermal rather than cold will always be available when required. Chip-scale clocks would allow powerful ad hoc communications networks to be signals © Chronos Technology Limited atoms to achieve an accuracy of around one A recent Royal Academy of Engineering established in the field for military purposes and enable data to be passed more reliably second in 30,000 years. report, Global Navigation Space Systems: and securely via precision time stamping, data authentication and temporal windows. glass vapour cell filled with caesium vapour, The last few years have seen the reliance and vulnerabilities, stated that chip- • Secure communications emergence of chip-scale atomic clocks that More generally, chip-scale clocks can potentially allow messages to be retrieved from in which caesium atoms are first excited by scale atomic clocks represent a “clear shining a laser through the cell with a are much smaller and lighter, and require less paradigm shift” in technological innovation – data streams without the need for protocol ‘headers’ and ‘footers’, enabling messages to power than their traditional counterparts. The National Physical Laboratory’s caesium be transmitted with greater speed and security. wavelength tuned to almost match one of a view shared across wider UK government, fountain atomic clock ‘NPL-CsF2’ is a room-sized the atoms’ optical absorptions. When the These devices incorporate a core ‘physics industry and academia. Earlier this year, the instrument that is used as the primary frequency • Efficient communications package’ of less than 1 cm3, with the standard for the measurement of time in the microwave modulation frequency of the laser UK Defence Science and Technology UK and contributes to International Atomic Chip-scale clocks allow the radio spectrum to be used more efficiently, for example via complete clock system and associated © National light is fully resonant with the caesium Laboratory (Dstl) issued a major call for Time and Universal Coordinated Time ultra-wideband spread-spectrum approaches, and help mitigate the high attendant electronics fitting inside the volume of a Physical Laboratory data loss of undersea communications. hyperfine clock transition, it changes the proposals to solicit novel ideas from research matchbox, and can provide accuracies in the Oil prospecting amount of light that is absorbed by the and industry in the area of miniaturised • region of one second in at least 300 years. The UK firm Chronos recently integrated the The oil and gas industry is already using chip-scale atomic clocks to explore new fields vapour. By monitoring the amount of light atomic clocks, with an aim to reach new Such portable atomic clocks will benefit Symmetricom CSAC into a printed circuit and maximise recovery from existing fields, either in geophone and hydrophone sensor absorption using a sensitive photo-detector, commercially viable clock and sensor systems numerous civilian and military technologies, board along with a GPS receiver such that the arrays for seismic exploration or in gradiometers and gravimeters for non-seismic the microwave resonance condition can be for the UK’s defence and security sectors and will possibly lead to new ones. An atomic satellite signal can be used to reference or exploration. observed and this information used to lock clock incorporated into a mobile phone, for during the next five to ten years. ‘discipline’ the CSAC, and other suppliers are the local oscillator to the hyperfine transition instance, would allow greater connectivity to designing devices around the miniature unit. and thus derive a ‘one second’ tick. networks and larger data-handling rates by COMMERCIAL REALITY However, clocks need to be developed that The atomic processes taking place inside enabling data packets to be more tightly offer even greater short-term stability so that the vapour cell are based on a quantum Chip-scale atomic clocks (CSACs) have synchronised. Incorporated into a satnav the time spent waiting for devices to be mechanical phenomenon called coherent already become commercial reality in the US device, chip-scale clocks would allow faster synchronised can be reduced. Now that it has population trapping (CPT) – see Atomic thanks to major investments by the Defence acquisition by the receiver of the Global been shown that atomic clocks can be built interrogation. The concept dates back to the and Advanced Research Projects Agency Positioning System (GPS) signal and better small, the time and frequency community is 1970s, but it took the confluence of various interpolation during periods of signal drop- (DARPA). Last year, the US firm Symmetricom being spurred on to create smaller, cheaper technologies to allow the breakthrough in out, to provide more reliable navigation – see launched a device with a volume of less than devices. The principles underlying these CPT-based miniaturised atomic clocks as 3 Applications for miniature atomic clocks. 17 cm that weighs only 35 g and consumes miniature clocks are similar to those for their demonstrated by the Symmetricom device. In fact, one of the key attractions of less than 120 mW of power, yet has a short- larger counterparts, but inevitable trade-offs For instance, micro-electromechanical miniature atomic clocks is their ability to term stability corresponding to one second against size, weight and power result in a less (MEMS) fabrication methods are ideal for provide a local alternative to GPS satellites, in 3,000 years. Other chip-scale clocks with accurate device. fabricating a suitably small vapour cell, while which provide the atomic timing signals for comparable performances have been A CSAC comprises three basic parts: a advances in semiconductor processing International stock exchanges don’t use the internet as a source for timing because of the risk of most applications. A GPS outage – whether it reported in the scientific literature but have core atomic physics package, a microwave tampering by hackers and other malicious users. Such networks are typically synchronised to GPS enable pinhead-sized lasers tailored to the is accidental, intentional from the use of GPS not yet been designed with mass production local oscillator and some electronic control time and are hence susceptible to GPS outage. Other non-GPS based networks use a precision time wavelength needed to optically stimulate the protocol with accuracy of the order of 10 microseconds jamming hardware, or the result of natural in mind. circuitry. The physics package is typically a caesium-133 vapour.

26 INGENIA INGENIA ISSUE 57 DECEMBER 2013 27 COMPACT ATOMIC CLOCKS EMERGING TECHNOLOGY

An increasing number of optical atomic clocks using laser- cooled trapped ions or atoms are demonstrating frequency uncertainties better than those achievable with the primary caesium fountain clocks

PHYSICS PACKAGE OPTIONS A second approach, aimed at improving on develop a high-accuracy miniaturised atomic the precision of existing CSAC devices, makes clock that exploits either microwave or ATOMIC TIMEKEEPING Two compact clock configurations show use of laser-cooled trapped ions or atoms. optical transitions. International atomic time (TAI) and particular promise. In the first, which is already These devices are more complex than thermal Indeed, an increasing number of optical the leap-second-adjusted version, being employed in commercial devices, a caesium architectures because they require atomic clocks using laser cooled trapped ions coordinated universal time (UTC), are cloud of caesium atoms in a vapour cell is additional laser systems to cool, prepare and or atoms are demonstrating frequency provided by the Bureau Internationale heated to a few tens of degrees above probe the atoms or ions. However, since the uncertainties better than those achievable with des Poids et Mesures (BIPM) in Paris, ambient temperature and CPT employed to cooled atoms or ions move at speeds of the primary caesium fountain clocks. The from the weighted average of time kept interrogate the system. The CPT technique metres per second rather than several technology is likely to lead to a redefinition of by over 200 commercial atomic clocks avoids the need to directly probe the atoms hundreds of metres per second as in the case the SI second within the next decade and has (caesium clocks and hydrogen masers) with microwaves at 9.2 GHz. This is desirable of hot atoms, spectral (Doppler) broadening of come about due to the availability of a device from around 70 national laboratories because the ~ 3 cm wavelength of the the atomic reference transition due to the called a femtosecond frequency comb, which worldwide. microwave radiation limits the minimum size atoms’ thermal motion is significantly reduced. allows microwave frequencies to be directly Data files from these clocks are possible for a compact clock, whereas the CPT This gives rise to superior short-term stability derived from the optical clock frequency. transmitted daily or weekly to the laser probe has a much smaller optical over the non-cooled arrangement. In addition, Currently, the combs are tabletop devices, but BIPM. The data include measured time wavelength and beam waist. Chip-scale ion-trap technology has advanced there is significant research underway to offsets of each clock from that country’s vapour cells generally comprise a MEMS cell considerably in recent years, with photo- develop micro-resonator versions that would A silica-on-silicon micro-trap, also shown with input laser light, could form the basis of a miniature UTC and regular time transfer data with an internal dimension of around 100 μm, lithographic techniques allowing traps to be lead to miniature optical clock variants in the atomic clock. The diode laser beams necessary for cooling and quantum state preparation are launched from optical fibres through a window above the wafer trap © National Physical Laboratory between the national timescales via while other possibilities such as alkali-filled built into 350μm-thick silicon wafers, and is medium term. two-way satellite transfer and GPS. The hollow-core fibres are also under investigation. the focus for efforts at NPL and elsewhere to this ancillary infrastructure to realise a applications in the aerospace, defence and BIPM-constructed timescale is further COST CONSIDERATIONS compact system solution. security and environmental sectors. ‘calibrated’ by data reported to BIPM for ATOMIC INTERROGATION Excited state 2 The price of a miniature atomic clock Ideally, a coordinated programme across For more about this programme see: the same period from caesium fountain Coherent population trapping (CPT) currently starts at around $1,500, inhibiting government, academia and industry is needed www.npl.co.uk/science-technology/time- primary standards operated at a few of describes a particular type of quantum the wide-scale civilian uptake of the to explore miniature atomic and quantum frequency/ the national standards laboratories. mechanical interaction between a technology. Military applications, meanwhile, configurations that can lead to versatile and photon and a multi-level atomic tend to have specific requirements in terms of compact positioning, navigation and timing BIOGRAPHIES structure. Caesium’s atomic structure can instrumentation. Sir Peter Knight FRS is the immediate past President of the Institute of Physics and Ground state 1 short-term frequency stability, operating be pictured as a simple 3-level system ENERGY temperature range and stability to vibrations Peering through a crystal ball, the Emeritus Professor of Quantum Optics at Imperial College London, where his research comprising the two hyperfine ground 9.192631770 GHz and accelerations. It is highly unlikely that a capabilities afforded by miniaturised atomic interests include quantum information processing and strong field physics. He is a Fellow (for caesium-133) states (0 and 1) and one excited state (2). Ground state 0 single commercial product will meet the clocks in combination with quantum inertial of the Royal Society, and of the Institute of Physics and of the Optical Society of America The laser light is split into two specific needs of every user. sensors that measure accumulated rotations (where he was also President). different optical frequencies or light fields Current and prior investment in atomic and/or gravity gradients open a new world of using microwave modulation. If the clock research and related quantum opportunities. There could come a time when Dr John Christensen FIET is Group Leader of the Time and Frequency group at NPL. He difference in frequency between the two light fields exactly equals the splitting between technologies has provided a strong a fully autonomous underwater vehicle could worked within government for 31 years, providing technical assurance to, and governance the two ground states, the atoms can be pumped or ‘trapped’ into a coherent foundation on which to develop new circumnavigate the world with a precision of a of, a variety of pure and applied research programmes in the areas of components, superposition of these two states (a quantum mechanical state where the atom is partly in compact clock technologies, but the cost of few centimetres for an extended period of sensors, guidance and navigation. He is working on engineering applications leading to level 0 and partly in level 1) via the coupling to level 2. A particular superposition of the designing and manufacturing the physics years, without the need for satellite-based the miniaturisation atomic clocks and quantum inertial sensors. ground states can therefore be made to appear ‘dark’ to the two laser light fields, since the package represents a commercial barrier. timing signals. probability of absorbing the laser photons approaches zero and the coherent dark state no This cold atom technology is currently Over the past two years, the UK Ministry of Professor Patrick Gill FInstP is a Senior Fellow at NPL and head of the Time and longer couples to the excited state. laboratory-based with a high infrastructure Defence and Dstl, together with NPL, have Frequency group there. He is concerned with the UK realisation of the primary SI unit of The reduced absorption gives rise to a spectrally-narrow increase in laser light overhead, for example, with the need to worked to enable a coordinated compact- time, the second, and research into future optical clocks. Professor Gill joined NPL in 1975 transmitted through the atomic sample as the microwave resonance condition is met, laser cool atoms or ions to micro-Kelvin clock research programme. Successful after completing his DPhil in laser research at the University of Oxford. which is then used to provide a precise atomic reference signal to which the temperatures in very high vacuum outcomes will lead to atomic timekeepers and The authors would like to thank Matthew Chalmers, freelance writer specialising in physics, for approximately 9.2 GHz modulation frequency can be electronically stabilised. enclosures. The challenge is to miniaturise other quantum sensors that will suit a range of his help in writing this article.

28 INGENIA INGENIA ISSUE 57 DECEMBER 2013 29