Mismatched dimensions in a tensor product with CNOT

In summary, the conversation is about the application of a CNOT gate to a state α|0>|00> + β|1>|00> in Nielson and Chuang's Quantum Computation book. The result is α|0>|00> + β|1>|10>. The person is using the drudge method and encounters a problem due to the mismatch in dimensions between the 4x4 CNOT matrix and the 8x1 vector. They realize that the solution is to use the tensor product of CNOT with the identity matrix for one of the particles and the identity matrix with CNOT for the other one.
  • #1
nomadreid
Gold Member
1,670
204
I am working through an explanation in Nielson and Chuang's Quantum Computation book where they apply a CNOT gate to a state α|0>|00> + β|1>|00>. (The notation here is |0> = the column vector (1,0) and |1>=(0,1), while |00> = |0>|0>, and |a>|b>=|a>⊗|b>, ⊗ being the tensor (outer) product. I am ignoring the constant factor here.) The result is α|0>|00> + β|1>|10>.
But I am working this through via the drudge method, that is, converting everything to old-fashioned matrix formulation, and I run into a problem: the CNOT matrix is a 4x4 matrix, and α|0>|00> + β|1>|00> is a 8 x 1 vector. Using ordinary matrix multiplication, this is a mismatch in dimensions. It appears to me that they are saying that
CNOT (α|0>|00>)= α|0>[CNOT (|00>)]. I am confused.
 
  • #3
I believe I have the missing link now. Instead of purely the CNOT matrix, I need the tensor product of CNOT with the identity (4 x 4) matrix for one of the particles, and the identity tensor product with CNOT for the other one.
 

1. What is a tensor product?

A tensor product is a mathematical operation that combines two or more vectors or matrices to create a new vector or matrix. It is commonly used in quantum mechanics to represent the state of a quantum system.

2. How does a tensor product result in mismatched dimensions?

When performing a tensor product on two vectors or matrices, the resulting vector or matrix will have dimensions equal to the product of the dimensions of the original vectors or matrices. If the dimensions of the original vectors or matrices do not match, the resulting tensor product will have mismatched dimensions.

3. What is a CNOT gate in quantum computing?

A CNOT (controlled-NOT) gate is a two-qubit gate in quantum computing that applies a NOT operation on the target qubit if and only if the control qubit is in the |1> state. It is commonly used in quantum logic gates and quantum error correction codes.

4. How does a CNOT gate affect tensor products with mismatched dimensions?

When a CNOT gate is applied to a tensor product with mismatched dimensions, the resulting tensor product will also have mismatched dimensions. This can cause errors in quantum algorithms and must be carefully accounted for in quantum computing operations.

5. How can mismatched dimensions in a tensor product with CNOT be resolved?

To resolve mismatched dimensions in a tensor product with CNOT, additional qubits can be added to the system to ensure that all the dimensions match. Another approach is to use different types of quantum gates, such as Toffoli or Fredkin gates, that do not result in mismatched dimensions when applied to tensor products.

Similar threads

  • Linear and Abstract Algebra
Replies
4
Views
2K
  • Advanced Physics Homework Help
Replies
1
Views
1K
  • Linear and Abstract Algebra
Replies
2
Views
928
  • Linear and Abstract Algebra
Replies
1
Views
824
  • Linear and Abstract Algebra
Replies
4
Views
881
  • Quantum Physics
Replies
1
Views
954
  • Advanced Physics Homework Help
Replies
1
Views
781
  • Quantum Physics
Replies
6
Views
2K
  • Linear and Abstract Algebra
Replies
1
Views
3K
  • Advanced Physics Homework Help
Replies
7
Views
908
Back
Top