QuBit is a library to support Quantum Superpositions in C++. This allows each CQuBit variable to hold a number of different values at the same time. Normal mathematical operations can be performed on a QuBit, but they affect every value in the QuBit, at the same time. Binary Operations on two QuBits cause a result based on every possible outcome.

Watch the lecture at FOSDEM 2010

Example of QuBit

a = { 1, 2, 3 };b = a + 10;b = { 11, 12, 13 };
a = { 2, 4, 6 }; b = { 10 ,100 ,1000 };c = a * b;c = { 20, 40, 60, 200, 400, 600, 2000, 4000, 6000 };
This is pseudo-code for demonstrative purposes only! The C++ comes later!

Each value can only exist in a QuBit once. There is no 'weighting' applied if a value is added several times.

Comparisons with a QuBit also occur with all the values at the same time. This means a search is simply,

qb = (1,2,3,4,5);
if qb eq 5 print "There's a '5' in the QuBit, somewhere!"

Quantum Stuff

The above examples demonstrate the basic functionality of QuBit. It's not significantly more complex than 'sets'. However, there are a few functions that take the extra step into 'gaga' land!

A QuBit can be in three possible states:

  • Disjunctive
  • Conjunctive
  • Collapsed


All states are considered when performing arithmetic (at the same time, remember!), but this not necessarily true of comparisons. A disjunctive QuBit will consider that if any of the individual comparisons evaluate to 'true', the whole expression will evaluate to 'true'. In psuedo-code:

if any{10,15,20,25} < 20 then print "At least one of these numbers is lower than 20"


Here, a comparison will evaluate to 'true' if all it's individual values evaluate to 'true'. In psuedo-code:

if all{10,15,20,25} < 20 then print "All of these numbers are lower than 20"


This occurs ofter a comparison has been made. The QuBit has exhausted all it's superpositions to produce the result, and is consequently empty. It does, however, contain a 'true' or 'false' value, dependent on the result. (You'll see how this boolean result is obtained later)


A collapsed quantum state is useful - but not that useful! It might be useful to know that there is (at least) one number less than 20 in a particular case, but it would be more useful to know which numbers are less than 20. To find this information, we use the eigenstates of the QuBit. Pseudo-code, anyone?

v = any{10,15,20,25} < 20
if v then print "Those numbers less than 20 are " eigenstates(v)

Declaring and Using a QuBit

We now turn to the actual C++ code require to perform these Quantum miracles! QuBit is, itself, a template and is therefore created with:

CQuBit<float> q;

This declares a Quantum superposition called 'q', consisting of floating point numbers. To add or remove values to this variable, you may use the following functions:

 q.Add(10); // Add '10'
 q.Add(2); // Add '2'
 q.AddRange(100,200); // Add every number from '100' to '200', inclusive
 q.AddRange(10,11,0.1f); // Add every number from '10' to '11' in steps of '0.1'
 q.Remove(10.5f); // Remove the number '10.5'

You can also construct a QuBit, using the ranged constructor,
CQuBit<float> q(100,200); // exactly the same as example three above

Use of QuBit is the same as any other variable. Since all (see note below) operators are overloaded you can write natural looking code, such as 'a+b', or 'a/=2'. You can use binary operations (such as additional or multiplication) with either one CQuBit and one float, or two CQuBits, as you see fit. Note restrictions.

Quantum-Orientated Member Functions

Having created a quantum superposition (with either Add or AddRange), it is now time to use it. Looking at the examples above, we can find if any value is less than 20 with this code.

CQuBit<float> q(10,25,5);
CQuBit<float> ans;

 ans = q.Any() < 20;
 if (ans.GetBoolResult()) cout << "At least one of these numbers is lower than 20";

The QuBit can also be treated as a disjunctive with the code q.Any();, allowing you to re-write the above thus:

CQuBit<float> q(10,25,5);
CQuBit<float> ans;

 ans = q < 20;
 if (ans.GetBoolResult()) cout << "At least one of these numbers is lower than 20";

Although the results of the comparison (i.e. q < 20) is boolean, the C++ version of the library forces the result type to be a CQuBit. You then determine whether it collapsed to 'true' or 'false' with the 'GetBoolResult()' member function. This is to allow retrieval of the eigenstates (if only a bool were returned this information would get lost). Using eigenstatees to find those values less than 20, one can use:

CQuBit<float> q(10,25,5);
CQuBit<float> ans;

 ans = q < 20;
 if (ans.GetBoolResult()) cout << "These numbers are lower than 20: " << ans.Eigenstates();

The Eigenstates function returns a QuBit itself, so could be applied to further calculations.

Note:Eigenstates could have been implemented in a similar fashion to PERL, with a cache storing the result of the last comparison operation. However, as it didn't feel very 'C' to be using global variables in this manner, so I adopted a QuBit return type, and 'GetBoolResult' to retrieve the comparisons' result. (Technically speaking this might also be considered storing a global!)

All is used in exactly the same manner as above. Only the results differ (obviously!).

CQuBit<float> q(10,25,5);
CQuBit<float> ans;

 ans = q < 20;
 if (ans.GetBoolResult()) cout << "All of these numbers are lower than 20: ";


The output stream produces space delimitted list of the states, enclosed in braces.

CQuBit<float> q;
 cout << q;
produces { 15 16 17 18 19 20 }

The input stream, will read this format into the QuBit, and leave the stream pointer immediately after the closing brace.

Set-Orientated Member Functions

The Any and All methods have another function: set manipulation. Recall that QuBits is basically an implementation of set handling. Also recall, that Any is disjunctive. So, if any of {1,2,3} is combined with any of {4,5,6}, the result is a union; {1,2,3,4,5,6}. Written in code, this would be:

CQuBit<float> q1(1,3);
CQuBit<float> q2(4,6);
CQuBit<float> ans;

 ans.Any(q1, q2); // The result is {1,2,3,4,5,6}

Note: QuBit ignores the type of original quantum states. So in this case, q1 and q2 could be conjunctive or disjunctive, the answer would be identical in both cases. All collapsed QuBits are ignored

Well, Any is union, it should be obvious what All (the conjunctive state) should be! One brownie point to everyone who said 'intersection'. The resultant QuBit features only those values present in both of the original superpositions.

CQuBit<float> q1(1,6);
CQuBit<float> q2(4,5);
CQuBit<float> ans;

 ans.All(q1, q2); // The result is {4,5}

Sample Code

Here are a number of small examples (all available for download in the zip file below). I have given the PERL code also, to show linage, as well as to provide a parallel for those familar with the language, or the Quantum::Superpositions library available.

FunctionPERLC++ (using QuBit)
mineigenstates(any() <= all())(ql.Any() <= ql.All()).Eigenstates()
maxeigenstates(any() >= all())(ql.Any() >= ql.All()).Eigenstates()
IsPrime==2 || % all(2..sqrt()+1) != 0 if (i==2) return true;
prime.AddRange(2, sqrt(i)+1);
ans = (i % prime.All()) != 0;
return ans.GetBoolResult();

factor = / any(2..-1);
CQuBit<float> n, q;

n.AddRange(2, i-1);
q = i/n.Any();
q = q.Floor()==q;
return q.Eigenstates();


QuBits are only concerned with values. Therefore, the return value from post and pre decrement operators are identical. i.e. a++ and ++a, are the same as a++. This doesn't mean that a++ and ++a are different, just that the return type from a++ is a value (as it should be), but ++a is not a reference to the a variable, as you might expect.

a = {1,2,3}b = a++;a = {2,3,4}; b = {1,2,3};
a = {1,2,3}b = ++a;a = {2,3,4}; b = {2,3,4};

Some compilers (noticable Microsoft's!) refuse to compile this code as is. To help those poor unfortunates, I've removed a couple of the friend functions with an #if 0. Namely,

  • friend CQuBit<_T> operator+(const _T &a, const CQuBit<_T> &b)
  • friend CQuBit<_T> operator*(const _T &a, const CQuBit<_T> &b)
  • friend CQuBit<_T> operator-(const _T &a, const CQuBit<_T> &b)

These prevent compilation of code such as, q2 = 5 * q1. This means you'll have to re-write your code by swapping the arguments over, or by creating a QuBit containing one entry. The code works, however.

The set of operators catered for are:

++ (pre and post)-- (pre and post)<<=>>===!= 
Notes: >> & << are also handled for streams.
The array operator ([]) is overloaded to retrieve individual state values, i.e. if a={10,20,30}, then a[0]=10, a[1]=20 and a[2]=30.


The full source to QuBit is still available!