Building the Future with Reversible Circuits: A Quantum Perspective
- Quantum Valley
- Apr 23
- 3 min read
In a world where classical computing is approaching its physical limits, quantum computing isn’t just a promising frontier—it’s a necessity. In my recent talk we Dr. Gandhi, we delved into one of the foundational concepts that makes quantum computing so powerful: reversible circuits. Let’s break down why reversibility matters and how it’s reshaping the architecture of computation through quantum gates and IBM’s Qiskit platform.
Why Reversible Circuits?
Classical logic gates like AND and OR are irreversible—once the output is produced, you can't uniquely determine the inputs. This irreversibility leads to information loss, which, as Landauer and Feynman suggested back in 1972, directly translates to energy loss in the form of heat.
Quantum computing, however, operates entirely through reversible circuits. Every quantum operation preserves information. This one-to-one mapping between input and output states means that quantum circuits—unlike classical ones—can, in theory, perform computation with zero power dissipation.
“Our circuits are reversible in nature… such circuits will have zero power dissipation.” —Dr. Gandhi
From Bits to Qubits
Classical bits are binary—they're either 0 or 1. Quantum bits (qubits) can be in a superposition of both 0 and 1. This is described by the quantum state:
where ∣c1∣2+∣c2∣2=1|c_1|^2 + |c_2|^2 = 1∣c1∣2+∣c2∣2=1.
These coefficients represent the probability of measuring the qubit in each state.
Superposition allows quantum computers to represent and compute on 2ⁿ states simultaneously—a phenomenon known as quantum parallelism.
Visualizing Quantum States
Qiskit, IBM’s open-source SDK for quantum computing, offers several tools to visualize quantum states:
Bloch Sphere: 3D representation of a single qubit.
Q-sphere: A more scalable 2D visualization that captures phase and multi-qubit states.
Statevector and probability histograms: Show outcomes after measurement.
Essential Quantum Gates
Dr. Gandhi’s session highlighted several single and multi-qubit gates:
Single Qubit: X, Y, Z, H (Hadamard), S, T, and rotation gates.
Multi-Qubit:
CNOT (Controlled-NOT): Flips the target if the control is 1.
Toffoli (CCX): Universal for classical computation.
Fredkin (CSWAP): Swaps qubits conditionally.
CZ: Applies a Z gate if the control is 1.
These gates allow you to build classical logic circuits (like AND, OR, XOR) in a reversible quantum form, providing a bridge between classical and quantum logic.
Deutsch to Shor: Quantum Algorithms in Action
The presentation also connected these building blocks to quantum algorithms:
Deutsch’s Algorithm: Showed early quantum speedup.
Shor’s Algorithm: Efficient integer factorization—breaking RSA.
Grover’s Algorithm: Speeds up database search from O(N) to O(√N).
Hands-On with IBM Qiskit Composer
Qiskit Composer allows drag-and-drop circuit building, automatic OpenQASM generation, and real-time visualization. Dr. Gandhi demonstrated how to:
Define qubits and classical bits.
Add gates.
Run simulations.
View output probabilities and state vectors.
Final Thoughts
Dr. Gandhi’s presentation wasn’t just a technical deep dive—it was a call to action. As quantum hardware and software evolve, understanding the reversible, low-energy, and parallel nature of quantum computing will be key to shaping tomorrow’s digital systems. If you're new to quantum computing, Qiskit offers a beginner-friendly way to explore. If you're experienced, reversible circuits open the door to optimization and innovation.
Ready to build the future? Start with a qubit.
Here are the slides from the talk: https://github.com/taposh/talks/blob/master/Building%20Reversible%20Circuits%20with%20Quantum%20Gates.pdf
Taposh Roy is the founder of Silicon Valley Quantum Computing group, director of the innovation & AI team at Kaiser Permanente and part of Harvard Medical School’s AI Bioethics program. These thoughts and recommendations are not from Kaiser Permanente, and Kaiser Permanente is not responsible for the content. He is the author of the books “Intelligent Governance” and “Medical Image Processing: with Deep Learning”. If you have questions Mr. Dutta Roy can be reached via linkedin
