Quantum operations of a trapped ion

Abstract

A method for utilising a trapped ion, e.g. a barium ion, for a quantum computing system, the trapped ion having an encoded quantum information unit, e.g. an encoded qubit, configurable to be in any one of a plurality of quantum states, the method comprising: a quantum operation process 150 that adjusts the quantum state of the quantum information unit, comprising the steps of: performing a check observation 110 of the trapped ion; and raising an error flag 112 in response to detecting the quantum information unit; and performing the corrective action 114 in response to the error flag. Quantum states comprise: an error state, a ground state; a metastable state manifold. The error state may comprise a loss state of the ion escaping entrapment. The check observation can be a fluorescence readout check and may comprise decay ground check. The quantum operation may comprise laser cooling. The method addresses errors in preparation and measurement of metastable qubits.

Description

Technical Field The present disclosure relates to a trapped ion system for quantum computing and in particular to a trapped ion system for quantum computing that is configured to encode a quantum information unit in any one of a plurality of quantum states. Background Quantum computing represents a transformative paradigm in information processing, promising exponential computational power over classical computing methods. The basic unit of information for quantum computing is the qubit. Traditional qubits, while powerful, are susceptible to environmental disturbances, limiting their coherence time and hindering the realization of scalable quantum processors. A trapped ion system is used to encode qubits in order to store the quantum information of the quantum computing system. A qubit comprises a pair of atomic states and can exist as a superposition of these two states. Metastable qubits are a promising avenue to address these challenges. Unlike conventional qubits, metastable qubits exhibit extended stability times, allowing for more robust quantum computations. Achieving reliable state preparation and precise measurement of metastable qubits are fundamental aspects in harnessing their potential for practical quantum information processing. It would be advantageous to improve any one of the preparation, measurement, or computation of metastable qubits because metastable qubits directly impact the overall performance and reliability of a quantum system. Addressing errors in state preparation and measurement of metastable qubits is a key concern in the development of practical quantum computing systems. It is an object of the disclosure to address one or more of the above-mentioned limitations. Summary In a first aspect of the disclosure, there is provided a method for utilising a trapped ion for a quantum computing system, the trapped ion having an encoded quantum information unit configurable to be in any one of a plurality of quantum states, the method comprising: a quantum operation process that adjusts the quantum state of the quantum information unit, comprising the steps of: performing a check observation of the trapped ion; and raising an error flag in response to detecting the quantum information unit; and performing the corrective action in response to the error flag. Optionally, wherein the encoded quantum information unit is configurable to be in any one of a plurality of quantum states comprising: an error state; a ground state; a predetermined metastable state-manifold, the method comprising: a quantum operation process that adjusts the quantum state of the quantum information unit to a ground state and / or a predetermined metastable state-manifold comprising the steps of: performing a check observation of the trapped ion thereby detecting the quantum state; and raising an error flag in response to detecting the quantum information unit resides in the error state during the check observation thereby triggering the step of performing a corrective action; and performing the corrective action to adjust the quantum state from an error state to a predetermined ground state and / or a predetermined metastable state-manifold. Optionally, wherein the quantum operation process comprises: repeating a state loop comprising the steps of: performing the check observation, raising the error flag, and performing the corrective action until the quantum state is detected to be in the ground state or the predetermined metastable statemanifold, thereby closing the state loop. Optionally wherein the quantum operation process further comprises the step of: performing an optical pulse sequence to adjust the quantum state of the quantum information unit in response to the check observation. Optionally, wherein the check observations comprise a plurality of check observations. Optionally, wherein the quantum operation process further comprises the steps of: analysing the plurality of check observations; and determining a result distribution based on the plurality of check observations; and raising the error flag in response to the result distribution thereby triggering the step of performing the corrective action. Optionally, wherein the check observation comprises: performing a fluorescence readout check. Optionally, wherein the corrective action comprises: an optical process wherein the state of the qubit is adjusted from an error state or ground state to a predetermined metastable state. Optionally, wherein the predetermined metastable state-manifold comprises a first predetermined metastable state and second predetermined metastable state, the method comprising: the quantum operation process further comprising: a state preparation process that adjusts the quantum state of the quantum information unit to the first predetermined metastable state and the state preparation process comprising the steps of: performing a preparation ground check observation of the trapped ion thereby detecting the quantum state; and raising a preparation error flag in response to detecting the quantum state is in a ground state or error state during the preparation check observation thereby triggering the step of performing a preparation corrective action, performing the preparation corrective action to adjust the quantum state from the ground state or error state to a first predetermined metastable state. Optionally wherein the state preparation process further comprises: repeating a preparation state loop of the steps comprising: performing a preparation ground check observation, raising a preparation error flag and performing a preparation corrective action until the quantum state is detected to be in the predetermined metastable state, thereby closing the preparation state loop. Optionally wherein the state preparation process further comprises: performing a preliminary ground check observation of the trapped ion thereby detecting the quantum state; and raising a preliminary error flag in response to detecting the quantum state is not in the ground state during the preliminary check observation thereby triggering the step of performing a preliminary corrective action; and performing the preliminary corrective action to adjust the quantum state to the ground state. Optionally wherein the state preparation process further comprises: repeating a preliminary state loop of the steps comprising: performing a preliminary check observation, raising a preliminary error flag and performing a preliminary corrective action until the quantum state is detected to be in the ground state, thereby closing the preliminary state loop. Optionally, the quantum operation process further comprises: performing a laser cooling process to cool the trapped ion. Optionally wherein the quantum operation process further comprises the step of: performing computational operations utilizing an encoded qubit in a predetermined metastable state-manifold. Optionally, the computational operations adjusts the first predetermined metastable state to a second predetermined metastable state. Optionally wherein the computational operations further comprise the step of: performing a decay ground check observation of the trapped ion thereby detecting whether quantum state has decayed to the ground state during computational operation process. Optionally wherein the computational operations further comprise the step of: raising a decay error flag if the qubit is detected to be in error state or a ground state during computation operations thereby detecting that the qubit quantum state has decayed during computation; and performing a decay corrective action in response to the raising of a decay error flag. Optionally, wherein the decay corrective action comprises: discarding the encoded qubit and halting the computational operation in response to the discarding the qubit and subsequently restarting the method steps for preparing a trapped ion. Optionally, wherein the quantum operation process further comprises the steps of: a measurement process to provide a measurement result of the trapped ions quantum state, wherein the measurement process detects that the trapped ion is in the predetermined metastable state-manifold comprising the step of: performing a measurement ground check observation of the trapped ion thereby detecting the quantum state; and raising a measurement error flag in response to detecting the quantum state is in the ground state during the measurement ground check observation thereby triggering the step of performing a corrective action; and performing the measurement corrective action to adjust the measurement result. Optionally wherein the quantum operation wherein the predetermined metastable state-manifold comprises: a N number of predetermined metastable states and wherein the measurement process further comprises the steps of: the N number of measurement ground check observations; and the N number of optical pulse sequences. Optionally, wherein the quantum operation process further comprises the steps of: analyzing the N number of measurement ground check observations; and determining a result distribution based on N number of check observations; and raising the measurement error flag in response to the result distribution thereby triggering the step of performing the corrective action; and performing the measurement corrective action. Optionally wherein the trapped ion comprises a first trapped ion having a first encoded qubit configurable in one of a first plurality of quantum states and a second trapped ion having a second encoded qubit having one of a second plurality of quantum states wherein the quantum states of the first and second trapped ion are entangled and wherein the computational operations further comprise the step of: discarding the first encoded qubit triggers the discarding of the second encoded qubit in response to the discarding of the first encoded qubit. Optionally wherein the error state comprises: a loss state determined by the ion escaping entrapment within a well of a predetermined EM field and; the method further comprises the steps of: raising a loss error flag in response to detecting a loss state during any observation of the quantum state, responding to the loss error flag by reloading the trapped ion. In a second aspect of the disclosure, there is provided quantum computing system comprising: a controller configured to perform the method steps of any preceding claim; a photon detector arranged to process a photon count from an observation of a trapped ion and arranged to provide an observation signal to the controller, the observation signal having a latency value; and a photon emitter configured to emit photons and wherein; the controller provides an emitter control signal to the photon emitter and provide a detector control signal to the photon detector, thereby controlling the photon emitter and the photon detector. Optionally, wherein the control unit comprises; a field programable gate array suitable for real-time processing of photon counts. Optionally, wherein the photon detector comprises: a CMOS imaging sensor and / or; a photomultiplier tube coupled to the camera and / or; a superconducting nanowire arranged for photon counting in real-time coupled to the camera and the photomultiplier tube. Optionally, wherein the photon detector is configured to have a latency value of less than 31ms. Optionally, wherein the photon detector is configured to have a latency value between 0.1ms to 1ms. Optionally, wherein the trapped ion comprises a barium ion. Description of the drawings The disclosure is described in further detail below by way of example and with reference to the accompanying drawings, in which: figure 1 is a process workflow for a method of quantum operations comprising a state preparation process of a trapped ion system according to first embodiment of the present disclosure; figure 2a is a process workflow for a method of quantum operations comprising a cooling process and the state preparation of a trapped ion system according to second embodiment of the present disclosure; figure 2b is a process workflow for a method of quantum operations comprising the cooling process, the state preparation process and a computation operations of a trapped ion system according to third embodiment of the present disclosure; figure 2c is a process workflow for a method of quantum operations comprising the cooling process, the state preparation process, the computational operations and a measurement process according to a fourth embodiment of the present disclosure; figure 2d is a process workflow for the method of quantum operations comprising the cooling process, the state preparation process, the computational operations and a measurement process according to a fifth embodiment of the present disclosure; figure 2e is a process workflow for the quantum operations comprising the measurement process according to a sixth embodiment of the present disclosure; figure 3 is a trapped ion system for quantum computing that uses barium ions according to a seventh embodiment of the present disclosure; and figure 4 is a photon detector-control module 400 in accordance with a eighth embodiment of the present disclosure. Description The following text will refer to quantum information, qubits, and quantum manifolds. A general definition is provided. A quantum state-manifold may comprise N number of quantum states, where N is a positive integer and manifold refers to a construct that represents a complex, high-dimensional space in which each point in space corresponds to a quantum state. This manifold is defined within the context of quantum mechanics, where the quantum states are representations of the state of a quantum system. Each quantum state on the manifold is described by a wave function or a quantum state vector in a Hilbert space, adhering to the principles of quantum superposition and entanglement. The manifold's structure and topology are dictated by the quantum mechanical properties of the system it represents, including the potential for non-local interactions and the probabilistic nature of quantum measurements. The present disclosure relates, but not exclusively, to quantum operations comprising state preparation and measurement of metastable qubit states. An example of a quantum information unit is an encoded qubit. A qubit is a fundamental unit of information used in quantum computing. The qubit comprises a pair of atomic states, a first state and a second state and the qubit can exist as a superposition of these two states. The qubit is encoded with the first state and the second state through the trapped ion system. Encoding, in this context, refers to the process of setting the first state and the second state of the qubit. The qubits are often encoded into the ground states of the ionised atom, for example a Calcium ion, as ground states are the most stable energy states and have a long life-time. However, the use of ground state only qubits limits the capabilities of the quantum computer, as less quantum information can be stored as they have low qubit frequencies. Qubits for use in quantum computing should also be field-insensitive. Qubits that are field sensitive are inferior for storing quantum information as the atomic states of the qubit will change in response to fluctuations in the magnetic field being applied. The quantum operation methods of the present disclosure are not limited to the encoding of the qubits and may be utilised for alternative units of quantum information. For example, with n-level systems, wherein n is greater than or equal to 2 is an integer. For instance, the encoded qubit may be replaced by an encoded qudit wherein n = 10 in the present disclosure. A metastable state refers to an intermediate, long-lived, predetermined quantum state that persists for a significant duration before transitioning to another state. These states are crucial in the development of quantum memory and coherence, contributing to the robustness of quantum information processing. In the context of hyperfine states, which involve the coupling of electron and nuclear spins in an atom. A metastable state is an energy state of an atom or ion which is of higher energy than the ground state. A metastable state has a sufficiently long lifetime for quantum computing. Exploiting metastable hyperfine states for quantum algorithms and quantum memory applications, offers a platform for sustained quantum coherence and aiding in error correction strategies for the realization of more reliable and scalable quantum computations. An error state refers to any unintended quantum state during a state preparation process or quantum computing process. For example, the error state may be a non-predetermined metastable state, a non-predetermined metastable state-manifold or a non-ground state. The error state may refer to a loss state wherein a trapped ion is has escaped from the ions designated electromagnetic (EM) trap. Figure 1 is a first embodiment of the present disclosure of a method of quantum operations 150 for utilising a trapped ion 330 for a quantum computing system, the trapped ion 330 having an encoded qubit configurable to be in any one of a plurality of quantum states. The quantum operations 150 may comprise a state preparation process 100. The quantum states may be in an error state; a ground state; or a predetermined metastable state. The state preparation process 100 adjusts the quantum state of the qubit to a ground state or a predetermined metastable state-manifold. The predetermined metastable state-manifold may comprise a first predetermined metastable state, a second predetermined metastable state and / or N predetermined metastable states, where N is a positive integer. The state preparation process 100 starts at step 102 and proceeds to initiate any of the following steps: SI: Preliminary Check Observation 104 The preliminary check observation 104 is performed by a ground observation of a trapped ion 330 thereby detecting the quantum state of the trapped ion 330. The trapped ion 330 may be in any one of a plurality of quantum states. The quantum states may be in an error state; a ground state; or a predetermined metastable state-manifold. The ground check observation 104 checks if the quantum state of the trapped ion 330 is in a ground state, if the trapped ion 330 is not in a ground state then the ground check observation may raise a ground error flag 106. If the ground check observation 104 detects that the trapped ion 330 is in the ground state the state preparation process may proceed to S4: optical pumping process followed by S5: Optical Pulse Process 109. The ground check observation 104 may be performed by any state checking process, such as fluorescence readout check 104, 110, 210 performed by an photon detector 360. For example for trapped ions 330 encoded with qubits, users may employ a fluorescence readout checks 104, 110, 210 to examine the emitted light when qubits transition between energy levels. By precisely measuring the fluorescence signals emitted, a user can determine the quantum state of the trapped ion 330. This ground check observation 104 can advantageously lead to confirmation of the success of quantum operations, verifying entanglement, and detecting potential errors or decoherence. The Fluorescence readout 104, 110, 210 checks emitted photons, ensuring they align with the expected quantum behaviour. By leveraging fluorescence as a quantum diagnostic tool, users may gain insights into the coherence and fidelity of quantum states, for advancing the reliability and functionality of quantum computing systems. S2: Ground Error Flag 106 A ground error flag 106 may be raised in response to detecting the quantum state of the trapped ion 330, if the trapped ion 330 is observed to be in an error state during the ground check observation 104. Thereby triggering the step of performing a ground corrective action 108. Alternatively, a ground error flag 106 may be raised when there is no trapped ion present, for example, if an existing ion collided with a background gas particle and is then ejected from the ion trap. The ground error flag 106 may for example may be an electronic signal carrying instruction to perform a ground corrective action and may be recorded in an error log. To elaborate, the ground error flag 106 may be a predefined signal activated when a quantum computing system detects abnormal conditions, faults, or discrepancies. It serves as an indicator that prompts the quantum computing system to initiate corrective actions. These actions may include optical pumping processes, error recovery processes, rerouting tasks, logging errors for analysis, discarding of ions or halting operations to prevent further complications. In software, ground error flags 106 are embedded within code to handle exceptional conditions, ensuring that programs respond appropriately to unexpected situations. In hardware systems, error flags may be sensor readings or communication protocols. Advantageously utilizing error flags enhances system reliability by facilitating proactive error detection and enabling timely corrective interventions. S3: Ground corrective action 108 The ground corrective action 108 may adjust the quantum state of the trapped ion 330 from an error state or a metastable state-manifold to a ground state. A ground corrective action 108 following the detection of a ground error flag 104 may result in implementing optical pumping processes 103 (see below) to correct the identified deviation from the intended quantum state, to a ground state. This correction is typically achieved through quantum error correction techniques designed to mitigate the impact of errors caused by environmental factors or imperfections in quantum operations. For example, the ground corrective action 108 may perform a further optical pumping process as defined in S4: Optical Pumping Process 103 below. Alternatively, the ground corrective action 108 may perform ion loading, where an ion is prepared into an electromagnetic trap. If the error flag 106 indicates a deviation in the quantum state of a qubit, a corrective action may involve applying specific quantum gates or algorithms to restore the qubit to its intended state. Alternatively, to using optical pumping, quantum error correction codes, such as those based on stabilizer codes, may be utilized to detect and rectify errors by encoding information redundantly. Advantageously a corrective action can correct quantum state to a ground state ultimately enhancing the overall reliability of quantum computations. S4: Optical Pumping Process 103 The optical pumping process 103 is any process that prepares the trapped ion 330 to induce a predetermined state. For example, this process may be a known quantum optics method, whereby ions are manipulated to favourably align their electron spins, effectively transferring them to a lower energy state, typically the ground state. Consider an ensemble of alkali metal atoms, such as a barium ion, initially distributed across multiple magnetic sublevels. In one example, when the atom or ion is subjected to circularly polarized light the light triggers resonance between multiple magnetic ground-state sub levels and with transitions from those sub-levels to one or more short-lived excited states. The circularly polarized light is aligned with a static B-field direction, which is known to those skilled in the art, utilizing techniques such as sigma+ or sigma- polarization. For instance, in the presence of circularly polarized light with angular momentum corresponding to the hyperfine transition, atoms initially in higher-energy sublevels absorb photons and move to lower-energy sublevels. This preferential absorption of photons induces a net alignment of electron spins, gradually populating the lower magnetic sublevels. Continuous illumination maintains this alignment, effectively "pumping" electrons to the predetermined state. S5: Optical Pulse Process 109 An optical pulse process 109 may be performed to induce a metastable state of the trapped ion 330. By precisely tailoring the characteristics of optical pulses, such as their duration, frequency, and polarization, transitions between energy levels in atoms can be engineered, leading to the creation of hyperfine metastable states. This method is particularly valuable in quantum information processing, where the extended coherence times associated with hyperfine metastable states enhance the reliability of quantum operations 150. The optical pulse sequence 109 may be performed by a photon emitter (not shown in this diagram) to adjust the quantum state of the trapped ion 330 to a metastable state. The use of optical pulses processes 103 advantageously allows for precise control over the quantum dynamics of atomic systems. Through resonance conditions and careful timing of pulses, users can selectively drive transitions, effectively steering atoms into long-lived hyperfine metastable states. This precise control is crucial for the implementation of quantum gates and the creation of entangled states in quantum computing protocols. S6: Preparation Check Observation 110 The preparation check observation 110 is performed by the observation of a trapped ion 330 thereby detecting the quantum state of the trapped ion 330. The preparation check observation 110 may share any characteristics with the preliminary check observation 104 previously discussed. The preparation check observation 110 indirectly checks if the quantum state of the trapped ion 330 is in a predetermined metastable state-manifold by checking if the trapped ion 330 is in a ground state. If the trapped ion 330 is in a ground state then it is not in a predetermined metastable statemanifold and the preparation check observation 110 may raise a metastable error flag 107 If the preparation check observation 110 detects that the trapped ion 330 is in metastable state the state preparation process 100 may end 116. In one example, the preparation check observation 110 may be an indirect observation, and may be a repeat of the ground check observation 104 wherein if the result of observation is that the trapped ion is not in a ground state, it is inferred that the trapped ion is in a metastable state, and when observed to be in a ground state then a metastable error flag 112 is raised. The preparation check observation 110 may be performed by any state checking process, such as fluorescence readout check 104, 110, 210 performed by a photon detector 360 as discussed in SI 104. S7: Metastable Error Flag 112 The metastable error flag 112 may be raised in response to detecting the quantum state of the trapped ion 330 is in the error state or the ground state during the preparation check observation 110 thereby triggering the step of performing a metastable corrective action 114. A metastable error flag 112 may be raised in response to detecting the quantum state of the trapped ion 330, if the trapped ion 330 is observed to be in an error state or ground state during the check observation 110, thereby triggering the step of performing a metastable corrective action 114. The metastable error flag 112 may for example be an electronic signal carrying an instruction to perform a ground corrective action and may be recorded in an error log (not shown in this diagram). For further examples of error flags see the examples previously discussed under S2: Ground Error Flag 106. S8: Metastable Corrective Action 114 The metastable corrective action 114 may adjust the quantum state of the trapped ion 330 from an error state or a ground to a predetermined metastable state or a predetermined metastable state-manifold. The metastable corrective action 114 following the detection of a metastable error flag 110 may result in implementing optical pulse processes to correct the identified deviation from the intended quantum state, to a predetermined metastable state or a predetermined metastable state manifold. For example, the metastable corrective action 114 may perform a further optical pulse process as defined in S5 109. Alternatively, the metastable action 114 may perform an optical pumping process 103 followed by a further optical pulse process 109. This correction may include quantum error correction techniques designed to mitigate the impact of errors caused by environmental factors or imperfections in the optical pulse processes. Once the trapped ion 330 is observed to not be in a ground state or an error state the state preparation process ends 116. As previously discussed in S6 110, the observation of the metastable state may be an indirect observation, based on a ground check observation S104. Figure 2a illustrates a second embodiment of the present disclosure of a method of quantum operations 200 having a cooling process 202 and state preparation process. The quantum operations 200 may share any of characteristics of the quantum operation 100 previously described. SO: Cooling Process 202 The state preparation process 100 may be preceded by a cooling process 202. For instance, a suitable quantum cooling is a process that exploits quantum mechanical principles to reduce the thermal motion and energy of trapped ion 330. Utilizing techniques such as laser cooling, quantum cooling may achieve temperatures substantially close to absolute zero. In laser cooling, precise photon absorption and re-emission lower the kinetic energy of the trapped ion 330 cooling the system to extremely low temperatures. Quantum cooling advantageously enables the creation of ultracold environments for studying quantum phenomena, implementing quantum algorithms, and fostering the development of quantum computers and simulators with enhanced coherence and fidelity. Figure 2b illustrates a third embodiment of the present disclosure of a method of quantum operations 240 having the cooling process 202, the state preparation process 100 and a computational operations process 206, which may referred to as computational operations. The quantum operations 240 may share any of characteristics of the quantum operation 100, 200 previously described. The computational operations 206 utilises the encoded qubit of the trapped ion 330 in a predetermined metastable state and / or a predetermined metastable state-manifold. One non-limiting example of a quantum computational operation utilising an encoded qubit, is a quantum algorithm designed for factoring large numbers, a problem known to be exponentially hard for classical computers. By utilizing the principles of superposition and entanglement, a qubit can exist in multiple states simultaneously, representing various potential solutions. Through quantum parallelism, the algorithm performs simultaneous computations on these states. A quantum gate, such as the Hadamard gate, initiates superposition, enabling the qubit to exist in a blend of 0 and 1. Entanglement is then employed, linking the qubit with another qubit to create correlated states. Quantum operations, like the controlled-NOT gate, further manipulate these states. Upon measurement, the qubit collapses to a definite state, providing the solution to the factoring problem. This quantum parallelism empowers quantum algorithms to outperform classical counterparts, showcasing the transformative potential of qubits in computational tasks. Figure 2c illustrates a fourth embodiment of the present disclosure of a method of quantum operations 260 having a cooling process 202, a state preparation process 100, a computational operations 206 and a measurement process 242. The quantum operations 260 may share any of characteristics of the quantum operation 100, 200, 240 previously described. The measurement process may share any of characteristics of the quantum operation 100, 200 previously described. The measurement process 242 is a process to determine which predetermined metastable state from the N number of metastable states that’s exist within the metastable state-manifold. Figure 2d illustrates a fifth embodiment of the present disclosure of a method of quantum operations 280 wherein the computational operations 206 comprises a decay check observation 210, a decay error flag 212 and a decay corrective action 214. The quantum operations 280 may share any of characteristics of the quantum operation 100, 200, 240, 260 previously described. A computational operations start 208 initiates the sequence to begin computing operations 280 and perform the delay check observations 210. The computational operations 206 may utilize computational subspace states, such as 10,C>, 11,C>, 12,C> where the quantum information is encoded between the state preparation 100 and measurement process 242. In one example, during the computation operation 206 the trapped Ion 330 may be cooled and prepared in |0,M> (state preparation). The system is mapped into the computational subspace (|0,M> -> |0,C>). Target computation is performed on the computational subspace. The system is mapped into the predetermined metastable state-manifold (|0,C> -> |0,M>, 11,C> -> |1, M> etc) and measurement is performed. S9: Decay Check Observation 210 The decay check observation 210 may share any characteristics of the preliminary check observation 104 or the preparation check observation 110 previously described. The decay check observation 210 is performed by the observation of the trapped ion 330 thereby detecting the quantum state of the trapped ion 330. The delay check observation 210 checks if they ion 330 is in the ground state, thereby indirectly checking if the quantum state is currently within a predetermined metastable state or a predetermined metastable statemanifold. If the trapped ion 330 is detected in a ground state then premature decay has occurred during computational operations 206. The decay check observation 110 checks if the quantum state of the trapped ion 330 is in a metastable state, if the trapped ion 330 is not in a metastable state then then the decay check observation 210 may raise a S10: decay error flag 212. If the decay check observation 210 detects that the trapped ion 330 is in metastable state, the delay check observation may continue to perform further checks until computational operation end 216. The delay check observation 210 may be performed by any state checking process, such as fluorescence readout check 104, 110, 210 performed by a photon detector 360 as discussed in S2 104. S10: Decay Error Flag 212 A decay error flag 212 may be raised in response to detecting the quantum state of the trapped ion 330 is in the error state or the ground state during the decay check observation 210, thereby triggering the step of performing a decay corrective action 214. Raising the decay error flag 212 is performed during quantum operations. The decay error flag 212 may for example may be an electronic signal carrying instruction to perform a ground corrective action and may be recorded in an error log. For further examples of error flags see the examples previously defined under S2: Ground Error Flag 106. Sil: Decay Corrective Action 214 The decay corrective action 214 may discard the encoded qubit of the trapped ion 330. The decay corrective action 214 may comprise halting the computational operation 206 in response to the discarding of the qubit and may subsequently include restarting the method steps for preparing a trapped ion, wherein discarding the qubit may refer to discarding the measurement outcome on the qubit. In a further embodiment, the trapped ion 330 comprises a first trapped ion having a first encoded qubit configurable in one of a first plurality of quantum states and a neighbouring second trapped ion having a second encoded qubit having one of a second plurality of quantum states wherein the quantum states of the first and second trapped ion are in communication and wherein the computational operations may have a further corrective actions triggering the discarding the first encoded qubit triggers the discarding of the second encoded qubit in response to the discarding of the first encoded qubit. A method according to the present disclosure may perform multiple checks and also undertake corrective action, rather than discarding a state having an error. Existing implementations of metastable qubit state preparation and measurement only involve a few checks (while the present disclosure has a lot of checks), and the shots where some checks were unsuccessful are discarded. These methods have several advantages over that: By doing more checks, the disclosure can detect more errors, decreasing the probability that an unspotted error creeps into the computation By reacting to checks in real time, the disclosure can increase the effective computation speed of the quantum computer, by: Decreasing the number of shots which need to be discarded, and Aborting the shots as soon as the error occurs It will be appreciated that a shot is a discrete event indicating the realisation of one cycle of cooling-state preparation-computational operations process 220. For instance, the photon detector measurement may involve continuous scattering and fluorescence measurement, wherein if the ion is in a first state, NO photons are recorded on average at the detector, while if the ion is in a second state N1 photons are recorded on average into the detector. Typical numbers for a single qubit are NO ~ 1 and N1 ~ 20. Each shot, the detector records N photons, and a simple algorithm determines the probability of state 0 and 1. For example, if NO ~ 1 and N1 ~ 20 and N = 0 photons would be recorded a shot (i.e. no photons), and it could be inferred with high confidence that the ion was in a first state. If N ~ 30 photons was recorded, then it could be inferred with high confidence that the ion was in a second state. Figure 2e illustrates a fifth embodiment of the present disclosure of a method of quantum operations 282 wherein the computational operations 282 comprises the measurement process 242. The measurement process 242 may comprise: a measurement process start 284, a measurement check observation 286, a measurement error flag 287, a measurement corrective action 288, an optical pulse process 289, N Measurement Operations 290, a measurement analysis 293, a measurement result 294 and a measurement process end 295. The quantum operations 282 may share any of characteristics of the quantum operation 100, 200, 240, 260, 280 previously described. The measurement process 242 is a process to determine which predetermined metastable state from the N number of metastable states that’s exist within the metastable state-manifold. The measurement process 242 starts 284 and proceeds to initiate any of the following steps: Ml: Measurement Check Observation 286 The measurement check observation 286 may share any characteristics with the preliminary check observation 104, preparation check observation 110 and the decay check observation 210 previously described. The measurement check observation 286 is performed by a ground observation of a trapped ion 330 thereby detecting the quantum state of the trapped ion 330. The trapped ion 330 may be in any one of a plurality of quantum states. The quantum states may be in an error state; a ground state; or a predetermined metastable state-manifold. The measurement check observation 104 checks if the quantum state of the trapped ion 330 is in a ground state, if the trapped ion 330 is a ground state then the measurement check observation may raise a measurement error flag 287. If the measurement check observation 286 detects that the trapped ion 330 is not in the ground state and it may proceed M4: Optical Pulse Process 289. M2: Measurement error flag 287 The measurement error flag 287 may share any characteristics with the ground error flag 106, the metastable error flag 112, or the decay error flag 212 previously described. The measurement error flag 287 may be raised in response to detecting the quantum state of the trapped ion 330 if the trapped ion 330 is observed to be in an error state or ground state during the measurement check observation 286 thereby triggering the step of performing a measurement corrective action 288. Alternatively, a measurement error flag 287 may be raised when there is no ion present, for example, if an existing ion collided with a background gas particle and has been ejected from the ion trap. M3: Measurement corrective action 288 The measurement corrective action 288 may share any characteristics with the ground corrective action 108, the metastable corrective action 114, or the decay corrective action 214 previously described. The measurement corrective action 288 may adjust the quantum state of the trapped ion 330 from an error state or ground state to a predetermined metastable state-manifold and / or a predetermined metastable state. The measurement corrective action 288 following the detection of a measurement error flag 288 may result in implementing optical pumping processes to correct the identified deviation from the intended quantum state, a ground state. If the ground check observation 104 detects that the trapped ion 330 is not in the ground state and it may proceed M4: Optical Pulse Process 289. M4: Optical Pulse Process 289 The optical pulse process 289 may share any characteristics with the optical pulse process 109 previously described. The optical pulse process 2899 may be performed by a photon emitter to adjust the quantum state of the trapped ion 330. M4: Optical Pulse Process may proceed to step M5: N Measurement Operations. M5: N Measurement Operations 290 N Measurement Operations 290 may comprise N number of N check observations 291 and N number of optical pulse processes 292 wherein the N number is the previously described N number for the quantity of predetermined metastable states within a predetermined metastable state manifold. N check observations 291 may share any characteristics with the preliminary check observation 104, preparation check observation 110, the decay check observation 210, and measurement check observation previously. The N check observations may check that the trapped ion is within a ground state or an error state. N Optical Pulse Process 292 may share any characteristics with the M4: Optical pulse process 289 previously described. The N check observations 291 may provide a M6: Measurement analysis step 293 with data from the observation 291. The final in sequence N check observation 291 may proceed to a Measurement process end step 295. M6: Measurement Analysis 293 The measurement analysis step 293 may perform an aggregated analysis on the N checks observations, which may be direct ground observations of the trapped ion 330, to indirectly determine if the trapped ion 330 is in a 1st metastable state or 2nd metastable state of a predetermined metastable state-manifold. If during the measurement analysis 293 any of the N check observations 291 result in a ground state or an error state being detected then an M2: measurement error flag 288 may be raised, followed by a subsequent M3: measurement corrective action 287. The measurement analysis 294 may provide the results to a M7: measurement result step 294. M7: Measurement Result 294 The measurement result 294 may recorded data in a storage medium, such as a digital hard drive. In use, the measurement process 242 may: 1. Perform a ground check 286 if conducted to verify the ion is in metastable state-manifold. If it’s in the ground state, an error flag is raised. 2. A series of N optical pulses 292 and N ground checks 291 is performed, where N is the number of states in metastable state-manifold S, such as N=2 if measuring a qubit. a. The first optical pulse maps |0, M> -> G, the second optical pulse maps 11, M> -> G etc 3. The results of all N ground checks are analyses. The analysis either results the measurement result - is the outcomes of checks were consistent with no error - or an error flag - if the outcomes of the checks indicate an error. In further embodiments, examples of the corrective actions 108, 114 214, 288 are provided, including: 1. State preparation 100 a. Preliminary check observation 104 i. Example error: the ion 330 is not in G. ii. Example reason: ion 330 loss. iii. Example corrective action: reload ion 330. b. The Preparation check observation 110 i. Example error: the ion 330 is in the ground state. ii. Example reason: failure in the optical pulse process or failure of optical pumping. iii. Example corrective action: repeat optical pumping and optical pulse. 2. Computation operations 206 a. The decay check observation 210 i. Example error: the ion 330 is in the ground state and the computational subspace is not in ground state ii. Example reason: ion 330 decay. iii. Example corrective action: ignore measurement outcome. 3. Measurement Processes 242 a. Measurement analysis 293: i. Example error: the ion 330 is not in the ground state during every N check ii. Example reason: ion leakage outside of predetermined metastable state-manifold iii. Example corrective action: ignore measurement outcome, repump M -> G Figure 3 illustrates a fifth embodiment of a quantum computing system 300 for quantum computing operations. The quantum computing system 300 comprises an ion trap 330 in a vacuum chamber 320, a voltage source 380 coupled to the ion trap 330, a source of a electromagnetic field 350a 350b, one or more coherent photon sources, such as lasers 340, and a photon detector 360, which may be a fluorescence detector. In alternative embodiments, the coherent photon sources may comprise AC voltage sources coupled to antennas. The quantum computing system 300 comprises electrodes 310 which may in this example be an acousto-optic modulator for controlling the transmitted power of the one or lasers 340. The one or more lasers 340 serve a number of purposes, including the excitation photo-ionisation of neutral atoms into ions and trapping the ions in the ion trap. The trapped ion system is used to encode a qubit in order to store the quantum information of the quantum computer. Quantum information can be stored into other states of ions, and recent work has moved towards encoding the qubits into the long-lived metastable states. A metastable state is an energy state of an atom or ion which is of higher energy than the ground state and is stable enough that the ion will not transition to a more stable state (for example, the ground state) for long duration. For example, ground-state and metastable Zeeman qubits have been studied in the dissertation by Chris Crocker titled: ‘High Purity Single Photons Entangled with Barium Ions for Quantum Computing’. However, these Zeeman qubits cannot be made field-insensitive and hence acquire error (decohere) quickly, making them unsuitable for quantum computing purposes. Optical qubits have also been looked at where the qubit is encoded between two long-lived states (the ground and a metastable state). However when using these optical qubits in a trapped ion system, they require demanding narrow-linewidth lasers which can be difficult to scale making them inadequate for quantum computing purposes. Other work has looked at ground-state qubits with hyperfine structure. Hyperfine structure refers to the detailed splitting of energy levels for an atom caused by the interactions of the magnetic moments of the nucleus and the electrons in the atom. However, in the work of Dietrich et al. (2010) the ground-state hyperfine barium ions used as qubits exhibited large frequency splitting and an inability to sympathetically cool via another barium ion making them inappropriate for use in quantum computing. To make a method possible the system mustbe able to process photon counts and react to them in real time. That means that: 1) The photon detector must be capable of streaming result data to the control unit with low latency. 2) The control unit must be able to implement conditional logic (e.g. "If photon detector recorded more than X photons, do a laser pulse, but if it recorded less than X photons, do nothing”) on the photon detector output and send it to the laser with low latency. There is no strict definition of "low latency", but the slower the process, the more error-prone and slow the overall computation becomes. Given metastable qubit lifetime of a T_qubit, and overall control system latency TJatency (which is the time from the photon being emitted by the qubit to the subsequent pulse reaching the qubit), each error check introduces an error of 1 - exp(-T_latency / T_qubit), which is approximately equal to T_latency / T_qubit. For a Ba+ Ion, it would have a T_qubit = 31 s in which in the introduced error checks would be <le-3s, therefore the latency needs to be <31 ms. In practice, as multiple checks are performed it is advantageous to have several orders of magnitude better latency, e.g. approximately 0.1-1 ms. These orders of magnitude can be achieved in practice by appropriate photon detectors 360 FPGA-based control logic. Figure 4 illustrates a low latency photon detector-control module 400 in accordance with a fifth embodiment of the present disclosure. The photon-detector-control module 400 comprises the photon detector 360, the controller 390 coupled to the photon detector 360, the RF source 380 coupled to switches 404 and the controller 390, the switches 404 coupled to the electrodes 310. The photon detector 360 has been previously discussed. In this example the photon detector 360 may be referred to as a camera and may comprise any suitable camera with features such as a complementary metal-oxide-semiconductor (CMOS), charge coupled device (CCD) or electron-multiplying charge coupled device image sensors, a photomultiplier tube (PMT), an Avalanche Photodiodes (APD) and / or a superconducting nanowire singlephoton detector (SNSDP). The controller 390 comprises a computer processing unit (CPU) 402, a master field programable gate array (MFPGA) 410 and a frame grabber 412. The CPU 402 enables a user to control the state preparation or observation processes. The CPU 402 is connected in this example by a Universal Serial Bus (USB) to a Radio Frequency (RF) source 380. In this example in use, the photon detector-control module 400 measures predetermined metastable states by optical processes previously described. The user controls the process using the CPU 402, which through the USB connection determines the parameters for the RF sources 380 and camera 360 via Transmission control protocol (TCP) / Internet protocol (IP) uploads and detection parameters to the frame grabber 412. In this example a serial peripheral interface (SPI) bus communication link is provided between the frame grabber 412 and the master field programmable gate array FPGA 410. The master FPGA 410 controls one or more transistor-transistor logic (TTL) digital outputs, which provide can turn on or off the electromagnetic switches 402, which control the electrodes 310 thereby controlling the state of the trapped ions 330. The controller 390 may provide live feedback. The master FPGA 410 may trigger camera detection when live feedback is enabled by the addition of the frame grabber FPGA 412. The frame grabber 412 may receive real-time camera images through a camera-Link communication protocol, processes the images according to parameters uploaded via TCP / IP, and outputs the results in SPI form to the master FPGA 410. The latter is programmed to continue the sequence according to these results. Various improvements and modifications can be made to the above without departing from the scope of the disclosure.

Claims

1. A method for utilising a trapped ion for a quantum computing system, the trapped ion having an encoded quantum information unit configurable to be in any one of a plurality of quantum states, the method comprising:a quantum operation process that adjusts the quantum state of the quantum information unit, comprising the steps of:performing a check observation of the trapped ion; andraising an error flag in response to detecting the quantum information unit; andperforming a corrective action in response to the error flag.

2. The method for utilising the trapped ion of claim 1, wherein the encoded quantum information unit is configurable to be in any one of a plurality of quantum states comprising:an error state;a ground state;a predetermined metastable state-manifold, the method comprising:a quantum operation process that adjusts the quantum state of the quantum information unit to a ground state and / or a predetermined metastable state-manifold comprising the steps of:performing a check observation of the trapped ion thereby detecting the quantum state; andraising an error flag in response to detecting the quantum information unit resides in the error state during the check observation thereby triggering the step of performing a corrective action; andperforming the corrective action to adjust the quantum state from an error state to a predetermined ground state and / or a predetermined metastable state-manifold.

3. The method for utilising the trapped ion of claim 2, wherein the quantum operation process comprises:repeating a state loop comprising the steps of: performing the check observation, raising the error flag, and performing the corrective action until the quantum state is detected to be in the ground state or the predetermined metastable state-manifold, thereby closing the state loop.

4. The method for utilising the trapped ion of any preceding claim wherein the quantum operation process further comprises the step of:performing an optical pulse sequence to adjust the quantum state of the quantum information unit in response to the check observation.

5. The method for utilising the trapped ion of claim 2, wherein the check observations comprise:a plurality of check observations.

6. The method for utilising the trapped ion of claim 5, wherein the quantum operation process further comprises the steps of:analysing the plurality of check observations;determining a result distribution based on the plurality of check observations; andraising the error flag in response to the result distribution thereby triggering the step of performing the corrective action.

7. The method for utilising the trapped ion of any preceding claim wherein the check observation comprises:performing a fluorescence readout check.

8. The method for utilising the trapped ion of any of claims 2 to 7, wherein the corrective action comprises:an optical process wherein the state of the qubit is adjusted from an error state or ground state to a predetermined metastable state.

9. The method for utilising the trapped ion of any of claims 2-8, wherein the predetermined metastable state-manifold comprises a first predetermined metastable state and second predetermined metastable state, the method comprising:a state preparation process that adjusts the quantum state of the quantum information unit to the first predetermined metastable state and the state preparation process comprising the steps of:performing a preparation ground check observation of the trapped ion thereby detecting the quantum state; andraising a preparation error flag in response to detecting the quantum state is in a ground state or error state during the preparation check observation thereby triggering the step of performing a preparation corrective action; andperforming the preparation corrective action to adjust the quantum state from the ground state or error state to a first predetermined metastable state.

10. The method for utilising the trapped ion of claim 8 wherein the state preparation process further comprises:repeating a preparation state loop of the steps comprising: performing a preparation ground check observation, raising a preparation error flag and performing a preparation corrective action until the quantum state is detected to be in the predetermined metastable state, thereby closing the preparation state loop.

11. The method for utilising the trapped ion of claim 8 or 9 wherein the state preparation process further comprises:performing a preliminary ground check observation of the trapped ion thereby detecting the quantum state;raising a preliminary error flag in response to detecting the quantum state is not in the ground state during the preliminary check observation thereby triggering the step of performing a preliminary corrective action; andperforming the preliminary corrective action to adjust the quantum state to the ground state.

12. The method for utilising the trapped ion of claim 10 wherein the state preparation process further comprises:repeating a preliminary state loop of the steps comprising: performing a preliminary check observation, raising a preliminary error flag and performing a preliminary corrective action until the quantum state is detected to be in the ground state, thereby closing the preliminary state loop.

13. The method for utilising the trapped ion of any preceding claim wherein the quantum operation process further comprises: performing a laser cooling process to cool the trapped ion.

14. The method for utilising the trapped ion of claim 11 or 12 wherein the quantum operation process further comprises the step of:performing computational operations utilizing an encoded qubit in a predetermined metastable state-manifold.

15. The method for utilising the trapped ion of claim 13 wherein the computational operations adjusts the first predetermined metastable state to a second predetermined metastable state.

16. The method for preparing a trapped ion of claim 14 wherein the computational operations further comprise the step of:performing a decay ground check observation of the trapped ion thereby detecting whether quantum state has decayed to the ground state during computational operation process.

17. The method for preparing a trapped ion of claim 15 wherein the computational operations further comprise the step of:raising a decay error flag if the qubit is detected to be in error state or a ground state during computation operations thereby detecting that the qubit quantum state has decayed during computation; andperforming a decay corrective action in response to the raising of a decay error flag.18.The method for preparing a trapped ion of claim 15 wherein the decay corrective action comprises:discarding the encoded qubit and halting the computational operation in response to the discarding the qubit and subsequently restarting the method steps for preparing a trapped ion.

19. The method for utilising the trapped ion of any of claims 8 to 17 wherein the quantum operation process further comprises the steps of:a measurement process to provide a measurement result of the trapped ions quantum state, wherein the measurement process detects that the trapped ion is in the predetermined metastable state-manifold comprising the step of:performing a measurement ground check observation of the trapped ion thereby detecting the quantum state; andraising a measurement error flag in response to detecting the quantum state is in the ground state during the measurement ground check observation thereby triggering the step of performing a corrective action; andperforming the measurement corrective action to adjust the measurement result.

20. The method for utilising the trapped ion of any of claim 18 wherein the quantum operation wherein the predetermined metastable statemanifold comprises:a N number of predetermined metastable states and wherein the measurement process further comprises the steps of:the N number of measurement ground check observations; and the N number of optical pulse sequences.

21. The method for utilising the trapped ion of claim 5, wherein the quantum operation process further comprises the steps of: analyzing the N number of measurement ground check observations; determining a result distribution based on N number of check observations;raising the measurement error flag in response to the result distribution thereby triggering the step of performing the corrective action; andperforming the measurement corrective action.22.The method for utilising a trapped ion of any of claims 14 to 20 wherein the trapped ion comprises a first trapped ion having a first encoded qubit configurable in one of a first plurality of quantum states and a second trapped ion having a second encoded qubit having one of a second plurality of quantum states wherein the quantum states of the first and second trapped ion are entangled and wherein the computational operations further comprise the step of: discarding the first encoded qubit triggers the discarding of the second encoded qubit in response to the discarding of the first encoded qubit.

23. The method for utilising a trapped ion of any preceding claim wherein the error state comprises:a loss state determined by the ion escaping entrapment within a well of a predetermined EM field and;the method further comprises the steps of:raising a loss error flag in response to detecting a loss state during any observation of the quantum state; and responding to the loss error flag by reloading the trapped ion.

24. A quantum computing system comprising:a controller configured to perform the method steps of any preceding claim and;a photon detector arranged to process a photon count from an observation of a trapped ion and arranged to provide an observation signal to the controller, the observation signal having a latency value; anda photon emitter configured to emit photons and wherein;the controller provides an emitter control signal to the photon emitter and provide a detector control signal to the photon detector, thereby controlling the photon emitter and the photon detector.

25. The quantum computing system of claim 23, wherein the control unit comprises;a field programable gate array suitable for real-time processing of photon counts.

26. The quantum computing system of claim 24, wherein the photon detector comprises;a CMOS imaging sensor and / or;a photomultiplier tube coupled to the camera and / or;a superconducting nanowire arranged for photon counting in real-time coupled to the camera and the photomultiplier tube.

27. The quantum computing system of any of claims 23 to 25, wherein the photon detector is configured to have a latency value of less than 31ms.

28. The quantum computing system of any of claims 23 to 25, wherein the photon detector is configured to have a latency value between 0.1ms to 1ms.

29. The quantum computing system of any of claims 23 to 27 wherein the trapped ion comprises;a barium ion.

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