Patent Application
20240095567
The present disclosure relates to methods and systems for driving a set of qubits (quantum bits), e.g. as part of a quantum processor or other quantum device.
As background, US 2020/0125987 A1 explains that in quantum hardware, a mechanism to implement gate operations and measurements on the hardware is to generate microwave pulses and send the microwave pulses into the quantum computing system. The microwave pulses can cause the state of the quantum computing system to update in a defined way. In an ideal case, the effect of the pulses on one qubit is independent of what is occur ring on the other qubits. However, in operation, pulses sent down a line (e.g., through a port, also referred to as microwave gate) that are intended for one qubit can hit an unintended qubit. The pulse(s) received at the unintended qubit(s) is referred to as “crosstalk” or “control crosstalk”. When the unintended qubits have similar frequencies (as they do in a cross-resonance coupling design) the control crosstalk can cause a frequency shift of the qubit based on a physical process known as the AC-stark shift.
To reduce crosstalk, the prior art system of US 2020/0125987 A1 describes a control sequence that comprises a pulsing operation for a first qubit and at least a second qubit of a quantum circuit; and a coordination component that performs a calibration operation associated with a first pulse of a first channel of the first qubit of the pulsing operation and at least a second pulse of at least a second channel of the second qubit of the pulsing operation to selectively reduce a crosstalk in the quantum circuit. To generate respective pulses for respective qubits, the system comprises an electrical waveform generator that generates a single channel waveform; one or more configurable amplitude scalers that scale the single pulse to facilitate calibration of the qubits; and one or more configurable phase-shifters that implement frame changes at the qubits.
As further background, Watson et al. [Nature 555, 633-637 (2018)] describe a programmable two-qubit quantum processor in silicon. In this prior art, the coherent individual control of both qubits is achieved by patterning three cobalt micromagnets on top of the device. These micromagnets provide a magnetic-field gradient with a component that is perpendicular to the external magnetic field for electric dipole spin resonance (EDSR). Furthermore, the field gradient across the two dots results in qubit frequencies that are well separated (fQ1=18.4 GHz, fQ2=19.7 GHz), which allows the qubits to be addressed independently.
There remains a need for further improving the ability to individually manipulate physically adjacent qubits.
The small size and tight pitch of qubits, hosted in semiconductor quantum dots qubit devices, makes them one of the leading candidates for large-scale quantum computers. However, this tight pitch of qubits can also make them more susceptible to microwave crosstalk. In current multiqubit quantum dot devices, the qubit microwave drive is typically implemented using a shared electrode driving multiple qubits, where frequencies of many qubits are multiplexed. This causes unwanted crosstalk between the qubits, and limits the fidelity of quantum gates. Aspects of the present disclosure are directed to an architecture of gate-defined quantum dot based qubits, where microwave driving field for each qubit is created by a combination of a multiple driving gates. In this architecture, each of the qubits has one main dedicated microwave gate, and its unwanted effects on the neighboring qubits is cancelled out by applying appropriate microwave excitations to the neighboring gates. This can result in a more localized microwave electric field at the position of each of the qubits with their designated frequencies. The amplitudes and the phases can be calibrated, enabling high-fidelity quantum operations of a large-scale dense qubit array in silicon.
Aspects as described herein can be embodied as methods and systems for driving a set of qubits. In a quantum processing device, as described herein, a first qubit has a first driving frequency and a first microwave gate configured to apply a first microwave signal to the first qubit. A second qubit is arranged adjacent the first qubit and has a second driving frequency, which is substantially different from the first driving frequency. This in principle allows the qubits to be addressed independently using different frequencies. Furthermore a second microwave gate, separate from the first microwave gate, is configured to apply a second microwave signal to the second qubit. This allows to locally apply the respective microwaves to the respective qubit, which may further improve the qubits to be addressed independently. Electrical signals can be applied to the microwave gates, e.g. using a (voltage or current) signal generator. A first electrical signal comprising the first driving frequency is applied to the first microwave gate for applying the first microwave signal at the first driving frequency to the first qubit thereby driving the first qubit. Simultaneously a second electrical signal is applied to the second microwave gate. The second electrical signal comprises the first driving frequency shifted in phase and/or lowered in amplitude with respect to the first electrical signal such that the second microwave signal is generated with the first driving frequency to arrive in counterphase with the amplitude of the first microwave signal arriving the second qubit. In this way crosstalk of the first microwave signal to the second qubit can be at least partially compensated, preferably cancelled as much as possible. By the synergetic combination of using qubits with substantially different driving frequencies, locally applying the microwaves to respective qubits using separate gates, and compensating crosstalk by also applying the driving frequency of a respective qubit on a neighboring gate, e.g. in counterphase, the ability to individually manipulate physically adjacent qubits is greatly improved.
Aspects of the present disclosure are described e.g. with reference to gate-defined electron spin qubits, but the teachings can be applied also to other tightly-pitched qubits with electrical drive. For example, the teachings can be applied to make a dense array of quantum dot qubits, where at least the neighboring qubits have different frequencies, and each of the qubits has a dedicated microwave gate. Gate 1 to control qubit 1, gate 2 for qubit 2 and so on. When applying the microwave drive to gate 2 in order to drive qubit 2, there will be crosstalk where qubit 1 will also be influenced by the drive on gate 2, and experience off-resonant driving. In order to correct for this, a microwave tone can be applied also to gate 1 with the frequency of qubit 2, with a particular amplitude and phase, such that the electrical excitation at the frequency of qubit 2 is cancelled out at the position of qubit 1. Similarly, the microwave excitation at the frequency of qubit 1 is cancelled out at the position of qubit 2. In this way, each of the gates carries microwave excitation at the frequencies of multiple qubits, designed to deliver drive at the frequency of its designated qubit, while cancelling out the excitation at the frequencies of other qubits.
These and other features, aspects, and advantages of the apparatus, systems and methods of the present disclosure will become better understood from the following description, appended claims, and accompanying drawing wherein:
Terminology used for describing particular embodiments is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term “and/or” includes any and all combinations of one or more of the associated listed items. It will be understood that the terms “comprises” and/or “comprising” specify the presence of stated features but do not preclude the presence or addition of one or more other features. It will be further understood that when a particular step of a method is referred to as subsequent to another step, it can directly follow said other step or one or more intermediate steps may be carried out before carrying out the particular step, unless specified otherwise. Likewise it will be understood that when a connection between structures or components is described, this connection may be established directly or through intermediate structures or components unless specified otherwise.
The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, the absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. Embodiments may be described with reference to schematic and/or cross-section illustrations of possibly idealized embodiments and intermediate structures of the invention. In the description and drawings, like numbers refer to like elements throughout. Relative terms as well as derivatives thereof should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation unless stated otherwise.
Aspects can be embodied as a method or system for driving a set of qubits Q1,Q2. Typically, the set of qubits comprises at least a first qubit Q1 and a second qubit Q2. Also more than two qubits can be provided, e.g. as shown. For example, the system comprises three, four, five, six, or more qubits. In one embodiment, the first qubit Q1 is provided to have a first driving frequency F1, and a second qubit Q2, adjacent the first qubit Q1, is provided to have having a second driving frequency F2, which is substantially different from the first driving frequency F1. Also a third qubits Q3 can be provided adjacent the second qubit Q2 opposite the first qubit Q1, e.g. with still different driving frequency F3, or with the same or similar driving frequency F1 as the first qubit Q1. Preferably, at least adjacent qubits have substantially different driving frequencies. For example, the driving frequencies of adjacent qubits are different by at least 100 kHz, preferably at least 1 MHz, at least 10 MHz, or even more than 100 MHz. The larger the difference in driving frequency, the better can be the individual control/addressability of the respective qubits. On the other hand there can be limitations in the differences of driving frequency, e.g. due to the relative close proximity of the qubits. Typically, the optimal frequency separation of qubits may also depend on how fast they need to be driven to stay well within the coherence time. As an example, for SiGe devices driven with EDSR, a separations on the order of 100-200 MHz can be optimal, although also other frequencies may be used. In donor qubits which can have much higher coherence, the frequency separation can be lower, e.g. between 1-2 MHz. For trapped ions, the frequency separation it can be even less, e.g. around 100 kHz.
In some embodiments, a first microwave gate G1 is configured to apply a first microwave signal W1 to the first qubit Q1. In other or further embodiments, a second microwave gate G2 is configured to apply a second microwave signal W2 to the second qubit Q2. Preferably, the second microwave gate G2 is separate from the first microwave gate G1. In this way each gate can apply a respective signal (predominantly) to a respective qubit. In one embodiment, a first electrical signal E1 is applied to the first microwave gate G1 comprising the first driving frequency F1. In this way the first microwave signal W1 can be applied at the first driving frequency F1 to the first qubit Q1, for driving the first qubit Q1. As described herein, a second electrical signal E2 is applied to the second microwave gate G2 simultaneously with the applying of the first electrical signal E1 to the first microwave gate G1. Preferably, the second electrical signal E2 also comprises the first driving frequency F1 for generating the second microwave signal W2 with the first driving frequency F1. Most preferably, the second electrical signal E2 is tuned such that the second microwave signal W2 arrives in counterphase to the first microwave signal W1 at the second qubit Q2 for at least partially compensating, e.g. substantially cancelling, crosstalk of the first microwave signal W1 to the second qubit Q2.
Typically, the microwave signals are generated at frequencies on the order of GHz. This means that the wavelengths are typically on the order of centimeters, which is much higher than the distance between qubits (less than a micrometer, tens of nanometer). So, for the second microwave signal W2 to arrive in counterphase with the first microwave signal W1 it may be typically sufficient that the second electrical signal E2 is in counterphase with the first electrical signal E1 at least at the position of generating the microwaves, near the qubits (there could be phase differences at the point of generating the electrical signals depending on the respective path lengths to the respective signal generator).
In some embodiments, a respective state of the qubits is determined by a respective spin, e.g. electron spin. For example, the qubits are quantum-dot-based spin qubits. For example, each qubit is defined by the location of a respective quantum dot. These qubits can have substantial advantages over other types of qubits owing to their potential for all-electrical operation and ability to be integrated at high density onto an industrial platform. For example, the (Zeeman-split) spin-up and spin-down states of an electron can be used to represent 0 and 1 states of the qubit. Also other spin systems can be envisaged.
In some embodiments, a respective driving frequency F1,F2 of each qubit Q1,Q2 is determined by a respective energy difference between different spin states of the qubit Q1,Q2. For example, the energy difference between spin-up and spin-down states can be different for different qubits Q1,Q2. Accordingly a respective frequency F1,F2 for driving a respective qubit Q1,Q2 between spin states can be different. In one embodiment, the energy difference is determined by a respective magnetic field B1,B2 experienced by the respective qubit Q1,Q2. Typically, the stronger the magnetic field applied to a quantum spin system, the larger the splitting between energy levels of different spin states, and the higher the driving frequency to switch between these states.
In some embodiments, the qubits are disposed in a first magnetic field gradient d|B|/dx with a total or absolute value of the magnetic field |B| changing as function of a first direction X between the qubits Q1,Q2 such that the total magnetic field B1 experienced by the first qubit Q1 is smaller (or larger) than the total magnetic field B2 experienced by the second qubit Q1 by at least 100 kHz, preferably at least 1 MHz, at least 10 MHz, or even more than 100 MHz. Typically, the total magnetic field determines the driving frequency of the spin system, e.g. by affecting the Larmor precession of the spin.
In some embodiments, the qubits Q1,Q2 are configured to be electrically driven by the respective microwave signals W1,W2 causing displacement of the respective spin system, e.g. electron, in a second direction Z through a second magnetic field gradient dBy/dz. By using an oscillating electric field of microwaves to displace the spin system in a magnetic field gradient, in effect the spin system experiences a changing magnetic field which can drive a transition between the spin states. This is also referred to as electric dipole spin resonance (EDSR). In general, EDSR allows to flip the orientation of the magnetic moments through the use of electromagnetic radiation at resonant frequencies. Alternatively, it can also be envisaged to directly apply an oscillating magnetic field to each qubit, e.g. by means of a respective antenna acting as the microwave gate. However, this may be more difficult to control and typically needs higher currents.
Preferably, the first direction X, between the qubits, is different from the second direction Z, in which direction the qubits are predominantly driven. For example, the second direction Z is transverse or perpendicular to the first direction. In one embodiment, the second magnetic field gradient dBy/dz has a component in a third direction Y that changes as function of displacement in the second direction Z. For example, the third direction Y is perpendicular to both the first direction X and second direction Z. Alternatively, the first and second magnetic field gradients can also be in the same direction, e.g. by driving the spin system along the magnetic field gradient in the first direction X between the qubits Q1,Q2.
In some embodiments, each microwave gate G1,G2 comprises a respective (at least one) electrode adjacent each respective qubit Q1,Q2. Preferably, the electrodes are configured to emit a respective microwave signal W1,W2 transmitted to the respective qubit Q1,Q2. For example, by applying an oscillating voltage and charge to the electrode, a corresponding electric field can be generated driving the respective qubit in a direction away and/or towards the electrode while traversing a magnetic field gradient.
In a preferred embodiment, the first microwave gate G1 is closer to the first qubit Q1 than to the second qubit Q2. Most preferably, the second microwave gate G2 is also closer to the second qubit Q2 than to the first qubit Q1. For example, a first minimum distance D11 between an electrode forming the first microwave gate G1 and the first qubit Q1 is less than a second minimum distance D12 between the electrode and the second qubit Q2 by at least 30% (factor 1.3), preferably at least a factor two or at least a factor three, more preferably at least a factor five, most preferably at least a factor ten. The larger the distance of the second qubit Q2 to the electrode forming the first microwave gate G1, the smaller may be a relative intensity of the first microwave signal W1 experienced by the second qubit Q2 which is not the intended recipient of that signal. On the other hand, the distance cannot be too large because the qubits Q1,Q2 themselves may be relatively close together, e.g. to allow interaction there between. For example, the first minimum distance D11 is less than one micrometer, preferably, less than half a micrometer, more preferably less than hundred nanometer, most preferably less than fifty nanometer. For example, the second minimum distance D12 is more than fifty nanometer, preferably more than hundred nanometer, e.g. up to a micrometer, or more. In one embodiment, a distance between the first qubit Q1 and adjacent second qubit Q2 is less than one micrometer, preferably less than half a micrometer, more preferably less than two hundred nanometer, most preferably between ten nanometer and hundred nanometer. Typically, there can also be difference in angle at which the microwave arrives, which may give an extra difference in crosstalk.
In interpreting the appended claims, it should be understood that the word “comprising” does not exclude the presence of other elements or acts than those listed in a given claim; the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements; any reference signs in the claims do not limit their scope; several “means” may be represented by the same or different item(s) or implemented structure or function; any of the disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21156758.1 | Feb 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/NL2022/050071 | 2/11/2022 | WO |
