EP1690218A2

EP1690218A2 - Device for reinitialising a quantum bit device having two energy states

Info

Publication number: EP1690218A2
Application number: EP04805408A
Authority: EP
Inventors: Denis Vion; Daniel Esteve; Philippe Joyez; Hugues Pothier; Pierre-François ORFILA; Eddy Collin; Grégoire ITHIER; Cristian Urbina
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2003-11-07
Filing date: 2004-11-08
Publication date: 2006-08-16
Also published as: FR2862151B1; JP2007516610A; FR2862151A1; CA2453126A1; WO2005048183A3; WO2005048183A2; US6930318B2; CA2453126C; US20050098773A1

Abstract

The inventive device reinitialising of a quantum bit device or a Qubit having two states |0>and |1> associated with two energy levels E0 and E1, respectively which are always such that E0<E1, is used for reinitialising said quantum bit device in the state |0> and comprises dissipative means (21) which is arranged near the quantum bit device and is suitable for absorbing an energy quantity only when said quantity is equal to certain ΔEenv values and coupling means (31, 32) suitable for temporarily modifying an energy difference ΔE01=E1-E0 of the initial ΔE01 value thereof which is not absorbable by said dissipative means into a ΔE'01 value = ΔEenv which is absorbable by the dissipative means. Said device can be used for Qubits provided with different physical supports.

Description

Device for resetting a quantum bit device with two energy states

Technical Field The subject of the present invention is a device for resetting a quantum bit device having two states | 0> and 1 1> associated respectively with two energy levels E ₀ and Ei such as E ₀ <E ^ to reset this quantum bit device to the state | 0>. Quantum bit devices (hereinafter referred to as Qubits) can have very many physical supports: atoms or ions optically or electromagnetically controlled, polarized photons, nuclear spins electromagnetically controlled, integrated electronic devices including superconductors with Josephson junctions, or gas semiconductors two-dimensional electrons, etc. Regardless of these physical supports, a Qubit always constitutes a system with two distinct states noted here 1 0> and 1 1>. These two basic states correspond, in the majority of cases, to two different energy levels, and in rare cases (polarized photons for example) to two states not based on different energy levels. The invention applies to all Qubits whose basic states correspond to two different energy levels. Without loss of generality, we will associate in what follows the state | 0> at the lowest energy level, noted E ₀ , and state 1 1> at the highest energy level, noted Ei. The State | 0> is also called here ground state. The invention thus relates to all types of Qubits including the two states | 0> and 1 1> correspond to two energy levels E ₀ and Ei which are always different, including during the quantum calculation stages (writing, coherent evolution, and possibly projective measurement). State of the art

In a Qubit, information must be able to be entered, stored or modified in a controlled manner by coupling or not to other Qubits, then read. The inscription is the preparation of the Qubit in any coherent superposition a | θ> + £> | 1> of its two basic states, expression in which "a" and "b" are complex numbers called probability amplitudes. The coherent control of the quantum state of the Qubit is the control of the evolution over time of the amplitudes of probabilities "a" and "b".

The loss of control of these amplitudes of probability can take place in two ways: The first is depolarization, that is to say the uncontrolled increase in the modulus of one of the amplitudes of probability in favor of the modulus of the other. Depending on whether the modulus of the amplitude "a" of the fundamental level increases or decreases, depolarization is an excitation or a relaxation of the energy of the Qubit. The relaxation of energy is an intrinsically random process characterized by a characteristic time Ti called relaxation time. This relaxation time is defined as the average time it takes for the Qubit initially prepared in state 1 1> to be found in state 1 0>. The second way of losing control over the amplitudes of probabilities corresponds to the appearance of a random phase shift between the states | θ> and 1 1>. In other words, for a Qubit which does not interact with other Qubits, the ideal evolution which is expressed mathematically by a (t) | θ> + b (t) | 1> is in fact replaced by a partially random evolution of the type a (t) | 0> + b (t) e ^{lφ (t} ) 1 1> where φ (t) is a random phase. With regard to these two possible phenomena, the Qubit must maintain a coherent evolution for as long as possible. Before any registration operation on a Qubit, it is necessary to place it in a reference state which is generally its fundamental state | 0>. According to the current state of the art, this reinitialization operation is carried out by simply waiting, during a period of time equal to a few times the relaxation time Ti, for the Qubit to relax towards the state I0> with a very high probability. . Such a reset can be very slow since the very functioning of a Qubit requires maintaining its quantum coherence for as long as possible, and therefore having a relaxation time T | as long as possible. In other words, the more efficient a Qubit, the longer the relaxation time Ti and the longer its reset becomes.

SUMMARY OF THE INVENTION The invention aims precisely to solve this problem of slow reinitialization, and to allow an efficient Qubit to be reinitialized quickly, without significant degradation of its ability to evolve in a coherent manner. These aims are achieved in accordance with the invention by means of a device for resetting a quantum bit or Qubit device having two states | 0> and 1 1> associated respectively with two energy levels E ₀ and Ei, always such as E ₀ <Ei, intended to reinitialize this quantum bit device in the state | 0>, characterized in that it comprises first dissipative means placed close to the quantum bit device, capable of absorbing a quantity of energy only when this quantity is equal to certain values ΔEenv, and second means capable of temporarily modifying the difference in energy ΔE01 = E1 - EO, from its initial value ΔE01 which the first dissipative means cannot absorb, to a value ΔE'01 = ΔEenv capable of being absorbed by the first dissipative means. We can further define this invention by saying that the Qubit with two energy levels can be reset by a temporary increase in its probability of relaxation, obtained in whole or in part by temporary coupling to a dissipative device capable of absorbing its energy. of transition. Finally, we can further define this invention by saying that the Qubit with two energy levels can be reinitialized by a change in its transition frequency so as to couple it to a system having at least one degree of freedom including the quantum spectral density. to the Qubit transition frequency is high. With a Qubit whose two states | 0> and 1 1> correspond respectively to two energy levels E ₀ and Ei such that E ₀ <Ei such an operation can be achieved using a reset device according to the invention such as: - the first dissipative means are placed in the immediate energy environment of the quantum bit device and have at least one absorption peak whose value ΔE _in v is: • sufficiently distant from the transition energy ΔE ₀ ι = Ei - E ₀ of quantum bit device, to make negligible or zero any interaction between the quantum bit device and these first dissipative means, but • close enough to this transition energy so that the quantum bit device can be placed in operating conditions where the 'transition energy ΔE' ₀ ι becomes equal to or close to the value ΔE _in v of the absorption peak, so that there can temporarily be a strong energy coupling between the disp positive quantum bit and these first dissipative means, and - the second means are capable of modifying the transition energy ΔE ₀₁ = Ei - Eo of the quantum bit device, to bring it, during resets, to said value ΔE ' ₀ ι equal to or close to the value ΔE _8nv of the absorption peak. The first dissipative means having at least one absorption peak with value ΔE _in v can have a particular degree of freedom of characteristic energy ΔE _in v which can absorb the transition energy

ΔE ₀₁ = Ei - Eo of the quantum bit device when it is brought to the value ΔE ' ₀ ι equal to or close to the value ΔE _in v of the absorption peak. The first dissipative means having at least one absorption peak with a value ΔE _env may also have a collection of degrees of freedom whose quantum spectral density at the transition frequency v ₀ ι = ΔE ₀ ι / h (where h is the Planck constant) of the quantum bit device is large. In this case there are a multiplicity of very close ΔE _env values. The second means of the invention allow either to reduce to an extremely low value the coupling of the Qubit with its environment, in order to minimize the decoherence effect of this environment, to increase the relaxation time Ti to a very large value and to allow the Qubit to evolve in a coherent manner, either to couple the Qubit with its environment, in order to maximize decoherence by relaxation, that is to say to decrease the relaxation time Ti to a very low value, and to quickly reset the Qubit in the state | 0>. According to a particular embodiment, the second means comprise means generating specific reset instructions, these instructions being applied to the usual means for adjusting the quantum bit device, so as to temporarily bring the transition energy ΔE ₀ ι to the value ΔE ' ₀ ι equal to or close to the value ΔE _in v of the absorption peak. More specifically, the specific reset instructions can then have a particular value chosen outside the ranges used when the Qubit is allowed to evolve in a coherent manner, but this is not compulsory. According to another particular embodiment, the second means comprise specific means for adjusting the operating parameters of the quantum bit device, acting directly on the transition energy of the quantum bit device to temporarily bring the transition energy ΔE ₀ ι to a value ΔE ' ₀ ι equal to or close to the value ΔE _in v of the absorption peak, independently of the setting of the operating parameters of the quantum bit device as chosen to make it evolve in a coherent manner. Only the fundamental characteristics of a Qubit with two energy levels being implemented, the invention is applicable to any Qubit of this type, whatever its technology of realization . According to a first preferential embodiment technology, these

Qubits are electric, with Josephson superconducting junctions. Their coherent evolution can be ensured by construction by minimizing their number of degrees of freedom, by conferring on these Qubits certain properties of symmetry described further on in the rest of the present application, and by adjusting the external parameters controlling their transition frequency. The invention applies in particular to the preferential case where the electric Qubit comprises a physical support having a quantum behavior constituted by an integrated electronic superconducting device produced using a Cooper pair box with two Josephson junctions provided with a writing circuit comprising a gate electrode, a reading circuit comprising a Josephson reading junction, of Josephson energy substantially greater than that of the Josephson junctions of the Cooper box, and magnetic means capable of adjusting the phase du Qubit, in accordance with the article published in the journal Science 1069372 of May 04, 2002 "Manipulating the Quantum State of an Electrical Circuit" by D. Vion, A. Aassime, A. Cottet, P. Joyez, H. Pothier, C Urbina, D. Esteve, MH Devoret. Such a circuit, called Quantronium in the following, is read by the application of a current pulse capable of switching the voltage of the Josephson read junction. The superconducting loop of such a Qubit is constructed in such a way that its transition frequency voi can be adjusted to a stationary value with respect to the external parameters and disturbances, condition that one seeks to write information in the Qubit or let it evolve consistently. This adjustment is in practice produced by generating, using a magnetic loop located nearby, an adjustable magnetic flux acting on the δ phase of the Qubit. The read circuit, independent of the write circuit, is galvanically connected to the Qubit's superconductive loop. It comprises, in addition to the Josephson junction for reading, on the one hand means for applying during the reading phase a current pulse l of configurable duration and amplitude, and on the other hand means for detecting phase jumps of 2π at the terminals of the Josephson reading junction which appear, following the reading pulse, for one of the two states only. In order to study how the probability of relaxation of the Qubit varies during the application of this read pulse, measurements were carried out by generating time steps on the rising edge of this pulse. Note that when this plateau reaches certain current values, each of which fixes a certain Qubit transition energy, the probability of relaxation of this Qubit becomes high (the relaxation time Ti becomes small). These values correspond to resonances of the immediate environment of the Qubit, an environment which in this case is simply the electrical reading circuit. For this Quantronium, the implementation of the invention can present several methods. The first dissipative means of the reset device according to the invention comprise a dissipative device which is typically produced by a resonant circuit provided with a resistive element. More precisely, it can be achieved by the voluntary introduction of a resonance in the electrical reading circuit of the Qubit, the frequency of which is far from the value which the transition frequency of the Qubit takes when the latter is stationary with respect to -vis control parameters, but can be temporarily taken by the Qubit transition frequency during a reset. The second means or coupling means can include a generator of control signals which are applied to the adjustment means specific to the Qubit, for example: - a current pulse of specific value applied to the magnetic coil (preferential realization of the magnetic adjustment means) acting on the phase δ of the Qubit . - a specific value of current pulse applied to the usual control means that constitutes the read circuit. Preferably, the value of the magnetic flux in the superconductive loop and / or of the current in the read junction will be chosen one (s) value (s) which are never taken during nominal operation. As regards the current in the read junction, this may for example take during the reinitialization phase a value of sign opposite to the sign of the current pulse used for reading the state of the Qubit. More generally, all the means for adjusting the parameters acting on this circuit are capable of constituting the reset means, provided that specific input quantities are applied to them. The operation is then as follows in the case of a command generating a current pulse in the reading circuit: - when one wants to write an information in the Qubit or let it evolve in a coherent way, one regulates the magnetic flux passing through the Qubit's superconductive loop and the current of the read junction so that the Qubit's transition frequency takes on its stationary value. - when you want to reset the Qubit, you generate a reading current pulse temporarily bringing the Qubit transition frequency to the value equal to or very close to the value of the frequency of the voluntarily introduced resonance. We see that this operation is evidently transposed in the case of a current pulse in the coil acting on phase δ of the Qubit. These coupling means can also be means specific to the reset device, such as for example second magnetic means (for example produced by a magnetic coil) acting on the δ phase of the Qubit, a second coupling grid of the box with pairs of Cooper, a second loop passing through the Josephson reading junction and capable of sending a current pulse of an amplitude corresponding to a high probability of Qubit decoherence, etc. Again, the operation is deduced with obviousness from the previous case. Preferably, the value of the magnetic flux in the superconductive loop and / or of the current in the read junction will be chosen one (s) value (s) which are never taken during nominal operation. As regards the current in the read junction, this may for example take during the reinitialization phase a value of sign opposite to the sign of the current pulse used for reading the state of the Qubit. According to a second technology, the Qubits can have for physical support an atom or an ion, whose energetic state changes under the effect of the absorption or the emission of a photon of frequency voi corresponding to the energy of Qubit transition. Maintaining coherence can be ensured for example by placing this atom (rsp. Ion) in an electromagnetic cavity small enough so that it cannot absorb any photon at the frequency v ₀₁ . The reset device according to the invention consists, for example, in moving this atom (rsp. Ion) out of this cavity using appropriate electromagnetic fields so that it can spontaneously de-energize. More efficiently, for example, the atom (rsp. Ion) can be brought into a second cavity having an absorption mode at the frequency of the Qubit. Thus, in this case, the second means of the device according to the invention include means capable of varying the probability that the atom or ion absorbs energy, and more particularly, these means modify the dimensions of the space in which the atom or ion is confined. The first dissipative means of the device according to the invention are then constituted by the new dissipative space into which the atom or the ion is brought. According to a third technology, the Qubits can have for physical support a quantum dot produced in a two-dimensional electron gas, controlled by grid electrodes on or under the surface of this gas. The two energy states corresponding to the two Qubit states are generally two electronic states of the quantum dot separated by a transition energy ΔE ₀ -ι. In this example of a quick reset device according to the invention, the first dissipative means comprise a resonant electrical circuit provided with a resistive component, placed in the circuit of at least one gate electrode, and the second means comprise a field source. magnetic or grid voltage sources to apply a specific value of the grid voltage to the grid electrodes, in order to vary the size of the quantum dot so as to bring the transition energy ΔE01 to a value ΔE ' ₀ ι equal at or near the value ΔE _env of an absorption peak of the electric circuit controlling at least one of the electrodes defining the geometry of the quantum dot.

Brief description of the drawings Other characteristics and advantages of the invention will emerge from the following description of particular embodiments, given by way of examples, with reference to the appended drawings, in which: - Figure 1 represents the block diagram an electronic Qubit based on Josephson junctions, of the Quantronium type, - Figure 2 is a three-dimensional diagram illustrating the choice of the operating point of a quantum bit device according to the invention, - Figure 3 shows examples of timing diagrams of signals involved in the manipulation or measurement of states measurements of a quantum bit device according to the invention, - Figure 4A shows the temporal representation of a reading current pulse applied to Quantronium after it has been prepared in the state | 1>, a plate placed on the rising edge of the pulse to carry out tests that can be moved to different values of the current in the read junction, with an adjustable duration, - Figure 4B shows the variation in the probability that the Quantronium is measured in the state M>, as a function of the current of the plateau placed on the rising edge of the read pulse, with a duration of the plateau which is here fixed at 100 ns, - Figure 4C shows the variation of the probability that Quantronium will be measured in the state | l> as a function of the duration of the plateau, and this for two values of the plateau current, inducing two different values of the transition frequency of the Quantronium, - Figure 5 is a schematic representation of an example of a circuit whose means of reset consist of specific control elements of the read pulse, - Figure 6 is a schematic representation of an example of a circuit whose reset means consist of specific control elements of the phase δ of the Qubit to using a magnetic loop located nearby, - Figure 7 is a schematic representation of an ionic Qubit, highlighting the communication device with an electromagnetic cavity and the means to move the ion there, and - Figure 8 is a schematic representation of a Qubit at "Quantum dot" in a two-dimensional gas, highlighting an electrical circuit inducing both a modification of its size (and therefore of the Qubit transition frequency) and an absorption of its energy in a given frequency mode.

Detailed description of particular embodiments of the invention We will first describe the preferred embodiment of an electrical Qubit, and more particularly the case where this Qubit is a superconductive electrical circuit produced using a box of pairs of Cooper with two Josephson junctions, a write circuit comprising a gate electrode, a read circuit comprising a Josephson read junction, of Josephson energy substantially greater than that of the Josephson junctions of the Cooper pair box , and magnetic means capable of adjusting the phase of the Qubit, in accordance with the article published in the journal Science 1069372 of May 04, 2002 "Manipulating the Quantum State of an Electrical Circuit" by D. Vion, A. Aassime, A. Cottet , P. Joyez, H. Pothier, C. Urbina, D. Esteve, MH Devoret. Such a circuit, which will hereinafter be called Quantronium, is usually read by the application of a current pulse capable of switching the Josephson read junction. Referring to FIG. 1, one can further define such a Quantronium by the fact that: a) it comprises a superconducting island between two Josephson 101, 102 junctions of comparable Josephson EJI and Ej ₂ energy, these junctions being included in a type S superconducting loop, the loop being constructed in such a way that the transition frequency of the Qubit can be electrically adjusted to a stationary value of the external parameters and disturbances, b) it comprises an electrode 131 capacitively linked to the superconducting island, the configuration of the circuit makes it possible to adjust the transition frequency between the two states of the Qubit, this electrode being moreover able, during the writing phases, to place the Qubit in a coherent superposition of the two states of the Qubit, c) it comprises means 133, 135 of applying to the superconductive loop a magnetic flux φ, that the configuration of the circuit makes it able to adjust the transition frequency voi between the two states of the Qubit, d) it comprises a third Josephson 105 junction called reading, whose Josephson EJL energy is of the order of 50 times to 100 times, and preferably approximately 80 times, the Josephson energy EJI or E ₂ , of the Josephson junctions 101, 102 defined above, this Josephson junction of reading 105 then functioning as a superconductive short-circuit during the writing or holding phases, and functioning as a threshold detector during the reading phase, e) it includes a reading circuit, independent of the writing circuit, galvanically connected to the superconductive loop of the Qubit, and comprising, in addition to the junction ion Josephson for reading 105, on the one hand means 125 for applying during the reading step a current pulse lb of adjustable duration and amplitude, and on the other hand means 126 for detecting phase jumps at the terminals of the Josephson junction of reading 105 under the combined effect of the state of the Qubit and the reading impulse.

For this Quantronium, the preferred implementation of the invention comprises, in addition to points a) to e), the following points which appear in FIG. 5: f) a dissipative device 21, exhibiting dissipation only at one or a few specific frequencies, far from the nominal transition frequency of Quantronium (typically a few GHz) or its multiples or sub-multiples. It is generally carried out for example using a resonant circuit provided with a resistive element. This circuit 21, placed at the terminals of the Josephson reading junction 105, is represented diagrammatically in FIGS. 5 and 6 by an RLC circuit comprising a resistor 157, the value of which is not a construction imperfection but chosen voluntarily to a value capable of dissipating the energy of the Quantronium, a capacitor 159 and an inductor 158. g) coupling means 31 applying, to the control means usual that constitutes the reading circuit, a current pulse of specific value so as to modify the transition energy of the Qubit ΔE ₀ ι = Ei - Eo, to bring it, to a value ΔE ' ₀ ι ≈.ΔE _Θ nv-, this latter value corresponding to the resonant frequency of the resonant circuit 21 provided with a resistive element. Furthermore, the entire electrical circuit of a Quantronium is designed in such a way that its structure and operation are as symmetrical as possible. In addition, the Qubit is able to be placed in operating conditions which promote the maintenance of its consistency. These operating conditions which promote the maintenance of the coherence of the Qubit which is the subject of the invention are: 1) prior to its use and during the manipulation of the quantum state, the electrical adjustment of the transition frequency of the two quantum states to a stationary value with respect to the direct voltage V _g applied to the capacitor 103 of capacitance C _g on the electrode 131 not belonging to the charge island, and with respect to the magnetic flux passing through the Qubit , 2) during the writing phases (manipulation of the quantum state), the cancellation of all the currents flowing in the loop of the Qubit proper, this by a particular combination of a magnetic flux and a current in the read junction 105, 3) the read circuit is not permanently activated but only at certain times determined by read pulses l _b .

The charge island can, through the capacitor 103 of capacitance C _g which is connected to it, be polarized in charge. This is done by applying to the electrode 131 of the capacitor 103 not belonging to the island, a superposition: of a DC bias voltage V _g which in practice determines the resonance frequency of the loop that constitutes the Qubit. of a sequence of voltage oscillations u (t) at this resonant frequency, their amplitude and their number allowing the Qubit to be placed in a coherent superposition a | θ> + b | l> of its two quantum states. The Josephson junctions 101, 102 delimiting the island of charges are chosen so that their Josephson Ej energy is close to their Coulomb Ec energy. Each of them has an electrode that is part of the superconducting island, and an electrode that is not part of it. Between the electrodes of each of these junctions which do not belong to the island, the phase difference δ imposed by the reading current pulse causes currents which depend on its state to flow in the Qubit loop. These two Josephson junctions 101, 102 are chosen such that their energy Ej is very close to their energy E _c , which is generally achieved for junctions of very small dimensions. In addition, the Josephson energies Ej of the junctions 101, 102 have values as close as possible. The electrodes of the Josephson junctions not belonging to the island join to form a superconductive loop, the whole forming the Qubit proper. In reality, this loop also includes the third junction Josephson 105, but of much larger dimensions so that for the latter, the energy Josephson Ej is very large compared to the energy of Coulomb E _c . It is this characteristic which enables the read junction to behave, outside of the read periods, like a superconductive short circuit. In its nominal operation, corresponding to maximum maintenance of coherence, a Quantronium is adjusted as follows: The operation of an example device according to the invention will be explained using the block diagram in Figure 1. The load island is delimited by the insulators of the Josephson 101 and 102 junctions. These junctions 101 and 102 have very similar characteristics. They are very small (about 0.1 μm by 0.1 μm) and designed so that the EE _c ratio of each is between 1 and 3. The value of this EE _c ratio for the effective junction of the box pairs of Cooper 110, four times higher, is therefore between and

4 and 12. The superconductive loop closes with a reading junction 105 of much larger dimensions (approximately 3 μm by 0.3 μm), behaving almost like a short-circuit for the Qubit outside of the measurement steps. This is due to its Josephson E energy approximately 100 times greater than the Josephson energy of junctions 101 and 102. Furthermore, the EE _c ratio of this reading junction 105 is approximately 5000 to 10000 times higher than the EE _c ratio of junctions 101 and 102. This loop is subjected to an adjustable magnetic flux φ induced either by a permanent magnet, or by a coil 133 or an electric wire traversed by a current coming from a current source 135, placed nearby, possibly on the same substrate as the qubit. This flow φ makes it possible to adjust the coordinate δ of the operating point during the manipulation of the qubit. In particular, it makes it possible to compensate for a negative quiescent current in the read junction 105 by maintaining the phase δ at the preferred value. This combination of magnetic flux through the loop and negative quiescent current in the measurement junction 105 makes it possible to increase the discriminating power of the states 0 and 1 during reading, as will be indicated below. The circuit can be produced with any type of superconductive material

5 such as for example aluminum or niobium. The preferential production was carried out on an Si / Si0 ₂ substrate by vacuum deposition aluminum conductive films. The Josephson 101 and 102 junctions are made by two aluminum strips deposited along the same axis, but separated by a few hundred nanometers. These ribbons are oxidized to Al ₂ 0 ₂ . Then a third ribbon of short length is deposited at the level of the separation between the first two ribbons so that this ribbon covers the first two in two areas of approximately 0.1 μm x 0.1 μm each, thereby constituting the two Josephson junctions 101, 102. This third ribbon constitutes the island of the Cooper pair box, and therefore also the first armature 131 of the capacitor 103 of capacitance C _g . In parallel with this new ribbon, another ribbon is produced, which will constitute the second armature of the capacitor 103. The charge island thus formed is of small dimensions. The rest of the qubit loop proper is carried out in an analogous manner, but the dimensions of the Josephson junction of reading 105 are much larger, around 1 μm ² . The operating temperature of the circuit must be much lower than the transition energy of the Qubit divided by the Boltzmann constant kβ. It is in practice of the order of 50 mK, and is obtained with a He ₃ / He dilution refrigerator. The Qubit's transition frequency depends on the external parameters of the charge coupled to the island and the magnetic flux through the loop. A noise in electrical charge or in magnetic flux is therefore likely to vary this transition frequency during the manipulation of the Qubit, and therefore to induce a random phase shift responsible for the decoherence of its quantum state. The sensitivity to these noises is therefore minimal at the operating points Fi, F ₂ , F ₃ where the transition frequency v01 is stationary with respect to the external parameters. These operating points are therefore preferential. For a Qubit according to the invention the possible points are the saddle point noted Fi in Figure 2 (N _g = 0.5 and δ = 0) as well as the vertex point F ₂ or F ₃ to within 2π (N _g = 0 or 1 and δ = 0) of the three-dimensional graph. In nominal operation, corresponding to a maximum maintenance of coherence, the registration of a state or a coherent superposition of states in a Quantronium is carried out as follows: The phases of registration of Quantronium are located in the frame general operating conditions which correspond to maximum maintenance of coherence, namely: 1) before its use and during the manipulation of the quantum state, the electrical adjustment of the transition frequency of the two quantum states to a value stationary with respect to the DC voltage V _g applied to the capacitor 103 of capacitance C _g on the electrode 131 not belonging to the charge island, and with respect to the magnetic flux passing through the qubit, 2 ) during the writing phases (manipulation of the quantum state), the cancellation of all the currents flowing in the loop of the qubit proper, this by a particular combination of a magnetic flux and of a current in the reading junction 105. The writing is carried out on the one hand by acting on the degree of freedom of charge by applying to the gate electrode 131 of the box 110 (coupling by the capacitor 103 capacitance C _g ) a direct voltage V _g from a voltage source 121 to reach the optimal operating point N _g = 1/2, where N _g represents the reduced polarization charge of the island C _g V _g / 2e) and on the other hand, from a source 122, radiofrequency pulses u (t) at resonance with the Qubit of the box 110. The amplitude of the alternating voltage u (t) corresponds to a load coupled of the order of 0.01 times 2e. During the reading phases, a Quantronium proceeds as follows: To read the state of the qubit, a current generator 125 in parallel with the Josephson junction of reading 105 generates a pulse l (Figure 3) of intensity and duration adjustable (typically 100 ns). During a pulse, this current generator 125 induces across the terminals of the junction Josephson of reading 105 a superconductive phase difference which, with the superconductive phase difference δ at the terminals of the Cooper 100 pair box and with the magnetic flux φ passing through the loop, satisfies the following equation: δ = γ + φ / φo 2π to the nearest sign where φ ₀ is the quantum of magnetic flux. At the operating point Fi (N _g = 0 δ = 0) during the time of the read pulse, the phase γ undergoes a displacement which generates a displacement of the phase δ. The read junction 105 switches around γ = π / 2. During the reading, the current must be close to the critical current of the reading junction 105 so as to have transition rates respectively close to 0% and 100% for the two states of the qubit. The bias current l _b outside of the reading is chosen to maintain the circuit at the point F chosen such as the points F ^ F ₂ , F ₃ of Figure 2. Finally, the choice of the flux φ coupled to the superconductive loop can be optimized so that the loop currents i ₀ and associated with the states | θ> and | l>, revealed by the reading pulse, are as different as possible. An optimization of the reading of the state of the Qubit according to the invention is carried out by choosing the magnetic flux φ induced through the superconductive loop so that the loop currents i ₀ and H associated with states 10> and | l>, revealed by the reading pulse, are as different as possible. To compensate for this magnetic flux and maintain a zero phase δ corresponding to the operating point Fj, the reading current generator delivers, outside the reading sequences, not a zero quiescent current, but a quiescent current of the order of -0.25 I _bc , negative with respect to the direction of the reading pulses. The peak value I _bc of these pulses remains unchanged. According to this preferential optimization, the displacement in phase δ during the pulse of reading will be between π / 2 and π. In the nominal operation of this Qubit, corresponding to a maximum maintenance of coherence, the decoupling of the Qubit from the read junction is obtained as follows: The read loop has eigenmodes whose frequency must be as far as possible from the natural frequency of the Qubit loop proper to avoid relaxation of the Qubit toward its ground state | θ>. This separation between the natural frequencies of the Qubit and the read junction already exists due to the significant difference between the dimensions and characteristics of the Josephson read junction 105, and those of the Josephson junctions 101, 102 delimiting the island. It is voluntarily increased by the addition of one or more capacitors at the terminals of the Josephson junction of reading 105. In summary, the ability of a Quantronium to evolve in a coherent manner is obtained by respecting three conditions: a) the first condition aims to eliminate the effect of noise under load seen by the island. Unlike the Cooper box measured under load, it is possible according to the invention to choose the ratio between the Josephson energy parameters Ej and Coulomb energy E _c so as to be able simultaneously: - to ensure the anharmonicity of the spectrum of energies to form actually a Qubit, - make the transition frequency almost completely insensitive to noise under load, - manipulate the state of the Qubit by applying radiofrequency voltages to the gate 131 of the island. b) the second condition aims to limit the relaxation of the Qubit, which contributes to decoherence, when the latter is placed in its excited state conventionally noted 1 1>. Indeed, a Qubit can get excited by transferring its energy to its immediate environment, in this case here in the entire electrical circuit. More precisely, this transfer takes place towards the real part 157 of the equivalent electrical impedance 157, 158 as seen from the Qubit (Figures 5 and 6). This impedance, even minimized, cannot be strictly zero. For the energy transfer to be zero, it is therefore necessary to impose as a condition that no current flows in the superconductive loop of the Qubit. This condition of canceling the global current of the Qubit loop, resulting from the state of the Qubit or from the coherent superposition of its states, is obtained by precisely adjusting the magnetic flux to a zero value by means of an antagonistic flux applied to the Qubit loop by a B field applied either by a suitable permanent magnet or by an induction loop traversed by a control current. This condition must be checked during the writing and maintenance phases, whatever the state of the Qubit and therefore whatever the flow to be canceled. c) the third condition aims to limit the influence of environmental parameters on decoherence. If we denote by IΨ (t = 0)> the following superimposed state: | ψ (t = 0)> = | θ> + | 1> Ideally, this state evolves freely by relative phase shift of the components 10> and 1 1> at the transition frequency voi of the Qubit: IΨ (t)> = 10> + exp (ivo-ιt) | 1> If the transition frequency voi depends on external parameters which are represented by the generic variable x, any noise on this generic variable x results in a fluctuation of the transition frequency v ₀ ι and therefore by a random error on the relative phase shift of components 10> and 1 1>: This is the phenomenon of decoherence. The decoherence is therefore minimal when the transition frequency voi is stationary with respect to x, that is to say when the condition is fulfilled: dv ₀ ι / dx = 0 For the invention, the external parameters on which the transition frequency v ₀ ι depends, and which have been symbolized by the generic variable x, are essentially two in number: the DC voltage V _g applied to the capacitor 103 and the magnetic flux φ through the superconducting loop constituting the Qubit proper. The values of V _g and φ are therefore adjusted so that the two conditions are fulfilled simultaneously: dv ₀ ι / dV _g = 0 dv ₀ ι / dφ = 0 which minimizes the decoherence, in particular in the case of a state superimposed on the Qubit. The implementation of the reset device according to the invention will now be described in the context of an association with a Quantronium. The invention consists in generating a transient excursion of at least one of the three conditions mentioned above outside of the functional conditions, in order to place the Qubit on the contrary in conditions inducing an energy exchange in the vicinity of the natural frequency of this Qubit, in order to quickly obtain decoherence by relaxation. These conditions electrically correspond to a sending of energy in resonances of secondary degrees of freedom of the circuit, then to a dissipation of this energy by resistive way. The resonant dissipative device is placed at the terminals of the Josephson reading junction. It is represented schematically in FIG. 5 by an RLC circuit comprising a voluntarily dissipative resistor 157, a capacitor 159 and a choke 158. It can just as easily be represented in an equivalent manner by a choke, a capacitor and a resistance in parallel , because in reality at the frequencies considered the constants are distributed. Any printed circuit element simultaneously has inductive and capacitive characteristics which are almost impossible to locate physically, as is well known in microwave. In addition, these characteristics also depend on its immediate environment, to the point that its geometry, even given with precision, is not sufficient to determine its distributed characteristics. We will simply define its resonant frequency corresponding, as seen above, to a particular degree of freedom ΔE _e nv which can absorb the transition energy ΔE ₀ ι = Ei - E ₀ of the Qubit when it is brought to the value ΔE ' ₀ ι ≈ ΔE _in v In the particular case considered, this frequency is around 8.06 GHz. The means of temporary coupling to the Qubit for its reinitialization can have several variants. According to the preferred variant, a current pulse of specific value of the order of - 130 nA is applied, by the usual control means which constitutes the reading circuit, so as to reduce the relaxation time Ti to 59 ns, and bring at the end of this time the probability of reading the initial information at around 32%, or for a reset time of several times Ti a value very close to zero. A second preferred variant consists in using a second current generator 155 (module 31) at the terminals of the Josephson read junction, specific to the reset device according to the invention, in accordance with the diagram in Figure 5. The equivalence of these two variants goes without saying for an electronics engineer, it is equivalent to adding a second current generator or using the same current generator provided with a control input, specific or not, capable of receiving reset pulses. For clarity of the diagram, there is shown schematically the coupling means by additional means 155 to be applied during the reset step, a pulse of current I _'b. (Figures 5 and 6). The operation of the invention will be explained using FIGS. 4A to 4C. FIG. 4A represents, as a function of time, the amplitude of a pulse l _b of reading current applied to Quantronium at the terminals of the Josephson junction of reading 105 by the current generator 125. This pulse starts from a quiescent value of the order of -300 nA favorable for maintaining consistency. Beforehand, this Quantronium was prepared in state 1 1> at an instant identified by the vertical peak descending in dotted lines, marked “μw draws”. At a value arbitrarily represented at -130 nA, there is shown a stabilization of the current called the current plateau. Then the reading pulse continues until a value of the order of 400 nA; a plate was placed on the rising edge of the pulse to test the invention. Many values have been tested for the amplitude of this plateau, as well as for its duration. FIG. 4B shows, for all the values of this current plateau denoted Ipiateau, the probability that there is to read the state 1 1> prepared in the quantum bit at the time of the "μw draws". All these measurements were made with a duration of 100 ns from the Ipiateau current plateau. We note in this figure large variations in this probability. The higher this probability, as for the first upward vertical arrow, at the abscissa -320 nA, the more the information read approaches the information entered: the consistency is good and the relaxation time T ^ is worth 730 ns . The lower this probability, as for the second downward vertical arrow, at the abscissa -130 nA, the more the information read moves away from the recorded information associated with the state 1 1>: the quantum bit easily loses its coherence by relaxation, and the relaxation time Ti drops to 59 ns. For the two values of plateau current -320 nA and -130 nA, FIG. 4C shows the variation of the probability that Quantronium is measured in the state 1 1> as a function of the duration of the plateau. Now we have seen that a reading pulse induces at the terminals of the Josephson junction of reading 105 a difference in superconductive phase which, with the difference in superconductive phases δ at the terminals of the Cooper box 110 and with the magnetic flux φ crossing the loop, check the following equation: δ = γ + φ / φo 2π to the nearest sign where φ ₀ is the quantum of magnetic flux. At the operating point, for example according to FIG. 2, the point Fi (N _g = 0 δ = 0) during the time of this read pulse, the phase γ undergoes a displacement which generates a displacement of the phase δ. During the duration of the plateau, the operating point moves on the curve of FIG. 2 following a plane defined by N _g constant. Thus for example the point Fi becomes F'i during the duration of the current plateau Ipiateau- The transition frequency of the quantum bit which was ΔE ₀ ι = E-. - Eo at the operating point Fi, becomes ΔE ' ₀ ι = E'i - E' ₀ at the operating point F'-i. It then suffices to choose the frequency of the resonant circuit so that ΔE _in v ≈ ΔE ' ₀ -ι. This value of the current I water can in practice be obtained indifferently with specific instructions applied to the current generator 125 designed for the reading pulse, or by a second current generator l'b 155 connected in parallel for this purpose. As an indication, with a current of -130 nA, and a resonance frequency of the dissipative circuit of about 8.06 GHz, a relaxation time Ti less than 60 ns is obtained, and a probability of relaxation reaching 97% after 200 ns. It goes without saying that the longer the quantum device considered is able to maintain its consistency for a long time, the more advantageous the relaxation device according to the invention. FIG. 6 shows an alternative embodiment in which the coupling means or first means comprise a module 32 with a magnetic coil 140 placed in the vicinity of the Qubit and acting on its phase δ, and a generator 145 of amplitude current pulses suitable for temporarily bringing the transition energy ΔE ₀₁ to a value ΔE'oi equal to or close to the value ΔE _env of the peak absorption. We will now proceed, with reference to FIG. 7, to the description of a preferred embodiment of the invention implementing an ionic Qubit. The energy state of the atom or ion 1 changes under the effect of the absorption or the emission of a photon of frequency v ₀₁ corresponding to the transition energy of the Qubit. The condition which one seeks to write information in the Qubit or let it evolve in a coherent manner consists in placing it in an electromagnetic cavity 2 small enough so that it cannot absorb any photon at the frequency v ₀₁ . According to the invention, the dissipative device corresponds to an environment in which absorption or emission of photons can occur (cavity 4). The control device consists either of means capable of varying the dimensions of the space in which this atom or ion 1 is confined (such as for example a set of laser beams, or means generating an appropriate magnetic field), or by a laser or electromagnetic device moving it from a cavity 2 where it is confined in a significantly larger cavity 4 (or even an absence of cavity) so that it can spontaneously de-energize. Thus, in the case of an atom or ion 1 making a Qubit with two energy levels, the Qubit is either trapped using electromagnetic fields, or free to move due to its own speed. The electromagnetic cavity 2 has a very low absorption and therefore a high quality factor. Depending on the case, this cavity can have: a natural frequency equal to or close to the Qubit transition frequency if it is used for a coherent control of this Qubit, or to mediate an interaction with another Qubit which can enter the cavity later ; a significantly higher fundamental natural frequency than the Qubit transition frequency, in order to avoid any exchange of energy between the cavity and the Qubit. For quick reset, atom or ion 1 is moved

(reference 3) either under the effect of its own initial speed, or by application of a locomotor electromagnetic field, applied by means not shown in the drawing, to the area where this Qubit can quickly relax in its ground state. In the case where the atom or the ion moves under the effect of its initial speed, the first means of the device according to the invention which define a coupling device simply comprise means of temporary removal of the means which maintain this atom or this ion in the very low absorption cavity. In the case where this atom or this ion is moved by electromagnetic means, these means constitute the first means of the device according to the invention. The Qubit rapid relaxation zone can be inside another electromagnetic cavity 4 of low quality factor (high absorption), which defines the second dissipative means of the device according to the invention. One of the natural frequencies of the cavity 4 must be equal to or close to the frequency of the Qubit for the relaxation of the Qubit to take place quickly. The Qubit transition frequency can be brought to a value equal to or close to the low quality factor resonant mode chosen for reinitialization, using appropriate electromagnetic fields. In Figure 7, there is shown by way of example Helmhoitz coils 5 supplied by a current source 6 and adapted to temporarily vary the Qubit transition frequency by Zeeman effect. We will now proceed with reference to FIG. 8 to the description of a preferred embodiment with a quantum dot produced in a two-dimensional electron gas. According to a particular technology, Qubits can have for physical support a quantum dot realized in a two-dimensional gas of electrons, controlled by gate electrodes 12, 13 on or under the surface of this gas. The two energy states corresponding to the two Qubit states are generally two electronic states of the quantum dot separated by a transition energy ΔE ₀ ι, controlled by the voltage of the gate electrodes 12, 13: a negative voltage of higher amplitude. further confines the quantum dot, and increases the difference between the energy levels, therefore between the states E ₀ and Ei, therefore increases the transition energy ΔE ₀ ι. Generally, these gate electrodes 12, 13 are subjected to the same bias voltage. According to the invention, the dissipative device corresponds to a resonant electrical circuit 16 provided with a resistive component 161, placed in the circuit of at least one gate electrode 13. The control device consists for example of a magnetic field source or a specific value of the gate voltage applied to the electrodes, capable of bringing the transition energy ΔE ₀ ι to a value equal to or close to that of an absorption peak of the electric circuit controlling at least one of the electrodes 12, 13 defining the geometry of the quantum dot, which amounts to saying, in order to bring the transition energy ΔE ₀ ι to a value ΔE ' ₀ ι

≈ Δtenv- If we consider Figure 8, we can see that the quantum dot 11 is delimited by voltages applied by gate electrodes 12, 13, themselves supplied by adjustable gate voltage sources 14, 15 The grid electrodes 12 are used to define the region of the space where the quantum dot is located 11. The grid electrode 13 can be of the same type as the grid electrodes 12, but can optionally be dedicated to resetting the qubit. A voltage pulse can be temporarily applied to the electrode 13 by the voltage source 15 to modify the size of the quantum dot 11 and bring the Qubit transition frequency to a value equal to or close to that of a resonance of l outdoor environment. Circuit 17 comprising a current source 171 and a coil 172 is used to generate a magnetic field in the region of the quantum dot. This circuit can be used to apply a magnetic field pulse so as to bring the Qubit transition frequency to a value equal to or close to that of a resonance of the external environment. The resonant circuit 16 comprises a resistor 161, an inductor 162 and a capacitor 163 mounted in parallel and is interposed between the voltage source 15 and the gate electrode 13. The resonant mode of the gate control circuit 13 is capable of '' absorb the energy of the Qubit when the transition energy of the latter coincides with the resonant mode. This mode could also be found in one of the circuits of the gate electrodes 12 or even in the circuit 17.

Claims

claims

1- Device for resetting a quantum bit device 5 having two states | 0> and 1 1> associated respectively with two energy levels EO and E1, always such that EO <E1, intended to reset this quantum bit device to the state 1 0>, characterized in that it comprises first dissipative means placed near the quantum bit device, capable of absorbing a quantity of energy only when this quantity is equal to certain values ΔEenv, and second means capable of temporarily modifying the difference in energy ΔE01 = E1 - E0, from its initial value ΔE01 which cannot be absorbed by the first dissipative means, to a value ΔE'01 = ΔEenv capable of being absorbed by the first dissipative means.

J5 2- Device for resetting a quantum bit device according to claim 1, characterized in that the first dissipative means are placed in the immediate energy environment of the quantum bit device and have at least one absorption peak whose ΔEenv value is:

20 • sufficiently distant from the transition energy ΔE01 = E1 - E0 of the quantum bit device, to make negligible or zero any interaction between the quantum bit device and these first dissipative means, but • sufficiently close to this transition energy for that the quantum bit device can be placed in operating conditions where the transition energy ΔE'01 becomes equal to or close to the value ΔEenv of the absorption peak, so that there may be temporarily a strong energy coupling between the quantum bit device and these first 30 dissipative means, and in that the second means are capable of modifying the transition energy ΔE01 = E1 - EO of the quantum bit device, to bring it, during resets, to said value ΔE'01 equal to or close to the value ΔEenv absorption peak.

3- Device for resetting a quantum bit device according to claim 2, characterized in that the first dissipative means having at least one absorption peak with value ΔEenv have a particular degree of freedom capable of absorbing the transition energy ΔE01 = E1 - EO of the quantum bit device when it is brought to the value ΔE'01 equal to or close to the value ΔEenv of the absorption peak.

4- Device for resetting a quantum bit device according to claim 2, characterized in that the first dissipative means having at least one absorption peak with value ΔEenv have a collection of degrees of freedom whose quantum spectral density at the transition frequency v01 = ΔE01 / h (where h is the Planck constant) of the quantum bit device is large. 5- Device for resetting a quantum bit device according to any one of claims 2 to 4, characterized in that the second means comprise means generating specific reset instructions, these instructions being applied to the usual means for adjusting the quantum bit device, so as to temporarily bring the transition energy ΔE01 to the value ΔE'01 equal to or close to the value ΔEenv of the absorption peak.

6- Device for resetting a quantum bit device according to claim 5, characterized in that the specific reset instructions have a value chosen outside the ranges used when the quantum bit device is allowed to evolve consistently.

7- Device for resetting a quantum bit device according to any one of claims 2 to 4, characterized in that the second means comprise specific means for adjusting the operating parameters of the quantum bit device, acting directly on the transition energy of the quantum bit device to temporarily bring the transition energy ΔE01 to a value ΔE'01 equal to or close to the value ΔEenv of the absorption peak, independently of the setting of the operating parameters of the quantum bit device as chosen to make it evolve coherently.

8- Device for resetting a quantum bit device according to any one of claims 1 to 7, characterized in that the quantum bit device comprises a physical medium having a quantum behavior constituted by an integrated electronic device superconducting Josephson junctions (101, 102) and Cooper pair box (110), provided with a Josephson reading junction (105) activated by a current pulse.

9- Device for resetting a quantum bit device according to claims 5 and 8, characterized in that the second means comprise means generating specific reset instructions in the form of a current pulse, these instructions being applied to the Josephson reading junction (105), and in that the amplitude of the current pulse is chosen to maximize the relaxation of the quantum bit device. 10- Device for resetting a quantum bit device according to claim 9, characterized in that the amplitude of the current pulse is chosen outside the range used during a read operation.

11- Device for resetting a quantum bit device according to claims 5 and 8, characterized in that it comprises magnetic adjustment means acting on phase δ of the quantum bit device which comprise a magnetic coil and a generator. current pulses, and in that the second means comprise means generating specific reset instructions in the form of a current pulse of specific value applied to the magnetic coil acting on this phase δ of the quantum bit device.

12- Device for resetting a quantum bit device according to claim 11, characterized in that the second means comprise a second magnetic coil (140) placed in the vicinity of the quantum bit device and acting on its phase δ, and a generator (145) of current pulses of adequate amplitude to temporarily bring the transition energy ΔE01 to a value ΔE'01 equal to or close to the value ΔEenv of the absorption peak.

13- Device for resetting a quantum bit device according to claim 11, characterized in that the second means comprise a second coupling grid of the Cooper pair box and a generator of voltage pulses, of adequate amplitude to temporarily bring the transition energy ΔE01 to a value ΔE'01 equal to or close to the value ΔEenv of the absorption peak. 14- Device for resetting a quantum bit device according to claim 11, characterized in that the second means comprise a second circuit loop (31) passing through the Josephson read junction (105), and capable of sending in this Josephson junction for reading (105) a current pulse of an adequate amplitude to temporarily bring the transition energy ΔE01 to a value ΔE'01 equal to or close to the value ΔEenv of the absorption peak.

15- Device for resetting a quantum bit device according to any one of Claims 8 to 14, characterized in that the first dissipative means comprise a dissipative device (21) placed at the terminals of the Josephson read junction (105) and comprising a dissipative resistor (157), a capacitor (159) and an inductor (158).

16- Device for resetting a quantum bit device according to any one of claims 1 to 7, characterized in that the quantum bit device comprises a physical medium (1) having a quantum behavior constituted by the energy state d 'an atom or an ion.

17- Device for resetting a quantum bit device according to claim 16, characterized in that the second means comprise means capable of varying the probability that the atom or the ion absorbs energy.

18- Device for resetting a quantum bit device according to claim 16, characterized in that the means capable of varying the probability that the atom or the ion absorbs energy, modify the dimensions of space in which this atom or ion is confined. 19- Device for resetting a quantum bit device according to claim 16, characterized in that the second means comprise a device moving the atom or the ion of a cavity (2) where it is confined in a cavity ( 4) significantly larger.

20- Device for resetting a quantum bit device according to any one of Claims 1 to 7, characterized in that the quantum bit device comprises a physical medium (11) having a quantum behavior constituted by a quantum dot produced in a two-dimensional electron gas, controlled by gate electrodes (12, 13) on or under the surface of this gas.

21- Device for resetting a quantum bit device according to Claims 2 and 20, characterized in that the first dissipative means comprise a resonant electrical circuit (16) provided with a resistive component (161), placed in the circuit d '' at least one gate electrode (13), and the second means comprises a magnetic field source (17) or gate voltage sources (14, 15) for applying a specific value of the gate voltage to the gate electrodes (12, 13), in order to vary the size of the quantum dot so as to bring the transition energy ΔE01 to a value ΔE'01 equal to or close to the value ΔEenv of an absorption peak of the controlling electrical circuit at least one of the electrodes (12, 13) defining the geometry of the quantum dot.