April 23, 2019
      
                  
        
  Author(s)
  Emanuel H. Knill,   James R. van Meter
 
       
            
    
    
        The three-spin-1/2 decoherence-free subsystem defines a logical qubit protected from collective noise and supports exchange-only universal gates. Such logical qubits are well-suited for implementation with electrically-defined quantum dots. Exact exchange