CN116402153A - Updating method of threshold line for confirming quantum state of quantum bit - Google Patents
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Abstract
The invention discloses a method for updating a threshold line for confirming a quantum state of a quantum bit, and relates to the field of quantum measurement and control; comprising the following steps: obtaining a third set corresponding to a certain quantum state and an initial threshold line; dividing the third set into two clusters, namely a first cluster and a second cluster, by using the initial threshold line; obtaining expected coordinates corresponding to the first cluster and the second cluster, namely a first coordinate and a second coordinate respectively, determining a second included angle between a connecting line of the first coordinate and the second coordinate and a set coordinate axis of a coordinate system according to the first coordinate and the second coordinate, rotating the first coordinate and the second coordinate according to the second included angle, and updating the first coordinate and the second coordinate; and determining an updating threshold line according to the initial threshold line and the first coordinates or the second coordinates before and after updating. The invention can continuously update the threshold line according to the reading precision requirement of the quantum bit after providing the threshold dividing line used for quantum state resolution.
Description
The application is a divisional application with application date of 2019, 4 months and 24 days, application number of 201910333096.6 and patent name of a method for updating a threshold line for confirming quantum states of quantum bits.
Technical Field
The invention belongs to the field of quantum measurement and control, and particularly relates to a method for updating a threshold line for confirming a quantum state of a quantum bit.
Background
The qubit information refers to a quantum state of the qubit, wherein the basic quantum states are a state |0> and a state |1>, after the qubit is operated, the quantum state of the qubit is changed, and on the quantum chip, the execution result of the quantum state of the qubit, namely the execution result of the quantum chip, is reflected after the execution of the quantum chip, and the execution result is carried and transmitted by a qubit reading signal.
The rapid analysis of the quantum state of a qubit by a qubit read signal is a key work for understanding the execution performance of a quantum chip, and an acquisition method for confirming a threshold line of the quantum state of the qubit is provided in the patent filed on the same day, and is implemented by the following steps: preparing quantum bits into a first quantum state and a second quantum state, respectively carrying out repeated measurement on the first quantum state and the second quantum state to obtain coordinate point data of a plurality of quantum bit reading signals on an orthogonal plane coordinate system, respectively recording the coordinate point data as a first set and a second set, respectively carrying out Gaussian fitting on the coordinate points of the first set and the second set to obtain a first statistical center point coordinate and a second statistical center point coordinate of a Gaussian fitting graph corresponding to the first set and the second set respectively, and respectively corresponding to a first standard deviation and a second standard deviation; determining a first probability density distribution function in the first set and a second probability density distribution function in the second set respectively; and determining the fidelity function, and determining the corresponding threshold line as the optimal threshold line when the fidelity function takes the maximum value.
In an ideal case, the threshold line obtained by the above technical solution can meet the requirement of high precision in a short time, but the system for qubit read return signal processing can generate performance floating with time, and the performance parameters of the qubit itself can also change, at the moment, the threshold line cannot meet the requirement of high precision.
Disclosure of Invention
The invention aims to provide a method for updating a threshold line for confirming a quantum state of a quantum bit, which solves the defects in the prior art and can continuously update the threshold line according to the reading precision requirement of the quantum bit after providing a threshold dividing line for quantum state resolution.
The technical scheme adopted by the invention is as follows:
a method for updating a threshold line for validating a qubit quantum state, applied to a quantum chip, comprising: obtaining a third set corresponding to a certain quantum state and an initial threshold line; dividing the third set into two clusters, namely a first cluster and a second cluster, by using the initial threshold line; obtaining expected coordinates corresponding to the first cluster and the second cluster, namely a first coordinate and a second coordinate respectively, determining a second included angle between a connecting line of the first coordinate and the second coordinate and a set coordinate axis of a coordinate system according to the first coordinate and the second coordinate, rotating the first coordinate and the second coordinate according to the second included angle, and updating the first coordinate and the second coordinate; and determining an updating threshold line according to the initial threshold line and the first coordinates or the second coordinates before and after updating.
Optionally, the updated connection line between the first coordinate and the second coordinate is parallel to the I-axis.
Optionally, the determining the update threshold line according to the initial threshold line and the first coordinate or the second coordinate before and after updating includes: the updated threshold line expression is the sum of the updated set axis coordinate of the second coordinate and the initial threshold line expression minus the set axis coordinate of the second coordinate before updating.
Optionally, obtaining the initial threshold line includes: obtaining coordinate point data corresponding to a plurality of quantum bit reading signals respectively on an orthogonal plane coordinate system when the quantum bit is positioned in a first quantum state, and marking the coordinate point data as a first set R |0> The method comprises the steps of carrying out a first treatment on the surface of the Obtaining coordinate point data corresponding to the plurality of quantum bit reading signals on an orthogonal plane coordinate system when the quantum bit is positioned in the second quantum state, and marking the coordinate point data as a second set R |1> The method comprises the steps of carrying out a first treatment on the surface of the Wherein: the first quantum state and the second quantum state are known quantum states and are different from each other, wherein: the orthogonal plane coordinate system is set as an I-Q coordinate system; according to the first set R |0> And said second set R |1> An initial threshold line is determined.
Optionally, according to the first set R |0> And said second set R |1> Determining an initial threshold line, comprising: obtaining the first set R |0> And said second set R |1> A center point and a probability density distribution function corresponding to the center point and the probability density distribution function respectively; the sum of the integrals of the two probability density distribution functions distributed on a set space is used as a fidelity function, wherein the set space is two spaces divided by a threshold line; the threshold line is a vertical line connecting the two central points; and determining the corresponding threshold line as an initial threshold line when the fidelity function takes the maximum value.
Optionally, the first set R is obtained |0> And said second set R |1> The center points corresponding respectively include: for the first set R |0> And said second set R |1> Respectively fitting all coordinate points of the corresponding fitting graph to obtain a first statistical center point coordinate (I |0> ，Q |0> ) And a first standard deviation sigma 1 Second statistical center point coordinates (I |1> ，Q |1> ) And a second standard deviation sigma 2 。
Optionally, the fitting is a gaussian fitting.
Optionally, the first set R is obtained |0> And said second set R |1> The probability density distribution functions respectively corresponding to the probability density distribution functions comprise: according to the first statistical center point coordinates (I |0> ，Q |0> ) And the first standard deviation sigma 1 Determining a first probability density distribution function p (R) of all coordinate points of a set of quantum data in an I-Q coordinate system |0> ) According to the second statistical center point coordinates (I |1> ，Q |1> ) And the second standard deviation sigma 2 Determining a second probability density distribution function p (R) of all coordinate points of the other set of quantum data in the I-Q coordinate system |1> )。
Optionally, the first probability density distribution function p (R |0> ) And a second probability density distribution function p (R |1> ) The method comprises the following steps of:
The evaluation formula of the initial threshold line is:
optionally, according to the first set R |0> And said second set R |1> Determining an initial threshold line, further comprising: determining a first included angle between a connecting line of the two central points and any coordinate axis of a coordinate system in which the quantum data is spatially distributed according to the two central points; rotating and updating coordinate points of the two center points in the coordinate system according to the first included angle; and rotating and updating all coordinates of the two groups of quantum data according to the first included angle.
Optionally, rotating and updating coordinate points of the two center points in the coordinate system according to the first included angle includes: determining the first included angle as an I-axis included angle between the connecting line of the two center points and an I-Q coordinate system; the two center points are respectively the first statistical center point coordinates (I |0> ，Q |0> ) And a second statistical center point coordinate (I |1> ，Q |1> ) The method comprises the steps of carrying out a first treatment on the surface of the Clockwise rotating the two center points according to the first included angle; updating the first statistical center point coordinates (I |0> ，Q |0> ) And the second statistical center point coordinates (I |1> ，Q |1> ) Respectively (I' |0> ，Q′ |0> )、(I′ |1> ，Q′ |1> ) Wherein: q'. |0> ＝Q′ |1> 。
Optionally, when the updated first statistical center point coordinates (I' |0> ，Q′ |0> ) And updated said second statistical center point coordinates (I' |1> ，Q′ |1> ) When the longitudinal axes of (2) are equal, the threshold line is a vertical threshold line perpendicular to the I-axis; determining a placeThe threshold line corresponding to the fidelity function when taking the maximum value is the optimal threshold line, which comprises the following steps: when the space to the right of the vertical threshold line is the updated first statistical center point coordinate (I' |0> ，Q′ |0> ) The space at the left side of the vertical threshold line is the second statistical center point coordinate (I' |1> ，Q′ |1> ) If the space is located, determining that the threshold line corresponding to the maximum value of the fidelity function is an optimal threshold line, otherwise, determining that the threshold line corresponding to the minimum value of the fidelity function is an optimal threshold line; wherein: the sum of the maximum value of the fidelity function and the minimum value of the fidelity function is 1.
Optionally, the sine value of the first included angle θ is:
optionally, the evaluation formula of the initial threshold line is converted into the following formula by simplification:
and (3) making:
wherein: g l′ The functional image of (I') is monotonically decreasing and intersects the I-axis, then solving equation (3) can be converted into solving the following equation:
obtaining the solution of formula (4) as a real solution I '=a, and obtaining the expression of the optimal threshold line l' is: i' =a.
Optionally, obtaining the expected coordinates corresponding to the first cluster and the second cluster includes: and respectively carrying out non-weighted average on all coordinate point data in the first cluster and the second cluster to obtain corresponding expected coordinates.
Optionally, after determining the updated threshold line according to the initial threshold line and the second coordinates before and after updating, the method further includes: setting a termination condition, and repeatedly executing the steps by taking the updated threshold line as an initial threshold line: dividing the third set into two clusters by using the initial threshold line, wherein the two clusters are respectively updated first clusters and second clusters; counting n=n+1; respectively carrying out weight-free averaging on all coordinate point data in the updated first cluster and the updated second cluster to obtain corresponding expected coordinates, namely a first coordinate and a second coordinate, determining a second included angle between a connecting line of the first coordinate and the second coordinate and a set coordinate axis of a coordinate system according to the first coordinate and the second coordinate, rotating the first coordinate and the second coordinate by the second included angle, and updating the first coordinate and the second coordinate again; determining an updating threshold line according to the initial threshold line and the first coordinates or the second coordinates before and after updating; stopping execution until the termination condition is reached, and determining the updated threshold line as the optimal threshold line to be obtained.
Optionally, the setting the termination condition includes: setting a maximum execution number N, and stopping execution when n=n, wherein: the maximum execution number N is manually selected.
Optionally, the setting the termination condition includes: setting a first threshold, wherein: the first threshold is selected according to the actually required processing precision; and stopping execution when the maximum value of the distance between the first coordinates before and after updating and the distance between the second coordinates before and after updating is smaller than the first threshold value.
Compared with the prior art, coordinate point data corresponding to a plurality of quantum bit reading signals on an orthogonal plane coordinate system when quantum bits are in a first quantum state and a second quantum state are respectively obtained and marked as a first set and a second set, a first threshold line for analyzing and resolving the quantum bit reading signals is firstly determined according to original data, namely the first set and the second set, then a plurality of coordinate point data corresponding to the quantum bit reading signals on the orthogonal plane coordinate system when the quantum bits are in a certain quantum state are repeatedly obtained and marked as a third set, the third set at the moment is taken as a data basis of an update threshold line to be obtained, the first threshold line is taken as an initial threshold line, the third set is divided into two clusters which are respectively a first cluster and a second cluster by using the initial threshold line, and n=1 is counted; determining an updated threshold line from the initial threshold line, the first cluster, and the second cluster; returning to the executing step by taking the updated threshold line as an initial threshold line: dividing the third set into two clusters, namely a first cluster and a second cluster, by using the initial threshold line, and counting n=n+1; and stopping executing the step of determining the updated threshold line as the optimal threshold line to be obtained until the set termination condition is reached, repeatedly acquiring the third set at any time according to the situation after the first threshold line is obtained through the original data, and acquiring the updated threshold line by using the data of the third set and the first threshold line.
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FIG. 1 is a flow chart of a method of updating a threshold line for validating a qubit quantum state in accordance with an embodiment of the present invention.
Detailed Description
The embodiments described below by referring to the drawings are illustrative only and are not to be construed as limiting the invention.
With reference to fig. 1, the present invention provides a method for updating a threshold line for confirming a quantum state of a qubit, comprising the steps of:
obtaining coordinate point data corresponding to a plurality of quantum bit reading signals respectively on an orthogonal plane coordinate system when the quantum bit is positioned in a first quantum state, and marking the coordinate point data as a first set R |0> The method comprises the steps of carrying out a first treatment on the surface of the Obtaining coordinate point data corresponding to the plurality of quantum bit reading signals on an orthogonal plane coordinate system when the quantum bit is positioned in the second quantum state, and marking the coordinate point data as a second set R |1> The method comprises the steps of carrying out a first treatment on the surface of the Wherein: the first quantum state and the second quantum state are both of known quantitySub-states and are different from each other, wherein: the orthogonal plane coordinate system is set as an I-Q coordinate system;
according to the first set R |0> And said second set R |1> Determining a first threshold line;
obtaining coordinate point data corresponding to a plurality of quantum bit reading signals respectively on an orthogonal plane coordinate system when the quantum bit is in a certain quantum state, and marking the coordinate point data as a third set;
setting a termination condition by taking the first threshold line as an initial threshold line;
dividing the third set into two clusters, namely a first cluster and a second cluster, by using the initial threshold line, and counting n=1;
determining an updated threshold line from the initial threshold line, the first cluster, and the second cluster;
repeating the steps with the updated threshold line as an initial threshold line: dividing the third set into two clusters, namely a first cluster and a second cluster, by using the initial threshold line, and counting n=n+1;
stopping execution until the termination condition is reached, and determining the updated threshold line as the optimal threshold line to be obtained.
The third quantum state can be divided into two clusters by the initial threshold line, wherein the first quantum state is a state |0>, the second quantum state is a state |1>, and a certain quantum state can be determined to be a mixed state of the first quantum state and the second quantum state.
Compared with the prior art, the method has the advantages that coordinate point data corresponding to a plurality of quantum bit reading signals on an orthogonal plane coordinate system when quantum bits are in a first quantum state and a second quantum state are obtained respectively and recorded as a first set and a second set, the first set corresponds to a large number of quantum bit reading signals when the quantum bits are in the first quantum state, the second set corresponds to a large number of quantum bit reading signals when the quantum bits are in the second quantum state, the data of the first set and the data of the second set serve as original data, a first threshold line for the first quantum state and the second quantum state is determined according to the original data, then, a plurality of coordinate point data corresponding to the quantum bit reading signals when the quantum bits are in a certain quantum state are repeatedly obtained on the orthogonal plane coordinate system and recorded as a third set, and the third set at the moment serves as a data basis of an update threshold line to be obtained; taking the first threshold line as an initial threshold line, dividing the third set into two clusters, namely a first cluster and a second cluster, by using the initial threshold line, and counting n=1; determining updated threshold lines according to the initial threshold lines, the first clusters and the second clusters; returning to the executing step by taking the updated threshold line as an initial threshold line: dividing the third set into two clusters, namely a first cluster and a second cluster, by using the initial threshold line, and counting n=n+1; and stopping executing the step of determining the updated threshold line as the optimal threshold line to be obtained until the set termination condition is reached, repeatedly obtaining the third set at any time according to the situation after obtaining the first threshold line through the original data, and obtaining the next updated threshold line by using the data of the third set and the updated threshold line obtained by the previous calculation.
Example 1
Specifically, referring to fig. 1, an embodiment 1 of a method for updating a threshold line for confirming a qubit quantum state of the present invention includes the following steps:
step 10, obtaining coordinate point data corresponding to a plurality of quantum bit reading signals respectively on an orthogonal plane coordinate system when the quantum bit is in a first quantum state, and marking the coordinate point data as a first set R |0> The method comprises the steps of carrying out a first treatment on the surface of the Obtaining coordinate point data corresponding to the plurality of quantum bit reading signals on an orthogonal plane coordinate system when the quantum bit is positioned in the second quantum state, and marking the coordinate point data as a second set R |1> The method comprises the steps of carrying out a first treatment on the surface of the Wherein: the first quantum state and the second quantum state are two basic quantum states, and the basic quantum states are identified as psi=a|0 in the Hilbert space according to any quantum state>+b|1>In other words, wherein 0>And |1>Is two orthogonal basis vectors in Hilbert space, corresponding to two basesThe quantum state, a and b, are the amplitude corresponding to two basic quantum states, wherein: the orthogonal plane coordinate system is set as an I-Q coordinate system;
in a specific arrangement, the first quantum state may be a |0> state quantum state, the second quantum state may be a |1> state quantum state, or vice versa, and in this embodiment, the first quantum state is preferably |0>, the second quantum state is preferably |1>, the orthogonal plane coordinate system is set as an I-Q coordinate system, I is a horizontal axis, and Q is a vertical axis.
Wherein, the quantum bit is prepared into a first quantum state and repeated measurement is carried out to obtain coordinate point data of a plurality of quantum bit reading signals on an orthogonal plane coordinate system, and the coordinate point data is recorded as a first set R |0> In the process of (2), a qubit signal reading device reads a qubit quantum state to obtain a qubit reading signal, and the qubit reading signal passes through a data processing device, for example: analyzing the quantum bit reading signal to obtain coordinate point data, wherein the set of the coordinate point data is a first set to obtain a first set R |0> The data can be stored in the data storage area of the computer for later use or can be directly used for the next processing, in particular, according to the data processing flow decision of the preset data processing equipment, and the same is true, the second set R | The same process is also performed;
step 20, according to the first set R |0> And said second set R |1> Determining a first threshold line;
said first set R |0> And said second set R |1> Determining a first threshold line, specifically comprising the steps of:
step 201, performing gaussian fitting on all coordinate points in the first set and all coordinate points in the second set, to obtain first statistical center point coordinates (I |0> ，Q |0> ) And a second statistical center point coordinate (I |1> ，Q |1> ) Dividing intoFirst standard deviation sigma corresponding to each other 1 And a second standard deviation sigma 2 The method comprises the steps of carrying out a first treatment on the surface of the Wherein: for use in the I-Q coordinate system for locating the first statistical center point coordinate (I |0> ，Q |0> ) And the second statistical center point coordinates (I |1> ，Q |1> ) A straight line divided in two spaces is denoted as a threshold line, and the threshold line is perpendicular to the first statistical center point coordinate (I |0> ，Q |0> ) And a second statistical center point coordinate (I |1> ，Q |1> ) Is connected with the connecting line of the (a); the two spaces are respectively marked as a first space and a second space;
wherein the Gaussian fitting of all coordinate points in the first set and all coordinate points in the second set is accomplished by a computer, and the first set R is obtained by a computer program |0> And a second set R |0> Performing two-dimensional Gaussian fitting to obtain a two-dimensional Gaussian distribution graph, and obtaining first statistical center point coordinates (I |0> ，Q |0> ) And a second statistical center point coordinate (I |1> ，Q |1> ) First standard deviation sigma respectively corresponding to 1 And a second standard deviation sigma 2 ；
Step 202, according to the first statistical center point coordinates (I |0> ，Q |0> ) And the first standard deviation sigma 1 Determining a first probability density distribution function p (R) of all coordinate points in the first set in an I-Q coordinate system |0> ) According to the second statistical center point coordinates (I |1> ，Q |1> ) And the second standard deviation sigma 2 Determining a second probability density distribution function p (R) of all coordinate points in the second set in the I-Q coordinate system |1> )；
Wherein: specifically, the first probability density distribution function p (R |0> ) And a second probability density distribution function p (R |1> ) The method comprises the following steps of:
The above formula is a probability density distribution function corresponding to gaussian distribution, and the formula may be directly obtained after the first set and the second set are fitted by a computer, but the first probability density distribution function p (R |0> ) And a second probability density distribution function p (R |1> ) The derivation of (a) is not limited to this method.
Step 203, determining a fidelity function as the first probability density distribution function p (R |0> ) An integral function in the first space and the second probability density distribution function p (R |1 ) A sum of integral functions in the second space;
step 204, determining the corresponding threshold line as the optimal threshold line when the fidelity function takes the maximum value.
Wherein: the evaluation formula of the optimal threshold line is as follows:
wherein: in the formula, A is a first space, and B is a second space.
By solving the above equation, an expression about the first threshold line, that is, the optimal threshold line, can be obtained, and the sum of the fidelity of both sides of the threshold line can be maximized by satisfying the expression, so that the optimal threshold line required by the present invention can be obtained.
It should be noted that, the fidelity function takes the most value, which means that the fidelity function takes the maximum value or the minimum value only, where, when the first statistical center point coordinate (I |0> ，Q |0> ) Located at the second statistical center point coordinate (I |1> ，Q |1> ) On the right side, the fidelity function needs to take the maximum value, when the firstStatistical center point coordinates (I) |0> ，Q |0> ) Located at the second statistical center point coordinate (I |1> ，Q |1> ) On the left, the fidelity function needs to take a minimum.
On the premise of meeting the two-dimensional double-Gaussian distribution statistical model, the obtained current threshold line can be proved to be necessarily identical with the first statistical center point coordinate (I |0> ，Q |0> ) And the second statistical center point coordinates (I |1> ，Q |1> ) Vertical;
the proving process is as follows:
it is known that:
it is not a matter of course to assume that the expression of the last acquired optimal threshold line in the IQ coordinate system is:
aI+bQ+c=0, where ab+.0, b+.0, a 2 +b 2 ＝1
Obtaining an included angle phi between an optimal threshold line and an I axis, rotating all coordinate point data in a first set and a second set clockwise by an angle phi with a coordinate origin as a center under an IQ coordinate system, wherein the updated first statistical center point coordinate (I |0> ，Q |0> ) And the second statistical center point coordinates (I |1> ，Q |1> ) Is denoted as (I) |0>，new ，Q |0>，new )、(I |1>，new ，Q |1>，new ) The expression of the rotated optimal threshold line becomes q= -c, and we assume that in space q+-c; wherein: phi can be solved byThe formula is obtained:
at this time, the space divided by q= -c and the fidelity correspondence calculation formula are:
and the optimal threshold line is a straight line that maximizes fidelity, i.e.:
namely:
wherein:
the maximum optimization method of the multiple functions g (a, b, c) is as follows: because the maximum value must exist in the problem, we only need to solve all the resident points, and then find the maximum value point in the resident points.
In the constraint condition ab not equal to 0, b not less than 0, a 2 +b 2 For =1, the stationarity point can be solved using the lagrangian multiplier method:
where lambda is an auxiliary parameter. That is:
from the above equation set, it can be sorted that the dwell point satisfies: (Q) |0> -Q |1> )a＝(I |0> -I |1> ) b. While
Is the first statistical center point coordinate (I |0> ，Q |0> ) And the second statistical center point coordinates (I |1> ，Q |1> ) Slope k of the connection line o1 ，/>
Slope k, which is the optimal threshold line l That is k o1 k l -1, whereby the optimal threshold line must be perpendicular to the line of the first statistical center point coordinates and the second statistical center point coordinates, proving to end.
Then, in particular, based on the above important facts, in the actual process, in order to facilitate the solution of the threshold line, a method for simplifying the degree of freedom is generally adopted, and before the step 202, the following steps are further included:
step 2011, based on the first statistical center point coordinates (I |0> ，Q |0> ) And the second statistical center point coordinates (I |1> ，Q |1> ) Determining a first included angle between the connecting line of the first and the second coordinate axes of the I-Q coordinate system;
step 2012, according to the first included angleRotates and updates the first statistical center point coordinates (I |0> ，Q |0> ) And the second statistical center point coordinates (I |1> ，Q |1> )；
And step 2013, rotating and updating all coordinate points in the first set and all coordinate points in the second set according to the first included angle.
By adopting the technical scheme of the steps, the first included angle is obtained, and then the first statistical center point coordinate (I |0> ，Q |0> ) And the second statistical center point coordinates (I |1> ，Q |1> ) All coordinate points in the first set and all coordinate points in the second set are used for reducing the freedom degree of the fidelity function through rotation operation, and the calculation of the fidelity function maximum value in the later period is facilitated.
Specifically, the first included angle θ is determined as the first statistical center point coordinate (I |0> ，Q |0> ) And the second statistical center point coordinates (I |1> ，Q |1> ) The two connecting lines form an I-axis included angle with an I-Q coordinate system, and the first statistical center point coordinate (I |0> ，Q |0> ) And the second statistical center point coordinates (I |1> ，Q |1> ) And all coordinate points in the first set and all coordinate points in the second set, wherein: updated first statistical center point coordinates (I |0> ，Q |0> ) And the second statistical center point coordinates (I |1> ，Q |1> ) Respectively (I' |0> ，Q′ |0> )、(I′ |1> ，Q′ |1> ) And Q' |0> ＝Q′ |1> By a rotation operation, the obtained updated first statistical center point coordinates (I' |0> ，Q′ |0> ) And a second statistical center point coordinate (I' |1> ，Q′ |1> ) Is equal to the Q component of the first statistical center point coordinate (I) |0> ，Q |0> ) And the second statistical center point coordinates (I |1> ，Q |1> ) Perpendicular, so that the optimal threshold line can be parallel to the Q axis through the steps, and the calculation difficulty is reduced.
Specifically, the evaluation formula of the optimal threshold line is as follows:
the following transformations may be performed:
wherein: consider the actual physical meaning P 1 -P 3 > 0, then formula (2) converts to:
and (3) making:
wherein: g l′ The functional image of (I') is monotonically decreasing and intersects the I axis, then solving equation (3) can be converted to solving the following equation according to the integral property:
obtaining the solution of the formula (5) as a real solution I ' =a, and obtaining a first threshold line l ' with the expression of I ' =a.
In particular, if sigma 1 ＝σ 2 The expression for the current threshold line is available as:
through the steps, the current threshold line which is originally calculated is converted into a straight line which is perpendicular to the Q axis, namely the first threshold line through rotation operation, so that the difficulty of solving the original equation (1) is greatly simplified from the algorithm, and the threshold straight line acquisition efficiency is improved.
Step 30, obtaining coordinate point data corresponding to a plurality of quantum bit reading signals respectively on an orthogonal plane coordinate system when the quantum bit is in a certain quantum state, and marking the coordinate point data as a third set;
specifically, the quantum bit is in a certain quantum state without determination, and the measured coordinate point data corresponding to all the quantum bit reading signals is used as a third set and is combined with the first set R |0> And said second set R |1> And the same is stored in a computer for further processing;
step 40, setting a termination condition by taking the optimal threshold line, namely the first threshold line, as an initial threshold line;
specifically, in order to obtain the updated threshold line, an initial threshold line needs to be provided, and in this scheme, the optimal threshold line obtained in the foregoing step, that is, the first threshold line is used as the initial threshold line.
Step 50, dividing the third set into two clusters, namely a first cluster and a second cluster, by using the initial threshold line, and counting n=1;
step 60, determining an updated threshold line according to the initial threshold line, the first cluster and the second cluster;
it should be noted that, here, a k-means clustering method in machine learning is used, and the basic idea is that: initializing k different center points { mu } (1) ，…，μ (k) And then iteratively exchanging the two different steps until convergence. Step one, each training sample is assigned to the nearest center point μ (i) The represented cluster i. Step two, each center point mu (i) Updated as all training samples x in cluster i (j) Is a mean value of (c).
It should be noted that the quantum ratio is repeatedly obtainedSpecially at |0>State or |1>The data after signal analysis is read by the quantum bit in the state, and the first statistical center point coordinate (I |0> ，Q |0> ) And second statistical center point coordinates (I |1> ，Q |1> ) Will float, but the distance between the two central coordinates, i.e
Maintaining unchanged; second, the noise level of the system is not greatly changed and can still be approximate to sigma 1 Sum sigma 2 . Finally, the distribution of the quantum bit reading result on the i-q coordinate system still obeys the two-dimensional double Gaussian statistical distribution. On the premise that the above three are true, we can still adopt the rotation transformation method as in the embodiment 2 to transform the theoretical threshold straight line into the solution of a single variable, namely:
in the form of (a). Meanwhile, on the premise that the three are established, the mathematical demonstration of I ' -I ' can be carried out ' |0> Is a constant, and the numerical value is only sum
σ 1 Sigma (sigma) 2 In relation, the horizontal coordinate difference between the optimal threshold line l and the expected coordinate is a constant c.
The proving process is as follows:
the threshold line is known to be perpendicular to the line connecting the center point coordinates. And after the rotation operation, satisfies:
then if the first statistical center point coordinate (I |0> ，Q |0> ) And second statistical center point coordinates (I |1> ，Q |1> ) Floating changes, equivalent to the need to solve for:
due to
Unchanged, therefore, after rotation transformation, there is still:
also because the purpose of the rotation transformation is to make the central point coordinate line parallel to the I-axis, i.e. Q' |0> -Q′ |1> ＝Q″ |0> -Q″ |1> =0, therefore:
i.e. |I' |0> -I′ |1> |＝|I″ |0> -I″ |1> |＝m。
Respectively comparing the first statistical center point coordinates (I |0> ，Q |0> ) And second statistical center point coordinates (I |1> ，Q |1> ) The calculation process of the equation (4) before and after the floating change:
as can be seen from formulas (a) and (b), both equations are other than I' |0> And I' |0> Except for the differences, the rest are identical (note: taking into account the realityThe constraint is that the threshold line is between two center point coordinates, the sign does not affect the process and conclusion of the solution), so the solution should be exactly the same form: i '-I' |0> ＝I″-I″ |0> 。
The proof ends.
Based on the above facts, in combination with the K-means clustering method, specifically, the solution of the updated threshold line includes the following steps:
and respectively carrying out weight-free average on all coordinate point data in the first cluster and the second cluster to obtain corresponding expected coordinates, namely a first coordinate and a second coordinate, determining a second included angle of any coordinate axis of an I-Q coordinate system between a connecting line of the first coordinate and the second coordinate, rotating the second included angle and updating all coordinate point data in the first cluster and the second cluster, the first coordinate and the second coordinate, wherein: the updated connecting line of the first coordinate and the second coordinate is parallel to an I axis;
specifically, cluster
And->
All sample coordinates in (1) are respectively weighted-free averaged to obtain corresponding expected coordinates +.>
And +.>
Obtaining a second angle theta ', wherein the sine value of the second angle theta' is as follows:
will cluster
And->
The first coordinate and the second coordinate rotate clockwise by an angle theta' in an I-Q coordinate system by taking a coordinate origin as a center, so that the aim of carrying out subsequent calculation by utilizing the important property that the difference value of the horizontal coordinate of the optimal threshold line and the expected coordinate is constant is achieved;
specific: at this time, the updated threshold line l1 is obtained
The following formula will be satisfied:
Step 70, repeating the steps with the updated threshold line as an initial threshold line: dividing the third set into two clusters, namely a first cluster and a second cluster, by using the initial threshold line, and counting n=n+1;
as with the calculation step described above, the updated threshold line is continuously obtained, and it is expected that the distances between the first center and the second center, which respectively correspond to the first cluster and the second cluster, will be smaller and smaller, that is, the performances will be more and more converged, and then the corresponding updated threshold line will be obtained more accurately.
And step 80, stopping execution until the termination condition is reached, and determining the updated threshold line as the optimal threshold line to be obtained.
Since the threshold line will always be refreshed, we can obtain the relatively more accurate threshold line we need under a limited number of iterations by the termination conditions set by the previous steps.
Through the steps, the core idea of the K-means clustering algorithm is used, the large clusters are divided into two small clusters by using the threshold line, the rotation angle is determined according to the expected coordinates of the two small clusters, the large clusters are rotated clockwise, then the threshold line is redetermined, the redetermined threshold line is used for dividing the large clusters into the two small clusters, the two small clusters are executed in sequence, and after the termination condition is met, the execution is stopped, and it is expected that the distance between the expected center coordinates of the small clusters which are re-divided by the new threshold line and the expected center coordinates of the small clusters which are divided by the previous threshold line is smaller and smaller, namely, the distance between the expected center coordinates of the small clusters which are divided by the new threshold line and the expected center coordinates of the small clusters which are divided by the previous threshold line is more and more converged, and the updated threshold line is closer to the theoretical threshold dividing line, namely, the distance is more and more accurate.
Preferably, in the step 40, a first threshold line, which is an optimal threshold line, is set as an initial threshold line, and a termination condition is set; the termination condition specifically includes:
setting a maximum execution number N, and stopping execution when n=n, wherein: the maximum execution times N are manually selected, and the numerical value of the maximum execution times N can be determined according to the actual required running time; thus, the execution time can be effectively controlled.
Preferably, in the step 40, a first threshold line, which is an optimal threshold line, is set as an initial threshold line, and a termination condition is set; the setting of the termination condition specifically includes:
setting a first threshold, wherein: the first threshold is selected according to the actually required processing precision;
and stopping execution when the maximum value of the distance between the first coordinates before and after updating and the distance between the second coordinates before and after updating is smaller than the first threshold value.
Specifically, a first threshold e is set, where: the first threshold epsilon is selected according to the actually required processing precision; when (when)
And stopping execution.
The first threshold value epsilon is determined by human, and the physical meaning of the first threshold value epsilon is that when the distance between the expected center coordinates of the small clusters after the new threshold line is re-segmented and the expected center coordinates of the small clusters after the previous threshold line is segmented is smaller than the first threshold value epsilon, the execution is stopped, and the finally obtained threshold line meets the precision requirement.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
Claims (18)
1. A method for updating a threshold line for validating a qubit quantum state, applied to a quantum chip, comprising:
obtaining a third set corresponding to a certain quantum state and an initial threshold line;
dividing the third set into two clusters, namely a first cluster and a second cluster, by using the initial threshold line;
obtaining expected coordinates corresponding to the first cluster and the second cluster, namely a first coordinate and a second coordinate respectively, determining a second included angle between a connecting line of the first coordinate and the second coordinate and a set coordinate axis of a coordinate system according to the first coordinate and the second coordinate, rotating the first coordinate and the second coordinate according to the second included angle, and updating the first coordinate and the second coordinate;
and determining an updating threshold line according to the initial threshold line and the first coordinates or the second coordinates before and after updating.
2. The method of claim 1, wherein the updated first coordinate and the second coordinate link are parallel to an I-axis.
3. The method of claim 1, wherein the determining an updated threshold line with the initial threshold line and the first or second coordinates before and after updating comprises:
the updated threshold line expression is the sum of the updated set axis coordinate of the second coordinate and the initial threshold line expression minus the set axis coordinate of the second coordinate before updating.
4. The method of claim 1, wherein obtaining the initial threshold line comprises:
obtaining coordinate point data corresponding to a plurality of quantum bit reading signals respectively on an orthogonal plane coordinate system when the quantum bit is positioned in a first quantum state, and marking the coordinate point data as a first set R |0> The method comprises the steps of carrying out a first treatment on the surface of the Obtaining coordinate point data corresponding to the plurality of quantum bit reading signals on an orthogonal plane coordinate system when the quantum bit is positioned in the second quantum state, and marking the coordinate point data as a second set R |1> The method comprises the steps of carrying out a first treatment on the surface of the Wherein: the first quantum state and the second quantum state are known quantum states and are different from each other, wherein: the orthogonal plane coordinate system is set as an I-Q coordinate system; according to the first set R |0> And said second set R |1> An initial threshold line is determined.
5. The method according to claim 4, wherein, according to the first set R |0> And said second set R |1> Determining an initial threshold line, comprising:
obtaining the first set R |0> And said second set R |1> A center point and a probability density distribution function corresponding to the center point and the probability density distribution function respectively;
the sum of the integrals of the two probability density distribution functions distributed on a set space is used as a fidelity function, wherein the set space is two spaces divided by a threshold line; the threshold line is a vertical line connecting the two central points;
and determining the corresponding threshold line as an initial threshold line when the fidelity function takes the maximum value.
6. According to claimThe method of claim 4 or 5, wherein the first set R is obtained |0> And said second set R |1> The center points corresponding respectively include:
for the first set R |0> And said second set R |1> Respectively fitting all coordinate points of the corresponding fitting graph to obtain a first statistical center point coordinate (I |0> ，Q |0> ) And a first standard deviation sigma 1 Second statistical center point coordinates (I |1> ，Q |1> ) And a second standard deviation sigma 2 。
7. The method of claim 6, wherein the fit is a gaussian fit.
8. The method according to claim 6, wherein the first set R is obtained |0> And said second set R |1> The probability density distribution functions respectively corresponding to the probability density distribution functions comprise:
according to the first statistical center point coordinates (I |0> ，Q |0> ) And the first standard deviation sigma 1 Determining a first probability density distribution function p (R) of all coordinate points of a set of quantum data in an I-Q coordinate system |0> ) According to the second statistical center point coordinates (I |1> ，Q |1> ) And the second standard deviation sigma 2 Determining a second probability density distribution function p (R) of all coordinate points of the other set of quantum data in the I-Q coordinate system |1> )。
9. The method of claim 8, wherein the first probability density distribution function p (R |0> ) And a second probability density distribution function p (R |1> ) The method comprises the following steps of:
The evaluation formula of the initial threshold line is:
10. the method according to claim 5, wherein, according to the first set R |0> And said second set R |1> Determining an initial threshold line, further comprising:
determining a first included angle between a connecting line of the two central points and any coordinate axis of a coordinate system in which the quantum data is spatially distributed according to the two central points;
rotating and updating coordinate points of the two center points in the coordinate system according to the first included angle;
and rotating and updating all coordinates of the two groups of quantum data according to the first included angle.
11. The method of claim 10, wherein rotating and updating the coordinate points of the two center points in the coordinate system according to the first included angle comprises:
determining the first included angle as an I-axis included angle between the connecting line of the two center points and an I-Q coordinate system; the two center points are respectively the first statistical center point coordinates (I |0> ，Q |0> ) And a second statistical center point coordinate (I |1> ，Q |1> )；
Clockwise rotating the two center points according to the first included angle;
updating the first statistical center point coordinates (I |0> ，Q |0> ) And the second statistical center point coordinates (I |1> ，Q |1> ) Respectively (I' |0> ，Q′ |0> )、(I′ |1> ，Q′ |1> ) Wherein: q'. |0> ＝Q′ |1> 。
12. The method according to claim 11, wherein when the updated first statistical center point coordinates (I' |0> ，Q′ |0> ) And updated said second statistical center point coordinates (I' |1> ，Q′ |1> ) When the longitudinal axes of (2) are equal, the threshold line is a vertical threshold line perpendicular to the I-axis;
determining that the threshold line corresponding to the fidelity function taking the maximum value is the optimal threshold line comprises the following steps:
when the space to the right of the vertical threshold line is the updated first statistical center point coordinate (I' |0> ，Q′ |0> ) The space at the left side of the vertical threshold line is the second statistical center point coordinate (I' |1> ，Q′ |1> ) If the space is located, determining that the threshold line corresponding to the maximum value of the fidelity function is an optimal threshold line, otherwise, determining that the threshold line corresponding to the minimum value of the fidelity function is an optimal threshold line; wherein: the sum of the maximum value of the fidelity function and the minimum value of the fidelity function is 1.
13. The method according to claim 10 or 11, wherein the sine value of the first angle θ is:
14. the method according to claim 10 or 11, characterized in that the evaluation formula of the initial threshold line is converted by simplification into the following formula:
and (3) making:
wherein: g l′ The functional image of (I') is monotonically decreasing and intersects the I-axis, then solving equation (3) can be converted into solving the following equation:
obtaining the solution of formula (4) as a real solution I '=a, and obtaining the expression of the optimal threshold line l' is: i' =a.
15. A method according to any one of claims 1-3, wherein obtaining the desired coordinates for the first cluster and the second cluster comprises:
and respectively carrying out non-weighted average on all coordinate point data in the first cluster and the second cluster to obtain corresponding expected coordinates.
16. The method of claim 15, wherein determining an updated threshold line with the initial threshold line and the second coordinates before and after updating further comprises:
setting a termination condition, and repeatedly executing the steps by taking the updated threshold line as an initial threshold line: dividing the third set into two clusters by using the initial threshold line, wherein the two clusters are respectively updated first clusters and second clusters; counting n=n+1;
respectively carrying out weight-free averaging on all coordinate point data in the updated first cluster and the updated second cluster to obtain corresponding expected coordinates, namely a first coordinate and a second coordinate, determining a second included angle between a connecting line of the first coordinate and the second coordinate and a set coordinate axis of a coordinate system according to the first coordinate and the second coordinate, rotating the first coordinate and the second coordinate by the second included angle, and updating the first coordinate and the second coordinate again;
determining an updating threshold line according to the initial threshold line and the first coordinates or the second coordinates before and after updating;
stopping execution until the termination condition is reached, and determining the updated threshold line as the optimal threshold line to be obtained.
17. The method of claim 16, wherein the setting of the termination condition comprises:
setting a maximum execution number N, and stopping execution when n=n, wherein: the maximum execution number N is manually selected.
18. The method of claim 16, wherein the setting of the termination condition comprises:
setting a first threshold, wherein: the first threshold is selected according to the actually required processing precision;
and stopping execution when the maximum value of the distance between the first coordinates before and after updating and the distance between the second coordinates before and after updating is smaller than the first threshold value.
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