Protection of quantum information in small clusters of qubits
Lead Research Organisation:
UNIVERSITY COLLEGE LONDON
Department Name: Physics and Astronomy
Abstract
Modern digital electronics has reached an important junction. The traditional way of delivering ever stronger computing power by simple miniaturisation is no longer possible. One potential avenue for future electronics lies in Quantum Computing which can potentially deliver enormous computational power for certain tasks. Over the last decade, improvements in the materials, design and new architectures for realising qubits have led to an impressive increase of their coherence time. Yet, further improving coherence is imperative to achieving a fault tolerant quantum processor. We propose an approach to enhance qubit coherence by orders of magnitude, based on storing quantum information in the lowest energy states of short qubit chains. This encoding is protected from major sources of decoherence due to a high degree of spatial symmetry supported by long range interactions. In this project we will apply these principles to both Rydberg atoms and superconducting circuits which are architectures that have the required properties to support this approach. The project will also develop protocols to couple, control, readout and benchmark the qubits. Finally, this project aims to reach a level of technological maturity such that this approach will have near term applications in today's quantum computing industry.
Organisations
People |
ORCID iD |
| Marzena Szymanska (Principal Investigator) |
| Title | Superconducting Qubit Design, Application Number: 2515373.5 |
| Description | [6] The invention is defined by the appended claims. [7] According to a first aspect, there is provided circuitry for representing a unit of quantum information, the circuitry comprising: a plurality of superconducting islands comprising: four or more first superconducting islands, and a common superconducting island; and a plurality of Josephson junctions arranged to couple particular superconducting islands to one another, the plurality of Josephson junctions comprising: a plurality of first Josephson junctions, a plurality of second Josephson junctions, and a plurality of third Josephson junctions; wherein the plurality of first superconducting islands are arranged in a closed loop such that each first superconducting island is coupled to respective nearest neighbour first superconducting islands via the first Josephson junctions, wherein the plurality of first superconducting islands are each coupled to one or more respective next-but-one nearest neighbour first superconducting islands via the second Josephson junctions, and wherein the plurality of first superconducting islands are each coupled to the common superconducting island via the third Josephson junctions. [8] As such, circuitry is provided for representing a unit of quantum information that, due to the symmetry relationships utilised in the design of the qubit, namely the coupling of particular superconducting islands to one another, is protected from noise and decoherence, leading to longer decoherence times. [9] It should be understood that the unit of quantum information may be a qubit, but in other cases may be a qutrit or qudit. The circuitry Josephson junctions of the circuitry may include two pieces of superconducting material (e.g. wire), belonging to respective superconducting islands of the circuit, separated by a barrier of insulating material. Furthermore, in some cases the common superconducting island may be electrically grounded. The superconducting islands may each be a Cooper pair box and where each of the plurality of first superconducting islands may correspond to a quantum object having a local quantum degree of freedom of a phase of a potential of the respective superconducting islands. The circuitry may have a plurality of quantum states each corresponding to a respective potential energy minima of a particular superposition of clockwise and anti-clockwise current states. [10] In some examples, the four or more first superconducting islands consist of an even number of superconducting islands, such that the circuitry may benefit from the symmetry protection described herein. [11] In some implementations, a charging energy of each of the first Josephson junctions is less than a charging energy of each of the third Josephson junctions, and/or a ratio of a Josephson energy of each of the first Josephson junctions to a Josephson energy of each of the third Josephson junctions is greater than or equal to 0.6, and/or a ratio of a Josephson energy of each of the second Josephson junctions to a Josephson energy of each of the third Josephson junctions is greater than or equal to 0.6. Accordingly, the level of protection from noise and decoherence may be particularly high. [12] In some examples, the circuitry comprises six or more first superconducting islands. As such, the level of symmetry protection provided may be further increased beyond that afforded by a closed loop of four superconducting islands. In some cases the plurality of Josephson junctions further comprises a plurality of fourth Josephson junctions; wherein the plurality of first superconducting islands are each coupled to one or more respective next-but-two nearest neighbour first superconducting islands via the fourth Josephson junctions. As such, the level of symmetry protection provided may be yet further increased. [13] According to a second aspect, there is provided an apparatus comprising: the circuitry as described herein; and a resonator coupled to the circuitry for initialisation and readout of the unit of quantum information. As such, the aforementioned circuitry representing a unit of quantum information may be operated so as to be used within a quantum computing system. In some cases, the apparatus may further be considered to comprise an electromagnet arranged to apply a magnetic field to the circuitry. [14] According to a third aspect there is provided: a method for providing a system representing a unit of quantum information: providing a circuitry representing a unit of quantum information as described herein; applying an external magnetic field to the circuitry to set a magnetic flux threading each of a plurality of first circuity loops to one half magnetic flux quantum and a magnetic flux threading each of a plurality of second circuity loops to a half-integer magnetic flux quantum; wherein the plurality of first circuity loops are each a closed circuity loop comprising a particular first Josephson junction and two third Josephson junctions neighbouring the particular first Josephson junction; wherein the plurality of second circuitry loops are each a closed circuitry loop comprising a particular second Josephson junction and two first Josephson junctions neighbouring the particular second Josephson; and setting a gate charge of each of the first superconducting islands to one half Cooper pair. As such, the system may be controlled to improve the symmetry protection provided to the qubit represented by the circuitry. [15] In some cases, the method may further comprise: using a resonator, executing a stimulated Raman adiabatic passage procedure to stimulate a transition between states of the unit of quantum information represented by the circuitry. As such, the state of the qubit may be effectively controlled despite the increased symmetry protection afforded to the circuitry. In some examples, the stimulated Raman adiabatic passage procedure comprises: applying, using the resonator, a first pulse at a first microwave frequency to stimulate excitation of the unit of quantum information represented by the circuitry from a first state to a higher excited state; and applying, using the resonator, a second pulse at a second microwave frequency to stimulate excitation of the unit of quantum information represented by the circuitry from a second state to the higher excited state. As such, the qubit may be initialised to the second state. According to some examples, the second pulse may have a larger peak amplitude than the first pulse, and/or the second pulse has an amplitude peak that is later in time than an amplitude peak of the first pulse. [16] According to some examples, the method further comprises: performing a qubit initialisation process. As such, the qubit may be placed reliably in a particular state, as is usually required when utilised within a quantum computing system. The qubit initialisation process may comprises: adiabatically increasing a magnetic flux threading each of the first and second circuity loops from a respective first value to a respective second value during a first time period; maintaining the magnetic flux threading each of the first and second circuity loops at the respective second value for a second time period; and adiabatically decreasing the magnetic flux threading each of the first and second circuity loops to the respective first value during the first time period. |
| IP Reference | |
| Protection | Patent / Patent application |
| Year Protection Granted | 2025 |
| Licensed | No |