Measuring very low pressures

Abstract

A vacuum pressure sensor comprises and ion trap device, or potential well, is connected to a vacuum chamber to trap ions found within the very low-pressure gas from within the vacuum chamber. A coher

Description

FIELD OF THE INVENTION The field of the invention relates to very low pressure measurements. BACKGROUND Measuring very low pressures accurately is challenging. Extractor ion gauges for example can measure down to about 2 x 10-12 mbar but there are currently no conventional gauges that can measure pressures below these values. SUMMARY A first aspect provides a vacuum pressure sensor comprising: an ion-trap device; a coherence detector configured to detect decay of coherence of qubits within said ion-trap device; and circuitry configured to determine a change in pressure from said detected decay of coherence. It was recognised that the coherence state of a trapped ion is very sensitive to changes in pressure, temperature and magnetic field. Thus, if the coherence state could be measured and magnetic field and temperature kept substantially stable, variations in the coherence state would provide an indication of the pressure. Trapped ion devices operate at very low pressures and thus, pressure indications at very low pressures could be achieved. Ion trap devices confine and suspend in free space ions or charged atomic particles using electromagnetic fields. Qubits are related to the electronic states of each ion in the trap. The residual pressure in the ion trap affects the lifetime and the coherence of qubits. This is because the pressure is proportional to the number / density of particles in a system that the trapped ions or emitted photons can interact with. These collisions will progressively disrupt the state of either of the above. Thus, in this way the relationship between the coherence of information and pressure can be calibrated against and used as the basis of pressure measurement in the region of and below <2e-12 mbar. Thereby allowing accurate pressure measurements at far lower pressures than have previously been possible. The coherent time of a qubit is the time that the qubit maintains its coherence for. Where there are many qubits the coherence time may be a decay time as the qubits lose their coherence and the degree of coherence or the quality of coherence of the system decreases. Thus, detecting the decay or decay time of qubits will provide an indication of the pressure of a system. There are different measurement techniques allowing the inference of coherence of a system and in some embodiments, said coherence detector comprises a Ramsey spectrometer. In some embodiments the coherence detector comprises a Ramsey spectrometer using a spin echo technique. In other embodiments, said coherence detector comprises an optical detector configured to detect luminescence emitted by the qubits. The decay time of the luminescence emitted by the qubits is related to a decay in coherence of the qubits. In some embodiments the optical detector may be a photon detector. In some embodiments, said circuitry is configured to receive a signal indicative of one or more gas species for which a vacuum pressure measurement is to be made and to determine said pressure in dependence upon said gas species and said detected decay in coherence. It is not only the pressure of the gas present that affects the coherence life time or decay, the type of gas does too. In this regard each collision with a gas particle may affect the coherence. Different gases have different mean free paths and different molecular weights and radii, all of which may affect the change in coherence. The circuitry may be calibrated for particular gases, or gas - 3- mixtures, such that change in coherence can be equated to a particular change in pressure. In some embodiments, said pressure sensor is able to measure pressures below 2 x 10'12mbar. Ion traps function at very low pressures and the change in coherence of qubits is very sensitive to collisions, making very low pressures and very low changes in pressure accurately measurable by such a device. In some embodiments, said coherence detector is configured to monitor coherence during operation of said ion trap device and said circuitry is configured to output at least one signal in response to detecting a change in pressure above a predetermined threshold. The measurement of pressure may be continuous or periodic and may occur during operation of the ion trap device. An ion trap device may be used for different functions and to operate well requires a very low pressure. Having a coherence detector and suitably calibrated circuitry allows the pressure within the ion trap device to be measured during its operation and changes in pressure detected. This can be useful to determine where pressure changes might occur allowing a signal to be generated in response thereto. The signal may comprise an alert signal alerting an operator to a potential fault and / or it may comprise a control signal for controlling a vacuum pump providing the vacuum within the ion trap. In some embodiments, said ion trap device comprises an ion trap quantum computer (IQC). An IQC requires a very low pressure to operate effectively. Low pressure measurements by external gauges may affect coherence and be inappropriate to use during operation of the IQC. Embodiments allow the coherence detector and circuitry to provide an intrinsic measurement of pressure of the vacuum surrounding the IQC. This allows changes in pressure to be determined and the operator alerted and / or vacuum pump operation triggered. A further aspect provides a method of measuring a pressure below 2 X 10-12 mbar comprising: evacuating an ion-trap device to an extreme high vacuum; detecting decay of coherence of qubits within said ion-trap device; and determining a pressure of said extreme high vacuum from said detected decay of coherence. An extreme high vacuum may be one with a pressure below 10-11 mbar. In some embodiments, said step of detecting decay of coherence of qubits comprises using Ramsey spectroscopy said decay of coherence of said qubits. In other embodiments, said step of detecting decay of coherence of qubits comprises detecting the decay of luminescence emitted by the qubits. In some embodiments, said method further comprises receiving a signal indicative of one or more gas species for which a pressure measurement is to be made, said step of determining said pressure doing so in dependence upon said gas species and said decay in coherence. In some embodiments, said method comprises continually detecting decay in coherence during operation of said ion trap device and generating and outputting at least one signal in response to detecting a change in pressure above a predetermined threshold. In some embodiments, said at least one signal comprises: an alert signal and / or a control signal for controlling a vacuum pump. Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: Figure 1 schematically shows a pressure sensor according to an embodiment; Figures 2 schematically shows an ion trap quantum computing device according to an embodiment; Figure 3 schematically shows a flow diagram illustrating steps in a method of measuring very low pressures according to an embodiment; and Figure 4 schematically shows a flow diagram illustrating steps in a method of measuring very pressure within an ion trap quantum computer. DESCRIPTION OF THE EMBODIMENTS Before discussing the embodiments in any more detail, first an overview will be provided. The coherence state of the qubits of a trapped ion is very sensitive to changes in P and T and magnetic field. Thus, if the coherence state of the qubits of the trapped ions is determined and magnetic field and temperature are maintained substantially constant, detected variations in the coherence state provide an indication of the P even at very low Ps. As the name suggests, the qubits are ions trapped by electric fields and manipulated with lasers. Trapped ions have relatively long coherence times, which means that the qubits are long-lived. Moreover, they can easily interact with their neighbours. In practice, for a trapped ion QC quantum computer system, ‘spare ions’ are held in "storage zones" which can replace a lost ion: i.e. ions that are knocked out of the trap which is the lifetime. In this regard the residual pressure of the system in which an Ion-trapped Quantum Computer (IQC) is placed affects the lifetime and the coherence of quantum information transferred within the IQC itself. This is because the pressure is proportional to the number I density of particles in a system that the trapped ions or emitted photons can interact with. These collisions will progressively disrupt the state of either of the above. Qubits are stored in stable electronic states of each ion, and quantum information can be transferred through the collective quantized motion of the ions in a shared trap (interacting through the Coulomb force). The relationship between the coherence of information or the coherence time of a qubit and pressure can be calibrated against and used as the basis of pressure measurement in the region of <2e-12 mbar. This allows accurate pressure measurements at far lower pressures than have previously been possible. Previously cold atom machines have been used to provide low pressure measurements. An ion trap device can operate at higher temperatures than a cold atom machine where trapped atoms are cooled by lasers to very low temperatures. This makes the ion trap device less expensive. The operating principle of such a device is set out below: The molecular volume collision rate R = (n / 2). v / X For N2 at 293K at 1 e-12 mbar X = mean free path = 7e7 m n = number density = 3e10 / m3 v = thermal velocity = 471 m / s Here R ~ 1 e5 / sec If it is assumed that in order to measure a change in P, there should be enough ‘interactions’ to cause an impact on coherence, then the lower limit of the gauge can be determined from the value of R where there is no longer any impact on coherence (however it is chosen to measure this impact / decoherence). For H2at 293K at 1e-12 mbar X = mean free path = 1,22e8 m n = number density = 2.4e10 / m3 v = thermal velocity = 1761 m / s R~ 1,76e5 / sec Figure 1 schematically shows a pressure sensor 5 configured to measure the pressure within a very high vacuum chamber 10. Pressure sensor 5 comprises an ion-trap device 20 that is arranged in fluid communication with the high vacuum chamber 10. In this embodiment, it is mounted on a flange to the side of the high vacuum chamber, in other embodiments it may be mounted within the high vacuum chamber 10 itself. There is a coherence detector 22 configured to determine the coherence of qubits of the trapped ions within the ion-trap device 20 and to output a signal indicating variations in the coherence to circuitry 30. The coherence detector 20 may be a Ramsey spectrometer, where a first pulse prepares the ion trap device by putting the ions in a coherent superposition between the ground state and excited state. The second pulse probes the remaining coherence of the system. This technique is known as Ramsey spectroscopy. It consists of two resonant tt / 2 pulses and a variable delay time, t, between them. Circuitry 30 is configured to convert the coherence measurements to pressure values. In this regard, circuitry 30 may be calibrated for the particular gas species within the high vacuum chamber and may be able to provide an accurate measurement of pressures to pressures below 2 x 10'12mbar. Figure 2 shows an ion-trap quantum computing device IQC 21 for which pressure measurements are continually taken. In this embodiment, the pressure within the IQC 21 is reduced to a suitable low operating pressure by vacuum pump 40. Vacuum pump 40 which may be a non-evaporable getter NEG pump is then turned off as electric fields, magnetic fields and mechanical vibrations all affect the coherence of an IQC. The IQC may then be operational and a coherence detector within the IQC may determine the coherence of the ions within the IQC. Coherence detector may output a signal to circuitry 30 which is configured to determine changes in pressure within the IQC 21 by determining changes or decay in the coherence of the qubits. Circuitry 30 will determine when the pressure changes by more than a predetermined amount and generate a signal in response to this. This signal may be an alert signal and / or it may be a control signal for the vacuum pump. Where it is a control signal for the vacuum pump 40 it will indicate to the vacuum pump that it should turn on and reduce the vacuum again. It may also send a control signal to the IQC indicating that the vacuum pump is to turn on and thus the IQC will lose coherence. Figure 3 shows a flow diagram illustrating steps in a method for measuring a low pressure according to an embodiment. In an initial step S10 an ion trap is arranged in fluid communication with a vacuum chamber whose pressure is to be measured and is below 2 X 10'12mbar. This may be done by mounting the ion trap device within the vacuum chamber or attaching it to a side of the chamber. The coherence decay of qubits within the ion-trap device is then detected using a coherence detector which may for example be a Ramsey spectroscopy. At step S30 a signal is received indicative of a gas species within the vacuum chamber, step S30 may be performed before step S20 and at step S40 a pressure of the vacuum chamber is determined from the determined coherence decay and the received signal indicative of the gas species. In this regard, the circuitry may be calibrated for different gas species and the coherence decay to pressure calibration curve will depend on the gas species. Figure 4 shows a method of continually evaluating changes in pressure of an ion-trap device such as an IQC during its operation. In an initial step S110 the ion-trap is evacuated to a pressure below 2 X 10'12mbar using a vacuum pump. The vacuum pump is then turned off. At step 120 coherence decay of qubits within the ion trap is detected. This step may be performed periodically or continuously. At step S130 the pressure of the ion trap is determined from these coherence times. At step D5 it is determined whether the pressure has risen above a predetermined value. If it has not, then step S120 and step S130 are repeated. If it has then a signal is generated at step S140 and output. This signal may be an alert signal to an operator and / or it may be a control signal to control the vacuum pump to evacuate the ion-trap device. Embodiments may provide the following advantages: • Pressure measurement at <2e-12 mbar; • When measuring pressure in an ion trap quantum computer no or very limited added equipment, magnetic field, source of outgassing is required; • Allow pressure measurement at very low XHV <1e-15 mbar and hence could calibrate RGAs (residual gas analysers) at their lowest partial pressure resolution; • Can be calibrated for different gases; • Can perform continuous pressure measurement intrinsic to an IQC. Extractor gauges (and RGAs) are not allowed continuous operation with an IQC since during operation magnetic fields are produced due to current flows intrinsic to their operation; • Could be a separate miniaturised ion trap device; • Would in principle be the lowest pressure measuring gauge currently available. Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing 5 from the scope of the invention as defined by the appended claims and their equivalents. REFERENCE SIGNS 5 pressure sensor 10 high vacuum chamber 20 ion trap device 5 21 ion-trap quantum computing device 22 coherence detector 30 circuitry 40 vacuum pump io

Claims

1. A vacuum pressure sensor comprising:an ion-trap device;a coherence detector configured to detect decay of coherence of qubits within said ion-trap device; andcircuitry configured to determine a change in pressure from said detected decay of coherence.

2. A vacuum pressure sensor according to claim 1, wherein said coherence detector comprises a Ramsey spectrometer.

3. A vacuum pressure sensor according to claim 1, wherein said coherence detector comprises an optical detector configured to measure changes in luminescence emitted by said qubits.

4. A vacuum pressure sensor according to any preceding claim, wherein said circuitry is configured to receive a signal indicative of one or more gas species for which a vacuum pressure measurement is to be made and to determine said pressure in dependence upon said gas species and said detected decay in coherence.

5. A vacuum pressure sensor according to any preceding claim, said pressure sensor being able to measure pressures below 2 x 10-12 mbar.

6. A vacuum pressure sensor according to any preceding claim, wherein said coherence detector is configured to monitor coherence during operation of said ion trap device and said circuitry is configured to output at least one signal in response to detecting a change in pressure above a predetermined threshold.

7. A vacuum pressure sensor according to claim 6, wherein said at least one signal comprises an alert signal8. A vacuum pressure sensor according to claim 6 or 7, wherein said at least one signal comprises a control signal for controlling a vacuum pump providing a vacuum being monitored by said pressure sensor.

9. A vacuum pressure sensor according to any preceding claim, wherein said pressure sensor comprises an ion trap quantum computer.

10. A method of measuring a pressure below 2 X 10-12 mbar comprising: evacuating an ion-trap device to an extreme high vacuum;detecting decay of coherence of qubits within said ion-trap device;determining a pressure of said extreme high vacuum from said detected decay of coherence.

11. A method according to claim 10, wherein said step of evacuating said ion trap device to an extreme high vacuum comprises placing said ion trap device in fluid communication with an extreme high vacuum chamber; and said step of determining comprises determining said pressure of said chamber.

12. A method according to claim 10 or 11, wherein said step of detecting decay of coherence of qubits comprises one of:using Ramsey spectroscopy to determine said decay of coherence of said qubits; ordetecting the decay of luminescence emitted by the qubits.

13. A method according to any one of claims 10 to 12, comprising receiving a signal indicative of one or more gas species for which a pressure measurement is to be made, said step of determining said pressure doing so in dependence upon said gas species and said decay in coherence.

14. A method according to any one of claims 10 to 13, comprising continually detecting decay of coherence of qubits during operation of said ion trap deviceand generating and outputting at least one signal in response to detecting a change in pressure above a predetermined threshold.

15. A method according to claim 14, wherein said at least one signal5 comprises: an alert signal and / or a control signal for controlling a vacuum pump.

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