METHOD FOR CREATING A QUANTUM DATA TOKEN

Description

[0001] The invention relates to a method for creating a quantum data token based on a system with a plurality of qubits, wherein a number (N) of qubits of the plurality of qubits are combined into an ensemble and form a unit of information, and wherein the qubits of the quantum data token are nuclear spins exposed to a magnetic field and the individual qubits of the ensemble are each brought into exactly one predetermined identical quantum state.

[0002] Data tokens generally represent a short sequence of data values ​​used as authentication, access and / or activation information in electronic data traffic.

[0003] Data tokens can be protected, for example, by encryption algorithms to prevent unauthorized use or copying. Such a data token can be transmitted openly, for example, via data networks.

[0004] Another option is to provide a data token on a physical carrier material that is physically transported to transmit the contained information. In this case, the data token functions as a kind of mechanically transportable and transferable key. However, such a key-like data token is only secure if unauthorized reading and copying can be prevented.

[0005] With the ability to store information in quantum systems, for example in the form of qubits, it is conceivable to build data tokens based on quantum information systems. These are then also referred to as "quantum data tokens" or "quantum tokens".

[0006] For example, US 9,361,962 B1 describes a quantum information system with a plurality of manipulable qubits based on a solid. Color centers in a diamond crystal are used as the qubits. These color centers are formed by adjacent crystal defects, where a vacancy is located next to a nitrogen vacancy. These special color centers are also called nitrogen vacancy (NV) centers. Compared to previous systems that require extremely low temperatures and complex vacuum systems to store and read individual qubits, NV centers offer a practically applicable method for implementing quantum information systems. The information is ultimately stored in the nuclear spin state of a lattice or foreign atom within the crystal.Under optimal conditions (low temperatures and high magnetic fields), relaxation times of the nuclear spins of atomic nuclei in the diamond lattice of up to 36 hours were achieved, which is already sufficient for many practical applications.

[0007] The articles "Coherent control of NV-centers in diamond in a quantum teaching lab", Vikas K. Sewani et al., arXiv.org, Cornell University Library, 201 OLIN Library Cornell University Itaca, NY 14853, July 26, 2020, and "Enhanced widefield quantum sensing with nitrogen-vacancy ensembles using diamond nanopillar arrays", DJ McCloskey et al., arXiv.org, Cornell University Library, 201 OLIN Library Cornell University Itaca, NY 14853, February 7, 2019, also describe quantum information systems with a plurality of manipulable qubits. A number of such qubits can be combined into an ensemble and can then be considered a unit of information of the type mentioned above.

[0008] The article "Quantum projection noise: Population fluctuations in two-level systems," WM Itano et al., Physical Review A, Vol. 47, No. 5, May 1, 1993, presents investigations of mercury and beta ions held in a high-frequency ion trap and prepared in various hyperfine states. Fluctuations in the population of these different states are observed, which are referred to as "quantum projection noise."

[0009] If individual qubits are used to store information, fundamental laws of quantum physics, especially the "No Cloning" theorem, guarantee that such a quantum token cannot be copied without destroying its stored information.

[0010] However, the manipulation required to create, detect, and control individual quantum states is technologically very demanding and requires high precision.

[0011] Starting from this state of the art, it is an object of the present invention to provide a method for creating a quantum token which is also protected by fundamental physical principles, cannot be read or copied without authorization, and which can nevertheless be created and read out easily using equipment.

[0012] This task is solved by a method for creating a quantum token with the features of the independent claim. Advantageous embodiments and further developments are the subject of the dependent claims.

[0013] In an inventive method of the type mentioned at the outset, the number of qubits in an ensemble is chosen such that a quantum state of a sub-semble, which has at most half the qubits of the ensemble, cannot be determined due to quantum projection noise.

[0014] Information is therefore not stored on the basis of a single qubit, but in an ensemble of multiple qubits. This significantly simplifies the preparation of the data token and also the technical readout of the information. To nevertheless prevent unauthorized reading or copying, the number of qubits forming an ensemble is chosen to be so small that the quantum projection noise prevents unauthorized reading or copying.

[0015] If the orientation of the quantum states set during preparation is known, the quantum state of each unit of information can be read out with a single measurement.

[0016] If, however, the orientation of the quantum states set during preparation is unknown, at least two measurements are required. By selecting the number of qubits in an ensemble of an information unit such that the quantum projection noise in a sub-semble with half the number of qubits is greater than the technically necessary accuracy required to read out this sub-semble, undetected unauthorized reading or copying is prevented. According to the invention, the quantum no-cloning theorem as a protective principle is replaced by a protection based on the fact that unauthorized copying of the quantum token becomes detectable through the quantum projection noise.

[0017] The qubits of the quantum data token are nuclear spins exposed to a magnetic field. During preparation, the nuclear spins are pivoted relative to a magnetic field axis to set and align them with the predetermined quantum state. Authorized reading of the quantum state is achieved by returning the nuclear spins to the magnetic field axis, which is possible if the orientation set during preparation is known.

[0018] In quantum mechanical terms, this means that with respect to the quantization axis given by the magnetic field, a basis state is measured which has no quantum projection noise.

[0019] If, on the other hand, the orientation is unknown, quantum projection noise prevents the determination of swivel angles with the precision that would be needed to guide the nuclear spins back to the magnetic field axis.

[0020] The quantum data token can, for example, comprise a crystal with a regular diamond lattice of carbon atoms, featuring NV centers, each containing a nitrogen impurity and a defect. In this case, the NV centers represent the qubits. Preparing and, in particular, aligning the qubits to establish the predetermined quantum state can be achieved by irradiating the crystal with microwave radiation and / or light. Preferably, the light is selectively directed onto specific areas of the crystal using a lens, with the qubits in these areas then forming an information unit of the data token. An information unit can, for example, be a near-surface cylinder of the crystal with a diameter of approximately 100–300 nanometers (nm) and a height of approximately 20–100 nm. An information unit can contain, for example, between 10 and 100,000 NV centers, forming the ensemble.

[0021] Several such information units can be arranged side-by-side near the surface of the crystal to store the information required for the data token. Preferably, different orientations are set for the qubits of the multiple information units to make a randomly successful copying by an attacker with a randomly chosen qubit orientation extremely unlikely.

[0022] The invention is explained in more detail below with reference to an exemplary embodiment and the accompanying figures. The figures show: Figure 1 shows a schematic representation of an atomic lattice on which a quantum data token can be based; and Figure 2 shows a device for generating and reading a quantum data token.

[0023] Figure 1Figure 1 schematically shows a section of a crystal 1, on the basis of which a quantum data token, hereinafter also called quantum token or data token, can be created and with which a patent-compliant method for creating and reading the data token can be used.

[0024] Crystal 1 is preferably a solid-state lattice of as regular a structure as possible, composed of lattice atoms 2, preferably carbon atoms. In crystal 1, individual lattice atoms 2 are replaced by a foreign atom 3, whereby a lattice atom 2 that would normally be adjacent to the foreign atom is missing – thus, instead of the corresponding lattice atom 2, a defect 4 is present.

[0025] In a crystal 1 consisting of carbon atoms in a diamond lattice, nitrogen is frequently used as the foreign atom 3, and the resulting combination of two coupled defects (foreign atom 3 and defect 4) is called an NV center. In the section of crystal 1 of the Figure 1Such an NV center is present. In a crystal 1 used as a data token according to the application, the number of NV centers, for example, ranges from 10 to 100,000 in a cube with an edge length of 100 nm, corresponding to a density of 1016 to 1020 x 1 / cm3. The density of the NV centers in crystal 1 can be controlled by appropriate doping with the foreign atoms 3 during the construction of crystal 1 from the lattice atoms 2. Alternatively, nitrogen-doped diamond can be used as the starting material, and defects can be created, for example, with femtosecond pulsed lasers or by electron bombardment.

[0026] In the Figure 1In the region of the defect, valence electrons 5 are shown, which determine the special optical properties of the NV center compared to the surrounding solid of crystal 1. These special optical properties are due to the electron energy levels of the valence electrons 5 of the NV center, which differ from those of the surrounding crystal lattice, which is why the NV center is also called the color center.

[0027] Through hyperfine interactions of the electron spins of the valence electrons 5 with the nuclear spins of the surrounding atoms, i.e., the lattice atoms 2 and / or the foreign atom 3, the energy levels and thus also the optical transitions of the NV centers are split. A nuclear spin state of the surrounding atoms can therefore be detected at the NV centers by optical spectroscopy.

[0028] Furthermore, by simultaneous irradiation with microwave energy, certain electron spin states in the valence electrons 5 can be manipulated (populated or depopulated), which couple with nuclear spin states and thus allow manipulation of the nuclear spin states of the surrounding lattice atoms 2 or the foreign atom 3.

[0029] The NV center thus becomes a quantum system whose state can be prepared and read out. In this sense, an NV center represents a single qubit in information technology terms.

[0030] For the use of crystal 1 as a registered quantum data token, a density of the NV centers is chosen such that a plurality of NV centers together form an ensemble of qubits that can be prepared and read out together.

[0031] Figure 2 shows an embodiment of a preparation and readout arrangement 10 for preparing and / or reading out the crystal 1 as a data token.

[0032] The preparation and selection arrangement 10 comprises a crystal holder 11 into which the crystal 1 is inserted. A magnet 111 is arranged in the area of ​​the crystal holder 11, which provides a magnetic field for the crystal 1 that is defined in its direction and magnitude.

[0033] Magnet 111 is schematically represented as a bar magnet. Magnet 111 can, for example, be a permanent magnet. In a further embodiment, to achieve a defined magnetic field strength and direction, an electromagnet, in particular in the form of one or more Helmholtz coil pairs, can be used.

[0034] Also in the area of ​​crystal 1, the crystal holder 11 has one or more microwave antennas 112, which are connected to a microwave generator 114 via a microwave conductor 113. During operation of the preparation and readout arrangement 10, a continuous microwave field or a pulse train of microwaves can be generated in the area of ​​crystal 1 via the microwave generator 114.

[0035] The microwaves allow the orientation of the qubits relative to the quantization axis to be rotated. In the image of a Bloch sphere, a quantum state is symbolized as a Bloch vector, pointing from the center towards the surface of the Bloch sphere. The Bloch vector, initially pointing along the quantization axis, is rotated by two angles by the microwaves. Information about the qubit orientation is required for authorized reading of the data token and should therefore be kept secret and transmitted or stored separately from the data token. The orientation of the qubits can be varied for different information units within a data token and also from data token to data token.

[0036] The crystal holder 11 is preferably designed in the form of a movable and / or swiveling sample table in order to be able to position the crystal 1 appropriately.

[0037] In front of the crystal 1, an objective lens 12 is arranged, through which stimulating light rays can be directed onto the crystal 1 and fluorescence radiation emitted by the crystal 1 can be detected.

[0038] The preparation and selection arrangement 10 includes an excitation optic 13, which comprises a laser 131. If a diamond crystal made of carbon atoms with nitrogen impurities is used as crystal 1, a laser with a wavelength of 514 nm (nanometers) can be used, for example, to excite transitions of the NV centers.

[0039] A polarizer, comprising, for example, a lambda / 4 wave plate and a polarizing beam splitter or, alternatively, an optical diode, is connected downstream of the laser 131 to prevent feedback into the laser. Light coupled out of the polarizer 132 is guided via various mirrors 133, an optical fiber 134 with corresponding input and output coupling 135, and another polarizer 136, and further via a beam splitter 122 and a mirror 121, onto the objective 12 and thus onto the crystal 1. The additional polarizer 136 is used because the optical fiber 134 used here does not retain polarization. In alternative embodiments of the preparation / readout arrangement 10, the optical fiber 134 and the polarizer 136 can be omitted. Polarization-retaining optical fibers can also be used as an alternative. The arrangement of the mirrors 133 and 121 and the beam splitter 122 is also exemplary.

[0040] Furthermore, the preparation and readout arrangement 10 includes a readout optic 14 with which fluorescence light emitted by the crystal 1 can be evaluated. The light emitted by the crystal 1 is directed by the objective 12 and the mirror 121 onto the beam splitter 122 and transmitted with at least a portion to the readout optic 14. When the previously described crystal 1 is used as a diamond lattice with nitrogen impurities, the fluorescence light emitted by the NV centers lies in the red spectral range.

[0041] The readout optics 14 comprise a possibly movable and replaceable filter 141 and a tube lens 142, which maps the light captured by the objective 12 into a camera 144 via a folding mirror 143.

[0042] The camera 144 serves for the precise lateral positioning of the crystal 1 and for setting the correct focus distance between the lens 12 and the crystal 1. To enable a qualitative analysis of the emitted fluorescence light, the fluorescence light can be directed into a fiber optic 145 and ultimately onto a photodiode 146 when the tilting mirror 143 is in a corresponding position. To achieve sufficient sensitivity, an avalanche photodiode can be used as the photodiode 146.

[0043] According to the application, for preparing and reading crystal 1, a density of NV centers is selected relative to the size of the area on crystal 1 captured by the objective 12 such that not a single NV center is analyzed or prepared as a single qubit, but rather an ensemble of a multitude of NV centers, i.e., a multitude of qubits. Each ensemble forms one information unit of the data token, thus representing, for example, one bit of information. Several such ensembles can, for example, be prepared side by side on the surface of crystal 1 and accordingly form several information units of the data token. Preferably, these information units are then also prepared using different orientations of the qubits.

[0044] A crystal 1 prepared in this way as a data token can be transported like a key and read out again at its destination, using a comparable preparation and readout arrangement 10, for example as in Figure 2 The following is shown and used. It must be taken into account that crystal 1 remains permanently exposed to a magnetic field in order to obtain the polarization information of the nuclear spins of the qubits.

[0045] The respective quantum state of an information unit on the data token represents a superposition of the quantum states of the individual qubits, i.e., the individual NV centers that belong to the ensemble.

[0046] The readout is achieved by back-rotating the nuclear spins onto the quantization axis, for example, using a suitable microwave pulse sequence. If the back-rotation is not performed correctly due to a lack of knowledge, the readout is subject to quantum projection noise.

[0047] If the orientation of the quantum states set during preparation is known, the quantum state of each information unit can be read out with a single measurement. To do this, the qubits are first rotated back onto the quantization axis, which is achieved using a suitable microwave pulse sequence. Then the state can be read out. No quantum projection noise occurs in this process.

[0048] If, however, the orientation of the quantum states set during preparation is unknown, two measurements are required, since a quantum state can be described by two angles of its Bloch vector. Because the qubits are not aligned along the quantization axis in this case, this measurement—unlike the authorized readout—is subject to quantum projection noise. Furthermore, since each measurement destroys the alignment of the ensemble, two measurements cannot be performed on the entire ensemble.

[0049] According to the application, the number N of qubits in an ensemble of an information unit is chosen such that the quantum projection noise in a sub-semble of half the number N / 2 qubits is greater than a technically achievable accuracy in reading the ensemble. The quantum projection noise causes the signal normalized to the number of qubits to scale proportionally with the noise ratio N (-1 / 2)< , i.e., proportionally with 1 / √N.

[0050] The number of qubits in an ensemble is therefore chosen such that the quantum projection noise would be too large when measuring on two sub-sembles.

[0051] By selecting the appropriate ensemble size, it is ensured that only a single meaningful measurement can be performed on the entire ensemble. If the orientation of the qubit states is unknown, a single measurement is insufficient to read the information content of the ensemble and thus the corresponding information unit of the data token. However, if the orientation is known, the qubits can first be rotated back onto the quantization axis, and then their state—that is, the desired information of the information unit—can be read out, bypassing the quantum projection noise.

[0052] To access the information contained in the data token even without knowing the qubit orientation, one could attempt to divide the ensemble into two or more parts and read these parts separately. However, due to the chosen size of the ensemble, each subunit is too small (i.e., has too few individual qubits) to copy the information with the required precision, considering the quantum projection noise. Furthermore, the information becomes noisier during the readout process and therefore cannot be copied accurately enough.

[0053] In summary, the chosen size of the ensemble, i.e., the number N of qubits or NV centers, prevents unauthorized reading and copying of the data token. Reference sign

[0054] 1 Crystal 2 Lattice atom 3 Foreign atom 4 Void 5 Valence electron 10 Preparation and selection setup 11Kristallhalter 111Magnet 112Mikrowellenantenne 113Mikrowellenleiter 114Mikrowellengenerator 12Objektiv 121Spiegel 122Strahlteiler 13Anregungsoptik 131Laser 132Polarisator 133Spiegel 134Lichtleiterfaser 135Einkopplung 136weiterer Polarisator 14Ausleseoptik 141Filter 142Tubuslinse 143Klappspiegel 144Kamera 145Faseroptik 146Photodiode

Claims

1. A method for creating a quantum data token based on a system comprising a plurality of qubits, wherein a number (N) of qubits from the plurality of qubits are combined into an ensemble and form an information unit, and wherein the qubits of the quantum data token are nuclear spins exposed to a magnetic field, and the individual qubits of the ensemble are each brought into exactly one predetermined identical quantum state, characterized in that the number (N) of qubits in an ensemble is selected such that a quantum state of a sub-ensemble comprising at most half of the qubits of the ensemble cannot be determined due to quantum projection noise, wherein the nuclear spins are rotated relative to a magnetic field axis to set the predetermined identical quantum state, wherein the quantum state is read out by returning the nuclear spins to the magnetic field axis, and wherein the quantum projection noise prevents sufficiently precise determination of the tilt angles required to return the nuclear spins to the magnetic field axis in the sub-ensemble.

2. A method according to claim 1, wherein the quantum data token is a crystal (1) with a regular diamond lattice of carbon atoms, which comprises NV centers, each comprising a nitrogen impurity atom (3) and a vacancy (4).

3. A method according to claim 2, wherein one qubit corresponds to one NV center and wherein the ensemble comprises the number (N) of NV centers.

4. A method according to claim 2 or 3, wherein the setting of the predetermined identical quantum state is achieved by irradiating the crystal (1) with microwave radiation and / or light.

5. A method according to claim 4, wherein the light is selectively irradiated onto regions of the crystal (1) using a lens (11), wherein the qubits in this region form an information unit.

6. A method according to any one of claims 2 to 5, wherein multiple of such information units are arranged side by side near the surface in the crystal (1).

7. A method according to claim 6, wherein different orientations are set for the qubits of the multiple information units.