Search

EP-4202793-B1 - METHOD FOR READING THE SPIN STATE OF A SYSTEM AND ASSOCIATED METHOD FOR DETERMINING RELIABILITY

EP4202793B1EP 4202793 B1EP4202793 B1EP 4202793B1EP-4202793-B1

Inventors

  • MORTEMOUSQUE, PIERRE-ANDRÉ
  • JADOT, BAPTISTE
  • MEUNIER, Tristan
  • URDAMPILLETA, MATIAS

Dates

Publication Date
20260506
Application Date
20211221

Claims (10)

  1. Method (100) for measuring the spin state of two charged particles (PC) being able to adopt a first spin state noted S, a second spin state noted T+, a third spin state noted T0 and a fourth spin state noted T-, the two charged particles (PC) being contained in a system, comprising a first quantum dot (QD1) and a second quantum dot (QD2) characterised by a first parameter Γ relative to the potential barrier (BPI) separating the two quantum dots (QD1,QD2)and a second parameter ε corresponding to the difference in energy between the fundamental state of the first quantum dot (QD1) and the fundamental state of the second quantum dot (QD2), the couple formed by the values of these two parameters defining an operating point of the system (SYS) as a function of which the system adopts a first charge state noted (1,1) wherein each quantum dot (QD1,QD2) contains a charged particle (PC), a second charge state noted (2,0) wherein the first quantum dot (QD1) contains two charged particles (PC) or a third charge state noted (0,2) wherein the second quantum dot (QD2) contains two charged particles (PC), the operating point of the system (SYS) being initially in a first operating point (P1) corresponding to the first charge state (1,1) of the system (SYS) and for which the first S, the second T+, the third T0 and the fourth T- spin states are eigenstates of the spin of the two charged particles (PC), the method (100) comprising: - a first step (1E1) of modification of the operating point of the system (SYS) during which the operating point is displaced from the first operating point (P1) to a second operating point (P2) corresponding to the second charge state (2,0) or to the third charge state (0,2), the energy level of the first spin state S and the second spin state T+ forming an avoided crossing during the displacement from the first operating point (P1) to the second operating point (P2), the modification of the operating point taking place non-adiabatically during the passage of the avoided crossing such that, during this step (1E1), the system (SYS) transits from the first charge state (1,1) to the charge state corresponding to the second operating point (P2) when the two charged particles (PC) contained in the system (SYS) are in the first spin state S and is maintained in the first charge state (1,1) for the other spin states; - a first step (1E2) of measuring the charge state of the system (SYS), the spin state of the two charged particles (PC) at the first operating point (P1) being the first spin state S if the charge state of the system (SYS) measured is equal to the charge state corresponding to the second operating point (P2), the operating point being once again displaced to the first operating point (P1); - a second step (1E3) of modification of the operating point of the system (SYS) during which the operating point is displaced from the first operating point (P1) to the second operating point (P2), the modification of the operating point taking place adiabatically during the passage of the avoided crossing such that, during this step, the system (SYS) transits from the first charge state (1,1) to the charge state corresponding to the second operating point (P2) when the two charged particles (PC) contained in the system (SYS) are in the second spin state T+ and is maintained in the first charge state (1,1) for the other spin states; - a second step (1E4) of measuring the charge state of the system, the spin state of the two charged particles (PC) at the first operating point (P1) being the second spin state T+ if the charge state of the system (SYS) measured is equal to the charge state corresponding to the second operating point (P1), the operating point being once again displaced to the first operating point (P1); - a third step (1E5) of modification of the operating point of the system during which the operating point is displaced from the first operating point (P1) to the second operating point (P2), the modification of the operating point taking place non-adiabatically during the passage of the avoided crossing; - a waiting step (1E6) at a waiting operating point (P3) corresponding to a charge state identical to that of the second operating point (P2) during a predefined time, step during which the system transits from the first charge state (1,1) to the charge state corresponding to the second operating point (P2) when the two charged particles (PC) are in the third spin state T0 and is maintained in the first charge state for the fourth spin state T-; - a third step (1E7) of measuring the charge state of the system (SYS), the spin state of the two particles at the first operating point (P1) being the third spin state T0 if the charge state of the system (SYS) measured is equal to the charge state corresponding to the second operating point (P2) and the fourth spin state T- if the charge state of the system measured is equal to the first charge state (1,1).
  2. Method (100) according to the preceding claim wherein the waiting operating point (P3) is different from the second operating point (P2), the method comprising, before the waiting step (1E6), a step of displacement of the operating point from the second operating point (P2) to the waiting operating point (P3).
  3. Method (100) according to one of the two preceding claims wherein each step (1E1, 1E3, 1E5) of modification of the operating point from the first operating point (P1) to the second operating point (P2) comprises: - a sub-step of modification from the first operating point (P1) to an intermediate operating point (P12), the intermediate operating point (P12) corresponding to the first charge state of the system (SYS), the avoided crossing (ST+) formed by the first spin state S and the second spin state T+ being crossed during this sub-step; - a sub-step of modification from the intermediate operating point (P12) to the second operating point (P2), the modification taking place non-adiabatically during this sub-step.
  4. Method according to one of the preceding claims wherein each step (1E2, 1E4, 1E7) of measuring the charge state is preceded by a step of displacement of the operating point from the second operating point (P2) or from the waiting operating point (P3) to a fourth operating point (P4) corresponding to a charge state identical to the second operating point (P2) and having a value of the first parameter Γ such that: Γ ≪ 1 τ mes where τ mes is the time constant associated with the measurement of the charge state of the system.
  5. Method for determining the fidelity of the measurement of a first spin state noted S of two charged particles (PC) being able to adopt said first spin state S, a second spin state noted T+, a third spin state noted T0 and a fourth spin state noted T-, the two charged particles (PC) being contained in a system, comprising a first quantum dot (QD1) and a second quantum dot (QD2) and characterised by a first parameter Γ relative to the potential barrier (BPI) separating the two quantum dots (QD1,QD2) and a second parameter ε corresponding to the difference in energy between the fundamental state of the first quantum dot (QD1) and the fundamental state of the second quantum dot (QD2), the couple formed by the values of these two parameters defining an operating point of the system (SYS) as a function of which the system can adopt a first charge state noted (1,1) wherein each quantum dot (QD1,QD2) contains a charged particle (PC), a second charge state noted (2,0) wherein the first quantum dot (QD1) contains two charged particles (PC) and a third charge state noted (0,2) wherein the second quantum dot (QD2) contains two charged particles (PC), the operating point of the system (SYS) being initially in a first operating point (P1) corresponding to the first charge state (1,1) of the system (SYS) and for which the first S, second T+, third T0 and fourth T- spin states are eigenstates of the spin of the two charged particles (PC), the method (100) comprising, for an initial population of spin states wherein the first spin state S is in the majority: - a step of modification of the operating point of the system (SYS) during which the operating point is displaced from the first operating point (P1) to a second operating point (P2) corresponding to the second charge state (2,0) or to the third charge state (0,2), the energy level of the first spin state S and the second spin state T+ forming an avoided crossing during the displacement from the first operating point (P1) to the second operating point (P2), the modification of the operating point comprising a predetermined number of non-adiabatic transitions back and forth on either side of the avoided crossing; - a step of measuring the charge state of the system (SYS) ; the steps being repeated from the same population of spin states for a plurality of numbers of transitions back and forth, the fidelity being determined from the evolution of the probability of measurement of a charge state corresponding to the second operating point (P2) as a function of the number of transitions back and forth.
  6. Method for determining the fidelity of the measurement of a second spin state noted T+ of two charged particles (PC) being able to adopt a first spin state S, the second spin state noted T+, a third spin state noted T0 and a fourth spin state noted T-, the two charged particles (PC) being contained in a system, comprising a first quantum dot (QD1) and a second quantum dot (QD2) and characterised by a first parameter Γ relative to the potential barrier (BPI) separating the two quantum dots (QD1,QD2) and a second parameter ε corresponding to the difference in energy between the fundamental state of the first quantum dot (QD1) and the fundamental state of the second quantum dot (QD2), the couple formed by the values of these two parameters defining an operating point of the system (SYS) as a function of which the system can adopt a first charge state noted (1,1) wherein each quantum dot (QD1,QD2) contains a charged particle (PC), a second charge state noted (2,0) wherein the first quantum dot (QD1) contains two charged particles (PC) and a third charge state noted (0,2) wherein the second quantum dot (QD2) contains two charged particles (PC), the operating point of the system (SYS) being initially in a first operating point (P1) corresponding to the first charge state (1,1) of the system (SYS) and for which the first S, second T+, third T0 and fourth T- spin states are eigenstates of the spin of the two charged particles (PC), the method (100) comprising, for an initial population of spin states wherein the second spin state T+ is in the majority: - a step of modification of the operating point of the system (SYS) during which the operating point is displaced from the first operating point (P1) to a second operating point (P2) corresponding to the second charge state (2,0) or to the third charge state (0,2), the energy level of the first spin state S and the second spin state T+ forming an avoided crossing during the displacement from the first operating point (P1) to the second operating point (P2), the modification of the operating point comprising a predetermined number of adiabatic transitions back and forth on either side of the avoided crossing; - a step of measuring the charge state of the system (SYS) ; the steps being repeated from the same population of spin states for a plurality of numbers of transitions back and forth, the fidelity being determined from the evolution of the probability of measurement of a charge state corresponding to the second operating point (P2) as a function of the number of transitions back and forth.
  7. Method for determining the fidelity of the measurement of a third spin state noted T0 of two charged particles (PC) being able to adopt a first spin state S, a second spin state noted T+, the third spin state noted T0 and a fourth spin state noted T-, the two charged particles (PC) being contained in a system, comprising a first quantum dot (QD1) and a second quantum dot (QD2) and characterised by a first parameter Γ relative to the potential barrier (BPI) separating the two quantum dots (QD1,QD2) and a second parameter ε corresponding to the difference in energy between the fundamental state of the first quantum dot (QD1) and the fundamental state of the second quantum dot (QD2), the couple formed by the values of these two parameters defining an operating point of the system (SYS) as a function of which the system can adopt a first charge state noted (1,1) wherein each quantum dot (QD1,QD2) contains a charged particle (PC), a second charge state noted (2,0) wherein the first quantum dot (QD1) contains two charged particles (PC) and a third charge state noted (0,2) wherein the second quantum dot (QD2) contains two charged particles (PC), the operating point of the system (SYS) being initially in a first operating point (P1) corresponding to the first charge state (1,1) of the system (SYS) and for which the first S, second T+, third T0 and fourth T- spin states are eigenstates of the spin of the two charged particles (PC), the method (100) comprising, for an initial population of spin states wherein the third spin state T0 is in the majority: - a step of modification of the operating point of the system during which the operating point is displaced from the first operating point (P1) to a second operating point (P2) corresponding to the second charge state (2,0) or to the third charge state (0,2), the energy level of the first spin state S and the second spin state T+ forming an avoided crossing during the displacement from the first operating point (P1) to the second operating point (P2), the modification of the operating point taking place non-adiabatically during the passage of the avoided crossing; - a waiting step at a waiting operating point (P3) corresponding to a charge state identical to that of the second operating point (P2) during a predefined time; - a step of measuring the charge state of the system (SYS); the steps being repeated from the same population of spin states for a plurality of predefined waiting times, the fidelity being determined from the evolution of the probability of measurement of a charge state corresponding to the second operating point (P2) as a function of the waiting times.
  8. Quantum device comprising at least two quantum dots (QD1,QD2) and means configured to implement a method (100) according to one of the preceding claims.
  9. Computer programme comprising instructions which, when the programme is executed by a device according to the preceding claim, lead it to implement the method according to one of claims 1 to 7.
  10. Computer readable data support, on which is recorded the computer programme according to the preceding claim, said computer programme comprising instructions which, when the programme is executed by a device according to the claim 8, lead the computer programme to implement the method according to one of claims 1 to 7.

Description

DOMAINE TECHNIQUE DE L'INVENTION Le domaine technique de l'invention est celui de l'informatique quantique. La présente invention concerne un procédé de lecture complète de l'état de spin de deux particules chargées contenues dans deux boites quantiques couplées, les particules chargées étant dans un état de spin arbitraire, et en particulier un procédé de lecture qui ne fait pas appel pour la lecture à des particules chargées contenues dans des boites quantiques extérieures au système considéré dont l'état initial est connu. Elle concerne également un procédé permettant de déterminer la fidélité de cette lecture. ARRIERE-PLAN TECHNOLOGIQUE DE L'INVENTION Lorsque l'on cherche à déterminer l'état d'un qubit de spin formé d'une particule chargée contenue dans une boite quantique, il est habituel d'avoir recours à une deuxième particule chargée contenue dans une deuxième boite quantique dont l'état de spin est connu de sorte à mettre en œuvre un procédé de lecture en deux étapes : une première étape de conversion spin/charge par la deuxième boite quantique et une deuxième étape de lecture de l'état de charge de la deuxième boite quantique. Aussi, il apparait de ce qui précède qu'il n'est à priori pas possible d'utiliser le principe classique de lecture de l'état de spin de deux particules chargées sans avoir recours à des particules chargées contenues dans des boites quantiques extérieures audit système dont l'état initial de spin est connu. Cependant, une telle solution présente l'inconvénient majeur de mobiliser quatre boites quantiques (ou trois boites quantiques avec un transfert de charge) alors que deux d'entre elles seulement (ceux du système dont on cherchera à connaitre l'état de spin) seront utilisés pour la réalisation de portes quantiques, les deux autres ne servant qu'à identifier l'état de spin du système une fois les portes quantiques réalisée. De plus, il est nécessaire de connaître l'état de spin des particules chargées contenues dans ces boites quantiques extérieures pour effectuer cette lecture. A titre d'exemple, le brevet US 10482388 B1 divulgue des procédés permettant uniquement de déterminer si l'état de spin des deux particules chargées est l'état de spin singulet, mais ne permet pas de différencier les différents états triplets. De plus, si l'on cherche à appliquer le même principe de lecture à un système à deux particules chargées contenues dans deux boites quantiques couplées et que l'on souhaite déterminer de manière complète l'état dudit système à l'aide des seules boites quantiques du système, on se heurte à la difficulté technique suivante : l'état de charge d'une boite quantique ne peut varier qu'entre deux valeurs alors que l'état de spin du système peut prendre quatre valeurs. Aussi, il n'est pas possible de « coder » dans l'état de charge les quatre états de spin possibles du système. Aussi, il existe un besoin d'un procédé de lecture de l'état de spin complet d'un système à deux particules chargées contenues dans deux boites quantiques couplées ne nécessitant pas l'usage de particules chargées contenue dans des boites quantiques extérieures au système considéré et dont l'état de spin initial est connu. RESUME DE L'INVENTION L'invention offre une solution aux problèmes évoqués précédemment, en permettant, à partir du seul système de particules chargées contenues dans deux boites quantiques couplées, de déterminer l'état de spin complet du système considéré sans connaissance a priori de cet état de spin, à partir de la mesure de l'état de charge du système. La présente demande est définie dans les revendications indépendantes. Les revendications dépendantes définissent des modes de réalisations spécifiques. Pour cela, un premier aspect de l'invention concerne un procédé de mesure de l'état de spin de deux particules chargées pouvant adopter un premier état de spin noté S, un deuxième état de spin noté T+, un troisième état de spin noté T0 et un quatrième état de spin noté T-, les deux particules chargées étant contenues dans un système, comprenant une première boite quantique et une deuxième boite quantique caractérisé par un premier paramètre Γ relatif à la barrière de potentiel séparant les deux boites quantiques et un deuxième paramètre ε correspondant à la différence en énergie entre l'état fondamental de la première boite quantique et l'état fondamental de la deuxième boite quantique, le couple formé par les valeurs de ces deux paramètres définissant un point de fonctionnement du système en fonction duquel le système adopte un premier état de charge noté (1,1) dans lequel chaque boite quantique contient une particule chargée, un deuxième état de charge noté (2,0) dans lequel la première boite quantique contient deux particules chargées ou un troisième état de charge noté (0,2) dans lequel la deuxième boite quantique contient deux particules chargée, le point de fonctionnement du système étant initialement dans un premier point de fonctionnement correspondant au premier ét