JP2000277723A - Quantum arithmetic element and manufacture of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simplify reading of an arithmetic result by extracting the result as a DC current signal, and dispensing with performing high-speed voltage operation or high-speed signal extraction. SOLUTION: Quantum bits formed of a quantum box electrode 101 and an counter electrode 102, or the superconducting box electrode 101 and the superconducting counter electrode are controlled by a gate voltage impressed to a gate electrode, and reading after the arithmetic operation of the quantum bits and preparation in the initial state before the arithmetic operation of the quantum bits can be realized by a probe electrode 106 which is connected through a tunnel barrier 107 with the quantum bits. After the end of reading of the Cooper electron pairs, the initial state is automatically prepared, and the same arithmetic operation is repeatedly performed so that the output can be obtained as a DC current signal, and reading can be simplified. Also the constitution of the reading circuit becomes simplified, and the number of wiring can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a quantum operation device used in a quantum computer related to a low-capacity Josephson coupling system and a method of manufacturing the same.

[0002]

2. Description of the Related Art Quantum computers are capable of solving problems, such as large natural number factorization, which could not be solved by a conventional classical computer due to the enormous amount of calculation. , Theoretically proposed (literature: SMIJ Journal of Computing, Vol. 26, p. 1484 (SIAM Journal of Computing, Vol. 26, p. 1484 (1
997))).

In this quantum computer, a quantum two-level system called a qubit is used to correspond to a bit in a classical computer. In performing the operation, a unitary transformation operation is performed on the group of qubits, and the qubit is read out after the operation is completed. As a quantum two-level system as a qubit, a superconducting quantum operation device can be used.

Heretofore, this type of superconducting quantum operation device has been disclosed, for example, in Los Alamos Preprint Server (http://xxx.lanl.gov/) cond-mat / 98080 on August 6, 1998.
67 and June 15, 1998, Physical Review
Bee Vol. 57, p. 15400 (Physical Review B, V
ol. 57, p. 15400 (1998)).

FIG. 5 is a circuit diagram showing an example of a conventional superconducting quantum operation device. The number of surplus Cooper pairs in the superconducting box electrode 501 is limited to 0 or 1 by the charging effect, and the two states are due to tunneling of the Cooper pair with the superconducting counter electrode 502 through the tunnel barrier 504. Coherently coupled. This functions as a qubit. The gate voltage applied to the gate electrode 503 acts on the box electrode 501 via the gate capacitance 505, and the surplus Cooper pairs in the superconducting box electrodes 501 in a plurality of rows collectively perform the operation on the qubit. .

In the figure, on the right side of the dotted line is a read circuit for reading out the state of the quantum bit. The readout circuit is constituted by a single electron transistor. The state of the number of charges in the qubit is read via the readout capacitor 506 to the island electrode 5.
07, the island electrode 507 and the tunnel barrier 511
Source electrode 508 and drain electrode 5 connected via
It is read as a change in the value of the current flowing during the period 09. Also,
The charge in the qubit of the island electrode 507 is
, A predetermined electric field is applied via the gate capacitor 512 to control the current of the drain electrode 509. In the above description, 1
Although a quantum operation circuit of bits has been described as an example, two arbitrary qubits can be made to interact with each other by arranging them in parallel and coupling the opposite electrodes of each qubit through an inductance. And arbitrary quantum operation between multiple bits becomes possible (August 6, 1998,
Los Alamos Preprint Server (http: //xxx.lanl.
gov /) cond-mat / 9808067).

[0007] Here, Cooper Pai
r) is a normal metal and only weak Coulomb repulsion acts between the electrons, and the individual electrons move independently. However, when a slight attractive interaction acts between the electrons, the individual electrons become This pair of electrons is called a Cooper pair because the energy is lower when two electrons with the same magnitude and opposite momentum form a pair than when they move freely. When the energy gained as a Cooper pair exceeds the energy of the thermal disturbance, many electrons are paired and condensed into one energy state, and this state becomes a superconducting state. Further, the phenomenon of perfect diamagnetism (Meissner effect) is explained because the condensed Cooper pairs all have the same phase, and the whole can be described by one wave function.

Further, in FIG.
2, a superconducting box electrode 501 and a gate electrode 503 form a single-electron tunneling (Single-Electron Tunneling) transistor.
T) The read capacity 506 is provided by providing a plurality of columns of transistors.
As a result, the operation result is read from the source electrode 508, the drain electrode 509, and the gate electrode 510 of the read transistor.

[0009]

In the conventional superconducting quantum operation device, when the operation result is read from the qubit, it is necessary to perform a high-speed and complicated operation and to take out a high-speed signal to the outside. Also, many wirings were required. During the quantum operation, the current flowing through the single-electron transistor acts as noise on the qubit side.
It is necessary to make the voltage between the drains zero so that no current flows.

Therefore, it is necessary to apply the source / drain voltage in a step function after the operation is completed. In addition, considering the effect of the potential of the island electrode 507 on the state of the qubit, the source voltage and the drain voltage must have the same magnitude and opposite signs. Further, in order to adjust the operating point of the single electron transistor, it is necessary to apply a voltage for adjustment to the gate electrode 510.

Furthermore, during the read operation, the state of the qubits is gradually lost due to the reaction of the read circuit, so that the read must be performed quickly before the read operation. However, the source-drain resistance of a single-electron transistor is as large as about 100 kΩ, and it is necessary to match impedance with a 50-Ω transmission line, which complicates the circuit configuration.

The present invention provides a simple adjustment of the configuration and operation of the above-described quantum operation device and a simple circuit configuration, which eliminates the need for high-speed voltage operation and high-speed signal extraction and makes it easy to read out the operation result. The task is to

[0013]

SUMMARY OF THE INVENTION The present invention provides a qubit structure in which a quantum box electrode and a counter electrode are coupled with a first tunnel barrier interposed therebetween, and a gate coupled with the quantum box electrode via a capacitance. An electrode, and a probe electrode coupled to the quantum box electrode via a second tunnel barrier, and the coherent oscillation time of the qubit through the first tunnel barrier is a carrier tunnel through the second tunnel barrier. It is characterized in that it is shorter than the relaxation time. Further, the thickness of the first tunnel barrier is smaller than the thickness of the second tunnel barrier. Further, the area of the first tunnel barrier is larger than the area of the second tunnel barrier.

The present invention also provides a technique relating to a quantum operator in which the quantum box electrode and the counter electrode are made of a superconductor, or the superconducting gap energy of the counter electrode is larger than that of the quantum box electrode. Further, in the realization of the second tunnel barrier of the quantum operator, a technique related to a method of manufacturing a quantum operator in which a tunnel barrier film is selectively overlapped with a tunnel barrier film provided in a first tunnel barrier forming step is provided.

Therefore, according to the present invention, the qubit formed by the quantum box electrode and the counter electrode or the superconducting box electrode and the superconducting counter electrode is controlled by the gate voltage applied to the gate electrode, and The probe electrodes coupled to each other with a tunnel barrier interposed therebetween enable readout of the qubit after operation and preparation of an initial state before the operation.

After the Cooper electron pair of the superconducting box electrode is read out, the initial state is automatically prepared. By repeating the same operation, an output can be obtained as a DC current signal. Be simple and easy. Further, in the present invention, the configuration of the readout circuit is simplified, and the number of electrodes can be reduced.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments according to the present invention will be described.
This will be described in detail with reference to the drawings.

FIG. 1 shows a typical example of the configuration of the present invention. In the figure, 101 is a superconducting box electrode made of a superconductor which is in a superconducting state at a low temperature, 102 is a counter electrode of a superconductor which can be called a source, 103 is a gate electrode which may be a superconductor, and 104 is a superconducting box electrode 101 and counter electrode 102
The tunnel barrier 105 formed of a thin thin film between the gate electrode 105 and the gate capacitance 105 between the gate electrode 103 and the superconducting box electrode 101.
6 is a superconductor probe electrode which also serves as a drain, 107
Is a tunnel barrier thicker than the tunnel barrier 104 between the superconducting box electrode 101 and the probe electrode 106.

The quantum operation device will be described in detail. In FIG. 1, a superconducting box electrode 101 made of a superconductor film formed on an insulating substrate is used as a tunnel barrier 1.
It is coupled to a counter electrode 102 made of a superconductor thin film with the electrode 04 interposed therebetween. The superconducting box electrode 101 and the capacitance 105
The gate electrode 103 is disposed via the superconducting box electrode and the tunnel electrode 107 and the probe electrode 106 via the tunnel barrier 107.
Are combined.

The gate potential applied to the gate electrode 103 controls the electrostatic potential of the superconducting box electrode 101, thereby controlling the tunneling of the Cooper pair via the tunnel barrier 104, that is, the transition of the state of the qubit. You.

The probe electrode 106 is positively biased, and only when there is an extra Cooper pair in the superconducting box electrode 101, two quasiparticle tunneling through the tunnel barrier 107 draws two electrons, and Observe the state of.

Here, the quasiparticle is a metal in a superconducting state, and many electrons form a Cooper pair and are condensed into one energy state.
It receives more than a certain amount of energy by external light irradiation and separates it into two electrons. Since the electrons in that state are different from the free electron state, they are called quasiparticles. In this case, in a tunnel junction in which both electrodes are superconductors, the quasiparticle current sharply increases at a voltage corresponding to the sum of the gap energies of both superconducting electrodes, and strong non-linearity occurs in the current-voltage characteristics. appear.

Next, the operation of the superconducting quantum operation device of the present invention will be described with reference to FIG. Here, a 1-bit quantum operation element is considered for simplicity. FIG. 2A shows how the number n of surplus Cooper pairs in the superconducting box electrode 101 depends on the gate voltage Vg, which is the bias voltage of the gate electrode 103, corresponding to the energies of the states 0 and 1. It is a thing. The point at which the two states degenerate is defined as Vg1. At the point Vg0 that is far from the degeneration gate voltage Vg1, the state 0
Is lower in energy than state 1, and by waiting at Vg = Vg0, the initial state n = 0 of the qubit can be prepared.

Next, consider applying a voltage pulse to the gate electrode 103 as shown in FIG. Time t
What was in state 0 before 0 oscillates coherently between state 0 and state 1 under the gate voltage of Vg1 for a pulse width Δt from time t0 +. Assuming that the Josephson energy of the tunnel barrier 104 is EJ, the oscillation cycle is [h / EJ], where Planck's constant is h. By controlling the pulse width, the transition of the quantum state can be arbitrarily controlled. This corresponds to a one-bit operation. After the end of the pulse, the state of the qubit is superimposed on state 0 and state 1 by the weight of the wave function corresponding to the operation result. Here, state 1 is the superconducting box electrode 10
From 1 to the positive side, V = (2Δ + Ec) / e to (2Δ +
Probe electrode 1 biased until 3Ec) / e
06 is unstable with respect to electron tunneling, and the electron tunneling probability Γ is: ΓΓ (eV + Ec) / e ² R (1) (R is the tunnel resistance of the probe junction, Ec is the box electrode (One electron charging energy) and the two electrons are
06 and relax to state 0.

On the other hand, state 0 is a stable state and nothing happens. After waiting for a time Td several times the probability Γ until the state 1 is completely relaxed, the initial state 0 is prepared again, and the same operation can be repeated by the next pulse.

By repeating this, the DC current flowing to the probe electrode 106 as an output becomes a signal proportional to the probability density that the state of the qubit is in the state 1 in the final state.

For example, when the result is completely in state 1, [2e
/ Td] can be observed. In this case, since the reading is only for reading the direct current, the signal line does not need to have a wide band characteristic. EJ >> hΓ needs to be satisfied so that the probability that a state is observed during the quantum operation can be ignored. Instead, there is no reaction from the probe junction to the qubit before the observation is made. When performing a quantum operation between a number of bits, the 1-bit elements are arranged in parallel, a plurality of superconducting box electrodes 101 are arranged in parallel, and the opposing electrodes 102 of the respective qubits are coupled via an inductance. Can be made to interact with each other, and an arbitrary quantum operation between multiple bits can be performed. Furthermore, from this quantum operation, it is possible to realize a quantum computer.

The superconducting quantum operation device shown in FIG.
As for the bias and control voltage, the state of the superconducting box electrode is read by the probe electrode and the gate electrode, and if the superconducting box electrodes are arranged in a plurality of rows, the operation result can be obtained. Compared to a simple quantum operation device,
It can be seen that there is no need for complicated control.

[0029]

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 3 is a top view of the embodiment of the present invention.
As the substrate, an insulator substrate 305, for example, a silicon substrate whose surface is oxidized is used. Each of the aluminum box electrode 301, the aluminum counter electrode 302, the aluminum probe electrode 306, and the aluminum gate electrode 303 is formed of a superconductor. FIG. 3 is a circuit diagram substantially corresponding to FIG. 1. The superconducting box electrode 101 is paired with the aluminum box electrode 301, and the other electrodes and barriers are the same.

Aluminum and niobium are used as materials. As the gate electrode 303, a normal conductive noble metal, gold, and platinum can be used separately. After the counter electrode 302 is deposited, oxygen is introduced again into the vacuum chamber to further oxidize the surface of the superconducting box electrode 301, and then the aluminum oxide tunnel barrier 304 and the aluminum oxide tunnel barrier 307 are oxidized. The probe electrode 306 is connected to the box electrode 30
Then, an aluminum oxide tunnel barrier 307 as a tunnel junction is formed by vapor deposition so as to slightly overlap with 1. Also,
The aluminum oxide tunnel barrier 304 and the aluminum oxide tunnel barrier 307 have the same width but different thicknesses. Therefore, in the plan view shown in FIG. 3, the width of the aluminum oxide tunnel barrier 307 is conceptually reduced. I'm making it big.

FIG. 4 shows an example of a manufacturing process of the device. 4 illustrates an example of manufacturing the quantum operation device illustrated in FIG. 3. The formation of the tunnel barriers 404 and 406 is performed as follows. First, through the mask 407,
After depositing the superconducting box electrode 401, oxygen or a mixed gas of 10% oxygen and argon is introduced into the vacuum chamber of the deposition apparatus to oxidize the surface. When the gate electrode 403 is an aluminum electrode, an aluminum oxide insulating film is formed on the surface. In the case of a niobium electrode, aluminum is thinly vapor-deposited on the surface in advance, and the aluminum is oxidized.

Next, the counter electrode 402 is deposited from different angles through the mask so as to slightly overlap the surface of the superconducting box electrode 401. The aluminum oxide sandwiched between the overlapping portions serves as a tunnel barrier, and a tunnel junction 404 is formed. The tunnel barrier on the probe electrode 405 side is
EJ >> hΓ is desirable to satisfy
After the counter electrode 402 is deposited, oxygen is introduced again into the vacuum chamber, and the surface of the superconducting box electrode 401 is further oxidized.
The probe electrode 405 is deposited so as to slightly overlap the superconducting box electrode 401 to form a tunnel junction 406. The three deposition processes from different angles including the front were shown.
It is a figure of the lower side of FIG. By providing a distance between the mask pattern 407 and the insulator substrate and vapor-depositing at a predetermined angle from the front, obliquely above, and the opposite side, the quantum operation device shown in FIG. 3 can be formed.

As another method for satisfying EJ >> hΓ, a material having a large superconducting gap, such as niobium, is used only for the counter electrode so that the area of the probe junction is smaller than that of the tunnel junction of the qubit. There is also a method of increasing EJ. The gate electrode 403 can be prepared in another deposition step before these steps,
The vapor deposition can be performed simultaneously with the above steps. By removing the metal adhering to the mask by lift-off,
The superconducting quantum operation device of the present invention is obtained.

[0034]

According to the present invention, the result can be taken out as a direct current signal, so that it is not necessary to operate a high-speed voltage or take out a high-speed signal, so that the operation result can be read out easily.

In addition, since the tunneling to the probe electrode directly connected to the qubit via a tunnel barrier can be taken out as an output without using a single electron transistor as the readout circuit, the structure of the readout circuit can be simplified. Thus, the number of wirings can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of the configuration of the present invention.

FIG. 2 is an explanatory diagram of the operation of the present invention.

FIG. 3 is a top view of the embodiment of the present invention.

FIG. 4 is an explanatory diagram of a method for manufacturing a device according to an example of the present invention.

FIG. 5 is a circuit diagram of a conventional superconducting qubit and its readout circuit.

[Explanation of symbols]

 101 Superconducting box electrode 102 Counter electrode 103 Gate electrode 104 Tunnel barrier 105 Gate capacitance 106 Probe electrode 107 Tunnel barrier 301 Aluminum box electrode 302 Aluminum counter electrode 303 Aluminum gate electrode 304 Aluminum oxide tunnel barrier 305 Insulator substrate 306 Aluminum probe electrode 307 Oxidation Aluminum tunnel barrier 401 Superconducting box electrode 402 Counter electrode 403 Gate electrode 404 Tunnel junction 405 Probe electrode 406 Tunnel junction 407 Mask pattern 501 Superconducting box electrode 502 Counter electrode 503 Gate electrode 504 Tunnel barrier 505 Gate capacitance 506 Read capacitance 507 Island electrode 508 Source electrode 509 Drain electrode 510 Gate electrode 511 Tunnel barrier 512 Gate capacitance

Claims (8)

[Claims] A qubit structure in which a quantum box electrode and a counter electrode are coupled with a first tunnel barrier interposed therebetween; a gate electrode coupled to the quantum box electrode via a capacitance; A probe electrode coupled through a second tunnel barrier, wherein a coherent oscillation time of the qubit through the first tunnel barrier is shorter than a carrier tunnel relaxation time through the second tunnel barrier. Quantum operation device. 2. The quantum operation device according to claim 1, wherein a thickness of said first tunnel barrier is smaller than a thickness of said second tunnel barrier. 3. The quantum operation device according to claim 1, wherein an area of the first tunnel barrier is larger than an area of the second tunnel barrier. 4. The quantum operation device according to claim 1, wherein said quantum box electrode and said counter electrode are made of a superconductor. 5. The quantum operation device according to claim 4, wherein a superconducting gap energy of said counter electrode is larger than a superconducting gap energy of said quantum box electrode. 6. A qubit structure in which a quantum box electrode and a counter electrode are coupled with a first tunnel barrier interposed therebetween, a gate electrode coupled to the quantum box electrode via a capacitance, A probe electrode coupled through two tunnel barriers, wherein the thickness of the first tunnel barrier is smaller than the thickness of the second tunnel barrier. 7. A qubit structure in which a quantum box electrode and a counter electrode are coupled with a first tunnel barrier interposed therebetween; a gate electrode coupled to the quantum box electrode via a capacitance; And a probe electrode coupled via two tunnel barriers, wherein the area of the first tunnel barrier is larger than the area of the second tunnel barrier. 8. A qubit structure in which a quantum box electrode and a counter electrode are coupled via a first tunnel barrier, a gate electrode coupled to the quantum box electrode via a capacitance, A method for manufacturing a quantum operation device including a probe electrode coupled via a second tunnel barrier, wherein the tunnel provided in the first tunnel barrier forming step is used to realize the second tunnel barrier. A method for manufacturing a quantum operation device, comprising selectively stacking a tunnel barrier film on a barrier film.