Patent Description:
Devices called atomic clocks have been known for several decades and are able to keep time with very high precision. Conventional atomic clocks use atoms in a gas phase that can undergo transitions that correspond in energy to electromagnetic radiation in the microwave part of the spectrum. In one example a tunable microwave cavity contains the gas and the cavity can be tuned such that the field in the cavity oscillates very stably at a frequency corresponding to the energy transition in question. Compact clocks, known as "chip-scale atomic clocks", have recently been developed that use a vapor of atoms such as caesium or rubidium. There have also been developments using oscillations at frequencies corresponding to the optical (visible) part of the electromagnetic spectrum.

The availability of very high stability frequency standards, and the time-keeping that they provide, is used in many fields, including the synchronization of communication networks and in positioning systems, such as the satellite-based global positioning system (GPS). Historically, atomic clocks have generally been quite large, delicate and have significant power requirements while operating. Thus there are the problems of providing compact, reliable, portable, low power atomic clocks.

Some proposals have been made regarding using endohedral fullerenes in a solid state atomic clock; see for example <CIT> or <CIT>. However, there are still problems regarding reducing environmental influence on the time-keeping; for example the clock transition frequency can vary with temperature of the material, which is undesirable for a stable clock. The reason for the temperature sensitivity is that the encapsulated atom in the center of the fullerene cage vibrates about its equilibrium position. If the temperature increases, then this thermal motion increases, so the electron wavefunction is distorted, this modifies the hyperfine coupling, and therefore alters the clock frequency. It is very difficult or impossible to actively stabilize the temperature with the precision needed to produce a stable clock of the desired precision.

The present invention aims to alleviate, at least partially, some or any of the above problems.

<CIT> is considered to represent the closest prior art which forms the basis of the pre-characterizing portion of claim <NUM>.

The present invention provides an oscillation device as defined in claim <NUM>.

Further optional aspects of the invention are defined in the dependent claims.

Embodiments of the invention will now be described, by way of non-limiting example, with reference to accompanying drawings, in which:.

<FIG> shows components of an oscillation device according to an embodiment of the invention. Such a device may sometimes be referred to as an "atomic clock" (or molecular clock); however, it should, of course, be noted that the term "atomic clock" is simply a convenient shorthand term for such devices. Firstly, they need not necessarily be "clocks". The heart of the device is an arrangement that can provide oscillations at a stable frequency. For this reasons, such devices may also be known as "frequency standards". By counting the oscillations of the standard frequency the clock function can be obtained because each oscillation represents a precise period of time. Secondly, the system undergoing the oscillations does not necessarily have to be "atomic" i.e. a single atom or atoms, but could also be ions, atomic clusters, molecular fragments, small molecules, crystalline defects or other suitable species. If the term "atomic clock" is used herein, this is purely for convenience; it is understood by the person skilled in the art to encompass all of the above terms/systems and further alternatives.

Referring to <FIG>, the core of the device is the medium <NUM> that comprises a system <NUM> which responds at particular frequencies.

An excitation device <NUM> both excites the system <NUM> of the medium <NUM> to cause it to undergo transitions which generate the time-keeping oscillations, and also probes the medium <NUM> such that the oscillations can be measured and the device controlled.

A detection device <NUM> is used to sense the response of the system <NUM> induced by the excitation device <NUM>. The output of the detection device <NUM> is fed to a controller <NUM>. The controller <NUM> produces a corrected output signal at output <NUM>, which is the clock signal or frequency standard, and the controller <NUM> controls the excitation device <NUM>.

Although the components in <FIG> are shown as separate items, they may, of course, be integrated; for example some or all of the components can be provided on a single, monolithic chip or integrated circuit, fabricated using techniques known from the fields of microlithography, nanotechnology, micro-electro-mechanical systems (MEMS) and/or nano-electro-mechanical systems (NEMS). Some or all of the components can also be provided with shielding from external influences, for example using a mumetal shield (not shown) to shield from magnetic fields and act as a Faraday cage to shield from electric fields.

Each of the components of <FIG> will now be described in more detail.

In this preferred embodiment, the medium <NUM> is made of condensed matter, such as a solid, whether crystalline or non-crystalline, or such as a glass or a polymer or other highly viscous material, or such as a liquid solution.

The medium <NUM> comprises a system <NUM> capable of undergoing transitions between states which have an energy difference corresponding to a particular oscillation frequency. In the preferred embodiment, the system <NUM> is a plurality of endohedral fullerenes.

The term "Fullerene" refers to a cage-like structure formed of carbon atoms and also known as carbon buckminster-fullerene or bucky-balls. The cage can be written as Cn, and the cage can be of various sizes; preferred embodiments include n = <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>, but this is not an exhaustive list. Some fullerenes such as C<NUM> and C<NUM> are spherical, but most are elongated. The diameter of the fullerene is typically of the order of <NUM>. The term fullerene used herein also encompasses derivates of the basic buckminster-fullerene cages.

The term "Endohedral" means that a species is located within the fullerene cage. According to one embodiment, the endohedral species is a single atom of an element. In some endohedral fullerene systems the endohedral species donates one or more electrons to the cage. Known examples of atomic endohedral species include Er, Gd, P, La, Lu, N, Sc, Tm, Y, Ho or Pr, in a variety of different size fullerene cages. Preferred endohedral species include any Group V element (N, P, As, Sb or Bi). One preferred embodiment is endohedral nitrogen in C<NUM> (i.e. a single nitrogen atom inside a carbon bucky-ball, written as N@C<NUM>). Diatomic endohedral species are also known, such as Er<NUM>, Ce<NUM>, Hf<NUM> or La<NUM>. Other preferred embodiments include trimetallic nitride templated endohedral metallofullerenes (TNT EMFs) of the form M<NUM>N@Cn where M can be one or more metal elements (for example Sc or Er, or a combination), and n is preferably <NUM>, but can take other values.

Endohedral fullerenes are attractive for use in a precision oscillator because the endohedral species is shielded from the environment by the carbon cage. This means that both the electron and nuclear spin lifetime and coherence time of the endohedral species can be very long which is advantageous for stable frequency operation.

The endohedral fullerenes can be embedded in a solid matrix, either in a random manner or in a specific pattern. Furthermore, the endohedral fullerenes may be provided within other structures, such as carbon nanotubes. A solid substrate can be provided to support the endohedral fullerenes and the matrix or other structures. The endohedral fullerenes may be in the form of a crystalline solid or powder, or may be deposited on a surface in a continuous layer or using a supramolecular template, or they may be in solution. The concentration may be diluted to reduce spin-spin dephasing and thereby increase Te2 (the electron spin coherence time). For example, a concentration of the order of <NUM><NUM> molecules of N@C<NUM> per millilitre (number density of molecules per cm<NUM>) or lower, provided that it is reasonably uniformly dispersed, typically provides a spin decoherence time that is not limited by dipole-dipole interactions. Higher concentrations can be used, but, at significantly higher concentrations, the decoherence time deteriorates. The invention is not limited to a particular concentration or range of concentrations.

Two preferred examples of endohedral fullerenes for use in this embodiment of the invention are N@C<NUM> and P@C<NUM>. However N@C<NUM> is the presently preferred choice because it offers superior spin properties and thermal stability and does not have the significant safety hazards associated with the production of P@C<NUM>, though P@C<NUM> is still one option. For N@C<NUM> the electron spin lifetime Te<NUM> can be as long as at least <NUM> at room temperature and a coherence time Te<NUM> of approximately <NUM>/<NUM> Te<NUM> has been obtained empirically (theoretically, Te<NUM> can be up to <NUM> Te<NUM>). The nuclear spin lifetime Tn<NUM> and coherence time Tn2 are also extremely long, for example at low temperature Tn<NUM> can be almost arbitrarily long (several hours at <NUM>).

Both N and P offer isotopes with nuclear spin I = ½. This nuclear spin value is preferred because it has only two possible values along any given axis, such as an axis imposed by an applied magnetic field, namely +½ and -½; this eliminates some sources of decoherence such as nuclear quadrupole broadening and carbon hyperfine broadening. Therefore, in a preferred embodiment, either one or both of the endohedral species and/or the carbon of the fullerene are isotopically purified forms.

An ion implantation method of producing N@C<NUM>, as an exemplary endohedral fullerene for use in embodiments of the invention, will now be described with reference to <FIG>. Approximately <NUM> of C<NUM> powder is put into an effusion cell <NUM> inside a vacuum chamber <NUM> evacuated at a pressure of ~<NUM>-<NUM> mbar via pump port <NUM>. The effusion cell <NUM> is heated at <NUM>. Under these conditions the C<NUM> is sublimed inside the chamber <NUM> and begins to condense onto a water-cooled copper target <NUM> placed above the effusion cell <NUM>. Cooling water is fed to and returned from the target <NUM> via tubing <NUM>. At the same time, the copper target <NUM> is bombarded with low energy nitrogen ions produced by a commercial ion source <NUM> supplied with an N<NUM> gas feed. Typical values for the beam energy and beam current are <NUM> eV and <NUM>-<NUM> mA, respectively. The orientation of the target <NUM> is such that it is located at <NUM>° angles to both the effusion cell <NUM> and the nitrogen ion source <NUM>.

After a few hours of operation, the copper target <NUM> is covered with a fullerene layer, a few tens of micrometers thick. A quartz crystal thickness monitor (not shown) is used to measure the growth of the fullerene film on the target <NUM> in such a manner that the rate of C<NUM> sublimation matches the nitrogen-ion bombardment rate.

The copper target <NUM> is subsequently immersed into an organic solvent such as CS<NUM> in order to extract the fullerenes. The resulting fullerene solution is ultrasonicated for a few minutes and filtered. Between <NUM> and <NUM> of N@C<NUM>/C<NUM> mixture is dissolved in CS<NUM>, while any insoluble material is discarded. The insoluble soot comprises polymerised fullerenes and destroyed fullerene cages.

The filtered solution is characterized by EPR (electron paramagnetic resonance spectroscopy). The molar ratio of N@C<NUM>/C<NUM> is typically found to be approximately <NUM>-<NUM>. The concentration of N@C<NUM>/C<NUM> is enriched by using a multi-stage chromatographic method. <NUM>N@C<NUM> or <NUM>N@C<NUM> can be produced selectively by using <NUM>N<NUM> or <NUM>N<NUM> gas to be ionised, respectively.

A methodology for the synthesis of P@C<NUM> is the same as for N@C<NUM>, with the difference that phosphine gas (PH<NUM>) is used as source gas for the production of phosphorous ions for implantation into the fullerene molecules. PH<NUM> is very flammable and extremely toxic, so specialised infrastructure is required to avoid potential laboratory contamination by such a poisonous material, and the equipment and handling techniques must comply with the relevant regulations.

Alternatively, other binary phosphorus compounds such as nitrides (e.g. P<NUM>N<NUM>) that are more stable and less toxic than PH<NUM> can be used. These compounds could be inserted as dopants during an arc discharge process for the synthesis of endohedral fullerenes. Under arc reactor conditions, these compounds decompose into their atomic constituents, so as to insert phosphorus into fullerene molecules.

In embodiments of the invention, the system undergoing transitions defines at least a first resonance frequency and a second resonance frequency. This can be implemented in several ways:.

Although techniques (ii) and (iii) above mention using a 'mixture', which is preferred for simplicity, in alternative embodiments it is equally possible to retain the different species in separate containers or in/on separate portions of the overall medium <NUM>.

The excitation device <NUM> comprises a source <NUM> of electric, magnetic or electro-magnetic oscillations at one or more frequencies. In the preferred embodiment, as discussed below, the frequencies correspond to the microwave part of the electro-magnetic spectrum, for example, tens of MHz, so the source <NUM> is a microwave source. The microwave source <NUM> in this embodiment is driven by two oscillators <NUM>, <NUM> to produce microwave excitations at two different frequencies. Each oscillator <NUM>, <NUM> can be an analogue oscillator or a digital synthesiser. There is a wide choice of known cavity design. Features from the field of electron spin resonance (ESR) measurement may be employed, for example standard ESR spectrometers use cylindrical split ring resonators. Another alternative is a microwave stripline resonator; this could even incorporate more than one resonant frequency by having striplines angled with respect to each other. In an alternative arrangement, each oscillator <NUM>, <NUM> is provided with a separate excitation source which it drives. In <FIG>, the excitation device is illustrated as comprising the oscillators <NUM>, <NUM>; however, the oscillators could alternatively comprise part of the controller <NUM>.

The detection device <NUM> detects absorption at the excitation frequencies, either by directly measuring the change in field strength, or by detecting a change in the transparency of the medium <NUM>. One example of a detection device is a microwave sensor. Another example is circuitry to detect the impedance of the resonant cavity - change in impedance implying change in absorption. A third example is optical detection. The detection device <NUM> may be separate from the excitation device <NUM>, as shown in <FIG>, or they may be integrated with each other. Furthermore, the detection device can comprise dedicated units for detecting each desired transition resonance frequency, or can be a common detector for all frequencies.

In the preferred implementation of the invention, the detection is performed using spin resonance by a spin resonance detection device. There are two approaches to using spin resonance for this purpose: continuous wave spin resonance and pulsed spin resonance. Using continuous wave spin resonance, detection is achieved by observing an absorption of the applied microwaves; this can be detected as a change in impedance of the resonant cavity containing the spin species. Using pulsed spin resonance, detection is achieved by observing the induction from a precessing magnetic moment in the sample; this can be achieved by applying a sequence of π and π/<NUM> pulses, and observing spin echo, as is done in the field of magnetic resonance imaging (MRI).

As explained above, the system <NUM> has a plurality of different energy states. A transition between two states has a characteristic resonance frequency f related to the energy difference E between those states by Planck's constant h: E = hf.

In a preferred embodiment of the invention with <NUM>N as the endohedral species in an endohedral fullerene such as N@C<NUM>, a plurality of different energy states arise from the different possible orientations of the <NUM>N electronic magnetic dipole moment with respect to its nuclear magnetic dipole. A transition of the N atom between these states arising from the magnetic dipole-dipole interaction is known as a hyperfine transition. In general atomic systems, these differences in energy level are due to the magnetic dipole-dipole interaction between the nuclear magnetic dipole moment and the electronic magnetic dipole moment, and a transition between states with different energy level is termed a 'magnetic dipole transition' or a 'spin resonance transition'. Of course, this is merely one exemplary system and form of transition.

Embodiments of the invention provide a system <NUM> that has at least two different transitions, that can be referred to as a clock transition CT and an auxiliary transition AT. These transitions define at least a first resonance frequency fA (clock transition), and a second resonance frequency fB (auxiliary transition). <FIG> shows a plot of two typical resonance frequencies (labelled as arising from the clock transition CT and from the Auxiliary transition AT) as a function of temperature. The vertical dashed line indicates the nominal operating temperature. It can be seen that the resonance frequencies change as the temperature deviates from the nominal operating temperature.

Referring back to <FIG>, the controller <NUM> controls the oscillators <NUM>, <NUM> such that their frequencies coincide with two resonance frequencies fA and fB of the system, as detected by the detection device <NUM> (detecting the response of the system <NUM> to the source <NUM> being driven by the oscillators <NUM>, <NUM>, with the output of the detection device being fed back to the controller to adjust the oscillators <NUM>, <NUM> as necessary). As the temperature of the system changes, the oscillators <NUM>, <NUM> will track the two resonance frequencies fA and fB, under the control of the controller <NUM> using the feedback from the detection device <NUM>. Thus the controller <NUM> obtains signals corresponding to the two resonance frequencies fA and fB, either based on control signals that it has sent to the excitation device <NUM>, or directly from the oscillators <NUM>, <NUM>. These signals can be analog or digital, and can be literally oscillations at frequencies fA and fB, or can be some other form of signal representative of those frequencies.

As illustrated schematically in <FIG>, the ratio of the two frequencies is unique for each temperature (as is the difference between the frequencies, provided the temperature dependency of the two transitions is not the same as each other). The controller <NUM> processes the signals corresponding to the two frequencies fA and fB (for example by taking their ratio) to obtain a value representative of the current temperature of the system <NUM>, or more particularly, representative of the temperature deviation from the nominal operating temperature. The controller then uses this value, together with a previously determined temperature dependence of the clock transition CT (a thermal coefficient of frequency change; such as the gradient or derivative of the clock transition frequency around the nominal operating temperature) in order to obtain a corrected value for the first resonance frequency - i.e. the frequency it would be at the nominal operating temperature - and then provides this corrected frequency as a signal at output <NUM> as a frequency standard or clock signal that is compensated against changes in temperature affecting the oscillation system <NUM>. The corrected frequency can be synthesized for the output <NUM>, for example by modifying the frequency signal from one of the oscillators <NUM>, <NUM>.

Effectively, monitoring a second resonance frequency of an auxiliary transition, in conjunction with the frequency of the clock transition, and taking their ratio or difference, provides a thermometer for assessing the temperature of the system, which can then be used as a feedback signal to compensate the frequency of the clock transition to correct for changes in temperature. Of course, this can all be done via signal processing, using dedicated or general purpose hardware, analog or digital; it is not necessary to obtain the actual value of the temperature of the system, and the output corrected (or compensated) signal can be different from any of the actual resonance frequencies, such as a multiple or fraction thereof, or a composite of the two resonance frequencies - but it is more stable with respect to variation in temperature than the actual resonance frequency of the system. The predetermined variation of the clock frequency with temperature can be stored for example as coefficient value or values, as an equation, or as a look-up table. The second resonance frequency (from the auxiliary transition) should vary with temperature in a different way from the first resonance frequency (from the clock transition). More than two transitions can be used in order to provide even more precise temperature correction.

The above embodiment of the invention compensates to stabilize the clock frequency against changes in temperature. Other embodiments of the invention can equally compensate against a different environmental influence or perturbation of the resonance frequency, for example changes in magnetic field. This works as described above, but with 'temperature' replaced by 'magnetic field'.

A generalized embodiment of the invention can compensate for multiple different perturbations or environmental influences on the resonance frequency of a clock transition. For example, by measuring the resonance frequency of three transitions, clock frequency corrections can be made for two different factors, such as temperature and magnetic field.

Claim 1:
An oscillation device comprising:
a system (<NUM>) capable of undergoing transitions between different energy states, the transitions defining at least a first resonance frequency and a second resonance frequency;
an excitation device (<NUM>) arranged to induce the system (<NUM>) to undergo such transitions;
a detection device (<NUM>) arranged to detect a response of the system caused by the excitation device, to produce an output; and
a controller (<NUM>) arranged to receive the output, to control the excitation device (<NUM>) to stimulate said transitions, and to obtain signals corresponding to at least the first and second resonance frequencies,
characterized in that:
the controller (<NUM>) is also arranged to process the obtained signals corresponding to at least the first and second resonance frequencies to produce a corrected output signal that is compensated against at least one environmental influence on the resonance frequencies of the system;
wherein the corrected output signal of the controller (<NUM>) is a corrected first resonance frequency, wherein the corrected output signal is obtained using the ratio or difference of the obtained signals corresponding to first and second resonance frequencies, in conjunction with a predetermined coefficient of change of the first resonance frequency with an environmental influence;
wherein the system (<NUM>) comprises at least one endohedral fullerene; and
wherein the environmental influence is at least one of temperature and magnetic field.